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Effect of multiphase flow on natural gas hydrate production in marine sediment
Journal Pre-proof Effect of multiphase flow on natural gas hydrate production in marine sediment Huiru Sun, Bingbing Chen, Mingjun Yang PII:
S1875-5100(19)30318-X
DOI:
https://doi.org/10.1016/j.jngse.2019.103066
Reference:
JNGSE 103066
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 10 May 2019 Revised Date:
30 July 2019
Accepted Date: 9 November 2019
Please cite this article as: Sun, H., Chen, B., Yang, M., Effect of multiphase flow on natural gas hydrate production in marine sediment, Journal of Natural Gas Science & Engineering, https://doi.org/10.1016/ j.jngse.2019.103066. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Effect of multiphase flow on natural gas hydrate production in marine sediment
2
Huiru Sun, Bingbing Chen, Mingjun Yang*
3
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of
4
Education, Dalian University of Technology, Dalian 116024, China
5 6
Abstract Natural gas hydrates (NGHs), regarded as an alternative future energy
7
source. Currently, tests for hydrate exploitation from marine sediment have been
8
performed in the Nankai Trough of Japan and the Shenhu area of the South China Sea.
9
Hydrate exploitation is influenced by water-gas flow in the sediment, and considering
10
the huge seawater reserves in hydrate accumulation areas, an experiment of
11
seawater-gas flow was performed to dissociate hydrate. The effects of seawater-gas
12
flow rates and initial hydrate saturation on methane hydrate (MH) production was
13
analyzed. The results showed that seawater-gas flow efficiently promotes hydrate
14
dissociation and inhibits hydrate reformation. Moreover, there was a faster heat and
15
mass transfer with increasing seawater flow rates and decreasing gas flow rates,
16
which enhanced the average MH dissociation rate. In addition, the variation time of
17
the flow channel increased with higher initial hydrate saturation. Additionally,
18
seawater-gas flow promotes MH dissociation stronger than deionized water-gas flow.
19
Keywords: methane hydrate dissociation; water flow erosion; seawater-gas flow; salt
20
ions; hydrate saturation
1
21
1. Introduction
22
Natural gas hydrates (NGHs) are typically found in continental permafrost and
23
marine sediment and they form from hydrocarbon gas and water under high pressure
24
and low temperature (Hiratsuka et al., 2015, Gao et al., 2018). The water molecules
25
interconnected through hydrogen bonding form cage structures to encage gas
26
molecules (Zheng et al., 2019, Saman et al., 2010, Liu et al., 2018). There are rich
27
natural gas hydrate reservoirs distributed around the world, in which the estimated
28
energy potential is far greater than the total amount of conventional coal, oil and
29
natural gas (Wang et al., 2017). NGHs are seen as potential sources of natural gas in
30
view of their high energy density, abundant reserves and nonpollution (Zhao et al.,
31
2014, Xu and Li, 2015). Therefore, researching hydrate formation and dissociation
32
kinetics and developing methods for natural gas recovery from hydrate reservoirs are
33
arousing wide concern in countries and among scientists all over the world in recent
34
years (Wang et al., 2016b, Song et al., 2015b).
35
For now, methods for large-scale production of natural gas from NGHs are still
36
evolving, and they are all based on changing reservoir pressure and temperature to
37
deviate from hydrate thermodynamic equilibrium (Ji et al., 2001). There are four
38
common methods for gas hydrate exploitation as follows. The depressurization
39
method, in which the reservoir pressure is decreased below the hydrate equilibrium
40
pressure at a specified temperature (Reagan et al., 2015, Li et al., 2012, Yang et al.,
41
2019b). The thermal stimulation method, in which the reservoir temperature is 2
42
increased above the hydrate equilibrium temperature via injecting hot water, hot brine
43
or steam (Song et al., 2015a, Nair et al., 2016, Chong et al., 2016b). The chemical
44
inhibitor stimulation method, in which chemicals are injected into the hydrate
45
reservoir to transform the hydrate pressure-temperature equilibrium and promote
46
hydrate dissociation (Li et al., 2011, Chong et al., 2016a). The CO2 replacement
47
method, in which liquid CO2 is injected into hydrate reservoirs to form CO2 hydrate
48
and replace methane gas (Yu et al., 2015, Brewer et al., 2014). The above methods all
49
have their own merits and demerits. The large heat loss of thermal stimulation, the
50
expensive cost and pollution environment of chemical inhibitors, and the low
51
efficiency of CO2 replacement restricts application of those three methods for
52
practical hydrate exploitation (Fitzgerald et al., 2014, Lee et al., 2010, Jung et al.,
53
2010). However, depressurization needs no external energy supply and energy
54
consumption is the least, so that was regarded as the most effective method to recover
55
natural gas from NGH reservoirs considering the economic cost (Chong et al., 2017a,
56
Zhao et al., 2014).
57
The depressurization method was first used for field-scale hydrate production at
58
the Mackenzie Delta Mallik site in the Northwest Territories, Canada, and then a
59
larger-scale test was conducted at the same site in 2007 and 2008 (Yang et al., 2016).
60
However, the hydrate dissociation is an endothermic reaction. When the pressure drop
61
is large, the sensible heat of the hydrate and heat transferred from the ambient
62
environment was not enough to sustain the faster hydrate dissociation reaction. So, 3
63
there may occur the hydrate reformation and ice generation in depressurization
64
process, which will obviously influent the gas production rate (Wang et al., 2018,
65
Zhao et al., 2015, Chen et al., 2015). Therefore, many experimental studies and
66
numerical modeling of hydrate dissociation and gas production using depressurization
67
were conducted. Konno et al. (Konno et al., 2014) performed the gas production test
68
on methane hydrate sediments using one-step and multistep depressurization. They
69
found that a suitable heat for hydrate-bearing sediment is the important factor to drive
70
hydrate dissociation. Zhan et al. (Zhan et al., 2018) used different quartz sand particle
71
sizes to simulate hydrate reservoirs, form methane hydrate, and dissociate hydrate via
72
the depressurization method. They analyzed and compared the effects of sediment
73
particle size on gas production rate and dissociation characteristics. Zhao et al. (Zhao
74
et al., 2012) conducted numerical simulations to investigate the water phase impact on
75
methane hydrate dissociation by depressurization in porous media. Their experimental
76
results showed that the movement of water plays a significant role in late stage
77
thermal conduction, and the gas production rate increased with higher water saturation
78
in late stage dissociation. Based on the available data in the Shenhu Area of the South
79
China Sea, Xiong et al. (Xiong et al., 2012) employed a 1D experimental setup to
80
study methane hydrate dissociation by depressurization, and they found that lower
81
dissociation pressure may lead to higher hydrate dissociation rate and higher
82
dissociation heat. Oyama et al. (Oyama et al., 2012) studied the depressurized
83
dissociation of natural methane-hydrate-bearing sediment with low permeability, and 4
84
they assessed two dissociation models making use of their experimental results. To
85
acquire kinetic hydrate dissociation data under different backpressures, depressurized
86
dissociation of methane hydrate was studied using magnetic resonance imaging (MRI)
87
by Wang et al. (Wang et al., 2015). For the other three hydrate dissociation methods,
88
Su et al. (Su et al., 2013) investigated the gas production potential from hydrate
89
sediment using thermal stimulation. They found that thermal stimulation was effective
90
at promoting gas release from hydrate sediment and enhanced gas production in the
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late stage of hydrate dissociation. Li et al. (Li et al., 2017) injected methanol into
92
methane-hydrate-bearing sediment to study dissociation behavior employing a
93
one-dimensional experimental device. They proposed new adjustment tactics for
94
experimental operation according to the injection rate ratio of methanol and water.
95
Jung et al. (Jung and Santamarina, 2010) measured the electrical resistance change
96
and relative stiffness in the CH4-CO2 replacement process, and found that the overall
97
hydrate remained solid and gas-hydrate-water phases coexisted for long periods of
98
time. Yuan et al. (Yuan et al., 2013) used a three-dimensional reactor to study
99
CO2-CH4 replacement in hydrate-bearing deposits with liquid CO2, and analyzed the
100
influence of different stratum conditions on the replacement process.
101
Whatever NGH exploitation methods are adopted, hydrate dissociation is always
102
accompanied by production and migration of water and gas. Chen et al. (Chen et al.,
103
2019b) proposed a water flow erosion method to promote hydrate dissociation and
104
investigated the effects of seawater flow on natural gas hydrate dissociation (Chen et 5
105
al., 2019a), whereas the effect of gas migration on hydrate dissociation was ignored.
106
In addition, deionized water-gas two-phase flow promotion of MH dissociation has
107
been studied, and hydrate reformation may appear under slower water-gas flow rates.
108
Therefore, based on those studies and considering the effect of seawater on hydrate
109
thermodynamic equilibrium, experiments of seawater-gas two-phase flow were
110
performed, and different seawater-gas flow rates effect on MH dissociation were
111
analyzed. In addition, the system pressure and temperature are always in the
112
thermodynamic stability region of the hydrate-bearing sediment core. Due to the huge
113
seawater reserves in hydrate accumulation areas, the method of seawater-gas flow to
114
dissociate hydrate is achievable, so the experimental results offer some reference and
115
guidance for future NGH exploration in marine environments.
116
2. Experiments
117
2.1 Apparatus and materials
118
Fig. 1 shows a sketch of the experimental setup designed to investigate MH
119
dissociation during seawater-gas two-phase flow. The experimental setup included an
120
MRI system (Varian, Inc., Palo Alto, CA, USA), a high-pressure vessel (polyimide),
121
three high-precision injection pumps (260D, Teledyne Isco Inc., Lincoln, NE, USA),
122
three thermostat baths (FL300 and FL 25, JULABO, Seelbach, Germany), two pressure
123
transducers (3510CF, Emerson Electric Co., Ltd., St. Louis, USA), a differential
124
pressure sensor (Nagano Keiki, Japan), and a data acquisition system. The MRI system
125
was used to visualize the change of MH distribution during seawater-gas flow, which 6
126
was operated at 400 MHz resonance frequency and 9.4 T magnet field. This MRI
127
system adopted a spin echo sequence to obtain two-dimensional proton
128
density-weighted images. The parameters of the sequence are as follows, which based
129
on Chen et al. (Chen et al., 2019a). Echo time was 4.39 ms, image data matrix was
130
128×128, and the field of view was 30 mm×30 mm (Chen et al., 2019a). Three injection
131
pumps were used to inject liquid, CH4 gas and to control the vessel pressure for
132
seawater-gas flow. The effective volume of injection pump was 260 ml, the accuracy of
133
flow control was ±1 µl·min-1, and accuracy of pressure control was ± 0.5%. The
134
temperatures of the pumps and MRI system were strictly controlled by thermostat baths,
135
and the stability of the thermostat baths was ±0.1
136
differential pressure sensor were used to collect pressure signals in the inlet and outlet
137
of the high-pressure vessel. The accuracy of the pressure transducers was ±0.05%, and
138
the accuracy of the differential pressure sensor was ±0.01%. The data acquisition
139
system included an A/D module (Advantech Co., Ltd., Milpitas, CA, USA) and a
140
computer, which was used to collect and dispose the pressure signals. Deionized water
141
and CH4 gas were used to form methane hydrate (MH). Seawater (configured by
142
deionized water) and CH4 gas were used in the MH dissociation process.
7
. Pressure transducers and a
143 144 145
Fig. 1 Schematic of the experimental system 2.2 Hydrate formation
146
A high-pressure vessel with inner diameter of 15 mm and length of 200 mm was
147
washed, dried, and filled with BZ-02 glass beads (As-One Co., Ltd., Japan). The
148
porosity of BZ-02 glass beads was 35.4%, the particle diameter was 0.177-0.250 mm
149
and the density of BZ-02 was 2.6 g·cm-3. After the leakage test, the vessel was
150
mounted in the center of the MRI magnetic body and vacuumed to expel free gas.
151
Next, the vessel was pressurized to 6000 kPa using water injection pump with a
152
constant injection rate of 20 ml·min-1 to fully saturate the porous media. After
153
maintaining the pressure stable for 1 h, the outlet valve was opened, and the vessel
154
pressure equaled to atmospheric pressure. Then, CH4 gas (Dalian Special Gases Co.,
155
Ltd., China, 99.99%) with 4000 kPa pressure was injected into the vessel to displace
156
the deionized water and obtain partly water-saturated porous media. Then, the vessel
157
was pressurized to 6000 kPa by CH4 gas injection with a constant injection rate of 20
8
158
ml·min-1 and maintained stable to form MH. During the whole process of hydrate
159
formation, the thermostat bath temperature was kept at 274.15 K. Moreover, the
160
images were obtained continuously via MRI for each experiment.
161
2.3 Seawater-gas two-phase flow process
162
The experimental parameters and results of twelve experimental cases are shown
163
in Table 1. Take Case 1 as an example to illustrate the seawater-gas two-phase flow
164
process. The seawater solution (NaCl solution with a mass fraction of 3.5%) was
165
prepared before the experiment. After hydrate formation finished, the outlet valve was
166
opened to interconnect backpressure pump to the vessel. Then, the pressure of
167
backpressure pump was decreased to 3200 kPa form 6000 kPa using backpressure
168
pump with a constant rate of -20 ml·min-1, and maintained that pressure for a while to
169
ensure that no MH dissociation appeared in the pressure adjustment process. When
170
the temperature is 274.15 K, the MH phase equilibrium pressure is 2850 kPa (Jr and
171
Koh, 2007). Therefore, the hydrate was always thermodynamically stable during the
172
backpressure adjustment process. Next, seawater (1ml·min-1) and CH4 (1ml·min-1)
173
were simultaneously injected into the vessel from the bottom to the top by seawater
174
injection pump and gas injection pump. The temperature of the flowing seawater and
175
CH4 gas was controlled at 273.95 K by thermostat bath to prevent hydrate dissociation
176
caused by temperature variation. Images of seawater-gas flow were recorded in
177
succession using an MRI system. In addition, after the seawater-gas flow process,
178
deionized water was used to wash the whole experimental system (pipelines, seawater 9
179
injection pump and high-pressure vessel) approximately 5~6 times till the
180
conductivity was lower than 0.69 µS·cm-1, which avoided residual seawater inhibiting
181
hydrate formation. The conductivities of seawater and deionized water were 2.85*104
182
µS·cm-1 and 0.69 µS·cm-1, respectively(Chen et al., 2019a).
183
Table 1. Experimental parameters and results Case 1 2 3 4 5 6 7 8 9 10 11 12
184 185 186 187 188 189 190
vw (ml/min) 1 2 3 3 4 10 5 5 5 5 5 5
vg (ml/min) 1 2 3 1 1 1 1 1 1 1 1 1
Pi (kPa) 6010 6015 6020 6010 6000 6005 6020 6011 6040 6060 6030 6050
Shi (%) 24.33 23.03 22.42 23.83 23.77 22.56 15.48 22.50 28.19 31.08 36.23 41.48
tapp (min) 80 46 38 24 14 10 8 12 14 14 20 26
tdis (min) 986 532 290 136 88 42 50 68 100 108 112 132
RV (%/min) 0.0269 0.0474 0.0890 0.2128 0.3212 0.7050 0.3686 0.4018 0.3278 0.3306 0.3938 0.3913
a
The symbols in this table are defined as follows: tapp is the flow channel appearance time; tdis is the flow channel disappearance time; Vw is the seawater flow rate during seawater-gas two-phase flow; Vg is the gas flow rate during seawater-gas two-phase flow; Shi is the initial MH saturation before hydrate dissociation (volume fraction); Pi is the MH formation pressure; and Rv is the average MH dissociation rate. All experiments were carried out at 274.15 K, the temperature of flowing seawater and gas was kept at 273.95 K, and the designed backpressure was 3200 kPa for all cases during the seawater-gas flow process.
191
In this study, the average MH dissociation rate was calculated by the flowing
192
equation (Wang et al., 2018), which can reflect the average water and gas production
193
rates.
194
Ri =
S hi − S h (i + ∆t )
(1)
∆t
10
195
where
196
and
197
the two changes of hydrate saturation.
198
3. Results and Discussion
S
Ri
is the MH dissociation rate at i min,
h i + ∆ t
S hi
is the hydrate saturation at i min
are the hydrate saturation at i + ∆t min. ∆t is the time interval between
199
Twelve experimental cases were performed to study the effects of different
200
seawater-gas flow rates and different initial hydrate saturations on MH dissociation.
201
The seawater-gas flow rate was expressed as the seawater flow rate-gas flow rate in
202
this study. In Cases 1~6, the MH saturation was controlled approximately the same
203
(22.5%~23.5%), and the seawater-gas flow rate of that was set to 1-1, 2-2, 3-3, 3-1,
204
4-1, and 10-1 ml·min-1, respectively. In Cases 7~12, the seawater-gas flow rates were
205
all set to 5-1 ml·min-1, and the MH saturation was controlled to approximately 15%,
206
23%, 28%, 31%, 36%, and 41%, respectively. The backpressure was controlled at
207
3200 kPa for all cases, and the temperature of flowing seawater and gas was set to
208
273.95
209
thermodynamically stable. The distribution of water and MH was recorded by the
210
MRI system.
211
3.1 Methane hydrate formation
K,
which
ensured
that
the
hydrate-bearing
sediment
core
was
212
The pressure and temperature were set to 6000 kPa and 274.15 K, and were kept
213
constant for the whole MH formation process for Cases 1~12. MH saturation,
214
expressed as the volume fraction of pore space occupied by hydrate, was computed by
215
mean intensity (MI) data acquired form MRI images (Chen et al., 2019a). Because 1 11
216
m3 of hydrate releases 0.8 m3 of liquid water when it is dissociated, and 1 volume of
217
liquid water forms 1.25 volumes of hydrate under standard temperature and pressure
218
(STP) (Jr and Koh, 2007). Hence, the MH saturation at the “i” minute can be
219
computed with the following equation (Chen et al., 2019b):
220
S hi = 1.25 ×
221
where
222
values in the images at t = 0 and t = i minutes, respectively. The details of hydrate
223
saturation for each case are shown in Table 1.
(I 0 − I i )× S w0 × 100 %
S w0
(2)
I0
represents the initial water saturation, and
I0
and
Ii
represent the MI
224
Because of a similar phenomenon, take Case 2 as an example to illustrate the
225
MH formation process. Fig. 2 shows the MI variation during MH formation process,
226
which reflected the amount of free water in porous media. Moreover, three MRI
227
images were used to visually show the liquid water distribution during different stages
228
of MH formation. Bright red and dark purple represent the brightest (more water) and
229
darkest (more hydrate) signals, respectively. As shown in Fig. 2, the MI was stable at
230
stage I and liquid water distribution in the MRI image was uniform, indicating there
231
was no hydrate formation and water saturation remained invariant. At stage II, the MI
232
first appeared to rapidly decline (points A~B) and then the downtrend became a slight
233
slowdown (points B~C). Meanwhile, the MRI image was darker than stage I. This
234
was because that when the MH starts to form, the rapid hydrate formation obviously
235
consumes the amount of liquid water and decreases the MI value. Then, MH
236
formation influences the contact between the gas phase and liquid water, so hydrate 12
237
formation rate slows down. When the hydrate formation is completed, the MI
238
gradually comes to stability at stage III and the image becomes the darkest. From the
239
MRI images, it can be observed that MH formation is nonuniform in porous media
240
because there is always migration of water and gas in the MH formation process.
241
Moreover, MI was nonzero after MH formation completion, indicating that residual
242
water still exists in porous media.
243 244 245
Fig. 2 Variations of MI and water distribution during MH formation process 3.2 Effects of different seawater-gas flow rates on MH dissociation
246
In our previous study (Yang et al., 2019a), when the deionized water-gas flow
247
rate was 1-1 ml·min-1, MH did not dissociate and even the hydrate reformation
248
phenomenon appeared during the deionized water-gas flow. However, MH always
249
dissociates during the seawater-gas flow process even if the seawater-gas flow rate is
250
slower, which suggests the seawater-gas flow obviously promotes MH dissociation 13
251
and inhibits MH reformation. There are two reasons explaining this phenomenon. The
252
first is the seawater solution ion effect that forms an electric field between positive
253
and negative ions to destroy the cluster structure of water molecules. When the water
254
molecules form a lattice, they overcome the electric field and further shift the hydrate
255
phase equilibrium curve to the left (Yang et al., 2012, Chong et al., 2017b), as shown
256
in Fig. 3 the MH hydrate phase equilibrium curve under the seawater and deionized
257
water experimental condition. Therefore, compared to deionized water-gas flow
258
process, hydrate stability decreases and easily dissociates in the seawater-gas flow
259
process. The second is that the bonds between water and ions are Coulombic that
260
stronger than hydrogen bonding or van der Waals forces (Yang et al., 2012),
261
resulting in water molecules were attracted by ions and further reduce the water
262
molecular activity (Atik et al., 2006). Moreover, the accumulation of Cl- decreases gas
263
molecule solubility in seawater. The phenomenon of salting-out appeared (Chen et al.,
264
2017, Kim et al., 2008) with a negative effect on hydrate reformation. Based on the
265
above discussion, recycling seawater-gas flow was effective for gas hydrate
266
production.
14
267 268
Fig. 3 MH phase equilibrium curves under the seawater and deionized water
269
experimental condition
270
Fig. 4 shows the variation of water distribution during MH dissociation with
271
continuous seawater-gas flow, in which the seawater-gas flow rates were 1-1 (Case 1),
272
2-2 (Case 2), 3-3 (Case 3), 3-1 (Case 4), 4-1 (Case 5), 5-1 (Case 8), and 10-1
273
ml·min-1(Case 6), respectively. Moreover, bright green signals represent liquid water,
274
and dark blue signals are MH and a small quantity of methane gas. As shown in Fig. 4,
275
MH dissociated gradually with the seawater-gas flow process. At 2 min, the
276
hydrate-bearing sediment core was saturated by seawater, and the dark blue image
277
suggested there was no hydrate dissociation. With continuous seawater-gas flow, the
278
images showed a partial bright green area, illustrating hydrate dissociated and water
279
saturation increased. Then, the bright area gradually enlarged along the
280
seawater-gas-hydrate three phase interface until the whole image became bright.
15
281
When the whole image became bright (corresponding to the last image), the MH
282
dissociation was finished and the water saturation in porous media reached maximum.
283
Fig. 4 also showed that when the seawater-gas flow rate was 1-1, 2-2 and 3-3
284
ml·min-1 (Cases 1-3), the duration of hydrate dissociation was relatively long. When
285
the flow rate was 10-1 ml·min-1 (Case 6), the duration was the shortest for
286
dissociating hydrate. That is, the higher seawater flow rate and lower gas flow rate
287
will accelerate MH dissociation process. The reasons are addressed below. The first
288
was that seawater salt invades the hydrate-dissociated water due to the concentration
289
difference, and the salt contamination resulted in a better heat conduction (Sun and
290
Mohanty, 2006, Chong et al., 2015). The second was the faster heat transfer during
291
seawater-gas flow process. Because the hydrate dissociation is an endothermic
292
reaction process, a lot of heat can be absorbed when the hydrate suddenly dissociated
293
and caused a rapid decline of hydrate-bearing sediment temperature, even caused the
294
hydrate reformation and ice generation. During the continuous seawater-gas flow
295
process, the temperature of flowing seawater and gas (273.95 K) was constant, which
296
can provide continuous heat transfer and keep the stability of sediment temperature.
297
Therefore, the plummeting temperature of hydrate-bearing sediment cores was not
298
appeared, which is benefit for the hydrate dissociation process, and further preventing
299
the hydrate reformation and ice generation. The third was that increasing the seawater
300
flow rate and decreasing the gas flow rate will increase the chemical potential
16
301
difference, accelerate the seawater phase mass transfer and further provide higher
302
driving force for dissociating hydrate.
303 304
Fig. 4 Variations of water distribution during MH dissociation process with different
305
seawater-gas flow rates
306
Fig. 5 shows the variations of MI during MH dissociation with different
307
seawater-gas flow rates, which reflected the amount variations of free water in porous
308
media. As shown in Fig. 5, the MI increased to approximately 1200 within a few
309
minutes at points A~B. This was because that after hydrate formation process, little
310
amount free water distributed in sediment, the large amount of free gas, sand and
311
hydrate caused the lowest value of MI. Then, when the seawater phase flow into the
312
sediment, the increase of the amount of free water caused the sharp increase of MI
313
Next, the MI appeared a sharp change under the seven flow rates, in which the MI 17
314
suddenly decrease and then gradually increase. The reasons for the phenomenon are
315
as follows. Firstly, due to the Jamin effect (Wang et al., 2016a, Yang et al., 2019a),
316
some parts of free gas still distributed under the area of field view (FOV), these parts
317
of free gas will be further displaced into the FOV by seawater. Because the MRI
318
system can only identify the 1H containing in liquid, so the gas gathered in FOV
319
caused the MI decrease. Secondly, with the continuous seawater flow, the hydrate will
320
gradually dissociate and the free water will gradually obtain the released pore space,
321
and ultimately caused the MI gradually increase. In Fig. 5 (a), when the seawater-gas
322
flow rate was 1-1 ml·min-1, the upward tendency of MI was the slowest at points B~
323
C1. In Fig. 5 (b), there has a rapidly increase of MI at points B~C (C4, C5), which
324
indicated the MH dissociation under the seawater-gas flow rates of 3-1 and 4-1
325
ml·min-1 was significantly faster than the results of in Fig. 5(a). Moreover, with the
326
flow rate further increased (5-1 and 10-1 ml·min-1), the variation of MI transforms
327
into the tension trend at points B~C (C6, C7) because of the quickening MH
328
dissociation. Finally, when the hydrate was dissociated completely, the amount of
329
water in the pore space was invariable and the MI was stable. In addition, the higher
330
seawater flow rate and the lower gas flow rate, the faster for MI reached stability.
331
Combined Figs. 4 and 5, jointly proves that seawater-gas flow can effectively promote
332
MH dissociation.
18
333 334
Fig. 5 Variations of MI during MH dissociation process with different seawater-gas
335
flow rates
336
The average MH dissociation rate is a main parameter reflecting the average gas
337
and water production rates. Fig. 6 shows comparisons of average MH dissociation rate
338
with different seawater-gas flow rates. As shown in Fig. 6, the average MH
339
dissociation rate is a function of the seawater-gas flow rate ratio and pour seawater
340
flow rate, which increased with the increase of flow rate ratio and pore seawater flow
341
rate. When the seawater-gas flow rate was 1-1, 2-2 and 3-3 ml·min-1, the MH
342
dissociation rate increased slowly. Moreover, there was a fast-increasing trend for the
343
MH dissociation rate with increasing seawater flow rate and decreasing gas flow rate.
344
When the flow rate was highest (10-1 ml·min-1), it induced the largest average MH
345
dissociation rate of 0.705 %/min. According to the trend of average MH dissociation
346
rate, the seawater flow rate was the main factor promoting hydrate dissociation, and
347
increasing the gas flow rate retards the dissociation process. That is, MH dissociation
348
from seawater-gas flow increased with higher flow rate ratio and seawater flow rate.
19
349
The reason is faster heat and mass transfer in hydrate-bearing sediment cores with
350
increasing seawater-gas flow rate ratio and seawater flow rate, providing greater
351
driving force to promote hydrate dissociation. So, it can be inferred that the
352
seawater-gas flow rate of 10-1 ml·min-1 in Case 6 effectively promotes MH
353
dissociation.
354 355
Fig. 6 Comparisons of average MH dissociation rates with different seawater-gas flow
356
rates
357
3.3 MH flow process dissociation behavior with different initial MH saturation
358
To investigate MH dissociation with different initial hydrate saturation, six
359
experiments were carried out with initial MH saturations of 15% (Case 7), 23% (Case
360
8), 28% (Case 9), 31% (Case 10), 36% (Case 11), and 41% (Case 12). Fig. 7 shows
361
the variations of water distribution and MI during MH dissociation induced by
20
362
seawater-gas flow with different initial hydrate saturation, in which the seawater-gas
363
flow rate was 5-1 ml·min-1. As shown in Fig. 7 (a), with the same seawater-gas flow
364
rate, the time to dissociate MH (corresponding to the second image) and the time to
365
dissociate MH completely (corresponding to the last image) all increased with
366
increasing initial hydrate saturation. Combined with the MI variation during MH
367
dissociation in Fig. 7 (b) a sharp MI change first appeared, and the reason have been
368
discussed in section 3.2. Then, it increased gradually with hydrate dissociation, and
369
took more time for the MI to reach stability with higher initial hydrate saturation. In
370
addition, when the hydrate dissociated completely, the MI remains stable and the
371
maximum MI value decreases slightly with increasing initial hydrate saturation. This
372
is because that there was a relatively slow hydrate dissociation process under higher
373
initial saturation that caused more gas dissolving in liquid water and further decreased
374
the maximum value of the MI. Fig. 7 suggest that the duration of hydrate dissociation
375
was longest under higher initial hydrate saturation. There are three reasons for this
376
phenomenon. The first is hydrate imperviousness (Liu et al., 2016), higher initial
377
hydrate saturation maintains hydrate-bearing sediment core stability better. The
378
second was that MH occupies more pore space in porous media under higher initial
379
hydrate saturation, which decreases the fluid flow space and water phase relative
380
permeability, further influencing the heat and mass transfer. The third was that there
381
has a higher chemical potential difference between the seawater and hydrate phases
382
when the initial hydrate saturation is lower. As reported (Sean et al., 2007), the 21
383
chemical potential difference was the main reason for hydrate dissociation in marine
384
environment above hydrate phase equilibrium, which was defined as the methane
385
concentration dissolved in the aqueous. During seawater-gas flow process, though
386
little methane gas dissolved in flowing seawater, the chemical potential difference still
387
enough for inducing hydrate dissociation. And the seawater-gas flow erosion was a
388
long-time and continuous process, thereby, the residual free water and gas will be
389
displaced at last. When the hydrate saturation is lower, the blocking effect of hydrate
390
on seawater flow was relatively small, so the displace process of free water will be
391
faster, which caused the chemical potential difference increase and accelerated the
392
hydrate dissociation.
393 394
Fig. 7 Variations of water distribution (a) and MI (b) during MH dissociation induced
395
by seawater-gas flow with different initial hydrate saturation
396
The average dissociation rate is an important parameter in hydrate exploitation
397
and it is the ratio of hydrate saturation to dissociation duration (Wang et al., 2018).
398
Fig. 8 shows the comparisons of average MH dissociation rate induced by 22
399
seawater-gas flow with different initial hydrate saturation. As shown in Fig. 8, the
400
MH dissociation rate has no obvious linear relationship with initial hydrate saturation,
401
and the initial hydrate saturation has a mild effect under the same seawater-gas flow
402
rate. This is because the MH dissociation rate is controlled by hydrate saturation and
403
dissociation duration. Even though hydrate dissociates quickly, the average
404
dissociation rate will not increase significantly due to lower hydrate saturation. In
405
contrast, when hydrate saturation is higher, there also is no high MH dissociation rate
406
because of the relatively long dissociation duration. Furthermore, the occurrence
407
structure of hydrate may influence the dissociation behavior. Because the pore-filling
408
hydrate has more surface area to retard fluid flow, there was a smaller water phase
409
permeability than grain-coating hydrate (Dai and Seol, 2014), which may influence
410
the MH dissociation rate, but this phenomenon was less evident in this study.
411
23
412
Fig. 8 Comparisons of average MH dissociation rate with different initial hydrate
413
saturation
414
3.4 Comparisons of seawater/deionized water-gas flow effects on MH dissociation
415
The flow channel appearance and disappearance time represents the time at
416
which the MH begins to dissociate and the end of MH dissociation. The flow channel
417
appearance time was defined as the moment that a small bright area appeared, as
418
shown in the second images in Fig. 4 and Fig 7 (a). The flow channel disappearance
419
time is defined as the moment that the whole image became bright, as shown the last
420
images in Fig. 4 and Fig 7 (a). The shorter the appearance and disappearance times,
421
the faster the hydrate dissociation. Fig. 9 shows comparisons of flow channel
422
appearance and disappearance times between seawater-gas and deionized water-gas
423
flow. As shown in Fig. 9, the flow channel appearance and disappearance time
424
obviously decreased with increasing water flow rate both in seawater-gas and
425
deionized water-gas flow condition. Moreover, the flow channel appearance and
426
disappearance time in deionized water-gas flow was longer than that of in
427
seawater-gas flow under the same flow rate, especially in the deionized water-gas
428
flow rate of 2-2 ml·min-1. The reasons for the above phenomenon are as follows.
429
Firstly, under the same temperature, Na+ and Cl- can obviously improve the methane
430
hydrate phase equilibrium pressure (as shown in Fig. 3), which induced a stronger
431
promotion effect in seawater-gas flow on hydrate dissociation than the deionized
432
water-gas flow. Secondly, due to the existence of ions, the CH4 gas solubility in 24
433
seawater than that of in deionized water (Chen et al., 2017, Kim et al., 2008), which
434
caused the larger chemical potential difference between seawater-hydrate phase and
435
further provided the stronger driving force for MH dissociation. Thirdly, in our
436
previous work (Chen et al., 2019b, Chen et al., 2019a, Yang et al., 2019a), we found
437
that the water flow rate was the main reason for hydrate dissociation above the
438
hydrate phase equilibrium. The higher water flow rate will accelerate hydrate
439
dissociation rate by increasing the chemical potential difference and enhancing heat
440
and mass transfer process. So, compared to others flow rate (3-3, 3-1, 4-1, 5-1
441
ml·min-1), the lower water flow rate in the deionized water-gas flow rate of 2-2
442
ml·min-1 provided a smaller driving force for hydrate dissociation and induced a huge
443
number of disappearance time.
444 445
Fig. 9 Comparisons of flow channel appearance and disappearance time of between
446
seawater-gas and deionized water-gas flow
25
447
Fig. 10 shows comparisons of average MH dissociation rates between the
448
seawater-gas and deionized water-gas flow processes. The experimental result and
449
details of deionized water-gas flow process can be found in published article (Yang et
450
al., 2019a). As shown in Fig. 10, the average MH dissociation rates with five flow
451
rates were compared, including 2-2, 3-3, 3-1, 4-1, and 5-1 ml·min-1. In two
452
experimental conditions (seawater/deionized water-gas flow), the MH dissociation
453
rate increased with the increase of water flow rate and the decrease of gas flow rate,
454
and the MH dissociation rate in the seawater-gas flow process was far greater than
455
that in deionized water-gas flow under the same flow rate. This phenomenon
456
indicated that the seawater-gas flow experimental conditions promote MH
457
dissociation more significantly than deionized water-gas flow. The reason was that the
458
Cl- implants into the water cage and attracts H+ from water molecules, which breaks
459
the hydrogen bonding structure of the water cage and accelerates hydrate dissociation
460
(Chong et al., 2015).In addition, because salt reduces methane gas solubility (Yang
461
and Xu, 2007), the injection and hydrate-dissociated gases form bubbles in seawater
462
(Chong et al., 2015). In contrast, the methane gas dissolved more uniformly in
463
deionized water. Therefore, there was larger methane concentration between the
464
seawater and hydrate phases during the seawater-gas flow process, which obviously
465
increased the chemical potential difference, and provided a crucial driving force for
466
hydrate dissociation.
26
467 468
Fig. 10 Comparisons of average MH dissociation rate between seawater-gas and
469
deionized water-gas flow
470
3.5 The application and shortage of seawater-gas flow inducing MH dissociation
471
This study verified the promotion effect of seawater-gas flow on hydrate
472
dissociation. The seawater-gas flow can increase the chemical potential difference,
473
accelerate the heat and mas transfer process, and inhibit hydrate reformation, which
474
have great application potential in actual hydrate production tests. In nature, four main
475
classes (Class 1, Class 2, Class 3, Class 4) of methane hydrate deposits are found
476
(Moridis et al., 2004). For Class 1 deposits (upper hydrate layer and under free gas
477
layer), the gas phase ratio was much higher than that of water phase during hydrate
478
production process. For Class 2 deposits (upper hydrate layer and under free water
479
layer), the water phase ratio was much higher than that of gas phase during hydrate
480
production process. For Class 3 and Class 4 deposits (no free water and gas layer), the 27
481
hydrate production process will always companied company with the huge production
482
of water and gas. It is surely that the issue of water production is a sticky point in the
483
hydrate production tests. Therefore, in the actual hydrate exploitation tests, we can
484
utilize the hydrate dissociated-water and can extra inject some seawater into the
485
hydrate deposits to induce hydrate dissociation. The seawater flow rate can be
486
controlled by adjusting the pressure difference between the production wells, and the
487
gas recovery rate also can be adjusted to achieve the seawater-gas two-phase flow. In
488
addition, considering to the hydrate production efficiency, the seawater-gas two-phase
489
flow can be used to combine with other production methods (depressurization) for
490
real applications, and which will be investigated in our follow-up work.
491
However, there are also exist some limitations by using seawater to dissociate
492
hydrate in real applications. Firstly, there will be need huge seawater injection amount
493
if the water flow erosion method was performed to recovery natural gas in the field
494
hydrate production test. Secondly, in order to avoid the reservoir collapse caused by
495
fast hydrate dissociation, the suitable seawater flow rate and flow direction of
496
seawater flow should be verified in the real hydrate production test. Thirdly, seawater
497
flow may induce outflow sand and influent the continuous gas production, this issue
498
also needs solve. Therefore, the results obtained from our test section translatable to
499
the real case still exist some difference. The current study was a fundamental study,
500
we will further investigate the effect of water-gas two-phase flow on hydrate
501
dissociation by increasing the experimental scale. 28
502
4. Conclusion
503
A seawater-gas flow experiment was carried out and the effect of seawater-gas
504
flow rate and initial hydrate saturation on MH dissociation was analyzed. The
505
experimental conclusions are as follows:
506
(1) When the flow rate was 1-1 ml·min-1, the seawater-gas flow process promotes
507
MH dissociation because of the ion and salting-out effects, whereas MH was not
508
dissociated and hydrate reformation even appeared in the deionized water-gas flow
509
process. It was shown that seawater-gas flow achieves gas production from hydrate
510
reservoirs and effectively inhibits hydrate reformation under slower flow rates.
511
(2) With the same initial hydrate saturation, the seawater-gas flow rate ratio and
512
pour seawater flow rate were the crucial factor for MH dissociation. The higher
513
seawater flow rate and lower gas flow rate cause higher average MH dissociation
514
rates due to the acceleration of heat and mass transfer. In addition, salt ions reduce gas
515
solubility and water activity further promoting hydrate dissociation.
516
(3) With the same seawater-gas flow rate, the time to induce MH to dissociate
517
and the total time for completing MH dissociation all increased with increasing initial
518
hydrate saturation. According to the trend of the average MH dissociation rate under
519
different initial hydrate saturation, hydrate saturation has a mild effect on the average
520
MH dissociation rate.
521
(4) Seawater-gas flow promotes MH dissociation stronger than deionized
522
water-gas flow. During seawater-gas flow, the duration of hydrate dissociation 29
523
decreased and the average MH dissociation rate increased with higher seawater-gas
524
flow rate ratio and higher seawater flow rate. Moreover, the average MH dissociation
525
rate in seawater-gas flow was higher than in deionized water-gas flow.
526
Acknowledgments
527
This study was financially supported by grants from the National Natural
528
Science Foundation of China (51436003, 51822603 and 51576025), the National Key
529
Research and Development Plan of China (2017YFC0307303 and 2016YFC0304001),
530
the Fok Ying-Tong Education Foundation for Young Teachers in Higher Education
531
Institutions of China (161050) and the Fundamental Research Funds for the Central
532
Universities of China (DUT18ZD403).
533
Conflict of interest
534
None declared
535
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effects on gas production from methane hydrate by depressurization.
699
International Journal of Heat and Mass Transfer, 77, 529-541.
700
ZHAO, J. F., YE, C. C., SONG, Y. C., LIU, W. G., CHENG, C. X., LIU, Y., ZHANG,
701
Y., WANG, D. Y. & RUAN, X. K. 2012. Numerical simulation and analysis
702
of water phase effect on methane hydrate dissociation by depressurization.
703
Industrial & Engineering Chemistry Research, 51, 3108-3118.
704
ZHAO, J. F., ZHU, Z. H., SONG, Y. C., LIU, W. G., ZHANG, Y. & WANG, D. Y.
705
2015. Analyzing the process of gas production for natural gas hydrate using
706
depressurization. Applied Energy, 142, 125-134.
707
ZHENG, J. N., CHENG, F. B., LI, Y. P., LV, X. & YANG, M. J. 2019. Progress and
708
trends in hydrate based desalination (HBD) technology: A review. Chinese
709
Journal of Chemical Engineering.
710 711 36
712
37
713
Effect of multiphase flow on natural gas hydrate production in marine sediment
714
Huiru Sun, Bingbing Chen, Mingjun Yang*
715
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of
716
Education, Dalian University of Technology, Dalian 116024, China
717 718
Abstract Natural gas hydrates (NGHs), regarded as an alternative future energy
719
source. Currently, tests for hydrate exploitation from marine sediment have been
720
performed in the Nankai Trough of Japan and the Shenhu area of the South China Sea.
721
Hydrate exploitation is influenced by water-gas flow, and considering the huge
722
seawater reserves in hydrate accumulation areas, an experiment of seawater-gas flow
723
was performed to dissociate hydrate. The effects of seawater-gas flow rates and initial
724
hydrate saturation on methane hydrate (MH) production was analyzed. The results
725
showed that seawater-gas flow efficiently promotes hydrate dissociation and inhibits
726
hydrate reformation. Moreover, there was a faster heat and mass transfer with
727
increasing seawater flow rates and decreasing gas flow rates, which enhanced the
728
average MH dissociation rate. In addition, the variation time of the flow channel
729
increased with higher initial hydrate saturation. Additionally, seawater-gas flow
730
promotes MH dissociation stronger than deionized water-gas flow.
731
Keywords: methane hydrate dissociation; water flow erosion; seawater-gas flow; salt
732
ions; hydrate saturation
38
733
1. Introduction
734
Natural gas hydrates (NGHs) are typically found in continental permafrost and
735
marine sediment and they form from hydrocarbon gas and water under high pressure
736
and low temperature (Hiratsuka et al., 2015, Gao et al., 2018). The water molecules
737
interconnected through hydrogen bonding form cage structures to encage gas
738
molecules (Zheng et al., 2019, Saman et al., 2010, Liu et al., 2018). There are rich
739
natural gas hydrate reservoirs distributed around the world, in which the estimated
740
energy potential is far greater than the total amount of conventional coal, oil and
741
natural gas (Wang et al., 2017). NGHs are seen as potential sources of natural gas in
742
view of their high energy density, abundant reserves and nonpollution (Zhao et al.,
743
2014, Xu and Li, 2015). Therefore, researching hydrate formation and dissociation
744
kinetics and developing methods for natural gas recovery from hydrate reservoirs are
745
arousing wide concern in countries and among scientists all over the world in recent
746
years (Wang et al., 2016b, Song et al., 2015b).
747
For now, methods for large-scale production of natural gas from NGHs are still
748
evolving, and they are all based on changing reservoir pressure and temperature to
749
deviate from hydrate thermodynamic equilibrium (Ji et al., 2001). There are four
750
common methods for gas hydrate exploitation as follows. The depressurization
751
method, in which the reservoir pressure is decreased below the hydrate equilibrium
752
pressure at a specified temperature (Reagan et al., 2015, Li et al., 2012, Yang et al.,
753
2019b). The thermal stimulation method, in which the reservoir temperature is 39
754
increased above the hydrate equilibrium temperature via injecting hot water, hot brine
755
or steam (Song et al., 2015a, Nair et al., 2016, Chong et al., 2016b). The chemical
756
inhibitor stimulation method, in which chemicals are injected into the hydrate
757
reservoir to transform the hydrate pressure-temperature equilibrium and promote
758
hydrate dissociation (Li et al., 2011, Chong et al., 2016a). The CO2 replacement
759
method, in which liquid CO2 is injected into hydrate reservoirs to form CO2 hydrate
760
and replace methane gas (Yu et al., 2015, Brewer et al., 2014). The above methods all
761
have their own merits and demerits. The large heat loss of thermal stimulation, the
762
expensive cost and pollution environment of chemical inhibitors, and the low
763
efficiency of CO2 replacement restricts application of those three methods for
764
practical hydrate exploitation (Fitzgerald et al., 2014, Lee et al., 2010, Jung et al.,
765
2010). However, depressurization needs no external energy supply and energy
766
consumption is the least, so that was regarded as the most effective method to recover
767
natural gas from NGH reservoirs considering the economic cost (Chong et al., 2017a,
768
Zhao et al., 2014).
769
The depressurization method was first used for field-scale hydrate production at
770
the Mackenzie Delta Mallik site in the Northwest Territories, Canada, and then a
771
larger-scale test was conducted at the same site in 2007 and 2008 (Yang et al., 2016).
772
However, the hydrate dissociation is an endothermic reaction. When the pressure drop
773
is large, the sensible heat of the hydrate and heat transferred from the ambient
774
environment was not enough to sustain the faster hydrate dissociation reaction. So, 40
775
there may occur the hydrate reformation and ice generation in depressurization
776
process, which will obviously influent the gas production rate (Wang et al., 2018,
777
Zhao et al., 2015, Chen et al., 2015). Therefore, many experimental studies and
778
numerical modeling of hydrate dissociation and gas production using depressurization
779
were conducted. Konno et al. (Konno et al., 2014) performed the gas production test
780
on methane hydrate sediments using one-step and multistep depressurization. They
781
found that a suitable heat for hydrate-bearing sediment is the important factor to drive
782
hydrate dissociation. Zhan et al. (Zhan et al., 2018) used different quartz sand particle
783
sizes to simulate hydrate reservoirs, form methane hydrate, and dissociate hydrate via
784
the depressurization method. They analyzed and compared the effects of sediment
785
particle size on gas production rate and dissociation characteristics. Zhao et al. (Zhao
786
et al., 2012) conducted numerical simulations to investigate the water phase impact on
787
methane hydrate dissociation by depressurization in porous media. Their experimental
788
results showed that the movement of water plays a significant role in late stage
789
thermal conduction, and the gas production rate increased with higher water saturation
790
in late stage dissociation. Based on the available data in the Shenhu Area of the South
791
China Sea, Xiong et al. (Xiong et al., 2012) employed a 1D experimental setup to
792
study methane hydrate dissociation by depressurization, and they found that lower
793
dissociation pressure may lead to higher hydrate dissociation rate and higher
794
dissociation heat. Oyama et al. (Oyama et al., 2012) studied the depressurized
795
dissociation of natural methane-hydrate-bearing sediment with low permeability, and 41
796
they assessed two dissociation models making use of their experimental results. To
797
acquire kinetic hydrate dissociation data under different backpressures, depressurized
798
dissociation of methane hydrate was studied using magnetic resonance imaging (MRI)
799
by Wang et al. (Wang et al., 2015). For the other three hydrate dissociation methods,
800
Su et al. (Su et al., 2013) investigated the gas production potential from hydrate
801
sediment using thermal stimulation. They found that thermal stimulation was effective
802
at promoting gas release from hydrate sediment and enhanced gas production in the
803
late stage of hydrate dissociation. Li et al. (Li et al., 2017) injected methanol into
804
methane-hydrate-bearing sediment to study dissociation behavior employing a
805
one-dimensional experimental device. They proposed new adjustment tactics for
806
experimental operation according to the injection rate ratio of methanol and water.
807
Jung et al. (Jung and Santamarina, 2010) measured the electrical resistance change
808
and relative stiffness in the CH4-CO2 replacement process, and found that the overall
809
hydrate remained solid and gas-hydrate-water phases coexisted for long periods of
810
time. Yuan et al. (Yuan et al., 2013) used a three-dimensional reactor to study
811
CO2-CH4 replacement in hydrate-bearing deposits with liquid CO2, and analyzed the
812
influence of different stratum conditions on the replacement process.
813
Whatever NGH exploitation methods are adopted, hydrate dissociation is always
814
accompanied by production and migration of water and gas. Chen et al. (Chen et al.,
815
2019b) proposed a water flow erosion method to promote hydrate dissociation and
816
investigated the effects of seawater flow on natural gas hydrate dissociation (Chen et 42
817
al., 2019a), whereas the effect of gas migration on hydrate dissociation was ignored.
818
In addition, deionized water-gas two-phase flow promotion of MH dissociation has
819
been studied, and hydrate reformation may appear under slower water-gas flow rates.
820
Therefore, based on those studies and considering the effect of seawater on hydrate
821
thermodynamic equilibrium, experiments of seawater-gas two-phase flow were
822
performed, and different seawater-gas flow rates effect on MH dissociation were
823
analyzed. In addition, the system pressure and temperature are always in the
824
thermodynamic stability region of the hydrate-bearing sediment core. Due to the huge
825
seawater reserves in hydrate accumulation areas, the method of seawater-gas flow to
826
dissociate hydrate is achievable, so the experimental results offer some reference and
827
guidance for future NGH exploration in marine environments.
828
2. Experiments
829
2.1 Apparatus and materials
830
Fig. 1 shows a sketch of the experimental setup designed to investigate MH
831
dissociation during seawater-gas two-phase flow. The experimental setup included an
832
MRI system (Varian, Inc., Palo Alto, CA, USA), a high-pressure vessel (polyimide),
833
three high-precision injection pumps (260D, Teledyne Isco Inc., Lincoln, NE, USA),
834
three thermostat baths (FL300 and FL 25, JULABO, Seelbach, Germany), two pressure
835
transducers (3510CF, Emerson Electric Co., Ltd., St. Louis, USA), a differential
836
pressure sensor (Nagano Keiki, Japan), and a data acquisition system. The MRI system
837
was used to visualize the change of MH distribution during seawater-gas flow, which 43
838
was operated at 400 MHz resonance frequency and 9.4 T magnet field. This MRI
839
system adopted a spin echo sequence to obtain two-dimensional proton
840
density-weighted images. The parameters of the sequence are as follows, which based
841
on Chen et al. (Chen et al., 2019a). Echo time was 4.39 ms, image data matrix was
842
128×128, and the field of view was 30 mm×30 mm (Chen et al., 2019a). Three injection
843
pumps were used to inject liquid, CH4 gas and to control the vessel pressure for
844
seawater-gas flow. The effective volume of injection pump was 260 ml, the accuracy of
845
flow control was ±1 µl·min-1, and accuracy of pressure control was ± 0.5%. The
846
temperatures of the pumps and MRI system were strictly controlled by thermostat baths,
847
and the stability of the thermostat baths was ±0.1
848
differential pressure sensor were used to collect pressure signals in the inlet and outlet
849
of the high-pressure vessel. The accuracy of the pressure transducers was ±0.05%, and
850
the accuracy of the differential pressure sensor was ±0.01%. The data acquisition
851
system included an A/D module (Advantech Co., Ltd., Milpitas, CA, USA) and a
852
computer, which was used to collect and dispose the pressure signals. Deionized water
853
and CH4 gas were used to form methane hydrate (MH). Seawater (configured by
854
deionized water) and CH4 gas were used in the MH dissociation process.
44
. Pressure transducers and a
855 856 857
Fig. 1 Schematic of the experimental system 2.2 Hydrate formation
858
A high-pressure vessel with inner diameter of 15 mm and length of 200 mm was
859
washed, dried, and filled with BZ-02 glass beads (As-One Co., Ltd., Japan). The
860
porosity of BZ-02 glass beads was 35.4%, the particle diameter was 0.177-0.250 mm
861
and the density of BZ-02 was 2.6 g·cm-3. After the leakage test, the vessel was
862
mounted in the center of the MRI magnetic body and vacuumed to expel free gas.
863
Next, the vessel was pressurized to 6000 kPa using water injection pump with a
864
constant injection rate of 20 ml·min-1 to fully saturate the porous media. After
865
maintaining the pressure stable for 1 h, the outlet valve was opened, and the vessel
866
pressure equaled to atmospheric pressure. Then, CH4 gas (Dalian Special Gases Co.,
867
Ltd., China, 99.99%) with 4000 kPa pressure was injected into the vessel to displace
868
the deionized water and obtain partly water-saturated porous media. Then, the vessel
869
was pressurized to 6000 kPa by CH4 gas injection with a constant injection rate of 20
45
870
ml·min-1 and maintained stable to form MH. During the whole process of hydrate
871
formation, the thermostat bath temperature was kept at 274.15 K. Moreover, the
872
images were obtained continuously via MRI for each experiment.
873
2.3 Seawater-gas two-phase flow process
874
The experimental parameters and results of twelve experimental cases are shown
875
in Table 1. Take Case 1 as an example to illustrate the seawater-gas two-phase flow
876
process. The seawater solution (NaCl solution with a mass fraction of 3.5%) was
877
prepared before the experiment. After hydrate formation finished, the outlet valve was
878
opened to interconnect backpressure pump to the vessel. Then, the pressure of
879
backpressure pump was decreased to 3200 kPa form 6000 kPa using backpressure
880
pump with a constant rate of -20 ml·min-1, and maintained that pressure for a while to
881
ensure that no MH dissociation appeared in the pressure adjustment process. When
882
the temperature is 274.15 K, the MH phase equilibrium pressure is 2850 kPa (Jr and
883
Koh, 2007). Therefore, the hydrate was always thermodynamically stable during the
884
backpressure adjustment process. Next, seawater (1ml·min-1) and CH4 (1ml·min-1)
885
were simultaneously injected into the vessel from the bottom to the top by seawater
886
injection pump and gas injection pump. The temperature of the flowing seawater and
887
CH4 gas was controlled at 273.95 K by thermostat bath to prevent hydrate dissociation
888
caused by temperature variation. Images of seawater-gas flow were recorded in
889
succession using an MRI system. In addition, after the seawater-gas flow process,
890
deionized water was used to wash the whole experimental system (pipelines, seawater 46
891
injection pump and high-pressure vessel) approximately 5~6 times till the
892
conductivity was lower than 0.69 µS·cm-1, which avoided residual seawater inhibiting
893
hydrate formation. The conductivities of seawater and deionized water were 2.85*104
894
µS·cm-1 and 0.69 µS·cm-1, respectively(Chen et al., 2019a).
895
Table 1. Experimental parameters and results Case 1 2 3 4 5 6 7 8 9 10 11 12
896 897 898 899 900 901 902
vw (ml/min) 1 2 3 3 4 10 5 5 5 5 5 5
vg (ml/min) 1 2 3 1 1 1 1 1 1 1 1 1
Pi (kPa) 6010 6015 6020 6010 6000 6005 6020 6011 6040 6060 6030 6050
Shi (%) 24.33 23.03 22.42 23.83 23.77 22.56 15.48 22.50 28.19 31.08 36.23 41.48
tapp (min) 80 46 38 24 14 10 8 12 14 14 20 26
tdis (min) 986 532 290 136 88 42 50 68 100 108 112 132
RV (%/min) 0.0269 0.0474 0.0890 0.2128 0.3212 0.7050 0.3686 0.4018 0.3278 0.3306 0.3938 0.3913
a
The symbols in this table are defined as follows: tapp is the flow channel appearance time; tdis is the flow channel disappearance time; Vw is the seawater flow rate during seawater-gas two-phase flow; Vg is the gas flow rate during seawater-gas two-phase flow; Shi is the initial MH saturation before hydrate dissociation (volume fraction); Pi is the MH formation pressure; and Rv is the average MH dissociation rate. All experiments were carried out at 274.15 K, the temperature of flowing seawater and gas was kept at 273.95 K, and the designed backpressure was 3200 kPa for all cases during the seawater-gas flow process.
903
In this study, the average MH dissociation rate was calculated by the flowing
904
equation (Wang et al., 2018), which can reflect the average water and gas production
905
rates.
906
Ri =
S hi − S h (i + ∆t )
(1)
∆t
47
907
where
908
and
909
the two changes of hydrate saturation.
910
3. Results and Discussion
S
Ri
is the MH dissociation rate at i min,
h i + ∆ t
S hi
is the hydrate saturation at i min
are the hydrate saturation at i + ∆t min. ∆t is the time interval between
911
Twelve experimental cases were performed to study the effects of different
912
seawater-gas flow rates and different initial hydrate saturations on MH dissociation.
913
The seawater-gas flow rate was expressed as the seawater flow rate-gas flow rate in
914
this study. In Cases 1~6, the MH saturation was controlled approximately the same
915
(22.5%~23.5%), and the seawater-gas flow rate of that was set to 1-1, 2-2, 3-3, 3-1,
916
4-1, and 10-1 ml·min-1, respectively. In Cases 7~12, the seawater-gas flow rates were
917
all set to 5-1 ml·min-1, and the MH saturation was controlled to approximately 15%,
918
23%, 28%, 31%, 36%, and 41%, respectively. The backpressure was controlled at
919
3200 kPa for all cases, and the temperature of flowing seawater and gas was set to
920
273.95
921
thermodynamically stable. The distribution of water and MH was recorded by the
922
MRI system.
923
3.1 Methane hydrate formation
K,
which
ensured
that
the
hydrate-bearing
sediment
core
was
924
The pressure and temperature were set to 6000 kPa and 274.15 K, and were kept
925
constant for the whole MH formation process for Cases 1~12. MH saturation,
926
expressed as the volume fraction of pore space occupied by hydrate, was computed by
927
mean intensity (MI) data acquired form MRI images (Chen et al., 2019a). Because 1 48
928
m3 of hydrate releases 0.8 m3 of liquid water when it is dissociated, and 1 volume of
929
liquid water forms 1.25 volumes of hydrate under standard temperature and pressure
930
(STP) (Jr and Koh, 2007). Hence, the MH saturation at the “i” minute can be
931
computed with the following equation (Chen et al., 2019b):
932
S hi = 1.25 ×
933
where
934
values in the images at t = 0 and t = i minutes, respectively. The details of hydrate
935
saturation for each case are shown in Table 1.
(I 0 − I i )× S w0 × 100 %
S w0
(2)
I0
represents the initial water saturation, and
I0
and
Ii
represent the MI
936
Because of a similar phenomenon, take Case 2 as an example to illustrate the
937
MH formation process. Fig. 2 shows the MI variation during MH formation process,
938
which reflected the amount of free water in porous media. Moreover, three MRI
939
images were used to visually show the liquid water distribution during different stages
940
of MH formation. Bright red and dark purple represent the brightest (more water) and
941
darkest (more hydrate) signals, respectively. As shown in Fig. 2, the MI was stable at
942
stage I and liquid water distribution in the MRI image was uniform, indicating there
943
was no hydrate formation and water saturation remained invariant. At stage II, the MI
944
first appeared to rapidly decline (points A~B) and then the downtrend became a slight
945
slowdown (points B~C). Meanwhile, the MRI image was darker than stage I. This
946
was because that when the MH starts to form, the rapid hydrate formation obviously
947
consumes the amount of liquid water and decreases the MI value. Then, MH
948
formation influences the contact between the gas phase and liquid water, so hydrate 49
949
formation rate slows down. When the hydrate formation is completed, the MI
950
gradually comes to stability at stage III and the image becomes the darkest. From the
951
MRI images, it can be observed that MH formation is nonuniform in porous media
952
because there is always migration of water and gas in the MH formation process.
953
Moreover, MI was nonzero after MH formation completion, indicating that residual
954
water still exists in porous media.
955 956 957
Fig. 2 Variations of MI and water distribution during MH formation process 3.2 Effects of different seawater-gas flow rates on MH dissociation
958
In our previous study (Yang et al., 2019a), when the deionized water-gas flow
959
rate was 1-1 ml·min-1, MH did not dissociate and even the hydrate reformation
960
phenomenon appeared during the deionized water-gas flow. However, MH always
961
dissociates during the seawater-gas flow process even if the seawater-gas flow rate is
962
slower, which suggests the seawater-gas flow obviously promotes MH dissociation 50
963
and inhibits MH reformation. There are two reasons explaining this phenomenon. The
964
first was the seawater solution ion effect that forms an electric field between positive
965
and negative ions to destroy the cluster structure of water molecules. When the water
966
molecules form a lattice, they overcome the electric field and further shift the hydrate
967
phase equilibrium curve to the left (Yang et al., 2012, Chong et al., 2017b), as shown
968
in Fig. 3 the MH hydrate phase equilibrium curve under the seawater and deionized
969
water experimental condition. Therefore, compared to deionized water-gas flow
970
process, hydrate stability decreases and easily dissociates in the seawater-gas flow
971
process. The second is that the bonds between water and ions are Coulombic that
972
stronger than hydrogen bonding or van der Waals forces (Yang et al., 2012),
973
resulting in water molecules were attracted by ions and further reduce the water
974
molecular activity (Atik et al., 2006). Moreover, the accumulation of Cl- decreases gas
975
molecule solubility in seawater. The phenomenon of salting-out appeared (Chen et al.,
976
2017, Kim et al., 2008) with a negative effect on hydrate reformation. Based on the
977
above discussion, recycling seawater-gas flow was effective for gas hydrate
978
production.
51
979 980
Fig. 3 MH phase equilibrium curves under the seawater and deionized water
981
experimental condition
982
Fig. 4 shows the variation of water distribution during MH dissociation with
983
continuous seawater-gas flow, in which the seawater-gas flow rates were 1-1 (Case 1),
984
2-2 (Case 2), 3-3 (Case 3), 3-1 (Case 4), 4-1 (Case 5), 5-1 (Case 8), and 10-1
985
ml·min-1(Case 6), respectively. Moreover, bright green signals represent liquid water,
986
and dark blue signals are MH and a small quantity of methane gas. As shown in Fig. 4,
987
MH dissociated gradually with the seawater-gas flow process. At 2 min, the
988
hydrate-bearing sediment core was saturated by seawater, and the dark blue image
989
suggested there was no hydrate dissociation. With continuous seawater-gas flow, the
990
images showed a partial bright green area, illustrating hydrate dissociated and water
991
saturation increased. Then, the bright area gradually enlarged along the
992
seawater-gas-hydrate three phase interface until the whole image became bright.
52
993
When the whole image became bright (corresponding to the last image), the MH
994
dissociation was finished and the water saturation in porous media reached maximum.
995
Fig. 4 also showed that when the seawater-gas flow rate were 1-1, 2-2 and 3-3
996
ml·min-1 (Cases 1-3), the duration of hydrate dissociation was
997
the flow rate was 10-1 ml·min-1 (Case 6), the duration was the shortest for
998
dissociating hydrate. That is, the higher seawater flow rate and lower gas flow rate
999
will accelerate MH dissociation process. The reasons are addressed below. The first is
1000
that seawater salt invades the hydrate-dissociated water due to the concentration
1001
difference, and the salt contamination resulted in a better heat conduction (Sun and
1002
Mohanty, 2006, Chong et al., 2015). The second was the faster heat transfer during
1003
seawater-gas flow process. Because the hydrate dissociation is an endothermic
1004
reaction process, a lot of heat can be absorbed when the hydrate suddenly dissociated
1005
and caused a rapid decline of hydrate-bearing sediment temperature, even caused the
1006
hydrate reformation and ice generation. During the continuous seawater-gas flow
1007
process, the temperature of flowing seawater and gas (273.95 K) was constant, which
1008
can provide continuous heat transfer and keep the stability of sediment temperature.
1009
Therefore, the plummeting temperature of hydrate-bearing sediment cores was not
1010
appeared, which is benefit for the hydrate dissociation process, and further preventing
1011
the hydrate reformation and ice generation. The third was that increasing the seawater
1012
flow rate and decreasing the gas flow rate will increase the chemical potential
53
relatively long. When
1013
difference, accelerate the seawater phase mass transfer and further provide higher
1014
driving force for dissociating hydrate.
1015 1016
Fig. 4 Variations of water distribution during MH dissociation process with different
1017
seawater-gas flow rates
1018
Fig. 5 shows the variations of MI during MH dissociation with different
1019
seawater-gas flow rates, which reflected the amount variations of free water in porous
1020
media. As shown in Fig. 5, the MI increased to approximately 1200 within a few
1021
minutes at points A~B. This was because that after hydrate formation process, little
1022
amount free water distributed in sediment, the large amount of free gas, sand and
1023
hydrate caused the lowest value of MI. Then, when the seawater phase flow into the
1024
sediment, the increase of the amount of free water caused the sharp increase of MI
1025
Next, the MI appeared a sharp change under the seven flow rates, in which the MI 54
1026
suddenly decrease and then gradually increase. The reasons for the phenomenon are
1027
as follows. Firstly, due to the Jamin effect (Wang et al., 2016a, Yang et al., 2019a),
1028
some parts of free gas still distributed under the area of field view (FOV), these parts
1029
of free gas will be further displaced into the FOV by seawater. Because the MRI
1030
system can only identify the 1H containing in liquid, so the gas gathered in FOV
1031
caused the MI decrease. Secondly, with the continuous seawater flow, the hydrate will
1032
gradually dissociate and the free water will gradually obtain the released pore space,
1033
and ultimately caused the MI gradually increase. In Fig. 5 (a), when the seawater-gas
1034
flow rate was 1-1 ml·min-1, the upward tendency of MI was the slowest at points B~
1035
C1. In Fig. 5 (b), there has a rapidly increase of MI at points B~C (C4, C5), which
1036
indicated the MH dissociation under the seawater-gas flow rates of 3-1 and 4-1
1037
ml·min-1 was significantly faster than the results of in Fig. 5(a). Moreover, with the
1038
flow rate further increased (5-1 and 10-1 ml·min-1), the variation of MI transforms
1039
into the tension trend at points B~C (C6, C7) because of the quickening MH
1040
dissociation. Finally, when the hydrate was dissociated completely, the amount of
1041
water in the pore space was invariable and the MI was stable. In addition, the higher
1042
seawater flow rate and the lower gas flow rate, the faster for MI reached stability.
1043
Combined Figs. 4 and 5, jointly proves that seawater-gas flow can effectively
1044
promote MH dissociation.
55
1045 1046
Fig. 5 Variations of MI during MH dissociation process with different seawater-gas
1047
flow rates
1048
The average MH dissociation rate is a main parameter reflecting the average gas
1049
and water production rates. Fig. 6 shows comparisons of average MH dissociation rate
1050
with different seawater-gas flow rates. As shown in Fig. 6, the average MH
1051
dissociation rate is a function of the seawater-gas flow rate ratio and pour seawater
1052
flow rate, which increased with the increase of flow rate ratio and pore seawater flow
1053
rate. When the seawater-gas flow rate were 1-1, 2-2 and 3-3 ml·min-1, the MH
1054
dissociation rate increased slowly. Moreover, there was a fast-increasing trend for the
1055
MH dissociation rate with increasing seawater flow rate and decreasing gas flow rate.
1056
When the flow rate was highest (10-1 ml·min-1), it induced the largest average MH
1057
dissociation rate of 0.705 %/min. According to the trend of average MH dissociation
1058
rate, the seawater flow rate was the main factor promoting hydrate dissociation, and
1059
increasing the gas flow rate retards the dissociation process. That is, MH dissociation
1060
from seawater-gas flow increased with higher flow rate ratio and seawater flow rate.
56
1061
The reason is faster heat and mass transfer in hydrate-bearing sediment cores with
1062
increasing seawater-gas flow rate ratio and seawater flow rate, providing greater
1063
driving force to promote hydrate dissociation. So, it can be inferred that the
1064
seawater-gas flow rate of 10-1 ml·min-1 in Case 6 effectively promotes MH
1065
dissociation.
1066 1067
Fig. 6 Comparisons of average MH dissociation rates with different seawater-gas flow
1068
rates
1069
3.3 MH flow process dissociation behavior with different initial MH saturation
1070
To investigate MH dissociation with different initial hydrate saturation, six
1071
experiments were carried out with initial MH saturations of 15% (Case 7), 23% (Case
1072
8), 28% (Case 9), 31% (Case 10), 36% (Case 11), and 41% (Case 12). Fig. 7 shows
1073
the variations of water distribution and MI during MH dissociation induced by
57
1074
seawater-gas flow with different initial hydrate saturation, in which the seawater-gas
1075
flow rate was 5-1 ml·min-1. As shown in Fig. 7 (a), with the same seawater-gas flow
1076
rate, the time to dissociate MH (corresponding to the second image) and the time to
1077
dissociate MH completely (corresponding to the last image) all increased with
1078
increasing initial hydrate saturation. Combined with the MI variation during MH
1079
dissociation in Fig. 7 (b) a sharp MI change first appeared, and the reason have been
1080
discussed in section 3.2.. Then, it increased gradually with hydrate dissociation, and
1081
took more time for the MI to reach stability with higher initial hydrate saturation. In
1082
addition, when the hydrate dissociated completely, the MI remains stable and the
1083
maximum MI value decreases slightly with increasing initial hydrate saturation. This
1084
is because that there was a relatively slow hydrate dissociation process under higher
1085
initial saturation that caused more gas dissolving in liquid water and further decreased
1086
the maximum value of the MI. Fig. 7 suggest that the duration of hydrate dissociation
1087
was longest
1088
phenomenon. The first is hydrate imperviousness (Liu et al., 2016), higher initial
1089
hydrate saturation maintains hydrate-bearing sediment core stability better. The
1090
second was that MH occupies more pore space in porous media under higher initial
1091
hydrate saturation, which decreases the fluid flow space and water phase relative
1092
permeability, further influencing the heat and mass transfer. The third was that there
1093
has a higher chemical potential difference between the seawater and hydrate phases
1094
when the initial hydrate saturation is lower. As reported (Sean et al., 2007), the
under higher initial hydrate saturation. There are three reasons for this
58
1095
chemical potential difference was the main reason for hydrate dissociation in marine
1096
environment above hydrate phase equilibrium, which was defined as the methane
1097
concentration dissolved in the aqueous. During seawater-gas flow process, though
1098
little methane gas dissolved in flowing seawater, the chemical potential difference still
1099
enough for inducing hydrate dissociation. And the seawater-gas flow erosion was a
1100
long-time and continuous process, thereby, the residual free water and gas will be
1101
displaced at last. When the hydrate saturation is lower, the blocking effect of hydrate
1102
on seawater flow was relatively small, so the displace process of free water will be
1103
faster, which caused the chemical potential difference increase and accelerated the
1104
hydrate dissociation. .
1105 1106
Fig. 7 Variations of water distribution (a) and MI (b) during MH dissociation induced
1107
by seawater-gas flow with different initial hydrate saturation
1108
The average dissociation rate is an important parameter in hydrate exploitation
1109
and it is the ratio of hydrate saturation to dissociation duration (Wang et al., 2018).
1110
Fig. 8 shows the comparisons of average MH dissociation rate induced by 59
1111
seawater-gas flow with different initial hydrate saturation. As shown in Fig. 8, the
1112
MH dissociation rate has no obvious linear relationship with initial hydrate saturation,
1113
and the initial hydrate saturation has a mild effect under the same seawater-gas flow
1114
rate. This is because the MH dissociation rate is controlled by hydrate saturation and
1115
dissociation duration. Even though hydrate dissociates quickly, the average
1116
dissociation rate will not increase significantly due to lower hydrate saturation. In
1117
contrast, when hydrate saturation is higher, there also is no high MH dissociation rate
1118
because of the relatively long dissociation duration. Furthermore, the occurrence
1119
structure of hydrate may influence the dissociation behavior. Because the pore-filling
1120
hydrate has more surface area to retard fluid flow, there was a smaller water phase
1121
permeability than grain-coating hydrate (Dai and Seol, 2014), which may influence
1122
the MH dissociation rate, but this phenomenon was less evident in this study.
1123
60
1124
Fig. 8 Comparisons of average MH dissociation rate with different initial hydrate
1125
saturation
1126
3.4 Comparisons of seawater/deionized water-gas flow effects on MH dissociation
1127
The flow channel appearance and disappearance time represents the time at
1128
which the MH begins to dissociate and the end of MH dissociation. The flow channel
1129
appearance time was defined as the moment that a small bright area appeared, as
1130
shown in the second images in Fig. 4 and Fig 7 (a). The flow channel disappearance
1131
time is defined as the moment that the whole image became bright, as shown the last
1132
images in Fig. 4 and Fig 7 (a). The shorter the appearance and disappearance times,
1133
the faster the hydrate dissociation. Fig. 9 shows comparisons of flow channel
1134
appearance and disappearance times between seawater-gas and deionized water-gas
1135
flow. As shown in Fig. 9, the flow channel appearance and disappearance time
1136
obviously decreased with increasing water flow rate both in seawater-gas and
1137
deionized water-gas flow condition. Moreover, the flow channel appearance and
1138
disappearance time in deionized water-gas flow was longer than that of in
1139
seawater-gas flow under the same flow rate, especially in the deionized water-gas
1140
flow rate of 2-2 ml·min-1. The reasons for the above phenomenon are as follows.
1141
Firstly, under the same temperature, Na+ and Cl- can obviously improve the methane
1142
hydrate phase equilibrium pressure (as shown in Fig. 3), which induced a stronger
1143
promotion effect in seawater-gas flow on hydrate dissociation than the deionized
1144
water-gas flow. Secondly, due to the existence of ions, the CH4 gas solubility in 61
1145
seawater than that of in deionized water (Chen et al., 2017, Kim et al., 2008), which
1146
caused the larger chemical potential difference between seawater-hydrate phase and
1147
further provided the stronger driving force for MH dissociation. Thirdly, in our
1148
previous work (Chen et al., 2019b, Chen et al., 2019a, Yang et al., 2019a), we found
1149
that the water flow rate was the main reason for hydrate dissociation above the
1150
hydrate phase equilibrium. The higher water flow rate will accelerate hydrate
1151
dissociation rate by increasing the chemical potential difference and enhancing heat
1152
and mass transfer process. So, compared to others flow rate (3-3, 3-1, 4-1, 5-1
1153
ml·min-1), the lower water flow rate in the deionized water-gas flow rate of 2-2
1154
ml·min-1 provided a smaller driving force for hydrate dissociation and induced a huge
1155
number of disappearance time.
1156 1157
Fig. 9 Comparisons of flow channel appearance and disappearance time of between
1158
seawater-gas and deionized water-gas flow
62
1159
Fig. 10 shows comparisons of average MH dissociation rates between the
1160
seawater-gas and deionized water-gas flow processes. The experimental result and
1161
details of deionized water-gas flow process can be found in published article (Yang et
1162
al., 2019a). As shown in Fig. 10, the average MH dissociation rates with five flow
1163
rates were compared, including 2-2, 3-3, 3-1, 4-1, and 5-1 ml·min-1. In two
1164
experimental conditions (seawater/deionized water-gas flow), the MH dissociation
1165
rate increased with the increase of water flow rate and the decrease of gas flow rate,
1166
and the MH dissociation rate in the seawater-gas flow process was far greater than
1167
that in deionized water-gas flow under the same flow rate. This phenomenon
1168
indicated that the seawater-gas flow experimental conditions promote MH
1169
dissociation more significantly than deionized water-gas flow. The reason was that the
1170
Cl- implants into the water cage and attracts H+ from water molecules, which breaks
1171
the hydrogen bonding structure of the water cage and accelerates hydrate dissociation
1172
(Chong et al., 2015).In addition, because salt reduces methane gas solubility (Yang
1173
and Xu, 2007), the injection and hydrate-dissociated gases form bubbles in seawater
1174
(Chong et al., 2015). In contrast, the methane gas dissolved more uniformly in
1175
deionized water. Therefore, there was larger methane concentration between the
1176
seawater and hydrate phases during the seawater-gas flow process, which obviously
1177
increased the chemical potential difference, and provided a crucial driving force for
1178
hydrate dissociation.
63
1179 1180
Fig. 10 Comparisons of average MH dissociation rate between seawater-gas and
1181
deionized water-gas flow
1182
3.5 The application and shortage of seawater-gas flow inducing MH dissociation
1183
This study verified the promotion effect of seawater-gas flow on hydrate
1184
dissociation. The seawater-gas flow can increase the chemical potential difference,
1185
accelerate the heat and mas transfer process, and inhibit hydrate reformation, which
1186
have great application potential in actual hydrate production tests. In nature, four main
1187
classes (Class 1, Class 2, Class 3, Class 4) of methane hydrate deposits are found
1188
(Moridis et al., 2004). For Class 1 deposits (upper hydrate layer and under free gas
1189
layer), the gas phase ratio was much higher than that of water phase during hydrate
1190
production process. For Class 2 deposits (upper hydrate layer and under free water
1191
layer), the water phase ratio was much higher than that of gas phase during hydrate
1192
production process. For Class 3 and Class 4 deposits (no free water and gas layer), the 64
1193
hydrate production process will always companied company with the huge production
1194
of water and gas. It is surely that the issue of water production is a sticky point in the
1195
hydrate production tests. Therefore, in the actual hydrate exploitation tests, we can
1196
utilize the hydrate dissociated-water and can extra inject some seawater into the
1197
hydrate deposits to induce hydrate dissociation. The seawater flow rate can be
1198
controlled by adjusting the pressure difference between the production wells, and the
1199
gas recovery rate also can be adjusted to achieve the seawater-gas two-phase flow. In
1200
addition, considering to the hydrate production efficiency, the seawater-gas two-phase
1201
flow can be used to combine with other production methods (depressurization) for
1202
real applications, and which will be investigated in our follow-up work.
1203
However, there are also exist some limitations by using seawater to dissociate
1204
hydrate in real applications. Firstly, there will be need huge seawater injection amount
1205
if the water flow erosion method was performed to recovery natural gas in the field
1206
hydrate production test. Secondly, in order to avoid the reservoir collapse caused by
1207
fast hydrate dissociation, the suitable seawater flow rate and flow direction of
1208
seawater flow should be verified in the real hydrate production test. Thirdly, seawater
1209
flow may induce outflow sand and influent the continuous gas production, this issue
1210
also needs solve. Therefore, the results obtained from our test section translatable to
1211
the real case still exist some difference. The current study was a fundamental study,
1212
we will further investigate the effect of water-gas two-phase flow on hydrate
1213
dissociation by increasing the experimental scale. 65
1214
4. Conclusion
1215
A seawater-gas flow experiment was carried out and the effect of seawater-gas
1216
flow rate and initial hydrate saturation on MH dissociation was analyzed. The
1217
experimental conclusions are as follows:
1218
(1) When the flow rate was 1-1 ml·min-1, the seawater-gas flow process promotes
1219
MH dissociation because of the ion and salting-out effects, whereas MH was not
1220
dissociated and hydrate reformation even appeared in the deionized water-gas flow
1221
process. It was shown that seawater-gas flow achieves gas production from hydrate
1222
reservoirs and effectively inhibits hydrate reformation under slower flow rates.
1223
(2) With the same initial hydrate saturation, the seawater-gas flow rate ratio and
1224
pour seawater flow rate were the crucial factor for MH dissociation. The higher
1225
seawater flow rate and lower gas flow rate cause higher average MH dissociation
1226
rates due to the acceleration of heat and mass transfer. In addition, salt ions reduce gas
1227
solubility and water activity further promoting hydrate dissociation.
1228
(3) With the same seawater-gas flow rate, the time to induce MH to dissociate
1229
and the total time for completing MH dissociation all increased with increasing initial
1230
hydrate saturation. According to the trend of the average MH dissociation rate under
1231
different initial hydrate saturation, hydrate saturation has a mild effect on the average
1232
MH dissociation rate.
1233
(4) Seawater-gas flow promotes MH dissociation stronger than deionized
1234
water-gas flow. During seawater-gas flow, the duration of hydrate dissociation 66
1235
decreased and the average MH dissociation rate increased with higher seawater-gas
1236
flow rate ratio and higher seawater flow rate. Moreover, the average MH dissociation
1237
rate in seawater-gas flow was higher than in deionized water-gas flow.
1238
Acknowledgments
1239
This study was financially supported by grants from the National Natural
1240
Science Foundation of China (51436003, 51822603 and 51576025), the National Key
1241
Research and Development Plan of China (2017YFC0307303 and 2016YFC0304001),
1242
the Fok Ying-Tong Education Foundation for Young Teachers in Higher Education
1243
Institutions of China (161050) and the Fundamental Research Funds for the Central
1244
Universities of China (DUT18ZD403).
1245
Conflict of interest
1246
None declared
1247
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73
Highlights • MH dissociation characteristics during seawater-gas flow process were visually studied. • Seawater-gas two-phase flow has a significant promotion effect on MH dissociation. • The MH dissociation rate increased with increasing seawater-gas flow rate ratio. • Initial saturation has mild effect on MH dissociation rate with the same flow rate.
Dear editor: We would like to submit the enclosed manuscript entitled " Effects of multiphase flow on natural gas hydrate production in marine sediment ". It is submitted to be considered for publication as a “Research paper" in your journal. The promotion effects of deionized water-gas flow on gas hydrate dissociation has been studied. Therefore, in consideration of the effect of seawater on hydrate thermodynamic equilibrium, the experiment of seawater-gas flow was performed to dissociate hydrate. In this paper, the effects of different seawater-gas flow rates and different initial hydrate saturation on methane hydrate (MH) production characteristics was analyzed and compared. Due to the huge seawater reserves in hydrate accumulation area, the method of seawater-gas flow to dissociate hydrate is easy to achieve, so the experimental results will offer some reference and guidance for future NGHs exploration in marine environment. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal. We have consulted the Guide for Authors in preparing the submitted manuscript and compliance with the Ethics in Publishing Policy as described in the Guide for Authors. The study is novel and article is well written. We believe the paper may be of particular interest to the readers of your journal. Correspondence and phone calls about the paper should be directed to Mingjun Yang at the following address, phone and fax number, and e-mail address: Name: Mingjun Yang Institute: Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology. Address: No.2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province, P.
R. China., 116024 Telephone: +86-411-84709093. Fax: +86-411-84708015. E-mail: [email protected]. Thank you very much for your considering our manuscript for potential publication. Sincerely yours, Minhjun Yang
Suggested Reviewers： Considering their contributions on hydrate based technology, the following professors are proposed as potential reviewers: (1) Amir H. Mohammadi, MINES ParisTech, CEP/TEP, Fontainebleau, France, E-mail: [email protected] (2) Praveen Linga, National University of Singapore, Singapore, E-mail: [email protected] (3) Xiaosen Li, Research Center of Gas Hydrate, Chinese Academy of Sciences, E-mail: [email protected]
S1875-5100(19)30318-X
DOI:
https://doi.org/10.1016/j.jngse.2019.103066
Reference:
JNGSE 103066
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 10 May 2019 Revised Date:
30 July 2019
Accepted Date: 9 November 2019
Please cite this article as: Sun, H., Chen, B., Yang, M., Effect of multiphase flow on natural gas hydrate production in marine sediment, Journal of Natural Gas Science & Engineering, https://doi.org/10.1016/ j.jngse.2019.103066. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Effect of multiphase flow on natural gas hydrate production in marine sediment
2
Huiru Sun, Bingbing Chen, Mingjun Yang*
3
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of
4
Education, Dalian University of Technology, Dalian 116024, China
5 6
Abstract Natural gas hydrates (NGHs), regarded as an alternative future energy
7
source. Currently, tests for hydrate exploitation from marine sediment have been
8
performed in the Nankai Trough of Japan and the Shenhu area of the South China Sea.
9
Hydrate exploitation is influenced by water-gas flow in the sediment, and considering
10
the huge seawater reserves in hydrate accumulation areas, an experiment of
11
seawater-gas flow was performed to dissociate hydrate. The effects of seawater-gas
12
flow rates and initial hydrate saturation on methane hydrate (MH) production was
13
analyzed. The results showed that seawater-gas flow efficiently promotes hydrate
14
dissociation and inhibits hydrate reformation. Moreover, there was a faster heat and
15
mass transfer with increasing seawater flow rates and decreasing gas flow rates,
16
which enhanced the average MH dissociation rate. In addition, the variation time of
17
the flow channel increased with higher initial hydrate saturation. Additionally,
18
seawater-gas flow promotes MH dissociation stronger than deionized water-gas flow.
19
Keywords: methane hydrate dissociation; water flow erosion; seawater-gas flow; salt
20
ions; hydrate saturation
1
21
1. Introduction
22
Natural gas hydrates (NGHs) are typically found in continental permafrost and
23
marine sediment and they form from hydrocarbon gas and water under high pressure
24
and low temperature (Hiratsuka et al., 2015, Gao et al., 2018). The water molecules
25
interconnected through hydrogen bonding form cage structures to encage gas
26
molecules (Zheng et al., 2019, Saman et al., 2010, Liu et al., 2018). There are rich
27
natural gas hydrate reservoirs distributed around the world, in which the estimated
28
energy potential is far greater than the total amount of conventional coal, oil and
29
natural gas (Wang et al., 2017). NGHs are seen as potential sources of natural gas in
30
view of their high energy density, abundant reserves and nonpollution (Zhao et al.,
31
2014, Xu and Li, 2015). Therefore, researching hydrate formation and dissociation
32
kinetics and developing methods for natural gas recovery from hydrate reservoirs are
33
arousing wide concern in countries and among scientists all over the world in recent
34
years (Wang et al., 2016b, Song et al., 2015b).
35
For now, methods for large-scale production of natural gas from NGHs are still
36
evolving, and they are all based on changing reservoir pressure and temperature to
37
deviate from hydrate thermodynamic equilibrium (Ji et al., 2001). There are four
38
common methods for gas hydrate exploitation as follows. The depressurization
39
method, in which the reservoir pressure is decreased below the hydrate equilibrium
40
pressure at a specified temperature (Reagan et al., 2015, Li et al., 2012, Yang et al.,
41
2019b). The thermal stimulation method, in which the reservoir temperature is 2
42
increased above the hydrate equilibrium temperature via injecting hot water, hot brine
43
or steam (Song et al., 2015a, Nair et al., 2016, Chong et al., 2016b). The chemical
44
inhibitor stimulation method, in which chemicals are injected into the hydrate
45
reservoir to transform the hydrate pressure-temperature equilibrium and promote
46
hydrate dissociation (Li et al., 2011, Chong et al., 2016a). The CO2 replacement
47
method, in which liquid CO2 is injected into hydrate reservoirs to form CO2 hydrate
48
and replace methane gas (Yu et al., 2015, Brewer et al., 2014). The above methods all
49
have their own merits and demerits. The large heat loss of thermal stimulation, the
50
expensive cost and pollution environment of chemical inhibitors, and the low
51
efficiency of CO2 replacement restricts application of those three methods for
52
practical hydrate exploitation (Fitzgerald et al., 2014, Lee et al., 2010, Jung et al.,
53
2010). However, depressurization needs no external energy supply and energy
54
consumption is the least, so that was regarded as the most effective method to recover
55
natural gas from NGH reservoirs considering the economic cost (Chong et al., 2017a,
56
Zhao et al., 2014).
57
The depressurization method was first used for field-scale hydrate production at
58
the Mackenzie Delta Mallik site in the Northwest Territories, Canada, and then a
59
larger-scale test was conducted at the same site in 2007 and 2008 (Yang et al., 2016).
60
However, the hydrate dissociation is an endothermic reaction. When the pressure drop
61
is large, the sensible heat of the hydrate and heat transferred from the ambient
62
environment was not enough to sustain the faster hydrate dissociation reaction. So, 3
63
there may occur the hydrate reformation and ice generation in depressurization
64
process, which will obviously influent the gas production rate (Wang et al., 2018,
65
Zhao et al., 2015, Chen et al., 2015). Therefore, many experimental studies and
66
numerical modeling of hydrate dissociation and gas production using depressurization
67
were conducted. Konno et al. (Konno et al., 2014) performed the gas production test
68
on methane hydrate sediments using one-step and multistep depressurization. They
69
found that a suitable heat for hydrate-bearing sediment is the important factor to drive
70
hydrate dissociation. Zhan et al. (Zhan et al., 2018) used different quartz sand particle
71
sizes to simulate hydrate reservoirs, form methane hydrate, and dissociate hydrate via
72
the depressurization method. They analyzed and compared the effects of sediment
73
particle size on gas production rate and dissociation characteristics. Zhao et al. (Zhao
74
et al., 2012) conducted numerical simulations to investigate the water phase impact on
75
methane hydrate dissociation by depressurization in porous media. Their experimental
76
results showed that the movement of water plays a significant role in late stage
77
thermal conduction, and the gas production rate increased with higher water saturation
78
in late stage dissociation. Based on the available data in the Shenhu Area of the South
79
China Sea, Xiong et al. (Xiong et al., 2012) employed a 1D experimental setup to
80
study methane hydrate dissociation by depressurization, and they found that lower
81
dissociation pressure may lead to higher hydrate dissociation rate and higher
82
dissociation heat. Oyama et al. (Oyama et al., 2012) studied the depressurized
83
dissociation of natural methane-hydrate-bearing sediment with low permeability, and 4
84
they assessed two dissociation models making use of their experimental results. To
85
acquire kinetic hydrate dissociation data under different backpressures, depressurized
86
dissociation of methane hydrate was studied using magnetic resonance imaging (MRI)
87
by Wang et al. (Wang et al., 2015). For the other three hydrate dissociation methods,
88
Su et al. (Su et al., 2013) investigated the gas production potential from hydrate
89
sediment using thermal stimulation. They found that thermal stimulation was effective
90
at promoting gas release from hydrate sediment and enhanced gas production in the
91
late stage of hydrate dissociation. Li et al. (Li et al., 2017) injected methanol into
92
methane-hydrate-bearing sediment to study dissociation behavior employing a
93
one-dimensional experimental device. They proposed new adjustment tactics for
94
experimental operation according to the injection rate ratio of methanol and water.
95
Jung et al. (Jung and Santamarina, 2010) measured the electrical resistance change
96
and relative stiffness in the CH4-CO2 replacement process, and found that the overall
97
hydrate remained solid and gas-hydrate-water phases coexisted for long periods of
98
time. Yuan et al. (Yuan et al., 2013) used a three-dimensional reactor to study
99
CO2-CH4 replacement in hydrate-bearing deposits with liquid CO2, and analyzed the
100
influence of different stratum conditions on the replacement process.
101
Whatever NGH exploitation methods are adopted, hydrate dissociation is always
102
accompanied by production and migration of water and gas. Chen et al. (Chen et al.,
103
2019b) proposed a water flow erosion method to promote hydrate dissociation and
104
investigated the effects of seawater flow on natural gas hydrate dissociation (Chen et 5
105
al., 2019a), whereas the effect of gas migration on hydrate dissociation was ignored.
106
In addition, deionized water-gas two-phase flow promotion of MH dissociation has
107
been studied, and hydrate reformation may appear under slower water-gas flow rates.
108
Therefore, based on those studies and considering the effect of seawater on hydrate
109
thermodynamic equilibrium, experiments of seawater-gas two-phase flow were
110
performed, and different seawater-gas flow rates effect on MH dissociation were
111
analyzed. In addition, the system pressure and temperature are always in the
112
thermodynamic stability region of the hydrate-bearing sediment core. Due to the huge
113
seawater reserves in hydrate accumulation areas, the method of seawater-gas flow to
114
dissociate hydrate is achievable, so the experimental results offer some reference and
115
guidance for future NGH exploration in marine environments.
116
2. Experiments
117
2.1 Apparatus and materials
118
Fig. 1 shows a sketch of the experimental setup designed to investigate MH
119
dissociation during seawater-gas two-phase flow. The experimental setup included an
120
MRI system (Varian, Inc., Palo Alto, CA, USA), a high-pressure vessel (polyimide),
121
three high-precision injection pumps (260D, Teledyne Isco Inc., Lincoln, NE, USA),
122
three thermostat baths (FL300 and FL 25, JULABO, Seelbach, Germany), two pressure
123
transducers (3510CF, Emerson Electric Co., Ltd., St. Louis, USA), a differential
124
pressure sensor (Nagano Keiki, Japan), and a data acquisition system. The MRI system
125
was used to visualize the change of MH distribution during seawater-gas flow, which 6
126
was operated at 400 MHz resonance frequency and 9.4 T magnet field. This MRI
127
system adopted a spin echo sequence to obtain two-dimensional proton
128
density-weighted images. The parameters of the sequence are as follows, which based
129
on Chen et al. (Chen et al., 2019a). Echo time was 4.39 ms, image data matrix was
130
128×128, and the field of view was 30 mm×30 mm (Chen et al., 2019a). Three injection
131
pumps were used to inject liquid, CH4 gas and to control the vessel pressure for
132
seawater-gas flow. The effective volume of injection pump was 260 ml, the accuracy of
133
flow control was ±1 µl·min-1, and accuracy of pressure control was ± 0.5%. The
134
temperatures of the pumps and MRI system were strictly controlled by thermostat baths,
135
and the stability of the thermostat baths was ±0.1
136
differential pressure sensor were used to collect pressure signals in the inlet and outlet
137
of the high-pressure vessel. The accuracy of the pressure transducers was ±0.05%, and
138
the accuracy of the differential pressure sensor was ±0.01%. The data acquisition
139
system included an A/D module (Advantech Co., Ltd., Milpitas, CA, USA) and a
140
computer, which was used to collect and dispose the pressure signals. Deionized water
141
and CH4 gas were used to form methane hydrate (MH). Seawater (configured by
142
deionized water) and CH4 gas were used in the MH dissociation process.
7
. Pressure transducers and a
143 144 145
Fig. 1 Schematic of the experimental system 2.2 Hydrate formation
146
A high-pressure vessel with inner diameter of 15 mm and length of 200 mm was
147
washed, dried, and filled with BZ-02 glass beads (As-One Co., Ltd., Japan). The
148
porosity of BZ-02 glass beads was 35.4%, the particle diameter was 0.177-0.250 mm
149
and the density of BZ-02 was 2.6 g·cm-3. After the leakage test, the vessel was
150
mounted in the center of the MRI magnetic body and vacuumed to expel free gas.
151
Next, the vessel was pressurized to 6000 kPa using water injection pump with a
152
constant injection rate of 20 ml·min-1 to fully saturate the porous media. After
153
maintaining the pressure stable for 1 h, the outlet valve was opened, and the vessel
154
pressure equaled to atmospheric pressure. Then, CH4 gas (Dalian Special Gases Co.,
155
Ltd., China, 99.99%) with 4000 kPa pressure was injected into the vessel to displace
156
the deionized water and obtain partly water-saturated porous media. Then, the vessel
157
was pressurized to 6000 kPa by CH4 gas injection with a constant injection rate of 20
8
158
ml·min-1 and maintained stable to form MH. During the whole process of hydrate
159
formation, the thermostat bath temperature was kept at 274.15 K. Moreover, the
160
images were obtained continuously via MRI for each experiment.
161
2.3 Seawater-gas two-phase flow process
162
The experimental parameters and results of twelve experimental cases are shown
163
in Table 1. Take Case 1 as an example to illustrate the seawater-gas two-phase flow
164
process. The seawater solution (NaCl solution with a mass fraction of 3.5%) was
165
prepared before the experiment. After hydrate formation finished, the outlet valve was
166
opened to interconnect backpressure pump to the vessel. Then, the pressure of
167
backpressure pump was decreased to 3200 kPa form 6000 kPa using backpressure
168
pump with a constant rate of -20 ml·min-1, and maintained that pressure for a while to
169
ensure that no MH dissociation appeared in the pressure adjustment process. When
170
the temperature is 274.15 K, the MH phase equilibrium pressure is 2850 kPa (Jr and
171
Koh, 2007). Therefore, the hydrate was always thermodynamically stable during the
172
backpressure adjustment process. Next, seawater (1ml·min-1) and CH4 (1ml·min-1)
173
were simultaneously injected into the vessel from the bottom to the top by seawater
174
injection pump and gas injection pump. The temperature of the flowing seawater and
175
CH4 gas was controlled at 273.95 K by thermostat bath to prevent hydrate dissociation
176
caused by temperature variation. Images of seawater-gas flow were recorded in
177
succession using an MRI system. In addition, after the seawater-gas flow process,
178
deionized water was used to wash the whole experimental system (pipelines, seawater 9
179
injection pump and high-pressure vessel) approximately 5~6 times till the
180
conductivity was lower than 0.69 µS·cm-1, which avoided residual seawater inhibiting
181
hydrate formation. The conductivities of seawater and deionized water were 2.85*104
182
µS·cm-1 and 0.69 µS·cm-1, respectively(Chen et al., 2019a).
183
Table 1. Experimental parameters and results Case 1 2 3 4 5 6 7 8 9 10 11 12
184 185 186 187 188 189 190
vw (ml/min) 1 2 3 3 4 10 5 5 5 5 5 5
vg (ml/min) 1 2 3 1 1 1 1 1 1 1 1 1
Pi (kPa) 6010 6015 6020 6010 6000 6005 6020 6011 6040 6060 6030 6050
Shi (%) 24.33 23.03 22.42 23.83 23.77 22.56 15.48 22.50 28.19 31.08 36.23 41.48
tapp (min) 80 46 38 24 14 10 8 12 14 14 20 26
tdis (min) 986 532 290 136 88 42 50 68 100 108 112 132
RV (%/min) 0.0269 0.0474 0.0890 0.2128 0.3212 0.7050 0.3686 0.4018 0.3278 0.3306 0.3938 0.3913
a
The symbols in this table are defined as follows: tapp is the flow channel appearance time; tdis is the flow channel disappearance time; Vw is the seawater flow rate during seawater-gas two-phase flow; Vg is the gas flow rate during seawater-gas two-phase flow; Shi is the initial MH saturation before hydrate dissociation (volume fraction); Pi is the MH formation pressure; and Rv is the average MH dissociation rate. All experiments were carried out at 274.15 K, the temperature of flowing seawater and gas was kept at 273.95 K, and the designed backpressure was 3200 kPa for all cases during the seawater-gas flow process.
191
In this study, the average MH dissociation rate was calculated by the flowing
192
equation (Wang et al., 2018), which can reflect the average water and gas production
193
rates.
194
Ri =
S hi − S h (i + ∆t )
(1)
∆t
10
195
where
196
and
197
the two changes of hydrate saturation.
198
3. Results and Discussion
S
Ri
is the MH dissociation rate at i min,
h i + ∆ t
S hi
is the hydrate saturation at i min
are the hydrate saturation at i + ∆t min. ∆t is the time interval between
199
Twelve experimental cases were performed to study the effects of different
200
seawater-gas flow rates and different initial hydrate saturations on MH dissociation.
201
The seawater-gas flow rate was expressed as the seawater flow rate-gas flow rate in
202
this study. In Cases 1~6, the MH saturation was controlled approximately the same
203
(22.5%~23.5%), and the seawater-gas flow rate of that was set to 1-1, 2-2, 3-3, 3-1,
204
4-1, and 10-1 ml·min-1, respectively. In Cases 7~12, the seawater-gas flow rates were
205
all set to 5-1 ml·min-1, and the MH saturation was controlled to approximately 15%,
206
23%, 28%, 31%, 36%, and 41%, respectively. The backpressure was controlled at
207
3200 kPa for all cases, and the temperature of flowing seawater and gas was set to
208
273.95
209
thermodynamically stable. The distribution of water and MH was recorded by the
210
MRI system.
211
3.1 Methane hydrate formation
K,
which
ensured
that
the
hydrate-bearing
sediment
core
was
212
The pressure and temperature were set to 6000 kPa and 274.15 K, and were kept
213
constant for the whole MH formation process for Cases 1~12. MH saturation,
214
expressed as the volume fraction of pore space occupied by hydrate, was computed by
215
mean intensity (MI) data acquired form MRI images (Chen et al., 2019a). Because 1 11
216
m3 of hydrate releases 0.8 m3 of liquid water when it is dissociated, and 1 volume of
217
liquid water forms 1.25 volumes of hydrate under standard temperature and pressure
218
(STP) (Jr and Koh, 2007). Hence, the MH saturation at the “i” minute can be
219
computed with the following equation (Chen et al., 2019b):
220
S hi = 1.25 ×
221
where
222
values in the images at t = 0 and t = i minutes, respectively. The details of hydrate
223
saturation for each case are shown in Table 1.
(I 0 − I i )× S w0 × 100 %
S w0
(2)
I0
represents the initial water saturation, and
I0
and
Ii
represent the MI
224
Because of a similar phenomenon, take Case 2 as an example to illustrate the
225
MH formation process. Fig. 2 shows the MI variation during MH formation process,
226
which reflected the amount of free water in porous media. Moreover, three MRI
227
images were used to visually show the liquid water distribution during different stages
228
of MH formation. Bright red and dark purple represent the brightest (more water) and
229
darkest (more hydrate) signals, respectively. As shown in Fig. 2, the MI was stable at
230
stage I and liquid water distribution in the MRI image was uniform, indicating there
231
was no hydrate formation and water saturation remained invariant. At stage II, the MI
232
first appeared to rapidly decline (points A~B) and then the downtrend became a slight
233
slowdown (points B~C). Meanwhile, the MRI image was darker than stage I. This
234
was because that when the MH starts to form, the rapid hydrate formation obviously
235
consumes the amount of liquid water and decreases the MI value. Then, MH
236
formation influences the contact between the gas phase and liquid water, so hydrate 12
237
formation rate slows down. When the hydrate formation is completed, the MI
238
gradually comes to stability at stage III and the image becomes the darkest. From the
239
MRI images, it can be observed that MH formation is nonuniform in porous media
240
because there is always migration of water and gas in the MH formation process.
241
Moreover, MI was nonzero after MH formation completion, indicating that residual
242
water still exists in porous media.
243 244 245
Fig. 2 Variations of MI and water distribution during MH formation process 3.2 Effects of different seawater-gas flow rates on MH dissociation
246
In our previous study (Yang et al., 2019a), when the deionized water-gas flow
247
rate was 1-1 ml·min-1, MH did not dissociate and even the hydrate reformation
248
phenomenon appeared during the deionized water-gas flow. However, MH always
249
dissociates during the seawater-gas flow process even if the seawater-gas flow rate is
250
slower, which suggests the seawater-gas flow obviously promotes MH dissociation 13
251
and inhibits MH reformation. There are two reasons explaining this phenomenon. The
252
first is the seawater solution ion effect that forms an electric field between positive
253
and negative ions to destroy the cluster structure of water molecules. When the water
254
molecules form a lattice, they overcome the electric field and further shift the hydrate
255
phase equilibrium curve to the left (Yang et al., 2012, Chong et al., 2017b), as shown
256
in Fig. 3 the MH hydrate phase equilibrium curve under the seawater and deionized
257
water experimental condition. Therefore, compared to deionized water-gas flow
258
process, hydrate stability decreases and easily dissociates in the seawater-gas flow
259
process. The second is that the bonds between water and ions are Coulombic that
260
stronger than hydrogen bonding or van der Waals forces (Yang et al., 2012),
261
resulting in water molecules were attracted by ions and further reduce the water
262
molecular activity (Atik et al., 2006). Moreover, the accumulation of Cl- decreases gas
263
molecule solubility in seawater. The phenomenon of salting-out appeared (Chen et al.,
264
2017, Kim et al., 2008) with a negative effect on hydrate reformation. Based on the
265
above discussion, recycling seawater-gas flow was effective for gas hydrate
266
production.
14
267 268
Fig. 3 MH phase equilibrium curves under the seawater and deionized water
269
experimental condition
270
Fig. 4 shows the variation of water distribution during MH dissociation with
271
continuous seawater-gas flow, in which the seawater-gas flow rates were 1-1 (Case 1),
272
2-2 (Case 2), 3-3 (Case 3), 3-1 (Case 4), 4-1 (Case 5), 5-1 (Case 8), and 10-1
273
ml·min-1(Case 6), respectively. Moreover, bright green signals represent liquid water,
274
and dark blue signals are MH and a small quantity of methane gas. As shown in Fig. 4,
275
MH dissociated gradually with the seawater-gas flow process. At 2 min, the
276
hydrate-bearing sediment core was saturated by seawater, and the dark blue image
277
suggested there was no hydrate dissociation. With continuous seawater-gas flow, the
278
images showed a partial bright green area, illustrating hydrate dissociated and water
279
saturation increased. Then, the bright area gradually enlarged along the
280
seawater-gas-hydrate three phase interface until the whole image became bright.
15
281
When the whole image became bright (corresponding to the last image), the MH
282
dissociation was finished and the water saturation in porous media reached maximum.
283
Fig. 4 also showed that when the seawater-gas flow rate was 1-1, 2-2 and 3-3
284
ml·min-1 (Cases 1-3), the duration of hydrate dissociation was relatively long. When
285
the flow rate was 10-1 ml·min-1 (Case 6), the duration was the shortest for
286
dissociating hydrate. That is, the higher seawater flow rate and lower gas flow rate
287
will accelerate MH dissociation process. The reasons are addressed below. The first
288
was that seawater salt invades the hydrate-dissociated water due to the concentration
289
difference, and the salt contamination resulted in a better heat conduction (Sun and
290
Mohanty, 2006, Chong et al., 2015). The second was the faster heat transfer during
291
seawater-gas flow process. Because the hydrate dissociation is an endothermic
292
reaction process, a lot of heat can be absorbed when the hydrate suddenly dissociated
293
and caused a rapid decline of hydrate-bearing sediment temperature, even caused the
294
hydrate reformation and ice generation. During the continuous seawater-gas flow
295
process, the temperature of flowing seawater and gas (273.95 K) was constant, which
296
can provide continuous heat transfer and keep the stability of sediment temperature.
297
Therefore, the plummeting temperature of hydrate-bearing sediment cores was not
298
appeared, which is benefit for the hydrate dissociation process, and further preventing
299
the hydrate reformation and ice generation. The third was that increasing the seawater
300
flow rate and decreasing the gas flow rate will increase the chemical potential
16
301
difference, accelerate the seawater phase mass transfer and further provide higher
302
driving force for dissociating hydrate.
303 304
Fig. 4 Variations of water distribution during MH dissociation process with different
305
seawater-gas flow rates
306
Fig. 5 shows the variations of MI during MH dissociation with different
307
seawater-gas flow rates, which reflected the amount variations of free water in porous
308
media. As shown in Fig. 5, the MI increased to approximately 1200 within a few
309
minutes at points A~B. This was because that after hydrate formation process, little
310
amount free water distributed in sediment, the large amount of free gas, sand and
311
hydrate caused the lowest value of MI. Then, when the seawater phase flow into the
312
sediment, the increase of the amount of free water caused the sharp increase of MI
313
Next, the MI appeared a sharp change under the seven flow rates, in which the MI 17
314
suddenly decrease and then gradually increase. The reasons for the phenomenon are
315
as follows. Firstly, due to the Jamin effect (Wang et al., 2016a, Yang et al., 2019a),
316
some parts of free gas still distributed under the area of field view (FOV), these parts
317
of free gas will be further displaced into the FOV by seawater. Because the MRI
318
system can only identify the 1H containing in liquid, so the gas gathered in FOV
319
caused the MI decrease. Secondly, with the continuous seawater flow, the hydrate will
320
gradually dissociate and the free water will gradually obtain the released pore space,
321
and ultimately caused the MI gradually increase. In Fig. 5 (a), when the seawater-gas
322
flow rate was 1-1 ml·min-1, the upward tendency of MI was the slowest at points B~
323
C1. In Fig. 5 (b), there has a rapidly increase of MI at points B~C (C4, C5), which
324
indicated the MH dissociation under the seawater-gas flow rates of 3-1 and 4-1
325
ml·min-1 was significantly faster than the results of in Fig. 5(a). Moreover, with the
326
flow rate further increased (5-1 and 10-1 ml·min-1), the variation of MI transforms
327
into the tension trend at points B~C (C6, C7) because of the quickening MH
328
dissociation. Finally, when the hydrate was dissociated completely, the amount of
329
water in the pore space was invariable and the MI was stable. In addition, the higher
330
seawater flow rate and the lower gas flow rate, the faster for MI reached stability.
331
Combined Figs. 4 and 5, jointly proves that seawater-gas flow can effectively promote
332
MH dissociation.
18
333 334
Fig. 5 Variations of MI during MH dissociation process with different seawater-gas
335
flow rates
336
The average MH dissociation rate is a main parameter reflecting the average gas
337
and water production rates. Fig. 6 shows comparisons of average MH dissociation rate
338
with different seawater-gas flow rates. As shown in Fig. 6, the average MH
339
dissociation rate is a function of the seawater-gas flow rate ratio and pour seawater
340
flow rate, which increased with the increase of flow rate ratio and pore seawater flow
341
rate. When the seawater-gas flow rate was 1-1, 2-2 and 3-3 ml·min-1, the MH
342
dissociation rate increased slowly. Moreover, there was a fast-increasing trend for the
343
MH dissociation rate with increasing seawater flow rate and decreasing gas flow rate.
344
When the flow rate was highest (10-1 ml·min-1), it induced the largest average MH
345
dissociation rate of 0.705 %/min. According to the trend of average MH dissociation
346
rate, the seawater flow rate was the main factor promoting hydrate dissociation, and
347
increasing the gas flow rate retards the dissociation process. That is, MH dissociation
348
from seawater-gas flow increased with higher flow rate ratio and seawater flow rate.
19
349
The reason is faster heat and mass transfer in hydrate-bearing sediment cores with
350
increasing seawater-gas flow rate ratio and seawater flow rate, providing greater
351
driving force to promote hydrate dissociation. So, it can be inferred that the
352
seawater-gas flow rate of 10-1 ml·min-1 in Case 6 effectively promotes MH
353
dissociation.
354 355
Fig. 6 Comparisons of average MH dissociation rates with different seawater-gas flow
356
rates
357
3.3 MH flow process dissociation behavior with different initial MH saturation
358
To investigate MH dissociation with different initial hydrate saturation, six
359
experiments were carried out with initial MH saturations of 15% (Case 7), 23% (Case
360
8), 28% (Case 9), 31% (Case 10), 36% (Case 11), and 41% (Case 12). Fig. 7 shows
361
the variations of water distribution and MI during MH dissociation induced by
20
362
seawater-gas flow with different initial hydrate saturation, in which the seawater-gas
363
flow rate was 5-1 ml·min-1. As shown in Fig. 7 (a), with the same seawater-gas flow
364
rate, the time to dissociate MH (corresponding to the second image) and the time to
365
dissociate MH completely (corresponding to the last image) all increased with
366
increasing initial hydrate saturation. Combined with the MI variation during MH
367
dissociation in Fig. 7 (b) a sharp MI change first appeared, and the reason have been
368
discussed in section 3.2. Then, it increased gradually with hydrate dissociation, and
369
took more time for the MI to reach stability with higher initial hydrate saturation. In
370
addition, when the hydrate dissociated completely, the MI remains stable and the
371
maximum MI value decreases slightly with increasing initial hydrate saturation. This
372
is because that there was a relatively slow hydrate dissociation process under higher
373
initial saturation that caused more gas dissolving in liquid water and further decreased
374
the maximum value of the MI. Fig. 7 suggest that the duration of hydrate dissociation
375
was longest under higher initial hydrate saturation. There are three reasons for this
376
phenomenon. The first is hydrate imperviousness (Liu et al., 2016), higher initial
377
hydrate saturation maintains hydrate-bearing sediment core stability better. The
378
second was that MH occupies more pore space in porous media under higher initial
379
hydrate saturation, which decreases the fluid flow space and water phase relative
380
permeability, further influencing the heat and mass transfer. The third was that there
381
has a higher chemical potential difference between the seawater and hydrate phases
382
when the initial hydrate saturation is lower. As reported (Sean et al., 2007), the 21
383
chemical potential difference was the main reason for hydrate dissociation in marine
384
environment above hydrate phase equilibrium, which was defined as the methane
385
concentration dissolved in the aqueous. During seawater-gas flow process, though
386
little methane gas dissolved in flowing seawater, the chemical potential difference still
387
enough for inducing hydrate dissociation. And the seawater-gas flow erosion was a
388
long-time and continuous process, thereby, the residual free water and gas will be
389
displaced at last. When the hydrate saturation is lower, the blocking effect of hydrate
390
on seawater flow was relatively small, so the displace process of free water will be
391
faster, which caused the chemical potential difference increase and accelerated the
392
hydrate dissociation.
393 394
Fig. 7 Variations of water distribution (a) and MI (b) during MH dissociation induced
395
by seawater-gas flow with different initial hydrate saturation
396
The average dissociation rate is an important parameter in hydrate exploitation
397
and it is the ratio of hydrate saturation to dissociation duration (Wang et al., 2018).
398
Fig. 8 shows the comparisons of average MH dissociation rate induced by 22
399
seawater-gas flow with different initial hydrate saturation. As shown in Fig. 8, the
400
MH dissociation rate has no obvious linear relationship with initial hydrate saturation,
401
and the initial hydrate saturation has a mild effect under the same seawater-gas flow
402
rate. This is because the MH dissociation rate is controlled by hydrate saturation and
403
dissociation duration. Even though hydrate dissociates quickly, the average
404
dissociation rate will not increase significantly due to lower hydrate saturation. In
405
contrast, when hydrate saturation is higher, there also is no high MH dissociation rate
406
because of the relatively long dissociation duration. Furthermore, the occurrence
407
structure of hydrate may influence the dissociation behavior. Because the pore-filling
408
hydrate has more surface area to retard fluid flow, there was a smaller water phase
409
permeability than grain-coating hydrate (Dai and Seol, 2014), which may influence
410
the MH dissociation rate, but this phenomenon was less evident in this study.
411
23
412
Fig. 8 Comparisons of average MH dissociation rate with different initial hydrate
413
saturation
414
3.4 Comparisons of seawater/deionized water-gas flow effects on MH dissociation
415
The flow channel appearance and disappearance time represents the time at
416
which the MH begins to dissociate and the end of MH dissociation. The flow channel
417
appearance time was defined as the moment that a small bright area appeared, as
418
shown in the second images in Fig. 4 and Fig 7 (a). The flow channel disappearance
419
time is defined as the moment that the whole image became bright, as shown the last
420
images in Fig. 4 and Fig 7 (a). The shorter the appearance and disappearance times,
421
the faster the hydrate dissociation. Fig. 9 shows comparisons of flow channel
422
appearance and disappearance times between seawater-gas and deionized water-gas
423
flow. As shown in Fig. 9, the flow channel appearance and disappearance time
424
obviously decreased with increasing water flow rate both in seawater-gas and
425
deionized water-gas flow condition. Moreover, the flow channel appearance and
426
disappearance time in deionized water-gas flow was longer than that of in
427
seawater-gas flow under the same flow rate, especially in the deionized water-gas
428
flow rate of 2-2 ml·min-1. The reasons for the above phenomenon are as follows.
429
Firstly, under the same temperature, Na+ and Cl- can obviously improve the methane
430
hydrate phase equilibrium pressure (as shown in Fig. 3), which induced a stronger
431
promotion effect in seawater-gas flow on hydrate dissociation than the deionized
432
water-gas flow. Secondly, due to the existence of ions, the CH4 gas solubility in 24
433
seawater than that of in deionized water (Chen et al., 2017, Kim et al., 2008), which
434
caused the larger chemical potential difference between seawater-hydrate phase and
435
further provided the stronger driving force for MH dissociation. Thirdly, in our
436
previous work (Chen et al., 2019b, Chen et al., 2019a, Yang et al., 2019a), we found
437
that the water flow rate was the main reason for hydrate dissociation above the
438
hydrate phase equilibrium. The higher water flow rate will accelerate hydrate
439
dissociation rate by increasing the chemical potential difference and enhancing heat
440
and mass transfer process. So, compared to others flow rate (3-3, 3-1, 4-1, 5-1
441
ml·min-1), the lower water flow rate in the deionized water-gas flow rate of 2-2
442
ml·min-1 provided a smaller driving force for hydrate dissociation and induced a huge
443
number of disappearance time.
444 445
Fig. 9 Comparisons of flow channel appearance and disappearance time of between
446
seawater-gas and deionized water-gas flow
25
447
Fig. 10 shows comparisons of average MH dissociation rates between the
448
seawater-gas and deionized water-gas flow processes. The experimental result and
449
details of deionized water-gas flow process can be found in published article (Yang et
450
al., 2019a). As shown in Fig. 10, the average MH dissociation rates with five flow
451
rates were compared, including 2-2, 3-3, 3-1, 4-1, and 5-1 ml·min-1. In two
452
experimental conditions (seawater/deionized water-gas flow), the MH dissociation
453
rate increased with the increase of water flow rate and the decrease of gas flow rate,
454
and the MH dissociation rate in the seawater-gas flow process was far greater than
455
that in deionized water-gas flow under the same flow rate. This phenomenon
456
indicated that the seawater-gas flow experimental conditions promote MH
457
dissociation more significantly than deionized water-gas flow. The reason was that the
458
Cl- implants into the water cage and attracts H+ from water molecules, which breaks
459
the hydrogen bonding structure of the water cage and accelerates hydrate dissociation
460
(Chong et al., 2015).In addition, because salt reduces methane gas solubility (Yang
461
and Xu, 2007), the injection and hydrate-dissociated gases form bubbles in seawater
462
(Chong et al., 2015). In contrast, the methane gas dissolved more uniformly in
463
deionized water. Therefore, there was larger methane concentration between the
464
seawater and hydrate phases during the seawater-gas flow process, which obviously
465
increased the chemical potential difference, and provided a crucial driving force for
466
hydrate dissociation.
26
467 468
Fig. 10 Comparisons of average MH dissociation rate between seawater-gas and
469
deionized water-gas flow
470
3.5 The application and shortage of seawater-gas flow inducing MH dissociation
471
This study verified the promotion effect of seawater-gas flow on hydrate
472
dissociation. The seawater-gas flow can increase the chemical potential difference,
473
accelerate the heat and mas transfer process, and inhibit hydrate reformation, which
474
have great application potential in actual hydrate production tests. In nature, four main
475
classes (Class 1, Class 2, Class 3, Class 4) of methane hydrate deposits are found
476
(Moridis et al., 2004). For Class 1 deposits (upper hydrate layer and under free gas
477
layer), the gas phase ratio was much higher than that of water phase during hydrate
478
production process. For Class 2 deposits (upper hydrate layer and under free water
479
layer), the water phase ratio was much higher than that of gas phase during hydrate
480
production process. For Class 3 and Class 4 deposits (no free water and gas layer), the 27
481
hydrate production process will always companied company with the huge production
482
of water and gas. It is surely that the issue of water production is a sticky point in the
483
hydrate production tests. Therefore, in the actual hydrate exploitation tests, we can
484
utilize the hydrate dissociated-water and can extra inject some seawater into the
485
hydrate deposits to induce hydrate dissociation. The seawater flow rate can be
486
controlled by adjusting the pressure difference between the production wells, and the
487
gas recovery rate also can be adjusted to achieve the seawater-gas two-phase flow. In
488
addition, considering to the hydrate production efficiency, the seawater-gas two-phase
489
flow can be used to combine with other production methods (depressurization) for
490
real applications, and which will be investigated in our follow-up work.
491
However, there are also exist some limitations by using seawater to dissociate
492
hydrate in real applications. Firstly, there will be need huge seawater injection amount
493
if the water flow erosion method was performed to recovery natural gas in the field
494
hydrate production test. Secondly, in order to avoid the reservoir collapse caused by
495
fast hydrate dissociation, the suitable seawater flow rate and flow direction of
496
seawater flow should be verified in the real hydrate production test. Thirdly, seawater
497
flow may induce outflow sand and influent the continuous gas production, this issue
498
also needs solve. Therefore, the results obtained from our test section translatable to
499
the real case still exist some difference. The current study was a fundamental study,
500
we will further investigate the effect of water-gas two-phase flow on hydrate
501
dissociation by increasing the experimental scale. 28
502
4. Conclusion
503
A seawater-gas flow experiment was carried out and the effect of seawater-gas
504
flow rate and initial hydrate saturation on MH dissociation was analyzed. The
505
experimental conclusions are as follows:
506
(1) When the flow rate was 1-1 ml·min-1, the seawater-gas flow process promotes
507
MH dissociation because of the ion and salting-out effects, whereas MH was not
508
dissociated and hydrate reformation even appeared in the deionized water-gas flow
509
process. It was shown that seawater-gas flow achieves gas production from hydrate
510
reservoirs and effectively inhibits hydrate reformation under slower flow rates.
511
(2) With the same initial hydrate saturation, the seawater-gas flow rate ratio and
512
pour seawater flow rate were the crucial factor for MH dissociation. The higher
513
seawater flow rate and lower gas flow rate cause higher average MH dissociation
514
rates due to the acceleration of heat and mass transfer. In addition, salt ions reduce gas
515
solubility and water activity further promoting hydrate dissociation.
516
(3) With the same seawater-gas flow rate, the time to induce MH to dissociate
517
and the total time for completing MH dissociation all increased with increasing initial
518
hydrate saturation. According to the trend of the average MH dissociation rate under
519
different initial hydrate saturation, hydrate saturation has a mild effect on the average
520
MH dissociation rate.
521
(4) Seawater-gas flow promotes MH dissociation stronger than deionized
522
water-gas flow. During seawater-gas flow, the duration of hydrate dissociation 29
523
decreased and the average MH dissociation rate increased with higher seawater-gas
524
flow rate ratio and higher seawater flow rate. Moreover, the average MH dissociation
525
rate in seawater-gas flow was higher than in deionized water-gas flow.
526
Acknowledgments
527
This study was financially supported by grants from the National Natural
528
Science Foundation of China (51436003, 51822603 and 51576025), the National Key
529
Research and Development Plan of China (2017YFC0307303 and 2016YFC0304001),
530
the Fok Ying-Tong Education Foundation for Young Teachers in Higher Education
531
Institutions of China (161050) and the Fundamental Research Funds for the Central
532
Universities of China (DUT18ZD403).
533
Conflict of interest
534
None declared
535
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SONG, Y. C., WANG, S. L., YANG, M. J., LIU, W. G., ZHAO, J. F. & WANG, S. R.
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2015b. MRI measurements of CO2 –CH4 hydrate formation and dissociation in
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porous media. Fuel, 140, 126-135.
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SU, Z., HUANG, L., WU, N. Y. & YANG, S. X. 2013. Effect of thermal stimulation
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on gas production from hydrate deposits in Shenhu area of the South China
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Sea. Science China Earth Sciences, 56, 601-610.
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SUN, X. F. & MOHANTY, K. K. 2006. Kinetic simulation of methane hydrate
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formation and dissociation in porous media. Chemical Engineering Science,
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61, 3476-3495.
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WANG, B., FAN, Z., WANG, P. F., LIU, Y., ZHAO, J. F. & SONG, Y. C. 2018.
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Analysis of depressurization mode on gas recovery from methane hydrate
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deposits and the concomitant ice generation. Applied Energy, 227, 624-633.
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WANG, J. Q., ZHAO, J. F., ZHANG, Y., WANG, D. Y., LI, Y. H. & SONG, Y. C.
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2016a. Analysis of the effect of particle size on permeability in 34
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hydrate-bearing porous media using pore network models combined with CT.
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Fuel, 163, 34-40.
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WANG, P. F., YANG, M. J., CHEN, B. B., ZHAO, Y. C., ZHAO, J. F. & SONG, Y.
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C. 2017. Methane hydrate reformation in porous media with methane
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migration. Chemical Engineering Science, 168, 344-351.
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WANG, S. L., YANG, M. J., WANG, P. F., ZHAO, Y. C. & SONG, Y. C. 2015. In
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situ observation of methane hydrate dissociation under different backpressures.
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Energy & Fuels, 29, 3251-3256.
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WANG, Y., FENG, J. C., LI, X. S., ZHANG, Y. & LI, G. 2016b. Large scale
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experimental evaluation to methane hydrate dissociation below quadruple
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point in sandy sediment. Applied Energy, 162, 372-381.
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XIONG, L. J., LI, X. S., WANG, Y. & XU, C. G. 2012. Experimental study on
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methane hydrate dissociation by depressurization in porous sediments.
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Energies, 5, 518-530.
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XU, C. G. & LI, X. S. 2015. Research progress on methane production from natural gas hydrates. RSC Advances, 5, 54672-54699. YANG, D. H. & XU, W. Y. 2007. Effects of salinity on methane gas hydrate system. Science in China Series D: Earth Sciences, 50, 1733-1745. YANG, M. J., SONG, Y. C., LIU, Y., LAM, W. H., LI, Q. P. & YU, X. C. 2012.
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Effects of halogen ions on phase equilibrium of methane hydrate in porous
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media. International Journal of Thermophysics, 33, 821-830.
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YANG, M. J., SUN, H. R., CHEN, B. B. & SONG, Y. C. 2019a. Effects of water-gas two-phase flow on methane hydrate dissociation in porous media. Fuel, 255. YANG, M. J., ZHE, F., ZHAO, Y. C., JIANG, L. L., ZHAO, J. F. & SONG, Y. C.
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2016. Effect of depressurization pressure on methane recovery from hydrate–
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gas–water bearing sediments. Fuel, 166, 419-426.
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YANG, M. J., ZHENG, J. N., GAO, Y., MA, Z. Q., LV, X. & SONG, Y. C. 2019b.
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Dissociation characteristics of methane hydrates in South China Sea sediments
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by depressurization. Applied Energy, 243, 266-273.
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YU, Z., XIONG, L. J., LI, X. S., CHEN, Z. Y. & XU, C. G. 2015. Replacement of
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CH4 in hydrate in porous sediments with liquid CO2 injection. Chemical
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Engineering & Technology, 37, 2022-2029.
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YUAN, Q., SUN, C. Y., LIU, B., WANG, X., MA, Z. W., MA, Q. L., YANG, L. Y.,
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CHEN, G. J., LI, Q. P., LI, S. & ZHANG, K. 2013. Methane recovery from
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natural gas hydrate in porous sediment using pressurized liquid CO2. Energy
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Conversion and Management, 67, 257-264.
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ZHAN, L., WANG, Y. & LI, X. S. 2018. Experimental study on characteristics of
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methane hydrate formation and dissociation in porous medium with different
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particle sizes using depressurization. Fuel, 230, 37-44.
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ZHAO, J. F., LIU, D., YANG, M. J. & SONG, Y. C. 2014. Analysis of heat transfer
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effects on gas production from methane hydrate by depressurization.
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International Journal of Heat and Mass Transfer, 77, 529-541.
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ZHAO, J. F., YE, C. C., SONG, Y. C., LIU, W. G., CHENG, C. X., LIU, Y., ZHANG,
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Y., WANG, D. Y. & RUAN, X. K. 2012. Numerical simulation and analysis
702
of water phase effect on methane hydrate dissociation by depressurization.
703
Industrial & Engineering Chemistry Research, 51, 3108-3118.
704
ZHAO, J. F., ZHU, Z. H., SONG, Y. C., LIU, W. G., ZHANG, Y. & WANG, D. Y.
705
2015. Analyzing the process of gas production for natural gas hydrate using
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depressurization. Applied Energy, 142, 125-134.
707
ZHENG, J. N., CHENG, F. B., LI, Y. P., LV, X. & YANG, M. J. 2019. Progress and
708
trends in hydrate based desalination (HBD) technology: A review. Chinese
709
Journal of Chemical Engineering.
710 711 36
712
37
713
Effect of multiphase flow on natural gas hydrate production in marine sediment
714
Huiru Sun, Bingbing Chen, Mingjun Yang*
715
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of
716
Education, Dalian University of Technology, Dalian 116024, China
717 718
Abstract Natural gas hydrates (NGHs), regarded as an alternative future energy
719
source. Currently, tests for hydrate exploitation from marine sediment have been
720
performed in the Nankai Trough of Japan and the Shenhu area of the South China Sea.
721
Hydrate exploitation is influenced by water-gas flow, and considering the huge
722
seawater reserves in hydrate accumulation areas, an experiment of seawater-gas flow
723
was performed to dissociate hydrate. The effects of seawater-gas flow rates and initial
724
hydrate saturation on methane hydrate (MH) production was analyzed. The results
725
showed that seawater-gas flow efficiently promotes hydrate dissociation and inhibits
726
hydrate reformation. Moreover, there was a faster heat and mass transfer with
727
increasing seawater flow rates and decreasing gas flow rates, which enhanced the
728
average MH dissociation rate. In addition, the variation time of the flow channel
729
increased with higher initial hydrate saturation. Additionally, seawater-gas flow
730
promotes MH dissociation stronger than deionized water-gas flow.
731
Keywords: methane hydrate dissociation; water flow erosion; seawater-gas flow; salt
732
ions; hydrate saturation
38
733
1. Introduction
734
Natural gas hydrates (NGHs) are typically found in continental permafrost and
735
marine sediment and they form from hydrocarbon gas and water under high pressure
736
and low temperature (Hiratsuka et al., 2015, Gao et al., 2018). The water molecules
737
interconnected through hydrogen bonding form cage structures to encage gas
738
molecules (Zheng et al., 2019, Saman et al., 2010, Liu et al., 2018). There are rich
739
natural gas hydrate reservoirs distributed around the world, in which the estimated
740
energy potential is far greater than the total amount of conventional coal, oil and
741
natural gas (Wang et al., 2017). NGHs are seen as potential sources of natural gas in
742
view of their high energy density, abundant reserves and nonpollution (Zhao et al.,
743
2014, Xu and Li, 2015). Therefore, researching hydrate formation and dissociation
744
kinetics and developing methods for natural gas recovery from hydrate reservoirs are
745
arousing wide concern in countries and among scientists all over the world in recent
746
years (Wang et al., 2016b, Song et al., 2015b).
747
For now, methods for large-scale production of natural gas from NGHs are still
748
evolving, and they are all based on changing reservoir pressure and temperature to
749
deviate from hydrate thermodynamic equilibrium (Ji et al., 2001). There are four
750
common methods for gas hydrate exploitation as follows. The depressurization
751
method, in which the reservoir pressure is decreased below the hydrate equilibrium
752
pressure at a specified temperature (Reagan et al., 2015, Li et al., 2012, Yang et al.,
753
2019b). The thermal stimulation method, in which the reservoir temperature is 39
754
increased above the hydrate equilibrium temperature via injecting hot water, hot brine
755
or steam (Song et al., 2015a, Nair et al., 2016, Chong et al., 2016b). The chemical
756
inhibitor stimulation method, in which chemicals are injected into the hydrate
757
reservoir to transform the hydrate pressure-temperature equilibrium and promote
758
hydrate dissociation (Li et al., 2011, Chong et al., 2016a). The CO2 replacement
759
method, in which liquid CO2 is injected into hydrate reservoirs to form CO2 hydrate
760
and replace methane gas (Yu et al., 2015, Brewer et al., 2014). The above methods all
761
have their own merits and demerits. The large heat loss of thermal stimulation, the
762
expensive cost and pollution environment of chemical inhibitors, and the low
763
efficiency of CO2 replacement restricts application of those three methods for
764
practical hydrate exploitation (Fitzgerald et al., 2014, Lee et al., 2010, Jung et al.,
765
2010). However, depressurization needs no external energy supply and energy
766
consumption is the least, so that was regarded as the most effective method to recover
767
natural gas from NGH reservoirs considering the economic cost (Chong et al., 2017a,
768
Zhao et al., 2014).
769
The depressurization method was first used for field-scale hydrate production at
770
the Mackenzie Delta Mallik site in the Northwest Territories, Canada, and then a
771
larger-scale test was conducted at the same site in 2007 and 2008 (Yang et al., 2016).
772
However, the hydrate dissociation is an endothermic reaction. When the pressure drop
773
is large, the sensible heat of the hydrate and heat transferred from the ambient
774
environment was not enough to sustain the faster hydrate dissociation reaction. So, 40
775
there may occur the hydrate reformation and ice generation in depressurization
776
process, which will obviously influent the gas production rate (Wang et al., 2018,
777
Zhao et al., 2015, Chen et al., 2015). Therefore, many experimental studies and
778
numerical modeling of hydrate dissociation and gas production using depressurization
779
were conducted. Konno et al. (Konno et al., 2014) performed the gas production test
780
on methane hydrate sediments using one-step and multistep depressurization. They
781
found that a suitable heat for hydrate-bearing sediment is the important factor to drive
782
hydrate dissociation. Zhan et al. (Zhan et al., 2018) used different quartz sand particle
783
sizes to simulate hydrate reservoirs, form methane hydrate, and dissociate hydrate via
784
the depressurization method. They analyzed and compared the effects of sediment
785
particle size on gas production rate and dissociation characteristics. Zhao et al. (Zhao
786
et al., 2012) conducted numerical simulations to investigate the water phase impact on
787
methane hydrate dissociation by depressurization in porous media. Their experimental
788
results showed that the movement of water plays a significant role in late stage
789
thermal conduction, and the gas production rate increased with higher water saturation
790
in late stage dissociation. Based on the available data in the Shenhu Area of the South
791
China Sea, Xiong et al. (Xiong et al., 2012) employed a 1D experimental setup to
792
study methane hydrate dissociation by depressurization, and they found that lower
793
dissociation pressure may lead to higher hydrate dissociation rate and higher
794
dissociation heat. Oyama et al. (Oyama et al., 2012) studied the depressurized
795
dissociation of natural methane-hydrate-bearing sediment with low permeability, and 41
796
they assessed two dissociation models making use of their experimental results. To
797
acquire kinetic hydrate dissociation data under different backpressures, depressurized
798
dissociation of methane hydrate was studied using magnetic resonance imaging (MRI)
799
by Wang et al. (Wang et al., 2015). For the other three hydrate dissociation methods,
800
Su et al. (Su et al., 2013) investigated the gas production potential from hydrate
801
sediment using thermal stimulation. They found that thermal stimulation was effective
802
at promoting gas release from hydrate sediment and enhanced gas production in the
803
late stage of hydrate dissociation. Li et al. (Li et al., 2017) injected methanol into
804
methane-hydrate-bearing sediment to study dissociation behavior employing a
805
one-dimensional experimental device. They proposed new adjustment tactics for
806
experimental operation according to the injection rate ratio of methanol and water.
807
Jung et al. (Jung and Santamarina, 2010) measured the electrical resistance change
808
and relative stiffness in the CH4-CO2 replacement process, and found that the overall
809
hydrate remained solid and gas-hydrate-water phases coexisted for long periods of
810
time. Yuan et al. (Yuan et al., 2013) used a three-dimensional reactor to study
811
CO2-CH4 replacement in hydrate-bearing deposits with liquid CO2, and analyzed the
812
influence of different stratum conditions on the replacement process.
813
Whatever NGH exploitation methods are adopted, hydrate dissociation is always
814
accompanied by production and migration of water and gas. Chen et al. (Chen et al.,
815
2019b) proposed a water flow erosion method to promote hydrate dissociation and
816
investigated the effects of seawater flow on natural gas hydrate dissociation (Chen et 42
817
al., 2019a), whereas the effect of gas migration on hydrate dissociation was ignored.
818
In addition, deionized water-gas two-phase flow promotion of MH dissociation has
819
been studied, and hydrate reformation may appear under slower water-gas flow rates.
820
Therefore, based on those studies and considering the effect of seawater on hydrate
821
thermodynamic equilibrium, experiments of seawater-gas two-phase flow were
822
performed, and different seawater-gas flow rates effect on MH dissociation were
823
analyzed. In addition, the system pressure and temperature are always in the
824
thermodynamic stability region of the hydrate-bearing sediment core. Due to the huge
825
seawater reserves in hydrate accumulation areas, the method of seawater-gas flow to
826
dissociate hydrate is achievable, so the experimental results offer some reference and
827
guidance for future NGH exploration in marine environments.
828
2. Experiments
829
2.1 Apparatus and materials
830
Fig. 1 shows a sketch of the experimental setup designed to investigate MH
831
dissociation during seawater-gas two-phase flow. The experimental setup included an
832
MRI system (Varian, Inc., Palo Alto, CA, USA), a high-pressure vessel (polyimide),
833
three high-precision injection pumps (260D, Teledyne Isco Inc., Lincoln, NE, USA),
834
three thermostat baths (FL300 and FL 25, JULABO, Seelbach, Germany), two pressure
835
transducers (3510CF, Emerson Electric Co., Ltd., St. Louis, USA), a differential
836
pressure sensor (Nagano Keiki, Japan), and a data acquisition system. The MRI system
837
was used to visualize the change of MH distribution during seawater-gas flow, which 43
838
was operated at 400 MHz resonance frequency and 9.4 T magnet field. This MRI
839
system adopted a spin echo sequence to obtain two-dimensional proton
840
density-weighted images. The parameters of the sequence are as follows, which based
841
on Chen et al. (Chen et al., 2019a). Echo time was 4.39 ms, image data matrix was
842
128×128, and the field of view was 30 mm×30 mm (Chen et al., 2019a). Three injection
843
pumps were used to inject liquid, CH4 gas and to control the vessel pressure for
844
seawater-gas flow. The effective volume of injection pump was 260 ml, the accuracy of
845
flow control was ±1 µl·min-1, and accuracy of pressure control was ± 0.5%. The
846
temperatures of the pumps and MRI system were strictly controlled by thermostat baths,
847
and the stability of the thermostat baths was ±0.1
848
differential pressure sensor were used to collect pressure signals in the inlet and outlet
849
of the high-pressure vessel. The accuracy of the pressure transducers was ±0.05%, and
850
the accuracy of the differential pressure sensor was ±0.01%. The data acquisition
851
system included an A/D module (Advantech Co., Ltd., Milpitas, CA, USA) and a
852
computer, which was used to collect and dispose the pressure signals. Deionized water
853
and CH4 gas were used to form methane hydrate (MH). Seawater (configured by
854
deionized water) and CH4 gas were used in the MH dissociation process.
44
. Pressure transducers and a
855 856 857
Fig. 1 Schematic of the experimental system 2.2 Hydrate formation
858
A high-pressure vessel with inner diameter of 15 mm and length of 200 mm was
859
washed, dried, and filled with BZ-02 glass beads (As-One Co., Ltd., Japan). The
860
porosity of BZ-02 glass beads was 35.4%, the particle diameter was 0.177-0.250 mm
861
and the density of BZ-02 was 2.6 g·cm-3. After the leakage test, the vessel was
862
mounted in the center of the MRI magnetic body and vacuumed to expel free gas.
863
Next, the vessel was pressurized to 6000 kPa using water injection pump with a
864
constant injection rate of 20 ml·min-1 to fully saturate the porous media. After
865
maintaining the pressure stable for 1 h, the outlet valve was opened, and the vessel
866
pressure equaled to atmospheric pressure. Then, CH4 gas (Dalian Special Gases Co.,
867
Ltd., China, 99.99%) with 4000 kPa pressure was injected into the vessel to displace
868
the deionized water and obtain partly water-saturated porous media. Then, the vessel
869
was pressurized to 6000 kPa by CH4 gas injection with a constant injection rate of 20
45
870
ml·min-1 and maintained stable to form MH. During the whole process of hydrate
871
formation, the thermostat bath temperature was kept at 274.15 K. Moreover, the
872
images were obtained continuously via MRI for each experiment.
873
2.3 Seawater-gas two-phase flow process
874
The experimental parameters and results of twelve experimental cases are shown
875
in Table 1. Take Case 1 as an example to illustrate the seawater-gas two-phase flow
876
process. The seawater solution (NaCl solution with a mass fraction of 3.5%) was
877
prepared before the experiment. After hydrate formation finished, the outlet valve was
878
opened to interconnect backpressure pump to the vessel. Then, the pressure of
879
backpressure pump was decreased to 3200 kPa form 6000 kPa using backpressure
880
pump with a constant rate of -20 ml·min-1, and maintained that pressure for a while to
881
ensure that no MH dissociation appeared in the pressure adjustment process. When
882
the temperature is 274.15 K, the MH phase equilibrium pressure is 2850 kPa (Jr and
883
Koh, 2007). Therefore, the hydrate was always thermodynamically stable during the
884
backpressure adjustment process. Next, seawater (1ml·min-1) and CH4 (1ml·min-1)
885
were simultaneously injected into the vessel from the bottom to the top by seawater
886
injection pump and gas injection pump. The temperature of the flowing seawater and
887
CH4 gas was controlled at 273.95 K by thermostat bath to prevent hydrate dissociation
888
caused by temperature variation. Images of seawater-gas flow were recorded in
889
succession using an MRI system. In addition, after the seawater-gas flow process,
890
deionized water was used to wash the whole experimental system (pipelines, seawater 46
891
injection pump and high-pressure vessel) approximately 5~6 times till the
892
conductivity was lower than 0.69 µS·cm-1, which avoided residual seawater inhibiting
893
hydrate formation. The conductivities of seawater and deionized water were 2.85*104
894
µS·cm-1 and 0.69 µS·cm-1, respectively(Chen et al., 2019a).
895
Table 1. Experimental parameters and results Case 1 2 3 4 5 6 7 8 9 10 11 12
896 897 898 899 900 901 902
vw (ml/min) 1 2 3 3 4 10 5 5 5 5 5 5
vg (ml/min) 1 2 3 1 1 1 1 1 1 1 1 1
Pi (kPa) 6010 6015 6020 6010 6000 6005 6020 6011 6040 6060 6030 6050
Shi (%) 24.33 23.03 22.42 23.83 23.77 22.56 15.48 22.50 28.19 31.08 36.23 41.48
tapp (min) 80 46 38 24 14 10 8 12 14 14 20 26
tdis (min) 986 532 290 136 88 42 50 68 100 108 112 132
RV (%/min) 0.0269 0.0474 0.0890 0.2128 0.3212 0.7050 0.3686 0.4018 0.3278 0.3306 0.3938 0.3913
a
The symbols in this table are defined as follows: tapp is the flow channel appearance time; tdis is the flow channel disappearance time; Vw is the seawater flow rate during seawater-gas two-phase flow; Vg is the gas flow rate during seawater-gas two-phase flow; Shi is the initial MH saturation before hydrate dissociation (volume fraction); Pi is the MH formation pressure; and Rv is the average MH dissociation rate. All experiments were carried out at 274.15 K, the temperature of flowing seawater and gas was kept at 273.95 K, and the designed backpressure was 3200 kPa for all cases during the seawater-gas flow process.
903
In this study, the average MH dissociation rate was calculated by the flowing
904
equation (Wang et al., 2018), which can reflect the average water and gas production
905
rates.
906
Ri =
S hi − S h (i + ∆t )
(1)
∆t
47
907
where
908
and
909
the two changes of hydrate saturation.
910
3. Results and Discussion
S
Ri
is the MH dissociation rate at i min,
h i + ∆ t
S hi
is the hydrate saturation at i min
are the hydrate saturation at i + ∆t min. ∆t is the time interval between
911
Twelve experimental cases were performed to study the effects of different
912
seawater-gas flow rates and different initial hydrate saturations on MH dissociation.
913
The seawater-gas flow rate was expressed as the seawater flow rate-gas flow rate in
914
this study. In Cases 1~6, the MH saturation was controlled approximately the same
915
(22.5%~23.5%), and the seawater-gas flow rate of that was set to 1-1, 2-2, 3-3, 3-1,
916
4-1, and 10-1 ml·min-1, respectively. In Cases 7~12, the seawater-gas flow rates were
917
all set to 5-1 ml·min-1, and the MH saturation was controlled to approximately 15%,
918
23%, 28%, 31%, 36%, and 41%, respectively. The backpressure was controlled at
919
3200 kPa for all cases, and the temperature of flowing seawater and gas was set to
920
273.95
921
thermodynamically stable. The distribution of water and MH was recorded by the
922
MRI system.
923
3.1 Methane hydrate formation
K,
which
ensured
that
the
hydrate-bearing
sediment
core
was
924
The pressure and temperature were set to 6000 kPa and 274.15 K, and were kept
925
constant for the whole MH formation process for Cases 1~12. MH saturation,
926
expressed as the volume fraction of pore space occupied by hydrate, was computed by
927
mean intensity (MI) data acquired form MRI images (Chen et al., 2019a). Because 1 48
928
m3 of hydrate releases 0.8 m3 of liquid water when it is dissociated, and 1 volume of
929
liquid water forms 1.25 volumes of hydrate under standard temperature and pressure
930
(STP) (Jr and Koh, 2007). Hence, the MH saturation at the “i” minute can be
931
computed with the following equation (Chen et al., 2019b):
932
S hi = 1.25 ×
933
where
934
values in the images at t = 0 and t = i minutes, respectively. The details of hydrate
935
saturation for each case are shown in Table 1.
(I 0 − I i )× S w0 × 100 %
S w0
(2)
I0
represents the initial water saturation, and
I0
and
Ii
represent the MI
936
Because of a similar phenomenon, take Case 2 as an example to illustrate the
937
MH formation process. Fig. 2 shows the MI variation during MH formation process,
938
which reflected the amount of free water in porous media. Moreover, three MRI
939
images were used to visually show the liquid water distribution during different stages
940
of MH formation. Bright red and dark purple represent the brightest (more water) and
941
darkest (more hydrate) signals, respectively. As shown in Fig. 2, the MI was stable at
942
stage I and liquid water distribution in the MRI image was uniform, indicating there
943
was no hydrate formation and water saturation remained invariant. At stage II, the MI
944
first appeared to rapidly decline (points A~B) and then the downtrend became a slight
945
slowdown (points B~C). Meanwhile, the MRI image was darker than stage I. This
946
was because that when the MH starts to form, the rapid hydrate formation obviously
947
consumes the amount of liquid water and decreases the MI value. Then, MH
948
formation influences the contact between the gas phase and liquid water, so hydrate 49
949
formation rate slows down. When the hydrate formation is completed, the MI
950
gradually comes to stability at stage III and the image becomes the darkest. From the
951
MRI images, it can be observed that MH formation is nonuniform in porous media
952
because there is always migration of water and gas in the MH formation process.
953
Moreover, MI was nonzero after MH formation completion, indicating that residual
954
water still exists in porous media.
955 956 957
Fig. 2 Variations of MI and water distribution during MH formation process 3.2 Effects of different seawater-gas flow rates on MH dissociation
958
In our previous study (Yang et al., 2019a), when the deionized water-gas flow
959
rate was 1-1 ml·min-1, MH did not dissociate and even the hydrate reformation
960
phenomenon appeared during the deionized water-gas flow. However, MH always
961
dissociates during the seawater-gas flow process even if the seawater-gas flow rate is
962
slower, which suggests the seawater-gas flow obviously promotes MH dissociation 50
963
and inhibits MH reformation. There are two reasons explaining this phenomenon. The
964
first was the seawater solution ion effect that forms an electric field between positive
965
and negative ions to destroy the cluster structure of water molecules. When the water
966
molecules form a lattice, they overcome the electric field and further shift the hydrate
967
phase equilibrium curve to the left (Yang et al., 2012, Chong et al., 2017b), as shown
968
in Fig. 3 the MH hydrate phase equilibrium curve under the seawater and deionized
969
water experimental condition. Therefore, compared to deionized water-gas flow
970
process, hydrate stability decreases and easily dissociates in the seawater-gas flow
971
process. The second is that the bonds between water and ions are Coulombic that
972
stronger than hydrogen bonding or van der Waals forces (Yang et al., 2012),
973
resulting in water molecules were attracted by ions and further reduce the water
974
molecular activity (Atik et al., 2006). Moreover, the accumulation of Cl- decreases gas
975
molecule solubility in seawater. The phenomenon of salting-out appeared (Chen et al.,
976
2017, Kim et al., 2008) with a negative effect on hydrate reformation. Based on the
977
above discussion, recycling seawater-gas flow was effective for gas hydrate
978
production.
51
979 980
Fig. 3 MH phase equilibrium curves under the seawater and deionized water
981
experimental condition
982
Fig. 4 shows the variation of water distribution during MH dissociation with
983
continuous seawater-gas flow, in which the seawater-gas flow rates were 1-1 (Case 1),
984
2-2 (Case 2), 3-3 (Case 3), 3-1 (Case 4), 4-1 (Case 5), 5-1 (Case 8), and 10-1
985
ml·min-1(Case 6), respectively. Moreover, bright green signals represent liquid water,
986
and dark blue signals are MH and a small quantity of methane gas. As shown in Fig. 4,
987
MH dissociated gradually with the seawater-gas flow process. At 2 min, the
988
hydrate-bearing sediment core was saturated by seawater, and the dark blue image
989
suggested there was no hydrate dissociation. With continuous seawater-gas flow, the
990
images showed a partial bright green area, illustrating hydrate dissociated and water
991
saturation increased. Then, the bright area gradually enlarged along the
992
seawater-gas-hydrate three phase interface until the whole image became bright.
52
993
When the whole image became bright (corresponding to the last image), the MH
994
dissociation was finished and the water saturation in porous media reached maximum.
995
Fig. 4 also showed that when the seawater-gas flow rate were 1-1, 2-2 and 3-3
996
ml·min-1 (Cases 1-3), the duration of hydrate dissociation was
997
the flow rate was 10-1 ml·min-1 (Case 6), the duration was the shortest for
998
dissociating hydrate. That is, the higher seawater flow rate and lower gas flow rate
999
will accelerate MH dissociation process. The reasons are addressed below. The first is
1000
that seawater salt invades the hydrate-dissociated water due to the concentration
1001
difference, and the salt contamination resulted in a better heat conduction (Sun and
1002
Mohanty, 2006, Chong et al., 2015). The second was the faster heat transfer during
1003
seawater-gas flow process. Because the hydrate dissociation is an endothermic
1004
reaction process, a lot of heat can be absorbed when the hydrate suddenly dissociated
1005
and caused a rapid decline of hydrate-bearing sediment temperature, even caused the
1006
hydrate reformation and ice generation. During the continuous seawater-gas flow
1007
process, the temperature of flowing seawater and gas (273.95 K) was constant, which
1008
can provide continuous heat transfer and keep the stability of sediment temperature.
1009
Therefore, the plummeting temperature of hydrate-bearing sediment cores was not
1010
appeared, which is benefit for the hydrate dissociation process, and further preventing
1011
the hydrate reformation and ice generation. The third was that increasing the seawater
1012
flow rate and decreasing the gas flow rate will increase the chemical potential
53
relatively long. When
1013
difference, accelerate the seawater phase mass transfer and further provide higher
1014
driving force for dissociating hydrate.
1015 1016
Fig. 4 Variations of water distribution during MH dissociation process with different
1017
seawater-gas flow rates
1018
Fig. 5 shows the variations of MI during MH dissociation with different
1019
seawater-gas flow rates, which reflected the amount variations of free water in porous
1020
media. As shown in Fig. 5, the MI increased to approximately 1200 within a few
1021
minutes at points A~B. This was because that after hydrate formation process, little
1022
amount free water distributed in sediment, the large amount of free gas, sand and
1023
hydrate caused the lowest value of MI. Then, when the seawater phase flow into the
1024
sediment, the increase of the amount of free water caused the sharp increase of MI
1025
Next, the MI appeared a sharp change under the seven flow rates, in which the MI 54
1026
suddenly decrease and then gradually increase. The reasons for the phenomenon are
1027
as follows. Firstly, due to the Jamin effect (Wang et al., 2016a, Yang et al., 2019a),
1028
some parts of free gas still distributed under the area of field view (FOV), these parts
1029
of free gas will be further displaced into the FOV by seawater. Because the MRI
1030
system can only identify the 1H containing in liquid, so the gas gathered in FOV
1031
caused the MI decrease. Secondly, with the continuous seawater flow, the hydrate will
1032
gradually dissociate and the free water will gradually obtain the released pore space,
1033
and ultimately caused the MI gradually increase. In Fig. 5 (a), when the seawater-gas
1034
flow rate was 1-1 ml·min-1, the upward tendency of MI was the slowest at points B~
1035
C1. In Fig. 5 (b), there has a rapidly increase of MI at points B~C (C4, C5), which
1036
indicated the MH dissociation under the seawater-gas flow rates of 3-1 and 4-1
1037
ml·min-1 was significantly faster than the results of in Fig. 5(a). Moreover, with the
1038
flow rate further increased (5-1 and 10-1 ml·min-1), the variation of MI transforms
1039
into the tension trend at points B~C (C6, C7) because of the quickening MH
1040
dissociation. Finally, when the hydrate was dissociated completely, the amount of
1041
water in the pore space was invariable and the MI was stable. In addition, the higher
1042
seawater flow rate and the lower gas flow rate, the faster for MI reached stability.
1043
Combined Figs. 4 and 5, jointly proves that seawater-gas flow can effectively
1044
promote MH dissociation.
55
1045 1046
Fig. 5 Variations of MI during MH dissociation process with different seawater-gas
1047
flow rates
1048
The average MH dissociation rate is a main parameter reflecting the average gas
1049
and water production rates. Fig. 6 shows comparisons of average MH dissociation rate
1050
with different seawater-gas flow rates. As shown in Fig. 6, the average MH
1051
dissociation rate is a function of the seawater-gas flow rate ratio and pour seawater
1052
flow rate, which increased with the increase of flow rate ratio and pore seawater flow
1053
rate. When the seawater-gas flow rate were 1-1, 2-2 and 3-3 ml·min-1, the MH
1054
dissociation rate increased slowly. Moreover, there was a fast-increasing trend for the
1055
MH dissociation rate with increasing seawater flow rate and decreasing gas flow rate.
1056
When the flow rate was highest (10-1 ml·min-1), it induced the largest average MH
1057
dissociation rate of 0.705 %/min. According to the trend of average MH dissociation
1058
rate, the seawater flow rate was the main factor promoting hydrate dissociation, and
1059
increasing the gas flow rate retards the dissociation process. That is, MH dissociation
1060
from seawater-gas flow increased with higher flow rate ratio and seawater flow rate.
56
1061
The reason is faster heat and mass transfer in hydrate-bearing sediment cores with
1062
increasing seawater-gas flow rate ratio and seawater flow rate, providing greater
1063
driving force to promote hydrate dissociation. So, it can be inferred that the
1064
seawater-gas flow rate of 10-1 ml·min-1 in Case 6 effectively promotes MH
1065
dissociation.
1066 1067
Fig. 6 Comparisons of average MH dissociation rates with different seawater-gas flow
1068
rates
1069
3.3 MH flow process dissociation behavior with different initial MH saturation
1070
To investigate MH dissociation with different initial hydrate saturation, six
1071
experiments were carried out with initial MH saturations of 15% (Case 7), 23% (Case
1072
8), 28% (Case 9), 31% (Case 10), 36% (Case 11), and 41% (Case 12). Fig. 7 shows
1073
the variations of water distribution and MI during MH dissociation induced by
57
1074
seawater-gas flow with different initial hydrate saturation, in which the seawater-gas
1075
flow rate was 5-1 ml·min-1. As shown in Fig. 7 (a), with the same seawater-gas flow
1076
rate, the time to dissociate MH (corresponding to the second image) and the time to
1077
dissociate MH completely (corresponding to the last image) all increased with
1078
increasing initial hydrate saturation. Combined with the MI variation during MH
1079
dissociation in Fig. 7 (b) a sharp MI change first appeared, and the reason have been
1080
discussed in section 3.2.. Then, it increased gradually with hydrate dissociation, and
1081
took more time for the MI to reach stability with higher initial hydrate saturation. In
1082
addition, when the hydrate dissociated completely, the MI remains stable and the
1083
maximum MI value decreases slightly with increasing initial hydrate saturation. This
1084
is because that there was a relatively slow hydrate dissociation process under higher
1085
initial saturation that caused more gas dissolving in liquid water and further decreased
1086
the maximum value of the MI. Fig. 7 suggest that the duration of hydrate dissociation
1087
was longest
1088
phenomenon. The first is hydrate imperviousness (Liu et al., 2016), higher initial
1089
hydrate saturation maintains hydrate-bearing sediment core stability better. The
1090
second was that MH occupies more pore space in porous media under higher initial
1091
hydrate saturation, which decreases the fluid flow space and water phase relative
1092
permeability, further influencing the heat and mass transfer. The third was that there
1093
has a higher chemical potential difference between the seawater and hydrate phases
1094
when the initial hydrate saturation is lower. As reported (Sean et al., 2007), the
under higher initial hydrate saturation. There are three reasons for this
58
1095
chemical potential difference was the main reason for hydrate dissociation in marine
1096
environment above hydrate phase equilibrium, which was defined as the methane
1097
concentration dissolved in the aqueous. During seawater-gas flow process, though
1098
little methane gas dissolved in flowing seawater, the chemical potential difference still
1099
enough for inducing hydrate dissociation. And the seawater-gas flow erosion was a
1100
long-time and continuous process, thereby, the residual free water and gas will be
1101
displaced at last. When the hydrate saturation is lower, the blocking effect of hydrate
1102
on seawater flow was relatively small, so the displace process of free water will be
1103
faster, which caused the chemical potential difference increase and accelerated the
1104
hydrate dissociation. .
1105 1106
Fig. 7 Variations of water distribution (a) and MI (b) during MH dissociation induced
1107
by seawater-gas flow with different initial hydrate saturation
1108
The average dissociation rate is an important parameter in hydrate exploitation
1109
and it is the ratio of hydrate saturation to dissociation duration (Wang et al., 2018).
1110
Fig. 8 shows the comparisons of average MH dissociation rate induced by 59
1111
seawater-gas flow with different initial hydrate saturation. As shown in Fig. 8, the
1112
MH dissociation rate has no obvious linear relationship with initial hydrate saturation,
1113
and the initial hydrate saturation has a mild effect under the same seawater-gas flow
1114
rate. This is because the MH dissociation rate is controlled by hydrate saturation and
1115
dissociation duration. Even though hydrate dissociates quickly, the average
1116
dissociation rate will not increase significantly due to lower hydrate saturation. In
1117
contrast, when hydrate saturation is higher, there also is no high MH dissociation rate
1118
because of the relatively long dissociation duration. Furthermore, the occurrence
1119
structure of hydrate may influence the dissociation behavior. Because the pore-filling
1120
hydrate has more surface area to retard fluid flow, there was a smaller water phase
1121
permeability than grain-coating hydrate (Dai and Seol, 2014), which may influence
1122
the MH dissociation rate, but this phenomenon was less evident in this study.
1123
60
1124
Fig. 8 Comparisons of average MH dissociation rate with different initial hydrate
1125
saturation
1126
3.4 Comparisons of seawater/deionized water-gas flow effects on MH dissociation
1127
The flow channel appearance and disappearance time represents the time at
1128
which the MH begins to dissociate and the end of MH dissociation. The flow channel
1129
appearance time was defined as the moment that a small bright area appeared, as
1130
shown in the second images in Fig. 4 and Fig 7 (a). The flow channel disappearance
1131
time is defined as the moment that the whole image became bright, as shown the last
1132
images in Fig. 4 and Fig 7 (a). The shorter the appearance and disappearance times,
1133
the faster the hydrate dissociation. Fig. 9 shows comparisons of flow channel
1134
appearance and disappearance times between seawater-gas and deionized water-gas
1135
flow. As shown in Fig. 9, the flow channel appearance and disappearance time
1136
obviously decreased with increasing water flow rate both in seawater-gas and
1137
deionized water-gas flow condition. Moreover, the flow channel appearance and
1138
disappearance time in deionized water-gas flow was longer than that of in
1139
seawater-gas flow under the same flow rate, especially in the deionized water-gas
1140
flow rate of 2-2 ml·min-1. The reasons for the above phenomenon are as follows.
1141
Firstly, under the same temperature, Na+ and Cl- can obviously improve the methane
1142
hydrate phase equilibrium pressure (as shown in Fig. 3), which induced a stronger
1143
promotion effect in seawater-gas flow on hydrate dissociation than the deionized
1144
water-gas flow. Secondly, due to the existence of ions, the CH4 gas solubility in 61
1145
seawater than that of in deionized water (Chen et al., 2017, Kim et al., 2008), which
1146
caused the larger chemical potential difference between seawater-hydrate phase and
1147
further provided the stronger driving force for MH dissociation. Thirdly, in our
1148
previous work (Chen et al., 2019b, Chen et al., 2019a, Yang et al., 2019a), we found
1149
that the water flow rate was the main reason for hydrate dissociation above the
1150
hydrate phase equilibrium. The higher water flow rate will accelerate hydrate
1151
dissociation rate by increasing the chemical potential difference and enhancing heat
1152
and mass transfer process. So, compared to others flow rate (3-3, 3-1, 4-1, 5-1
1153
ml·min-1), the lower water flow rate in the deionized water-gas flow rate of 2-2
1154
ml·min-1 provided a smaller driving force for hydrate dissociation and induced a huge
1155
number of disappearance time.
1156 1157
Fig. 9 Comparisons of flow channel appearance and disappearance time of between
1158
seawater-gas and deionized water-gas flow
62
1159
Fig. 10 shows comparisons of average MH dissociation rates between the
1160
seawater-gas and deionized water-gas flow processes. The experimental result and
1161
details of deionized water-gas flow process can be found in published article (Yang et
1162
al., 2019a). As shown in Fig. 10, the average MH dissociation rates with five flow
1163
rates were compared, including 2-2, 3-3, 3-1, 4-1, and 5-1 ml·min-1. In two
1164
experimental conditions (seawater/deionized water-gas flow), the MH dissociation
1165
rate increased with the increase of water flow rate and the decrease of gas flow rate,
1166
and the MH dissociation rate in the seawater-gas flow process was far greater than
1167
that in deionized water-gas flow under the same flow rate. This phenomenon
1168
indicated that the seawater-gas flow experimental conditions promote MH
1169
dissociation more significantly than deionized water-gas flow. The reason was that the
1170
Cl- implants into the water cage and attracts H+ from water molecules, which breaks
1171
the hydrogen bonding structure of the water cage and accelerates hydrate dissociation
1172
(Chong et al., 2015).In addition, because salt reduces methane gas solubility (Yang
1173
and Xu, 2007), the injection and hydrate-dissociated gases form bubbles in seawater
1174
(Chong et al., 2015). In contrast, the methane gas dissolved more uniformly in
1175
deionized water. Therefore, there was larger methane concentration between the
1176
seawater and hydrate phases during the seawater-gas flow process, which obviously
1177
increased the chemical potential difference, and provided a crucial driving force for
1178
hydrate dissociation.
63
1179 1180
Fig. 10 Comparisons of average MH dissociation rate between seawater-gas and
1181
deionized water-gas flow
1182
3.5 The application and shortage of seawater-gas flow inducing MH dissociation
1183
This study verified the promotion effect of seawater-gas flow on hydrate
1184
dissociation. The seawater-gas flow can increase the chemical potential difference,
1185
accelerate the heat and mas transfer process, and inhibit hydrate reformation, which
1186
have great application potential in actual hydrate production tests. In nature, four main
1187
classes (Class 1, Class 2, Class 3, Class 4) of methane hydrate deposits are found
1188
(Moridis et al., 2004). For Class 1 deposits (upper hydrate layer and under free gas
1189
layer), the gas phase ratio was much higher than that of water phase during hydrate
1190
production process. For Class 2 deposits (upper hydrate layer and under free water
1191
layer), the water phase ratio was much higher than that of gas phase during hydrate
1192
production process. For Class 3 and Class 4 deposits (no free water and gas layer), the 64
1193
hydrate production process will always companied company with the huge production
1194
of water and gas. It is surely that the issue of water production is a sticky point in the
1195
hydrate production tests. Therefore, in the actual hydrate exploitation tests, we can
1196
utilize the hydrate dissociated-water and can extra inject some seawater into the
1197
hydrate deposits to induce hydrate dissociation. The seawater flow rate can be
1198
controlled by adjusting the pressure difference between the production wells, and the
1199
gas recovery rate also can be adjusted to achieve the seawater-gas two-phase flow. In
1200
addition, considering to the hydrate production efficiency, the seawater-gas two-phase
1201
flow can be used to combine with other production methods (depressurization) for
1202
real applications, and which will be investigated in our follow-up work.
1203
However, there are also exist some limitations by using seawater to dissociate
1204
hydrate in real applications. Firstly, there will be need huge seawater injection amount
1205
if the water flow erosion method was performed to recovery natural gas in the field
1206
hydrate production test. Secondly, in order to avoid the reservoir collapse caused by
1207
fast hydrate dissociation, the suitable seawater flow rate and flow direction of
1208
seawater flow should be verified in the real hydrate production test. Thirdly, seawater
1209
flow may induce outflow sand and influent the continuous gas production, this issue
1210
also needs solve. Therefore, the results obtained from our test section translatable to
1211
the real case still exist some difference. The current study was a fundamental study,
1212
we will further investigate the effect of water-gas two-phase flow on hydrate
1213
dissociation by increasing the experimental scale. 65
1214
4. Conclusion
1215
A seawater-gas flow experiment was carried out and the effect of seawater-gas
1216
flow rate and initial hydrate saturation on MH dissociation was analyzed. The
1217
experimental conclusions are as follows:
1218
(1) When the flow rate was 1-1 ml·min-1, the seawater-gas flow process promotes
1219
MH dissociation because of the ion and salting-out effects, whereas MH was not
1220
dissociated and hydrate reformation even appeared in the deionized water-gas flow
1221
process. It was shown that seawater-gas flow achieves gas production from hydrate
1222
reservoirs and effectively inhibits hydrate reformation under slower flow rates.
1223
(2) With the same initial hydrate saturation, the seawater-gas flow rate ratio and
1224
pour seawater flow rate were the crucial factor for MH dissociation. The higher
1225
seawater flow rate and lower gas flow rate cause higher average MH dissociation
1226
rates due to the acceleration of heat and mass transfer. In addition, salt ions reduce gas
1227
solubility and water activity further promoting hydrate dissociation.
1228
(3) With the same seawater-gas flow rate, the time to induce MH to dissociate
1229
and the total time for completing MH dissociation all increased with increasing initial
1230
hydrate saturation. According to the trend of the average MH dissociation rate under
1231
different initial hydrate saturation, hydrate saturation has a mild effect on the average
1232
MH dissociation rate.
1233
(4) Seawater-gas flow promotes MH dissociation stronger than deionized
1234
water-gas flow. During seawater-gas flow, the duration of hydrate dissociation 66
1235
decreased and the average MH dissociation rate increased with higher seawater-gas
1236
flow rate ratio and higher seawater flow rate. Moreover, the average MH dissociation
1237
rate in seawater-gas flow was higher than in deionized water-gas flow.
1238
Acknowledgments
1239
This study was financially supported by grants from the National Natural
1240
Science Foundation of China (51436003, 51822603 and 51576025), the National Key
1241
Research and Development Plan of China (2017YFC0307303 and 2016YFC0304001),
1242
the Fok Ying-Tong Education Foundation for Young Teachers in Higher Education
1243
Institutions of China (161050) and the Fundamental Research Funds for the Central
1244
Universities of China (DUT18ZD403).
1245
Conflict of interest
1246
None declared
1247
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1248
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Highlights • MH dissociation characteristics during seawater-gas flow process were visually studied. • Seawater-gas two-phase flow has a significant promotion effect on MH dissociation. • The MH dissociation rate increased with increasing seawater-gas flow rate ratio. • Initial saturation has mild effect on MH dissociation rate with the same flow rate.
Dear editor: We would like to submit the enclosed manuscript entitled " Effects of multiphase flow on natural gas hydrate production in marine sediment ". It is submitted to be considered for publication as a “Research paper" in your journal. The promotion effects of deionized water-gas flow on gas hydrate dissociation has been studied. Therefore, in consideration of the effect of seawater on hydrate thermodynamic equilibrium, the experiment of seawater-gas flow was performed to dissociate hydrate. In this paper, the effects of different seawater-gas flow rates and different initial hydrate saturation on methane hydrate (MH) production characteristics was analyzed and compared. Due to the huge seawater reserves in hydrate accumulation area, the method of seawater-gas flow to dissociate hydrate is easy to achieve, so the experimental results will offer some reference and guidance for future NGHs exploration in marine environment. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal. We have consulted the Guide for Authors in preparing the submitted manuscript and compliance with the Ethics in Publishing Policy as described in the Guide for Authors. The study is novel and article is well written. We believe the paper may be of particular interest to the readers of your journal. Correspondence and phone calls about the paper should be directed to Mingjun Yang at the following address, phone and fax number, and e-mail address: Name: Mingjun Yang Institute: Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology. Address: No.2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province, P.
R. China., 116024 Telephone: +86-411-84709093. Fax: +86-411-84708015. E-mail: [email protected]. Thank you very much for your considering our manuscript for potential publication. Sincerely yours, Minhjun Yang
Suggested Reviewers： Considering their contributions on hydrate based technology, the following professors are proposed as potential reviewers: (1) Amir H. Mohammadi, MINES ParisTech, CEP/TEP, Fontainebleau, France, E-mail: [email protected] (2) Praveen Linga, National University of Singapore, Singapore, E-mail: [email protected] (3) Xiaosen Li, Research Center of Gas Hydrate, Chinese Academy of Sciences, E-mail: [email protected]