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Influence of gas sweep on methane recovery from hydrate-bearing sediments
Chemical Engineering Science 134 (2015) 727–736
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Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Influence of gas sweep on methane recovery from hydrate-bearing sediments Xiao-Hui Wang, Chang-Yu Sun n, Guang-Jin Chen, Ya-Nan He, Yi-Fei Sun, Yun-Fei Wang, Nan Li, Xiao-Xin Zhang, Bei Liu, Lan-Ying Yang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
H I G H L I G H T S
Gas sweep is in favor for reducing the partial pressure of methane in free gas. Methane recovery rate by N2 injection is much quicker than direct depressurization. The batch N2 injection mode is more suitable for controlling hydrate dissociation rate. The method of N2 sweep is quite suitable for low saturation hydrate reservoir.
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2015 Received in revised form 14 May 2015 Accepted 23 May 2015 Available online 3 June 2015
Gas sweep is favorable for reducing the partial pressure of methane in free gas during the production of methane from hydrate-bearing sediments. To evaluate the influence of gas sweep on methane recovery, pure N2 was injected into hydrate-bearing sediments using a scale-up three-dimensional apparatus. The influences of injection mode, hydrate saturation, and N2 injection rate were investigated. The experimental results suggest that the methane recovery rate by pure N2 sweep is much quicker than direct depressurization. The driving force for hydrate dissociation increases with the increase of the N2 mole fraction, which further promotes the decomposition of hydrates and guarantees a high gas production rate. In terms of N2 injection rate on gas production, the higher the gas production, the more N2 that is required, which means the increase of gas production rate is at the cost of injecting much more N2. Compared with the continuous mode, the batch injection mode is more suitable for controlling the hydrate dissociation rate and may be the lower risk way for hydrate exploitation. The gas sweep method may supply a new strategy that would be helpful to make low saturation hydrate reservoirs become a technically recoverable resource. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Hydrate exploitation Gas sweep Recovery efficiency Depressurization
1. Introduction Natural gas hydrates are non-stoichiometric crystalline solids formed by water and natural gas under low temperature and highpressure, which widely exist in subsea sediments and permafrost zones (Sloan and Koh, 2007). The gross reserves of organic carbon bound in natural gas hydrates is conservatively estimated to be double that of all known fossil fuels on the earth (Makogon, 2010; Boswell and Collett, 2011). Therefore, as an unconventional resource of natural gas, gas hydrates are considered as an important energy
n
Corresponding author. Fax: þ 86 1089733156. E-mail address: [email protected] (C.-Y. Sun).
http://dx.doi.org/10.1016/j.ces.2015.05.043 0009-2509/& 2015 Elsevier Ltd. All rights reserved.
source in the near future and have attracted much attention from researchers all over the world. However, it is an enormous challenge to extract natural gas from gas hydrates currently. Some methods for producing natural gas from hydrates have been proposed, such as depressurization, thermal stimulation, chemical inhibitors injection, and CO2 or mixed gas (CO2/N2) replacement. Gas hydrates production via thermal stimulation involves the increase of temperature above the hydrate stability region by technologies such as hot brine injection, steam or cyclic steam injection, which has been investigated experimentally (Tang et al., 2005; Tsimpanogiannis and Lichtner, 2007; Linga et al., 2009; Yang et al., 2010) and numerically (Moridis et al., 2002; Pooladi-Darvish, 2004; Tonnet and Herri, 2009). This method suffers from a vast loss of heat used to improve the temperature of hydrate-bearing geologic reservoir. Gas hydrate
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production via depressurization is considered to be the most economically promising technology for no charge extra energy (Ji et al., 2001; Moridis and Collett, 2003; Moridis and Sloan, 2007; Liang et al., 2010; Yang et al., 2012; Oyama et al., 2012). However, the temperature and driving force for hydrate dissociation drop in response to the depressurization of hydrate-bearing sediments, leading to a gradual decrease in the production rate, which is ultimately controlled by heat transfer toward the hydrate dissociation region (Lee et al., 2010; Seol and Myshakin, 2011; Li et al., 2014a). In addition, if the pressure difference between equilibrium and outlet is high, the pores will become plugged by hydrates regeneration (Nagao, 2012). Therefore, extra energy is still needed to sustain the hydrate dissociation in the depressurized operation. Injecting thermodynamic inhibitors can result in hydrate dissociation as the temperature and pressure conditions for hydrate stability are shifted (Sung et al., 2002). The most common thermodynamic inhibitors are alcohols (e.g., methanol, mono-ethylene glycol, and di-ethylene glycol) and salts (e.g., NaCl, CaCl2, KCl, and NaBr), however their use in the production of natural gas hydrates is limited by environmental impact and high economic costs. The injection of CO2 into hydrate-bearing sediments can cause the release of CH4 and the formation of CO2 hydrate within the CH4 hydrate stability field, which serves as a dual purpose for the recovery of an energy source and greenhouse-gas sequestration. The CO2–CH4 replacement has been systemically investigated including the feasibility of kinetics and thermodynamics (Zhao et al., 2012; Komatsu et al., 2013), replacement mechanism (Yoon et al., 2004; Bai et al., 2012; Qin and Kuhs, 2013), and influence of CO2 phase type (gaseous, liquid, or CO2 emulsion) (Ota et al., 2005, 2007; Yuan et al., 2012, 2013, 2014). However, the CO2–CH4 replacement method suffers from a low replacement rate for industrial production (Ota et al., 2005). In order for gas hydrates to become a producible energy resource, new technologies and approaches are still needed. Compared with gas or liquid CO2 replacement, methane recovery driven by gas mixture, for example, CH4–N2/CO2 (Park et al., 2006; Koh et al., 2012), CH4–air or CH4–CO2/air (Kang et al., 2014), has been investigated and seem to be more feasible in terms of cost and environmental protection, which had also been adopted by US DOE in 2012 to verify the capacity of CH4–CO2 replacement by injecting N2 (77%) þCO2 (23%) into hydrate-bearing sediments at Ignik-Sikumi field (NETL, 2012). However, at the initial stage of gas injection, gas mixtures would diffuse in hydrate reservoir firstly, rather than replace with methane immediately. In addition, the direct injection of large amounts of gas mixture may accompany with thermal stimulation for the sensible heat of gas. As water and hydrates coexist in the pore space, CO2 may react with free water, thus would likely to improve the concentration of N2 in the stratum. However, up to now, most researches only focus on studying the influence of CH4–CO2 replacement on gas production. To more closely examine the gas production from hydrate-bearing sediments by injecting gas mixtures (CO2, N2, air, etc.) in real scenarios in the field, further investigations will consider the effects of gas sweep in pore space and the sensible heat induced by gas mixtures, which may cause the dissociation of gas hydrates. On the other hand, the gas hydrate stability is controlled by conditions such as temperature, pressure, salinity, and gas composition (Li et al., 2014b). The first three parameters have been investigated to evaluate the capacity of gas production except for gas compositions. Recently, Kinnari et al. (2014) supplied some hydrate management strategies in practical gas and oil production systems, including gas sweep. In this work, to examine the capacity of changing gas composition on methane production, a series of experiments using a scale-up three-dimensional apparatus were conducted to investigate the influence of N2 sweep on methane recovery from hydrate-bearing sediments. Compared with CH4 and CO2, N2 is much more difficult to form hydrates
which can avoid blocking the mass transfer channels. It also has lower solubility in water, and is easily to be extracted out from the seafloor. Being the main compositions of coal-bed gas, methane– nitrogen mixtures can be separated by mature technologies, such as pressure swing adsorption (Fatehi et al., 1995) or adsorption by MOFs or ZIF-8 (Liu and Smit, 2009, 2010). The market price per unit volume of methane is approximately 10 times of nitrogen in China, which makes this method be feasible in terms of economy. Besides, the influence of operation mode (the batch injection mode and the continuous injection mode) and other operation factors, such as the N2 injection rate, the injection–production ratio, and hydrate saturation, were also investigated in this work.
2. Experimental section 2.1. Materials Methane and nitrogen with purity of 0.999 were supplied by the Beijing Beifen Gas Industry Corporation, China. The brine (NaCl) solution with salinity of 33.5 g/L was prepared in the laboratory. The hydrate-bearing sediments with a porosity of 0.387 were formed by 20/40 mesh quartz sands with an average diameter of 0.38 mm. 2.2. Apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1, which has been described in our previous work (Yuan et al., 2012, 2013, 2014). It mainly consists of a high-pressure reactor, a gas injection system, a cooling system, a gas collection system, a gas– water separation system, and a data acquisition system. All experiments are conducted in a high-pressure reactor constructed from stainless steel with an effective volume of 7.05 L (Φ300 mm 100 mm) and maximum working pressure of 16 MPa. The reactor is placed into a water bath containing ethylene glycol solution to maintain a constant temperature ranging from 253 to 353 K. Sixteen thermocouples, with an accuracy of 0.1 K, are inserted into the reactor from the top to detect the temperature distribution and variation during hydrate formation and gas sweep process, which are divided into four groups along the radial direction. The distribution of the thermocouples (T1–T16) in the reactor is shown in Fig. 2. Pressures are monitored by two pressure transducers with an accuracy of 0.02 MPa, which are mounted on the top and the bottom of the reactor, respectively. A Monitor and Control Generated System (MCGS) is used to collect and record data of temperature, pressure, and flow rate during the experiment. A filter is attached to the front of mass flow transducer to prevent water from affecting the accuracy of mass flow. 2.3. Procedures The experiments were conducted according to the following procedures, which are divided into two parts: the preparation of CH4 hydrate-bearing sediments and the N2 sweep process. 2.3.1. Preparation of hydrate-bearing sediments The method of CH4 hydrate preparation for each group of replacement experiment is the same as that used in our previous study (Yuan et al., 2012). First, a known amount of brine solution was cooled to 273.2 K and quartz sands were frozen to 267.2 K, and kept for 24 h. Then, the brine solution was injected into the sands, and stirred immediately and adequately, so that water can be in the form of fine ice particles homogeneously distributed in the sand. The ice–sand mixture was filled in the reactor before each experiment. To ensure that the water remained as ice in the
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Fig. 1. Schematic graph of the experimental apparatus: 1 – water bath; 2 – reactor; 3, 8, 11, 12, 13, 15, 17, 21, and 24 – valve; 4 – thermocouples; 6, and 14 – pressure transducer; 7 – pressure reducing valve; 9 – CH4 cylinder; 10 – N2 cylinder; 16, and 18–gas–water separator; 19 – filter; 20 – back-pressure regulator; 5, 22, and 25– mass flow transducer; 23 – gas collection cylinder; 26 – computer.
Reactor
5
6
7
8
1
2
3
4
16
15
14
13
12
11
10
9
Reactor parameter Depth: 100 mm Radius: 150 mm
Thermocouples parameters Number
9
5
10
6 11
7 8
4 3
12
16 15
2
14 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Depth,mm
82 82 82 82 58 58 58 58 34 34 34 34 10 10 10 10
Radius,mm
132 99 66 33 132 99 66 33 132 99 66 33 132 99 66 33
1
Fig. 2. Distribution of the thermocouples in the reactor.
sand after being loaded into the reactor, the temperature of the reactor was held at 272.7 K to prevent water migrating to the surface of sediments during the formation of hydrate (Mekala et al., 2014). After that, the reactor was vacuumed for 20 min and pressurized with methane from the top to the desired experimental pressure followed by starting the MCGS to record the experimental parameters every minute. If the pressure in the reactor was kept for stable for 24 h, the process of hydrate formation was considered to be finished.
2.3.2. Sweep process of gaseous N2 After the preparation of hydrate-bearing sediments, the temperature of water bath was increased to 275.2 K to simulate the geologic temperature of hydrate reservoir. The pressure in the reactor was slowly decreased to the equilibrium pressure of CH4 hydrate at the corresponding experimental temperature by a backpressure regulator. Once reached, valves 7 and 12 in Fig. 1 were
opened, and N2 was injected into the reactor continually from the bottom. The free gaseous CH4 in the pore space of the sediments was brought out by N2 leading to the increase of mole ratio of N2 in gas phase and the dissociation of methane hydrate. At a higher mole ratio of N2, the pressure to keep the hydrate stable is much higher. The released gas mixture was collected by a collection cylinder (40.3 L) and the compositions of gas phase in the reactor and in the gas collection cylinder were analyzed by a gas chromatograph (Agilent 6890 A) every 0.5 h for continual injection mode or every 12 h for batch injection mode, respectively. During the N2 sweep process, to investigate the influence of N2 on gas production, the pressure in the reactor was kept constant. When the composition of N2 in the reactor was higher than 95%, valves 8, 11, 12, and 13 were closed, and the sweep process was considered to be complete. Afterward, the temperature of the reactor was increased to 293 K to promote the dissociation of residual hydrate, and held for 12 h. The mole fractions of CH4/N2 in the reactor and the gas collection cylinder were analyzed by gas chromatograph,
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which were marked as yiG;end and yiCC;end , respectively. The corresponding number of moles of CH4 in the reactor and the gas 4 4 cylinder were named as nCH and nCH , respectively. G;end CC;end
Table 1 The initial conditions for N2 sweep process. Run Salinity (g/L)
Pressure (MPa)
Temperature (K)
1 2 3 4
3.1 3.1 3.1 3.1
275.2 275.2 275.2 275.2
2.4. Calculation of the amount of dissociated hydrate At any given time, the total number of moles (nT;t ) of methane in the system remains constant and is equal to that at time zero (nT;0 ) or at the end of experiment (nT;end ). The system includes the high-pressure reactor, the gas collection cylinder, and the connecting pipeline. niT;0 ¼ niG;0 þ niH;0 ¼ niG;end þ niCC;end
ð1Þ
where nG is the number of moles of free gas in the reactor; nH is the number of moles of gas trapped in hydrate in the reactor; nCC is the number of moles in the gas collection cylinder, in which the volume of gas collection cylinder contains that of connecting pipeline. The superscript i refers to methane. The number of mole that is released by hydrate dissociation can be calculated as follows: ΔniH;t ¼ niCC;t þ niG;t niG;0
ð2Þ
In above equations, the number of moles of gas is calculated by: ni ¼ yi
PV zRT
ð3Þ
where z is the compressibility factor calculated by BWRS equation of state, and the composition of the gas phase is determined by gas chromatograph. During the sweep process, the methane recovery ratio (MRR) is calculated as follows: MRR ¼
niCC;t niT;0
100%
ð4Þ
The hydrate decomposition ratio (HDR) is defined as, HDR ¼
ΔniH;t niH;0
100%
ð5Þ
At the same time, to evaluate the economic value of this method, the injection–production ratio (IPR) is calculated: IPR ¼
N2 2 nN CC;t þ nG;t 4 nCH CC;t
ð6Þ
N2 2 where nN CC;t and nG;t are the number of moles of N2 in gas collection cylinder and in gas phase of the reactor at any time, respectively; 4 nCH CC;t is the number of moles of CH4 in gas collection cylinder. Injection–production ratio directly affects the pressure maintenance and the productivity of reservoirs.
3. Results and discussion Four groups of experiments were performed to investigate the influences of operation mode, N2 injection rate, and hydrate saturation on the recovery of methane from hydrate-bearing sediments by the N2 sweep. The initial conditions for the N2 sweep process in four experimental runs are listed in Table 1. 3.1. The continuous mode In experimental runs 1–3, the continuous mode was adopted for hydrate exploitation by the N2 sweep. Run 2 was taken as an example to introduce the experimental phenomena. In run 2, the N2 sweep process followed the preparation of methane hydrate. N2 was injected into the reactor from the bottom of the reactor at the
33.5 33.5 33.5 33.5
Hydrate saturation (%) 18.00 17.81 27.64 21.18
Injection rate (mL/s)
16.7 50 50 50
Injection mode
continuous continuous continuous batch
rate of 50 mL/s with the continuous mode, meanwhile, gas mixture was exhausted from the top of reactor (See in Fig. 1). During this process, the total pressure of the reactor was kept constant controlled by a back-pressure regulator. When N2 traversed from the bottom to the outlet of the reactor, CH4 in gas phase was brought out, causing the mole fraction of methane in gas phase quickly decrease. Figs. 3 and 4 show the hydrate dissociation results and temperature variation by the N2 sweep for experimental run 2, respectively. According to Fig. 3b, the mole fraction of CH4 in the reactor quickly reduces to 0.1 in 2 h, with the variation tendency leveling out and eventually reaching 0.028. Because the experimental pressure was maintained at 3.1 MPa during the whole process, the methane partial pressure decreases with the increase of mole fraction of N2 in gas phase, and the fugacity difference of methane between gas phase and hydrate phase increases with the continuous introduction of N2. Hydrate dissociation therefore took place along with the N2 sweep process by destroying the equilibrium conditions and increasing the driving force. According to Fig. 3c, in first 1.5 h, the hydrate decomposition ratio is much higher than methane recovery ratio. The reason is that once methane gas was taken out by the flowing N2 from the reactor, hydrate would be forced to dissociate to make up the lost methane of gas phase to maintain the phase equilibrium, which leading to a higher hydrate decomposition ratio at this stage. As the amount of methane from hydrate dissociation was lower than that taken out by N2, the driving force would increase further. Therefore, the continuous dissociation of hydrate caused the decrease of the temperature of the hydrate sediments which was even below 273.2 K as shown in Fig. 4. Ice layers may also form and thicken with the dissociation of hydrate. However, there is some difference for the increased temperature variation trend between the locations at T1–T4 with T5–T16 at the beginning stage. The former four thermocouples were placed at the bottom of the reactor, and the temperatures directly decreased because the sensible heat of N2 cannot compensate for the heat consumed by the hydrate dissociation. For the latter twelve thermocouples, the temperatures initially increased under the action of sensible heat supplied by the injection gas, then decreased, meaning that the hydrate dissociation took place from the injection well to the recovery well. Further study of the variation of sediments permeability and flow obstructions at this stage is required as the hydrate may regenerate and ice forms during the depressurization process. After 1.5 h, methane gas had to transfer from hydrate phase to gas phase through the ice layer and was accompanied by the reduction of hydrate reserves. This resulted in the decrease of the hydrate dissociation rate after approximately 1.5 h, and the hydrate decomposition ratio is conversely lower than the methane recovery ratio as seen in Fig. 3c. After this time, the methane recovery ratio was up to 0.9, and gas composition in the reactor changed slowly. To extract the rest of methane gas, more N2 was required. According to Fig. 3d, the injection–production ratio gradually becomes larger, especially after 2 h. With the injection–production ratio increasing, the mole fraction of methane in the gas collection cylinder linearly decreases as shown in Fig. 3a, which means more N2 is needed
0.8
1.0
0.7
0.8
0.6
Mole fraction
Mole fraction
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0.5 0.4
731
0.6
CH N
0.4 0.2
0.3 0.0
1
2 Time (h)
3
0
4
100
100
90
90
80
80
70
70
Methane recovery ratio Hydrate decomposition ratio
60
60 50
0
1
2 Time (h)
1
2
3
4
Time (h)
3
4
50
4.0
Injection-production ratio
0
Hydrate decomposition ratio (%)
Methane recovery ratio (%)
0.2
3.5 3.0 2.5 2.0 1.5 1.0
0.5
1.0
1.5
2.0 2.5 Time (h)
3.0
3.5
4.0
Fig. 3. Experimental results for run 2 by continuous injection mode with hydrate saturation of 17.81% and salinity of 33.5 g/L when at 3.1 MPa, 275.2 K, and injection rate of 50 mL/s: (a) gas composition in gas collection cylinder with elapsed time; (b) gas composition of gas phase in reactor with elapsed time; (c) variation of hydrate decomposition ratio and methane recovery ratio with elapsed time; and (d) injection–production ratio with elapsed time.
to recovery the residual methane (less than 10 mol%). Taking the economical efficiency into consideration, the enduring time for this set of experiment should be constrained to 2 h. As hydrate dissociation is an endothermic reaction, many researchers (Moridis and Sloan, 2007; Liang et al., 2010; Song et al., 2015; Zhao et al., 2015) have reported that when the method of depressurization is used to recover methane trapped in hydrates, the temperatures and the driving force of hydrate dissociation will drop in response to the depressurization of hydrate-bearing sediments, thus leading to the gradual decrease of gas production rate. Another series of experiments (runs 5–8) with a depressurization method at different production pressures were conducted to compare with the N2 sweep method. Experimental parameters of hydrate dissociation by the depressurization method are listed in Table 2 and the results are shown in Fig. 5. The methane recovery ratio for run 2 by N2 sweep when at 3.1 MPa was also plotted for comparison. It can be seen that the recovery ratio of methane by N2 sweep can reach 90% in first 1.5 h (run 2), which is much quicker than that by single depressurization. In addition, during the N2 sweep process, the mole fraction of N2 gradually increases with the elapsed time, leading to the increase of the driving force for hydrate dissociation and further promotes the decomposition of hydrate. The N2 works as diluent gas toward the dissociation of gas hydrates and directly affect the gas compositions to increase gas production rate. Fig. 6 shows the hydrate formation conditions for pure CH4 and CH4 þN2 gas mixtures calculated by a Chen-Guo hydrate model (Chen and Guo, 1998). For the depressurization method, the ambient conditions will change along with the arrow direction 2 in Fig. 6; whereas in the N2 sweep method, the driving force for hydrate dissociation
will increase in the arrow direction 1. Therefore, to ensure continuous gas production by maintaining the reservoir temperature at a certain range, a combined production method coupling the depressurization with other production methods, such as N2 sweep, would be more useful and effective. Considering the cost used to separate the recovery gas, the fraction of methane extracted from the hydrate reservoir should be above a certain value (such as, 0.3) depending on the price of natural gas. In this case, the driving force calculated can reach 5.41 MPa as shown in Fig. 6, which is much larger than the direct depressurization. 3.2. The batch mode The batch injection mode is also widely used in the oil industry to enhance oil recovery. Using this mode, the hydrate decomposition by hot water or inhibitor injection has been investigated (Koh et al., 2012; Yuan et al., 2013). In this work, the batch injection mode was also extended to hydrate recovery by N2 sweep. For experimental run 4 using the batch mode, the system pressure initially decreased to 3.1 MPa, followed by the continuous injection of N2 into the reactor from the bottom until the pressure was raised to 5.0 MPa. Once achieved, all valves were closed and the system was kept at this state for 12 h. After this process, gas compositions in the reactor and gas collection cylinder were measured by a gas chromatograph. Thereafter, the gas in the reactor was released until the pressure was reduced to 3.1 MPa, and the next cycle of N2 sweep started. The pressure curves for each cycle in run 4 and the hydrate dissociation results are shown in Figs. 7 and 8, respectively. Compared with the continuous mode, the variation tendencies of gas compositions in gas collection
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Fig. 4. Temperature variations at different locations in the reactor during N2 sweep in run 2.
16
Table 2 Experimental parameters of hydrate dissociation by the depressurization method.
20–40 20–40 20–40 20–40
Methane recovery ratio (%)
5 6 7 8
Quartz sand (mesh)
Salinity (g/L)
Hydrate saturation (%)
33.5 33.5 33.5 33.5
17.92 18.03 17.86 17.62
Initial temperature (K) 275.2 275.2 275.2 275.2
14
Pressure (MPa)
12
Pressure (MPa)
Run
0.5 1.0 1.5 2.5
10 8
100
4
90
2
80
0
70
2
260
264
268
272
276
280
284
288
Temperature (K)
60 50
Fig. 6. Hydrate formation conditions for pure CH4 and CH4 þ N2 gas mixtures (T ¼ 275.2 K) calculated by the Chen-Guo hydrate model (1998).
0.5 MPa, run 5 1.0 MPa, run 6 1.5 MPa, run 7 2.5 MPa, run 8 3.1 MPa, run 2
40 30 20 10 0 -1
1
6
0
1
2
3
4 5 Time (h)
6
7
8
9
Fig. 5. Gas recovery ratio versus elapsed time by the depressurization method under different production pressures (runs 5–8) or by the N2 sweep method (run 2).
cylinder and in the reactor are more smooth in the whole process, as shown in Fig. 8a and b. Hydrate also began to dissociate after the injection of N2 from the bottom of the reactor until a new
balance was attained. The production rate is ultimately controlled by heat transfer toward the hydrate dissociation region. For the batch mode, there is enough time for heat transfer from water bath to hydrate dissociation zone, so the decomposed water can be in form of free water, which would help methane transfer from the surface of hydrate dissociation zone to gas phase. According to Fig. 8c, the hydrate decomposition ratio is higher than the ratio of methane recovery in whole process, which differing from the continuous mode as shown in Fig. 3c. According to Fig. 8d, the IPR nearly increases linearly with the elapsed time, and the IPR of the continuous mode is larger than that of the batch mode, suggesting that the batch mode is more economical. Meanwhile, a small
X.-H. Wang et al. / Chemical Engineering Science 134 (2015) 727–736
number of wells are needed for the batch mode in practice field production. As shown in Fig. 7, in the first four cycles after N2 injection, the system pressure continues to rise, while it remains constant for the following five cycles. This means the hydrate dissociation takes place in the first four cycles of N2 sweep, while hydrate has almost totally decomposed in the following five cycles. It should be noted that there is a turning point in the first cycle, the pressure increases firstly, then turns to decrease. In the first cycle of N2 sweep, after N2 was injected into the reactor from the bottom, the distribution of N2 was non-uniform and the concentration of N2 at bottom was high enough to promote hydrate dissociation and
5500
induce the increase of system pressure. With the continuous increase of pressure, the distribution of N2 becomes uniform. The temperature of the ambient sediment decreases due to the endothermic reaction of hydrate dissociation, resulting in hydrate regeneration and decrease of pressure. In addition, after thorough hydrate dissociation, it is unnecessary to continue with the injection of N2, such as the fifth to ninth cycles in run 4. However, in this work, to compare with the continuous mode, it was not stopped after the fourth cycle until the mole fraction of methane from the exhausted gas was above 98%. When gas sweep by the batch mode is used to hydrate production, the total pressure should be below the equilibrium pressure in the first or second gas injection cycle to avoid hydrate regeneration. The N2 sweep cycle could be stopped after hydrate was totally decomposed and the residual gas can be extracted directly. The batch injection mode can be used to control the dissociation rate of hydrate and may be an economical and lower risk way for hydrate exploitation.
5000
3.3. The influence of sweep rate on gas production
6000
Pressure (KPa)
733
4500
The injection rate of N2 is an important factor because it directly affects the composition of gas phase. Experimental runs 1 and 2 were performed to examine the influence of injection rate, in which the gas flow rate is 16.7 mL/s (run 1) and 50 mL/s (run 2), respectively, with the same other experimental parameters. The comparison of different sweep rates on gas production is shown in Fig. 9. As shown in Fig. 9a, the methane recovery ratio in run 2 is higher than that in run 1 during the whole process. The time needed for methane recovery ratio reaching 90% for run 2 is nearly half that for run 1, although the hydrate saturation is almost the same for these two runs. In addition, there is a turning point at
4000 3500 3000 cycle: 1
2
5
3
4
6
5
6
7
7
8
9
8
9
10
Time (day) Fig. 7. Variation of pressure curves for each cycle during the gas injection stage in run 4 using the batch method.
0.8 1.0
CH4
0.7
N2
Mole fraction
Mole fraction
0.8
0.6 0.5 0.4
CH4
0.6
N2
0.4 0.2
0.3 0.0
2
4 6 Cycle times
8
10
100
100
80
80
60
60 Methane recovery ratio Hydrate decomposition ratio
40
20
0
2
4 6 Cycle times
8
40
10
20
0
2
4 6 Cycle times
0
2
4 6 Cycle times
8
10
3.0
Injection-production ratio
0
Hydrate decomposition ratio (%)
Methane recovery ratio (%)
0.2
2.5 2.0 1.5 1.0 0.5
8
10
Fig. 8. Experimental results for run 4 by batch injection mode with hydrate saturation of 21.18% and salinity of 33.5 g/L when at 3.1 MPa, 275.2 K, and injection rate of 50 mL/ s: (a) gas composition in gas collection cylinder at different cycle times, (b) gas composition of gas phase in reactor at different cycle times, (c) variation of hydrate decomposition ratio and methane recovery ratio at different cycle times, and (d) injection–production ratio at different cycle times.
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the recovery ratio of methane for run 2 reaches 87.5%, while it only reaches 55% for run 3. The reason may be that when N2 is continuously injected into the reactor, the mole fraction of N2 in gas phase gradually increases and hydrate begins to dissociate. In the first stage, the methane recovery ratio is mainly governed by hydrate dissociation, so the gas production rate of run 2 is the same as run 3. The rapid dissociation of hydrate in first stage causes the temperature of reservoir decrease to be below the freezing point, so the water from hydrate dissociation exists in the form of ice. Therefore, the gas production rate becomes lower after the first stage as shown in Fig. 10a, whereas in the second stage, the methane recovery ratio may mainly be governed by hydrate dissociation and methane transfer. In the third stage, the amount of un-decomposed hydrate was much lower while the effect of the mass transfer increases, resulting in lower gas production rate. As described above, the exploitation of hydrate by N2 sweep also needs external energy to maintain the gas production rate for hydrate reservoir with high saturation, similar to the depressurization method. In addition, as shown in Fig. 10b, when at the same IPR, the methane recovery ratio of run 2 is higher than run 3, meaning that gas sweep is an effective way for recovering methane gas from low saturation or widely-distributed hydratebearing deposits. In comparison, the driving force for hydrate decomposition is lower for the depressurization method; the thermal stimulation method needs to inject much heat; while the inhibitor injection method is limited by environmental protection. The method of N2 sweep is more suitable especially for low saturation hydrate reservoirs from the viewpoints of energy saving
1.5 h for the curve of methane recovery ratio of run 2. The methane recovery ratio quickly increases to 87.5% before this point; afterwards, the curve tends to be smooth. As shown in Fig. 6, with the increase of the mole fraction of N2, the driving force for hydrates dissociation increases accordingly. The faster injection rate of N2 will result in the higher N2 mole fraction in gas composition. However, the IPR of run 2 was nearly twice that of run 1 to acquire the same methane recovery ratio for the same saturation hydrate sediments, which means the increase of gas production rate is at the cost of injecting much more N2. 3.4. The influence of hydrate saturation on gas production
100
100
90
90
Mehtane recovery ratio (%)
Methane recovery ratio (%)
The saturation of seafloor hydrates mainly ranges from 10% to 60%, with some as high as 90%. Therefore, if the gas production method is suitable for hydrate exploitation is also dependent on the overall saturation of hydrate-bearing sediment. In this work, the influence of saturation of hydrate reservoir on methane recovery by N2 sweep were examined in runs 2 and 3 with hydrate saturation of 17.81% and 27.64%, respectively, with the same injection rate of 50 mL/s. Fig. 10 shows the comparison of gas production by N2 sweep for hydrate samples with different hydrate saturation. As shown from Fig. 10a, the gas recovery ratio in run 2 is higher than that in run 3 during the whole process. The curve of methane recovery ratio can be divided into three stages for run 3 while two stages for run 2 on basis of the gas production rate, with only in the first stage of run 3 having a gas production rate as quick as that of run 2. Meanwhile, at the end of this stage,
80 70 Run 1 Run 2
60 50
0
1
2
3 Time (h)
4
5
6
80 70 Run 1 Run 2
60 50
0
1
2 3 Injection-production ratio
4
Fig. 9. Comparison of different sweep rate on gas production for runs 1 and 2: (a) variation of methane recovery ratio with elapsed time, and (b) the relationship between methane recovery ratio and injection–production ratio.
Fig. 10. Comparison of gas production by the N2 sweep for runs 2 and 3 with different hydrate saturation: (a) variation of the methane recovery ratio with elapsed time, and (b) the relationship between methane recovery ratio and injection–production ratio.
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and gas production rate. Most nature gas hydrates are widely dispersed in low saturation and are not considered to be a viable resource with current technologies. Gas sweep may supply a new strategy that would be helpful to make this type of hydrate become a technically recoverable resource.
4. Conclusions A series of experiments were performed to study the exploitation of hydrate by gas (N2) sweep using a scale-up three-dimensional apparatus. The following conclusions were obtained: (i) The gas production rate by gas sweep is much quicker than single depressurization. During the N2 sweep process, the mole fraction of N2 gradually increases with the elapsed time. The driving force for hydrate dissociation increases with the increase of mole fraction of N2, which further promotes the decomposition of hydrates and guarantees a high gas production rate. (ii) Compared with the continuous mode, the gas compositions in gas collection cylinder and reactor using the batch injection mode decrease more slowly during the whole sweep process, suggesting that the batch injection mode can be used to control hydrate dissociation rate and may be the lower risk way for hydrate exploitation. (iii) The faster injection rate of N2 results in the higher N2 mole fraction in gas composition. The increase of gas production rate is at the cost of injecting much more N2. The experimental results also show that the method of N2 sweep is quite suitable for low saturation hydrate reservoir. The methane production from hydrate reservoir by gas sweep may supply a new strategy for high-dispersed hydrate resource.
Acknowledgments The financial support received from National 973 Project of China (No.2012CB215005) and National Natural Science Foundation of China (Nos.U1162205, 51376195) are gratefully acknowledged. References Bai, D.S., Zhang, X.R., Chen, G.J., Wang, W.C., 2012. Replacement mechanism of methane hydrate with carbon dioxide from microsecond molecular dynamics simulations. Energy Environ. Sci. 5, 7033–7041. Boswell, R., Collett, T.S., 2011. Current perspectives on gas hydrate resources. Energy Environ. Sci. 4, 1206–1215. Chen, G.J., Guo, T.M., 1998. A new approach to gas hydrate modelling. Chem. Eng. J. 71, 145–151. Fatehi, A.I., Loughlin, K.F., Hassan, M.M., 1995. Separation of methane–nitrogen mixtures by pressure swing adsorption using a carbon molecular sieve. Gas Sep. Purif. 9, 199–204. Ji, C., Ahmadi, G., Smith, D.H., 2001. Natural gas production from hydrate decomposition by depressurization. Chem. Eng. Sci. 56, 5801–5814. Kang, H., Koh, D.Y., Lee, H., 2014. Nondestructive natural gas hydrate recovery driven by air and carbon dioxide. Sci. Rep. 4, 6616. Kinnari, K., Hundseid, J., Li, X., Askvik, K.M., 2014. Hydrate management in practice. In: Proceedings of the Eighth International Conference on Gas Hydrates, China. Koh, D.Y., Kang, H., Kim, D.O., Park, J., Cha, M., Lee, H., 2012. Recovery of methane from gas hydrates intercalated within natural sediments using CO2 and a CO2/ N2 gas mixture. ChemSusChem 5, 1443–1448. Komatsu, H., Ota, M., Smith, R.L., Inomata, H., 2013. Review of CO2 CH4 clathrate hydrate replacement reaction laboratory studies – properties and kinetics. J. Taiwan Inst. Chem. Eng. 44, 517–537. Lee, J., Park, S., Sung, W., 2010. An experimental study on the productivity of dissociated gas from gas hydrate by depressurization scheme. Energy Convers. Manag. 51, 2510–2515.
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Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Influence of gas sweep on methane recovery from hydrate-bearing sediments Xiao-Hui Wang, Chang-Yu Sun n, Guang-Jin Chen, Ya-Nan He, Yi-Fei Sun, Yun-Fei Wang, Nan Li, Xiao-Xin Zhang, Bei Liu, Lan-Ying Yang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
H I G H L I G H T S
Gas sweep is in favor for reducing the partial pressure of methane in free gas. Methane recovery rate by N2 injection is much quicker than direct depressurization. The batch N2 injection mode is more suitable for controlling hydrate dissociation rate. The method of N2 sweep is quite suitable for low saturation hydrate reservoir.
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2015 Received in revised form 14 May 2015 Accepted 23 May 2015 Available online 3 June 2015
Gas sweep is favorable for reducing the partial pressure of methane in free gas during the production of methane from hydrate-bearing sediments. To evaluate the influence of gas sweep on methane recovery, pure N2 was injected into hydrate-bearing sediments using a scale-up three-dimensional apparatus. The influences of injection mode, hydrate saturation, and N2 injection rate were investigated. The experimental results suggest that the methane recovery rate by pure N2 sweep is much quicker than direct depressurization. The driving force for hydrate dissociation increases with the increase of the N2 mole fraction, which further promotes the decomposition of hydrates and guarantees a high gas production rate. In terms of N2 injection rate on gas production, the higher the gas production, the more N2 that is required, which means the increase of gas production rate is at the cost of injecting much more N2. Compared with the continuous mode, the batch injection mode is more suitable for controlling the hydrate dissociation rate and may be the lower risk way for hydrate exploitation. The gas sweep method may supply a new strategy that would be helpful to make low saturation hydrate reservoirs become a technically recoverable resource. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Hydrate exploitation Gas sweep Recovery efficiency Depressurization
1. Introduction Natural gas hydrates are non-stoichiometric crystalline solids formed by water and natural gas under low temperature and highpressure, which widely exist in subsea sediments and permafrost zones (Sloan and Koh, 2007). The gross reserves of organic carbon bound in natural gas hydrates is conservatively estimated to be double that of all known fossil fuels on the earth (Makogon, 2010; Boswell and Collett, 2011). Therefore, as an unconventional resource of natural gas, gas hydrates are considered as an important energy
n
Corresponding author. Fax: þ 86 1089733156. E-mail address: [email protected] (C.-Y. Sun).
http://dx.doi.org/10.1016/j.ces.2015.05.043 0009-2509/& 2015 Elsevier Ltd. All rights reserved.
source in the near future and have attracted much attention from researchers all over the world. However, it is an enormous challenge to extract natural gas from gas hydrates currently. Some methods for producing natural gas from hydrates have been proposed, such as depressurization, thermal stimulation, chemical inhibitors injection, and CO2 or mixed gas (CO2/N2) replacement. Gas hydrates production via thermal stimulation involves the increase of temperature above the hydrate stability region by technologies such as hot brine injection, steam or cyclic steam injection, which has been investigated experimentally (Tang et al., 2005; Tsimpanogiannis and Lichtner, 2007; Linga et al., 2009; Yang et al., 2010) and numerically (Moridis et al., 2002; Pooladi-Darvish, 2004; Tonnet and Herri, 2009). This method suffers from a vast loss of heat used to improve the temperature of hydrate-bearing geologic reservoir. Gas hydrate
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production via depressurization is considered to be the most economically promising technology for no charge extra energy (Ji et al., 2001; Moridis and Collett, 2003; Moridis and Sloan, 2007; Liang et al., 2010; Yang et al., 2012; Oyama et al., 2012). However, the temperature and driving force for hydrate dissociation drop in response to the depressurization of hydrate-bearing sediments, leading to a gradual decrease in the production rate, which is ultimately controlled by heat transfer toward the hydrate dissociation region (Lee et al., 2010; Seol and Myshakin, 2011; Li et al., 2014a). In addition, if the pressure difference between equilibrium and outlet is high, the pores will become plugged by hydrates regeneration (Nagao, 2012). Therefore, extra energy is still needed to sustain the hydrate dissociation in the depressurized operation. Injecting thermodynamic inhibitors can result in hydrate dissociation as the temperature and pressure conditions for hydrate stability are shifted (Sung et al., 2002). The most common thermodynamic inhibitors are alcohols (e.g., methanol, mono-ethylene glycol, and di-ethylene glycol) and salts (e.g., NaCl, CaCl2, KCl, and NaBr), however their use in the production of natural gas hydrates is limited by environmental impact and high economic costs. The injection of CO2 into hydrate-bearing sediments can cause the release of CH4 and the formation of CO2 hydrate within the CH4 hydrate stability field, which serves as a dual purpose for the recovery of an energy source and greenhouse-gas sequestration. The CO2–CH4 replacement has been systemically investigated including the feasibility of kinetics and thermodynamics (Zhao et al., 2012; Komatsu et al., 2013), replacement mechanism (Yoon et al., 2004; Bai et al., 2012; Qin and Kuhs, 2013), and influence of CO2 phase type (gaseous, liquid, or CO2 emulsion) (Ota et al., 2005, 2007; Yuan et al., 2012, 2013, 2014). However, the CO2–CH4 replacement method suffers from a low replacement rate for industrial production (Ota et al., 2005). In order for gas hydrates to become a producible energy resource, new technologies and approaches are still needed. Compared with gas or liquid CO2 replacement, methane recovery driven by gas mixture, for example, CH4–N2/CO2 (Park et al., 2006; Koh et al., 2012), CH4–air or CH4–CO2/air (Kang et al., 2014), has been investigated and seem to be more feasible in terms of cost and environmental protection, which had also been adopted by US DOE in 2012 to verify the capacity of CH4–CO2 replacement by injecting N2 (77%) þCO2 (23%) into hydrate-bearing sediments at Ignik-Sikumi field (NETL, 2012). However, at the initial stage of gas injection, gas mixtures would diffuse in hydrate reservoir firstly, rather than replace with methane immediately. In addition, the direct injection of large amounts of gas mixture may accompany with thermal stimulation for the sensible heat of gas. As water and hydrates coexist in the pore space, CO2 may react with free water, thus would likely to improve the concentration of N2 in the stratum. However, up to now, most researches only focus on studying the influence of CH4–CO2 replacement on gas production. To more closely examine the gas production from hydrate-bearing sediments by injecting gas mixtures (CO2, N2, air, etc.) in real scenarios in the field, further investigations will consider the effects of gas sweep in pore space and the sensible heat induced by gas mixtures, which may cause the dissociation of gas hydrates. On the other hand, the gas hydrate stability is controlled by conditions such as temperature, pressure, salinity, and gas composition (Li et al., 2014b). The first three parameters have been investigated to evaluate the capacity of gas production except for gas compositions. Recently, Kinnari et al. (2014) supplied some hydrate management strategies in practical gas and oil production systems, including gas sweep. In this work, to examine the capacity of changing gas composition on methane production, a series of experiments using a scale-up three-dimensional apparatus were conducted to investigate the influence of N2 sweep on methane recovery from hydrate-bearing sediments. Compared with CH4 and CO2, N2 is much more difficult to form hydrates
which can avoid blocking the mass transfer channels. It also has lower solubility in water, and is easily to be extracted out from the seafloor. Being the main compositions of coal-bed gas, methane– nitrogen mixtures can be separated by mature technologies, such as pressure swing adsorption (Fatehi et al., 1995) or adsorption by MOFs or ZIF-8 (Liu and Smit, 2009, 2010). The market price per unit volume of methane is approximately 10 times of nitrogen in China, which makes this method be feasible in terms of economy. Besides, the influence of operation mode (the batch injection mode and the continuous injection mode) and other operation factors, such as the N2 injection rate, the injection–production ratio, and hydrate saturation, were also investigated in this work.
2. Experimental section 2.1. Materials Methane and nitrogen with purity of 0.999 were supplied by the Beijing Beifen Gas Industry Corporation, China. The brine (NaCl) solution with salinity of 33.5 g/L was prepared in the laboratory. The hydrate-bearing sediments with a porosity of 0.387 were formed by 20/40 mesh quartz sands with an average diameter of 0.38 mm. 2.2. Apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1, which has been described in our previous work (Yuan et al., 2012, 2013, 2014). It mainly consists of a high-pressure reactor, a gas injection system, a cooling system, a gas collection system, a gas– water separation system, and a data acquisition system. All experiments are conducted in a high-pressure reactor constructed from stainless steel with an effective volume of 7.05 L (Φ300 mm 100 mm) and maximum working pressure of 16 MPa. The reactor is placed into a water bath containing ethylene glycol solution to maintain a constant temperature ranging from 253 to 353 K. Sixteen thermocouples, with an accuracy of 0.1 K, are inserted into the reactor from the top to detect the temperature distribution and variation during hydrate formation and gas sweep process, which are divided into four groups along the radial direction. The distribution of the thermocouples (T1–T16) in the reactor is shown in Fig. 2. Pressures are monitored by two pressure transducers with an accuracy of 0.02 MPa, which are mounted on the top and the bottom of the reactor, respectively. A Monitor and Control Generated System (MCGS) is used to collect and record data of temperature, pressure, and flow rate during the experiment. A filter is attached to the front of mass flow transducer to prevent water from affecting the accuracy of mass flow. 2.3. Procedures The experiments were conducted according to the following procedures, which are divided into two parts: the preparation of CH4 hydrate-bearing sediments and the N2 sweep process. 2.3.1. Preparation of hydrate-bearing sediments The method of CH4 hydrate preparation for each group of replacement experiment is the same as that used in our previous study (Yuan et al., 2012). First, a known amount of brine solution was cooled to 273.2 K and quartz sands were frozen to 267.2 K, and kept for 24 h. Then, the brine solution was injected into the sands, and stirred immediately and adequately, so that water can be in the form of fine ice particles homogeneously distributed in the sand. The ice–sand mixture was filled in the reactor before each experiment. To ensure that the water remained as ice in the
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Fig. 1. Schematic graph of the experimental apparatus: 1 – water bath; 2 – reactor; 3, 8, 11, 12, 13, 15, 17, 21, and 24 – valve; 4 – thermocouples; 6, and 14 – pressure transducer; 7 – pressure reducing valve; 9 – CH4 cylinder; 10 – N2 cylinder; 16, and 18–gas–water separator; 19 – filter; 20 – back-pressure regulator; 5, 22, and 25– mass flow transducer; 23 – gas collection cylinder; 26 – computer.
Reactor
5
6
7
8
1
2
3
4
16
15
14
13
12
11
10
9
Reactor parameter Depth: 100 mm Radius: 150 mm
Thermocouples parameters Number
9
5
10
6 11
7 8
4 3
12
16 15
2
14 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Depth,mm
82 82 82 82 58 58 58 58 34 34 34 34 10 10 10 10
Radius,mm
132 99 66 33 132 99 66 33 132 99 66 33 132 99 66 33
1
Fig. 2. Distribution of the thermocouples in the reactor.
sand after being loaded into the reactor, the temperature of the reactor was held at 272.7 K to prevent water migrating to the surface of sediments during the formation of hydrate (Mekala et al., 2014). After that, the reactor was vacuumed for 20 min and pressurized with methane from the top to the desired experimental pressure followed by starting the MCGS to record the experimental parameters every minute. If the pressure in the reactor was kept for stable for 24 h, the process of hydrate formation was considered to be finished.
2.3.2. Sweep process of gaseous N2 After the preparation of hydrate-bearing sediments, the temperature of water bath was increased to 275.2 K to simulate the geologic temperature of hydrate reservoir. The pressure in the reactor was slowly decreased to the equilibrium pressure of CH4 hydrate at the corresponding experimental temperature by a backpressure regulator. Once reached, valves 7 and 12 in Fig. 1 were
opened, and N2 was injected into the reactor continually from the bottom. The free gaseous CH4 in the pore space of the sediments was brought out by N2 leading to the increase of mole ratio of N2 in gas phase and the dissociation of methane hydrate. At a higher mole ratio of N2, the pressure to keep the hydrate stable is much higher. The released gas mixture was collected by a collection cylinder (40.3 L) and the compositions of gas phase in the reactor and in the gas collection cylinder were analyzed by a gas chromatograph (Agilent 6890 A) every 0.5 h for continual injection mode or every 12 h for batch injection mode, respectively. During the N2 sweep process, to investigate the influence of N2 on gas production, the pressure in the reactor was kept constant. When the composition of N2 in the reactor was higher than 95%, valves 8, 11, 12, and 13 were closed, and the sweep process was considered to be complete. Afterward, the temperature of the reactor was increased to 293 K to promote the dissociation of residual hydrate, and held for 12 h. The mole fractions of CH4/N2 in the reactor and the gas collection cylinder were analyzed by gas chromatograph,
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which were marked as yiG;end and yiCC;end , respectively. The corresponding number of moles of CH4 in the reactor and the gas 4 4 cylinder were named as nCH and nCH , respectively. G;end CC;end
Table 1 The initial conditions for N2 sweep process. Run Salinity (g/L)
Pressure (MPa)
Temperature (K)
1 2 3 4
3.1 3.1 3.1 3.1
275.2 275.2 275.2 275.2
2.4. Calculation of the amount of dissociated hydrate At any given time, the total number of moles (nT;t ) of methane in the system remains constant and is equal to that at time zero (nT;0 ) or at the end of experiment (nT;end ). The system includes the high-pressure reactor, the gas collection cylinder, and the connecting pipeline. niT;0 ¼ niG;0 þ niH;0 ¼ niG;end þ niCC;end
ð1Þ
where nG is the number of moles of free gas in the reactor; nH is the number of moles of gas trapped in hydrate in the reactor; nCC is the number of moles in the gas collection cylinder, in which the volume of gas collection cylinder contains that of connecting pipeline. The superscript i refers to methane. The number of mole that is released by hydrate dissociation can be calculated as follows: ΔniH;t ¼ niCC;t þ niG;t niG;0
ð2Þ
In above equations, the number of moles of gas is calculated by: ni ¼ yi
PV zRT
ð3Þ
where z is the compressibility factor calculated by BWRS equation of state, and the composition of the gas phase is determined by gas chromatograph. During the sweep process, the methane recovery ratio (MRR) is calculated as follows: MRR ¼
niCC;t niT;0
100%
ð4Þ
The hydrate decomposition ratio (HDR) is defined as, HDR ¼
ΔniH;t niH;0
100%
ð5Þ
At the same time, to evaluate the economic value of this method, the injection–production ratio (IPR) is calculated: IPR ¼
N2 2 nN CC;t þ nG;t 4 nCH CC;t
ð6Þ
N2 2 where nN CC;t and nG;t are the number of moles of N2 in gas collection cylinder and in gas phase of the reactor at any time, respectively; 4 nCH CC;t is the number of moles of CH4 in gas collection cylinder. Injection–production ratio directly affects the pressure maintenance and the productivity of reservoirs.
3. Results and discussion Four groups of experiments were performed to investigate the influences of operation mode, N2 injection rate, and hydrate saturation on the recovery of methane from hydrate-bearing sediments by the N2 sweep. The initial conditions for the N2 sweep process in four experimental runs are listed in Table 1. 3.1. The continuous mode In experimental runs 1–3, the continuous mode was adopted for hydrate exploitation by the N2 sweep. Run 2 was taken as an example to introduce the experimental phenomena. In run 2, the N2 sweep process followed the preparation of methane hydrate. N2 was injected into the reactor from the bottom of the reactor at the
33.5 33.5 33.5 33.5
Hydrate saturation (%) 18.00 17.81 27.64 21.18
Injection rate (mL/s)
16.7 50 50 50
Injection mode
continuous continuous continuous batch
rate of 50 mL/s with the continuous mode, meanwhile, gas mixture was exhausted from the top of reactor (See in Fig. 1). During this process, the total pressure of the reactor was kept constant controlled by a back-pressure regulator. When N2 traversed from the bottom to the outlet of the reactor, CH4 in gas phase was brought out, causing the mole fraction of methane in gas phase quickly decrease. Figs. 3 and 4 show the hydrate dissociation results and temperature variation by the N2 sweep for experimental run 2, respectively. According to Fig. 3b, the mole fraction of CH4 in the reactor quickly reduces to 0.1 in 2 h, with the variation tendency leveling out and eventually reaching 0.028. Because the experimental pressure was maintained at 3.1 MPa during the whole process, the methane partial pressure decreases with the increase of mole fraction of N2 in gas phase, and the fugacity difference of methane between gas phase and hydrate phase increases with the continuous introduction of N2. Hydrate dissociation therefore took place along with the N2 sweep process by destroying the equilibrium conditions and increasing the driving force. According to Fig. 3c, in first 1.5 h, the hydrate decomposition ratio is much higher than methane recovery ratio. The reason is that once methane gas was taken out by the flowing N2 from the reactor, hydrate would be forced to dissociate to make up the lost methane of gas phase to maintain the phase equilibrium, which leading to a higher hydrate decomposition ratio at this stage. As the amount of methane from hydrate dissociation was lower than that taken out by N2, the driving force would increase further. Therefore, the continuous dissociation of hydrate caused the decrease of the temperature of the hydrate sediments which was even below 273.2 K as shown in Fig. 4. Ice layers may also form and thicken with the dissociation of hydrate. However, there is some difference for the increased temperature variation trend between the locations at T1–T4 with T5–T16 at the beginning stage. The former four thermocouples were placed at the bottom of the reactor, and the temperatures directly decreased because the sensible heat of N2 cannot compensate for the heat consumed by the hydrate dissociation. For the latter twelve thermocouples, the temperatures initially increased under the action of sensible heat supplied by the injection gas, then decreased, meaning that the hydrate dissociation took place from the injection well to the recovery well. Further study of the variation of sediments permeability and flow obstructions at this stage is required as the hydrate may regenerate and ice forms during the depressurization process. After 1.5 h, methane gas had to transfer from hydrate phase to gas phase through the ice layer and was accompanied by the reduction of hydrate reserves. This resulted in the decrease of the hydrate dissociation rate after approximately 1.5 h, and the hydrate decomposition ratio is conversely lower than the methane recovery ratio as seen in Fig. 3c. After this time, the methane recovery ratio was up to 0.9, and gas composition in the reactor changed slowly. To extract the rest of methane gas, more N2 was required. According to Fig. 3d, the injection–production ratio gradually becomes larger, especially after 2 h. With the injection–production ratio increasing, the mole fraction of methane in the gas collection cylinder linearly decreases as shown in Fig. 3a, which means more N2 is needed
0.8
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0.5 0.4
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Methane recovery ratio Hydrate decomposition ratio
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0
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Time (h)
3
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50
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0
Hydrate decomposition ratio (%)
Methane recovery ratio (%)
0.2
3.5 3.0 2.5 2.0 1.5 1.0
0.5
1.0
1.5
2.0 2.5 Time (h)
3.0
3.5
4.0
Fig. 3. Experimental results for run 2 by continuous injection mode with hydrate saturation of 17.81% and salinity of 33.5 g/L when at 3.1 MPa, 275.2 K, and injection rate of 50 mL/s: (a) gas composition in gas collection cylinder with elapsed time; (b) gas composition of gas phase in reactor with elapsed time; (c) variation of hydrate decomposition ratio and methane recovery ratio with elapsed time; and (d) injection–production ratio with elapsed time.
to recovery the residual methane (less than 10 mol%). Taking the economical efficiency into consideration, the enduring time for this set of experiment should be constrained to 2 h. As hydrate dissociation is an endothermic reaction, many researchers (Moridis and Sloan, 2007; Liang et al., 2010; Song et al., 2015; Zhao et al., 2015) have reported that when the method of depressurization is used to recover methane trapped in hydrates, the temperatures and the driving force of hydrate dissociation will drop in response to the depressurization of hydrate-bearing sediments, thus leading to the gradual decrease of gas production rate. Another series of experiments (runs 5–8) with a depressurization method at different production pressures were conducted to compare with the N2 sweep method. Experimental parameters of hydrate dissociation by the depressurization method are listed in Table 2 and the results are shown in Fig. 5. The methane recovery ratio for run 2 by N2 sweep when at 3.1 MPa was also plotted for comparison. It can be seen that the recovery ratio of methane by N2 sweep can reach 90% in first 1.5 h (run 2), which is much quicker than that by single depressurization. In addition, during the N2 sweep process, the mole fraction of N2 gradually increases with the elapsed time, leading to the increase of the driving force for hydrate dissociation and further promotes the decomposition of hydrate. The N2 works as diluent gas toward the dissociation of gas hydrates and directly affect the gas compositions to increase gas production rate. Fig. 6 shows the hydrate formation conditions for pure CH4 and CH4 þN2 gas mixtures calculated by a Chen-Guo hydrate model (Chen and Guo, 1998). For the depressurization method, the ambient conditions will change along with the arrow direction 2 in Fig. 6; whereas in the N2 sweep method, the driving force for hydrate dissociation
will increase in the arrow direction 1. Therefore, to ensure continuous gas production by maintaining the reservoir temperature at a certain range, a combined production method coupling the depressurization with other production methods, such as N2 sweep, would be more useful and effective. Considering the cost used to separate the recovery gas, the fraction of methane extracted from the hydrate reservoir should be above a certain value (such as, 0.3) depending on the price of natural gas. In this case, the driving force calculated can reach 5.41 MPa as shown in Fig. 6, which is much larger than the direct depressurization. 3.2. The batch mode The batch injection mode is also widely used in the oil industry to enhance oil recovery. Using this mode, the hydrate decomposition by hot water or inhibitor injection has been investigated (Koh et al., 2012; Yuan et al., 2013). In this work, the batch injection mode was also extended to hydrate recovery by N2 sweep. For experimental run 4 using the batch mode, the system pressure initially decreased to 3.1 MPa, followed by the continuous injection of N2 into the reactor from the bottom until the pressure was raised to 5.0 MPa. Once achieved, all valves were closed and the system was kept at this state for 12 h. After this process, gas compositions in the reactor and gas collection cylinder were measured by a gas chromatograph. Thereafter, the gas in the reactor was released until the pressure was reduced to 3.1 MPa, and the next cycle of N2 sweep started. The pressure curves for each cycle in run 4 and the hydrate dissociation results are shown in Figs. 7 and 8, respectively. Compared with the continuous mode, the variation tendencies of gas compositions in gas collection
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Fig. 4. Temperature variations at different locations in the reactor during N2 sweep in run 2.
16
Table 2 Experimental parameters of hydrate dissociation by the depressurization method.
20–40 20–40 20–40 20–40
Methane recovery ratio (%)
5 6 7 8
Quartz sand (mesh)
Salinity (g/L)
Hydrate saturation (%)
33.5 33.5 33.5 33.5
17.92 18.03 17.86 17.62
Initial temperature (K) 275.2 275.2 275.2 275.2
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Pressure (MPa)
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Pressure (MPa)
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288
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Fig. 6. Hydrate formation conditions for pure CH4 and CH4 þ N2 gas mixtures (T ¼ 275.2 K) calculated by the Chen-Guo hydrate model (1998).
0.5 MPa, run 5 1.0 MPa, run 6 1.5 MPa, run 7 2.5 MPa, run 8 3.1 MPa, run 2
40 30 20 10 0 -1
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9
Fig. 5. Gas recovery ratio versus elapsed time by the depressurization method under different production pressures (runs 5–8) or by the N2 sweep method (run 2).
cylinder and in the reactor are more smooth in the whole process, as shown in Fig. 8a and b. Hydrate also began to dissociate after the injection of N2 from the bottom of the reactor until a new
balance was attained. The production rate is ultimately controlled by heat transfer toward the hydrate dissociation region. For the batch mode, there is enough time for heat transfer from water bath to hydrate dissociation zone, so the decomposed water can be in form of free water, which would help methane transfer from the surface of hydrate dissociation zone to gas phase. According to Fig. 8c, the hydrate decomposition ratio is higher than the ratio of methane recovery in whole process, which differing from the continuous mode as shown in Fig. 3c. According to Fig. 8d, the IPR nearly increases linearly with the elapsed time, and the IPR of the continuous mode is larger than that of the batch mode, suggesting that the batch mode is more economical. Meanwhile, a small
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number of wells are needed for the batch mode in practice field production. As shown in Fig. 7, in the first four cycles after N2 injection, the system pressure continues to rise, while it remains constant for the following five cycles. This means the hydrate dissociation takes place in the first four cycles of N2 sweep, while hydrate has almost totally decomposed in the following five cycles. It should be noted that there is a turning point in the first cycle, the pressure increases firstly, then turns to decrease. In the first cycle of N2 sweep, after N2 was injected into the reactor from the bottom, the distribution of N2 was non-uniform and the concentration of N2 at bottom was high enough to promote hydrate dissociation and
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induce the increase of system pressure. With the continuous increase of pressure, the distribution of N2 becomes uniform. The temperature of the ambient sediment decreases due to the endothermic reaction of hydrate dissociation, resulting in hydrate regeneration and decrease of pressure. In addition, after thorough hydrate dissociation, it is unnecessary to continue with the injection of N2, such as the fifth to ninth cycles in run 4. However, in this work, to compare with the continuous mode, it was not stopped after the fourth cycle until the mole fraction of methane from the exhausted gas was above 98%. When gas sweep by the batch mode is used to hydrate production, the total pressure should be below the equilibrium pressure in the first or second gas injection cycle to avoid hydrate regeneration. The N2 sweep cycle could be stopped after hydrate was totally decomposed and the residual gas can be extracted directly. The batch injection mode can be used to control the dissociation rate of hydrate and may be an economical and lower risk way for hydrate exploitation.
5000
3.3. The influence of sweep rate on gas production
6000
Pressure (KPa)
733
4500
The injection rate of N2 is an important factor because it directly affects the composition of gas phase. Experimental runs 1 and 2 were performed to examine the influence of injection rate, in which the gas flow rate is 16.7 mL/s (run 1) and 50 mL/s (run 2), respectively, with the same other experimental parameters. The comparison of different sweep rates on gas production is shown in Fig. 9. As shown in Fig. 9a, the methane recovery ratio in run 2 is higher than that in run 1 during the whole process. The time needed for methane recovery ratio reaching 90% for run 2 is nearly half that for run 1, although the hydrate saturation is almost the same for these two runs. In addition, there is a turning point at
4000 3500 3000 cycle: 1
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Time (day) Fig. 7. Variation of pressure curves for each cycle during the gas injection stage in run 4 using the batch method.
0.8 1.0
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8
10
Fig. 8. Experimental results for run 4 by batch injection mode with hydrate saturation of 21.18% and salinity of 33.5 g/L when at 3.1 MPa, 275.2 K, and injection rate of 50 mL/ s: (a) gas composition in gas collection cylinder at different cycle times, (b) gas composition of gas phase in reactor at different cycle times, (c) variation of hydrate decomposition ratio and methane recovery ratio at different cycle times, and (d) injection–production ratio at different cycle times.
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the recovery ratio of methane for run 2 reaches 87.5%, while it only reaches 55% for run 3. The reason may be that when N2 is continuously injected into the reactor, the mole fraction of N2 in gas phase gradually increases and hydrate begins to dissociate. In the first stage, the methane recovery ratio is mainly governed by hydrate dissociation, so the gas production rate of run 2 is the same as run 3. The rapid dissociation of hydrate in first stage causes the temperature of reservoir decrease to be below the freezing point, so the water from hydrate dissociation exists in the form of ice. Therefore, the gas production rate becomes lower after the first stage as shown in Fig. 10a, whereas in the second stage, the methane recovery ratio may mainly be governed by hydrate dissociation and methane transfer. In the third stage, the amount of un-decomposed hydrate was much lower while the effect of the mass transfer increases, resulting in lower gas production rate. As described above, the exploitation of hydrate by N2 sweep also needs external energy to maintain the gas production rate for hydrate reservoir with high saturation, similar to the depressurization method. In addition, as shown in Fig. 10b, when at the same IPR, the methane recovery ratio of run 2 is higher than run 3, meaning that gas sweep is an effective way for recovering methane gas from low saturation or widely-distributed hydratebearing deposits. In comparison, the driving force for hydrate decomposition is lower for the depressurization method; the thermal stimulation method needs to inject much heat; while the inhibitor injection method is limited by environmental protection. The method of N2 sweep is more suitable especially for low saturation hydrate reservoirs from the viewpoints of energy saving
1.5 h for the curve of methane recovery ratio of run 2. The methane recovery ratio quickly increases to 87.5% before this point; afterwards, the curve tends to be smooth. As shown in Fig. 6, with the increase of the mole fraction of N2, the driving force for hydrates dissociation increases accordingly. The faster injection rate of N2 will result in the higher N2 mole fraction in gas composition. However, the IPR of run 2 was nearly twice that of run 1 to acquire the same methane recovery ratio for the same saturation hydrate sediments, which means the increase of gas production rate is at the cost of injecting much more N2. 3.4. The influence of hydrate saturation on gas production
100
100
90
90
Mehtane recovery ratio (%)
Methane recovery ratio (%)
The saturation of seafloor hydrates mainly ranges from 10% to 60%, with some as high as 90%. Therefore, if the gas production method is suitable for hydrate exploitation is also dependent on the overall saturation of hydrate-bearing sediment. In this work, the influence of saturation of hydrate reservoir on methane recovery by N2 sweep were examined in runs 2 and 3 with hydrate saturation of 17.81% and 27.64%, respectively, with the same injection rate of 50 mL/s. Fig. 10 shows the comparison of gas production by N2 sweep for hydrate samples with different hydrate saturation. As shown from Fig. 10a, the gas recovery ratio in run 2 is higher than that in run 3 during the whole process. The curve of methane recovery ratio can be divided into three stages for run 3 while two stages for run 2 on basis of the gas production rate, with only in the first stage of run 3 having a gas production rate as quick as that of run 2. Meanwhile, at the end of this stage,
80 70 Run 1 Run 2
60 50
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3 Time (h)
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5
6
80 70 Run 1 Run 2
60 50
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2 3 Injection-production ratio
4
Fig. 9. Comparison of different sweep rate on gas production for runs 1 and 2: (a) variation of methane recovery ratio with elapsed time, and (b) the relationship between methane recovery ratio and injection–production ratio.
Fig. 10. Comparison of gas production by the N2 sweep for runs 2 and 3 with different hydrate saturation: (a) variation of the methane recovery ratio with elapsed time, and (b) the relationship between methane recovery ratio and injection–production ratio.
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and gas production rate. Most nature gas hydrates are widely dispersed in low saturation and are not considered to be a viable resource with current technologies. Gas sweep may supply a new strategy that would be helpful to make this type of hydrate become a technically recoverable resource.
4. Conclusions A series of experiments were performed to study the exploitation of hydrate by gas (N2) sweep using a scale-up three-dimensional apparatus. The following conclusions were obtained: (i) The gas production rate by gas sweep is much quicker than single depressurization. During the N2 sweep process, the mole fraction of N2 gradually increases with the elapsed time. The driving force for hydrate dissociation increases with the increase of mole fraction of N2, which further promotes the decomposition of hydrates and guarantees a high gas production rate. (ii) Compared with the continuous mode, the gas compositions in gas collection cylinder and reactor using the batch injection mode decrease more slowly during the whole sweep process, suggesting that the batch injection mode can be used to control hydrate dissociation rate and may be the lower risk way for hydrate exploitation. (iii) The faster injection rate of N2 results in the higher N2 mole fraction in gas composition. The increase of gas production rate is at the cost of injecting much more N2. The experimental results also show that the method of N2 sweep is quite suitable for low saturation hydrate reservoir. The methane production from hydrate reservoir by gas sweep may supply a new strategy for high-dispersed hydrate resource.
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