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Experimental investigation on thermal cracking, permeability under HTHP and application for geothermal mining of HDR
Accepted Manuscript Experimental investigation on thermal cracking, permeability under HTHP and application for geothermal mining of HDR
Yangsheng Zhao, Zijun Feng, Yu Zhao, Zhijun Wan PII:
S0360-5442(17)30850-2
DOI:
10.1016/j.energy.2017.05.093
Reference:
EGY 10901
To appear in:
Energy
Received Date:
06 August 2016
Revised Date:
28 March 2017
Accepted Date:
14 May 2017
Please cite this article as: Yangsheng Zhao, Zijun Feng, Yu Zhao, Zhijun Wan, Experimental investigation on thermal cracking, permeability under HTHP and application for geothermal mining of HDR, Energy (2017), doi: 10.1016/j.energy.2017.05.093
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ACCEPTED MANUSCRIPT
Highlights 1. Performances of permeability, thermal cracking and microstructure in granite under high temperature and high pressure (HTHP) were investigated. 2. The related feature between thermal-cracking law and permeability under HTHP was revealed. 3. The influence of rock properties under HTHP on the sustainability of extracting Hot Dry Rock (HDR) geothermal energy system was demonstrated.
ACCEPTED MANUSCRIPT
1
Experimental investigation on thermal cracking, permeability
2
under HTHP and application for geothermal mining of HDR
3
Yangsheng Zhao1
Zijun Feng1
Yu Zhao 2,3*
Zhijun Wan4
4
1. Mining Technology Institute, Taiyuan University of Technology, Taiyuan, China 030024;
5
2. School of Civil Engineering, Chongqing University, Chongqing, China 40045;
6
3. Key Laboratory of New Technology for Construction of Cities in Mountain Area( Chongqing University ),
7
Ministry of Education, Chongqing, China 400030;
8 9
4. School of Mining Engineering, China University of Mining & Technology, Xuzhou, China 221008;
10
Abstract: Thermal cracking behavior of granite at high temperature and high pressure (HTHP) is the key
11
to the performance of Hot Dry Rock (HDR) geothermal energy extraction system. In this study,
12
permeability tests accompanying acoustic emission (AE) tests in granites are first conducted under HTHP
13
by 600℃ 20MN servo control rock triaxial testing machine. The test results show that granites, nearly
14
impermeable rocks, can show a striking increase of permeability by heating from the critical temperature.
15
The growth curve of granite permeability shows two phases because of the multi-period of thermal-
16
cracking in the heating process from room temperature to 500℃. The coupled effect of temperature and
17
pressure shows that critical temperature of permeability change decreases with increasing confining
18
pressure. Then, a detailed characterization of the sample microstructure is presented using Micro-CT
19
method. It is discovered that thermal cracking mainly occurs at grain boundaries in forms of inter-
20
granular microcracks along apparent weaknesses, and develops with increasing temperature. Meanwhile
21
intra-granular cracks are observed when heating to 500℃, indicating that thermal cracking in granite
22
under HTHP is induced by both intra-granular and inter-granular thermal stress. At last, experimental
23
stimulation and application for geothermal mining of HDR are discussed.
24 25
Key words: thermal cracking; permeability; AE event; Micro-CT; Hot Dry Rock
26 27 28 29
*Corresponding author: Yu Zhao, Prof, Ph.D. Tel: 86-023-65123363 Email: [email protected]
1
ACCEPTED MANUSCRIPT Nomenclature cw
30
specific heat of the water [L2T−2K−1]
cr
specific heat of the rock matrix
Tr
temperature of rock matrix
[L2T−2K−1]
[K]
[ML-3]
ρr
density of rock matrix
[K]
λr
thermal conductivity of rock-matrix
Trb
fracture surface temperature
W
source sink term of heat
ρw
density of water
Tw
water temperature
λw
thermal conductivity of water
T
temperature
t
time
b
crack aperture
[MLT−3K−1]
[K]
[T]
p [L]
kfi
[ML-3] [MLT−3K−1]
[K]
[ML-1T−3] [ML−1T−2]
water pressure in pore or crack water permeability coefficient
[LT-1]
1. Introduction
31
Hot Dry Rock (HDR) geothermal energy is mainly stored in granite. HDR technology inevitably involves
32
artificial enhancement of the permeability of the rock heat exchanger, colloquially known as geothermal
33
reservoir[1]. Thermal cracking plays an important role in exploitation of geothermal of HDR. Thermal stress
34
can significantly change physical properties through development of thermal cracks[2], which may enhance the
35
permeability, speed up the fracture propagation and finally trigger free convection. Therefore, it is of
36
considerable importance to understand how the geothermal reservoir will be modified by thermal cracking.
37
Focusing on rock’s thermal cracking, many works have been done. Researchers committed to studying
38
the influence of thermal cracking on physical properties in the early times. Laboratory studies showed that
39
thermal cracking could change rock permeability[3-5], strength[6], porosity[7], elastic moduli[8], and other
40
mechanical properties[9-11]. In addition, the transition of quartz crystals from phase
to phase
in granite at
41
high temperature was
42
slip[13]. Ghassemi found that the fracture aperture had a significant increase after the extraction of geothermal
43
energy[14]. Thermal stress can also cause formation of new cracks. Laboratory experiments attempting to
44
induce thermal cracking are presented by many scholars[15-16]. Although numerous experiments about
45
thermally treated rocks have been conducted to investigate the thermal-induced changes in mechanical
46
properties, the evolution of thermal cracks is still not completely understood.
observed[12].
It is generally believed that thermal stresses can cause fracture opening and
47
The reservoir stimulation determines the HDR system’s performance to a great extent[17], which can be
48
regarded as a function of the effective permeability between the wells. To investigate the creation and
49
evolution of the reservoir, abundant numerical simulations and experiments associated with reservoir
50
stimulation of HDR geothermal system have been carried out by various researchers[18-21]. Ghassemi coupled
51
flow and heat transport to thermo-poroelastic deformation in a discretely fractured reservoir and examined the
52
physical phenomena that govern fluid injection/extraction[14]. Zhao established a theoretical model of fractured
53
rock mass deformation, seepage, and heat transfer to simulate the extraction of HDR geothermal energy[22].
54
Siratovich presented a new methodology designed to replicate thermal stressing and subsequent cooling under
55
water saturated conditions[23]. Hadgu studied the influence of fracture orientation on production temperature in
56
low permeability geothermal system through thermal-hydrologic simulations[24].
57
It is more special for experimental investigation on thermal cracking to consider effects of high
58
temperature and high pressure. In particular, previous experiments were conducted under either low pressure
59
or unstressed state conditions. However, large amounts of HDR geothermal energies are located at depth of
60
3000-10000m below the surface[25], with an average temperature of 400℃-500℃. How do changes in high
61
temperature and high pressure influence the physical properties of rocks? And how do changes in physical 2
ACCEPTED MANUSCRIPT 62
properties of rocks influence the sustainability of extracting hot dry rock geothermal energy? This study tries
63
to provide insight to the mechanism of thermal cracking induced damage on permeability at a condition of
64
HTHP and assist in stimulation optimization.
65
2. Experimental investigation on rock permeability
66
2.1 Equipment
67
The permeability tests were carried out by 600℃ 20MN servo control rock triaxial testing machine with
68
HTHP (Figure 1) in China University of Mining and Technology. It composes of three parts including host
69
loading system, auxiliary system for sample assembly and measurement system. The stress, pressure,
70
temperature and other parameters can be controlled by the host loading system during testing. The axial and
71
lateral load of the triaxial test equipment can be loaded on the sample separately. The deformation of the tested
72
sample can be precisely measured by grating sensor with a precision of 0.005 mm. The maximum axial and
73
lateral loads both are 10 MN. The maximum axial pressure on sample is 318 MPa, while the lateral pressure is
74
250 MPa. The size of tested sample is Ф 200×400 mm, which is 64 times that of a standard sample. The
75
maximum heated temperature of testing equipment is not less than 600℃. The whole stiffness of the
76
equipment is greater than 9×1010 N/m.
77
The permeability test and heating process were conducted simultaneously. Nitrogen was used as flow
78
gas, and was controlled in the airintake by valve and barometer. Gas flow in the air outlet was measured by
79
soap-foam flowmeter and glass rotameter. The Figure 2 shows the flow chart of permeability measurement.
80
Due to the large scale of sample, the heating process was slowly preformed (5℃/h) to have an adequate
81 82
heating.
2.2 Sample preparation and test procedure
83
Three granite samples (NO.2, NO.4 and NO.7) for permeability tests were cored from Pingyi, Shandong
84
province in China, named “Luhui granite”. Samples were first produced into cylindrical roughcast by stone
85
processing machine and then polished carefully to the size of Ф 200×400 mm. They were heated from room
86
temperature to 500℃ at confining pressure of 75MPa, 25Mpa and 12.5Mpa, respectively. The goal of this
87
study is to replicate the thermal cracking process that may occur in a geothermal environment at depth of
88
3000m, 1000m and 500m.
89
The permeability test procedures are summarized as follows:
90
1) Measuring the size of sample and then installing it into the pressure chamber in the auxiliary system.
91
2) Loading the axial and confining pressure to predetermined value and then heating the sample manually
92
with the heating rate of 5℃/h.
93
3) Heating the sample to plateau temperature (50℃, 100℃, 150℃, …, 450℃, 500℃) and holding the
94
plateau temperature for 2 hours. Then injecting nitrogen and measuring the permeability of sample by means
95
of monitoring the gas flow rate.
96 97 98
4) Repeating the heating process to reach the next plateau temperature and then conducting the permeability test similarly, until accomplishing all permeability tests.
2.3 Results
99
The evolution of permeability is analyzed for samples successively, with the main results being
100
exemplified by sample NO.7. Permeability in different gas pressures has increased over the originally
101
observed value after thermal treatment, and shows the same varying trend at different gas pressure (Figure 3). 3
ACCEPTED MANUSCRIPT 102
It begins to increase when heating to 65℃~80℃, and gets its first peak value at the temperature of 150℃. For
103
the next heating process of temperature range from 150℃ to 350℃, the permeability changes a little. It slowly
104
decreases to a vale value area and then increases slightly. After 350℃, it increases sharply and reaches its
105
second peak value with the temperature at about 400℃~450℃. Thus, the growth curve of permeability can be
106
divided into two phases. At initial stage of thermal cracking action, the permeability shows a weak variation.
107
When the temperature rising over a certain value, defined as critical temperature, the permeability shows a
108
sharply increase. The variation of critical temperature against confining pressure is plotted in Figure. 4. It can
109
be seen that the critical temperature decreases rapidly when the confining pressure less than 30MPa, while it
110
tends toward steady value for confining pressure over 30 MPa. The most reasonable explanation may be that
111
intra-granular thermal stress is induced by shrinkage and decomposition of minerals from high confining
112
pressure[26]. Apart from the intra-granular thermal stresses, there are also inter-granular thermal stresses due to
113
the anisotropic expansion[27]. When these stresses exceed the local strength, microcracks are generated. As
114
confining pressure increasing, the pore radius decreases and finally reaches its minimum value. Therefore,
115
thermal expansion and deformation are completely restrained, and no more intra-granular thermal stresses are
116
induced. This indicates that inter-granular thermal stresses play a decisive role in thermal cracking, and critical
117
temperature are independent of confining pressure. In an isotropic, homogeneous granite, Chaki indicated that
118
500℃ could be the critical temperature of permeability changes under low confining pressure (0.8MPa)[12].
119
He observed a weak variation in permeability over the temperature range of 105-500 ℃, but a rapid increase
120
above 500℃, further supporting the validity of our experimental results.
121
The sensitivity of the permeability to temperature indicates that the permeable networks in thermal-
122
treated granite are likely to be multi-fracture dominated. Increasing permeability in a zone with a high
123
geothermal gradient will trigger free convection[28]. The test results illustrate that, for HDR geothermal system,
124
permeability can be enhanced through the simple application of thermal cracking.
125
3. AE experiment of rock thermal cracking
126
3.1 AE testing methodology
127
To investigate the effect of temperature on thermal cracking, AE response was detected by AE sensor
128
during the heating process. Four AE sensors were strictly installed on the top and bottom of the compression
129
chamber using magnetic stand of clock gauge. Signals of thermal cracking were recorded, including AE event
130 131
number, AE event energy, AE event duration and AE amplitude.
3.2 AE characteristics of rock thermal-cracking
132
The curve of AE response varying with temperature is plotted in Figure 5. From the figure we can obtain:
133
1) At the initial period of heating process (35℃ to 40℃), AE events occur with AE product
134
(multiplication of vibration amplitude and time) of 7000, indicating the beginning of thermal-cracking. The
135
recorded product of AE response mounts to 16000 when temperatures rise to 65℃, demonstrating that
136
intensive thermal cracking happens in this temperature stage.
137
2) The AE response is relative quiet at about 65℃ to 110℃. For the next heating period of 110℃ to 230
138
℃, the product value reaches 28000 and the average value is 13000. The long duration of AE response
139
manifests that tempestuous thermal cracking occurs during this temperature stage.
140
3) The AE response represents relative faintness with temperature ranging from 230℃ to 270℃.
141
4) The AE response is active with further temperature increasing from 270℃ to 340℃, while its average 4
ACCEPTED MANUSCRIPT 142
product value is lower than that of 110℃-230℃.
143
5) After AE response undergoing a plateau at about 340℃ to 400℃, it turns to active again during the
144
period of 400℃ to 500℃, and the product reaches the maximum value of 37000, representing the most
145
powerful cracking.
146
The above analyses demonstrate that a few intensive periods of thermal cracking occur during the heating
147
process. The simultaneous measurement of permeability shows that rock permeability represents a peak value
148
area in intensive thermal-cracking process (Figure 5). It slowly decreases to a vale value area in inactive period
149
of AE events. When the next thermal-cracking peak area comes out, the permeability increases again. After
150
experienced repeating thermal-cracking accumulation, the permeability is more and more high and thermal-
151
cracking is more intensive synchronously. Note that intensive thermal cracking occurs before 200℃ under
152
HTHP, which is contrasted to the experimental results in ref [4] where very little damage occurs below 300°C.
153
We considered that high confining pressure contributes to making such a great difference. With the increase of
154
temperature, the crystal water and bounded water escape, and porosity increases. Synchronously, the
155
interspace due to the loss of crystal and bounded water is compressed under high pressure. This will cause
156
internal frictions and thus generate AE events. Therefore, intensive thermal cracking is noticed at low
157
temperature.
158
4. Investigations on micro-structure of granitic thermal crack
159
Samples for micro-CT experiments are specially processed under condition of HTHP, and are carried out
160
by the μCT225kVFCB micro-CT experimental system designed by China Taiyuan University and
161
Technology. This investigation is essential to understand the effect of thermal treatment, as any macroscopic
162
property change to the samples should be observable at the microstructural level[23]. Samples are processed to
163
approximate circular cylinders with the size of Ф 2.7 mm × 20 mm. The amplification coefficient can be up to
164
105 times. Flat lattice of scanning is 2048×2048, and the size of scanning cell is 1.847µm. In order to analysis
165
internal meso-structure, we cut the sample into 1000 layers in longitudinal direction with each layer of
166 167
4.1 Main contents and meso-structure of granite
1.847µm to.
168
Prior to the experiment, Luhui granite is epigranular and dense. As shown in Table.1, Luhui granite is a
169
strongly heterogeneous brittle and hard rock, mainly consist of feldspar, quartz, and illite, et.al. Figure 6 shows
170
the meso-structure of granite in the room temperature using high-accuracy micro-CT. Crystal grain, boundary
171
of grain, binding material among grain and grain pore can be clearly differentiated. The component and
172
content of granite mineral are observed through X-ray diffract analysis spectrum, as shown in Figure 7. It can
173
be observed that the main mineral compositions (feldspar, quartz and illite) have almost the same proportion.
174
However, mechanical properties of feldspar, quartz and illite differ greatly, making granites with intensively
175 176
heterogeneous.
4.2 Evolution of thermal crack in the heating process
177
The microstructure of rock controls the macroscopic physical properties. In granite, the crack damage is
178
clearly evident in heated samples through the generation of cracks (Figure 8). The sizes of scanning cell and
179
sample are 1.09 µm×1.09 µm and 1.72 mm×1.447 mm, respectively.
180
Room temperature: As can been seen from Figure 8a, no obvious crack in micro scale is observed.
181
Basically, the cross section of the sample consists of three parts: high density area located in the right side with 5
ACCEPTED MANUSCRIPT 182
the X-ray absorption coefficient of 0.0302749, low density located in the middle with the coefficient of
183
0.0124444 and, sub-low density area located in the left side with the coefficient of 0.0166752. There are some
184
twisty lines formed by some connecting scanning cells with obvious lower density locating in the low density
185
area in the middle, which turn to be the first thermal cracking region.
186
200℃: It can be clearly seen from Figure 8b that the crystal particles are surrounded by a large majority
187
of microcracks in weakening lines when heated to 200℃. But a large-closed polygon crack around granitic
188
particles has not yet been formed. For example, the looked-like macro-long crack in the left upper side actually
189
consists of many disconnect microcracks, with the length of 23 µm, 34 µm, 45 µm, 18 µm, 32 µm, 46 µm, 104
190
µm and 18 µm (bottom to upper), respectively.
191
300℃: Microcracks further develop and propagate with creasing temperature. Large cracks can be
192
observed, and the crack length increased significantly (Figure 8c) when heating to 300℃. For example, the
193
length of crack on the border of the right side is 316 µm, and the upper right side one is 308 µm. Both of them
194
are rapidly developed, much greater than the length value of 200℃. Meanwhile the size of crack around the
195
granite crystal particles is also increased.
196
500℃: When heating to 500℃, microcracks develop to macrocracks cutting through the whole rock
197
sample, and closed polygonal cracks around each crystal grain are completely formed, leading to the formation
198 199
of mylonitic texture in granite (Figure 8d). Meanwhile, some intra-granular cracks begin to initiate.
4.3 Analysis on the characteristics of thermal crack under 500℃
200
Table 2~6 show the size of granite grain in different sections under 500℃. As shown in Table 3, the size
201
of rock grains in the 700th layer of x-y section can be clearly identified in the range of 0.04mm to 0.1mm, and
202
the equivalent radius of circle is in the range of 0.1 mm to 0.2 mm.
203
(1)As shown in Figure 9, CT scanning images of the 300th, 500th, 700th and 900th layers(x-y section)
204
depict that the thermal cracks are almost polygon distribution around weak plane of granite grains boundary.
205
The area of broken cell is 0.1-0.2 mm2, the equivalent radius of circle is 0.15-0.25 mm, which is close to the
206
size of granite grain. Due to thermal damage happening in the weak area of particle joint, the circle equivalent
207
radius of crack cell is 0.05mm larger than the size of granite grain (Table.2, Table.3 and Table.4).
208
(2)The CT scanning images of longitudinal sections (x-z section and y-z section) also demonstrate that
209
thermal cracking happens in the weak area of particle joint. As shown in Table 5 and Table.6, the sizes of
210
crack cell in the 512th and 1260th layer of X-Z section are both 0.1~0.3mm.
211
(3)Intra-granular cracks are observed under 500℃, such as the crack located at the bottom right corner in
212
700th layer (x-y section). In the middle part of 1260th layer (y-z section), a thermal crack unit also penetrated
213
a long and thin rock particle.
214
5. Geothermal mining of HDR mechanism
215
5.1 Concept of geothermal HDR system
216
The concept of HDR is to exploit energy resources from the earth by drilling wells into hot, crystalline
217
rock at great depth. A well is drilled first to inject cold water at high pressure to stimulate or hydraulically
218
fracturing the natural rock joints, thereby creating a geothermal reservoir. Injected cold water picks up heat
219
and returns to the surface via production well (Figure 10). The artificial geothermal reservoir forms the heart
220
of HDR energy extraction. In a low-permeability geothermal reservoir, heat is transported through rock matrix
221
which is slow due to the low thermal conductivity of rock. While in a good connectivity reservoir, heat is 6
ACCEPTED MANUSCRIPT 222 223
transported by a convention system.
5.2 Evolution of reservoir and heat transport
224
The implication of enhanced permeability tests indicates that the mechanism of reservoir stimulation
225
appears to be a complementary mix of high thermal cracking and interplay of regional stress fields. The
226
microscopic investigations clearly demonstrate the evolution of the reservoir under thermal-cracking process.
227
It is observed that thermal cracking occurs first at grain boundaries in forms of microcracks along apparent
228
weaknesses. These microcracks gradually connect and propagate under thermal stress, and eventually develop
229
to macrocracks when heated to high temperature. Microseismic monitoring can be used for detecting reservoir
230
development in suit. We use AE events to exploit the effects that thermal stress places on the reservoir in
231
laboratory environment. On one hand, the initiation of AE event indicates the temperature necessary for
232
fracture propagation and, on the other hand the extent of AE response reveals the rate of growth in fractures
233
and porosity. AE events were first monitored when heated to about 60℃, and after that a few intensive periods
234
were monitored, indicating new thermal cracks formation. Thus we can use these induced events to locate the
235
thermalised crack and follow its growth.
236
Permeability, the main mechanism to be envisaged for the creation of HDR reservoir, has been
237
significantly enhanced in our test. In general, granite has negligible porosity and permeability before heating.
238
With the onset of thermal cracking, the permeability is stimulated, increased by two orders of magnitude from
239
4.7×10-8 to 6.03×10-6 D of sample NO.7. Meanwhile, multiple peak behavior of AE response and
240
permeability are clear indications of the presence of multiple flow paths. The permeability evolves with
241
increased temperature is essential to relating this relationship to thermal stimulation optimization.
242
The heat transmission takes place both in the rock matrix and the fracture, and is transferred by
243
convection and conduction. We have previously reported a thermo-hydro-mechanical coupled model for
244
enhanced geothermal system[22]. In the model, fluid density is a function of the water pressure and temperature,
245
and rock mass is simplified as a fracture media consisting of fractures and matrix rock block of pores and
246
cracks. The conservation of thermal energy for the heat transport fluid flowing in the fractures:
247 248
cw
wTw 2 w2Tw cw w k fi p,i Tw ,i r Trb Tw t b
(1)
The conservation of thermal energy for the heat conduction in HDR (or matrix rock block):
r cr
249
Tr rTr ,ii W t
(2)
250
Where is density, c specific heat, T temperature, thermal conductivity, W source sink term of heat,
251
p water pressure. The subscript w, r and rb refer to the mobile fluid phase, rock matrix and fracture surface,
252
respectively. The conservation of thermal energy of the rock mass fracture describes the heat exchange process
253
with simultaneous conduction and convection, which is generally more accurate than discussing simple
254
conduction or convection.
255
6 Discussions
256
6.1 Thermal cracking
257
Thermal stress will be induced by changes of temperature in the inner of heterogeneous rock, regardless
258
of the mechanical state of granite. When the stresses exceed bearing capacity of inner grain and cement,
259
thermal cracking happens. The thermal expansion coefficients of multi-crystals mineral are completely 7
ACCEPTED MANUSCRIPT 260
different and all of them are a function of temperature. According to thermal elastic theory, thermal stress will
261
first occur in different crystals mineral and gets its maximum value in the grain boundary. Therefore, thermally
262
induced cracks in granite are mainly inter-granular cracks at initial stage of thermal cracking action. However,
263
intra-granular cracks begin to initiate at late stage of thermal cracking action. Intra-granular thermal stresses
264
can be induced by shrinkage and decomposition of minerals under high confining pressure. When these
265
stresses exceed the local strength, intra-granular cracks are generated. It is concluded that under low pressure
266
conditions, differential thermal expansion of adjacent minerals is considered responsible for the thermal
267
damage in granite at elevated temperatures. Under high pressure and high temperature conditions, however,
268
thermal cracking can occur both between adjacent crystalline grains (inter-granular thermal stress) and within
269
grains (intra-granular thermal stress).
270
Owing to the complex composing of granite and great difference in thermal expansion coefficients among
271
crystals grains, granites show multi-period of thermal-cracking under heating processes. Take NO.7 granite
272
sample as example, there exists four main thermal-cracking intensive stages before heating to 500℃, which
273 274
are 55℃ to 65℃, 110℃ to 230℃, 270℃ to 340℃and 400℃ to 500℃.
6.2 Permeability changes.
275
The variation of gas permeability against temperature of heat treatment experienced two phases, a
276
weak variation and an exponential rush. At initial stage of thermal cracking action, microcracks may be
277
generated but with low cracking density, and connected network has not been created. In this phase, the
278
permeability increased first and then it dropped to a vale value area. Numerous studies have observed
279
permeability reduction in granite under heating processes[29-31]. Xie and Zhao find that thermal
280
expansions of rock grains turn out to be outward deformation before heating to 150℃. But some of them
281
change to inward deformation over the temperature range of 150-350℃[32], which gives a reasonable
282
explanation of our varied permeability results. Obviously, outward expansions of rock grains increase the
283
porosity and connectivity of cracks, while inward expansions tend to close cracks thus decreasing the
284
permeability. Although microcracking started in granite at about 60 ℃, most of the mineral grains were
285
microcracked at late stage of thermal cracking action (about 350 ℃). In this stage, microcracks develop
286
to macrocracks, and intra-granular cracks are created which improve the connectivity of the crack
287 288
network significantly. As a result, permeability increased sharply. 6.3 Experimental stimulation and application for geothermal mining of HDR
289
Permeability test results show that granite, nearly impermeable rock, could show a striking increase
290
of permeability by heating from the critical temperature likely to be found in a geothermal environment at
291
great depth. Sun suggested that 400℃ could be a critical threshold of the thermal damage of granite
292
under unstressed state conditions[16]. While our experimental results showed that the value of critical
293
temperature deceases to 150℃ under confining pressure of 75MPa, which makes thermal stimulation
294
happen effortlessly. Although the temperature and pressure increase with the depth, which gives a higher
295
thermal efficiency, drilling costs and equipment costs become proportionately more significant.
296
Therefore, the depth of the reservoir would be significant. It is necessary to carry out extensive feasibility
297
studies before a site is selected for the development of HDR geothermal system. Based on the
298
experimental results that the critical temperature of granite changes little at confining pressure above
299
30MPa, we prefer that confining pressure of 30MPa could be a reliable value when employing thermal 8
ACCEPTED MANUSCRIPT 300
stimulation to enhance the permeability of reservoir. The development of thermal cracks on the
301
microscopic scale in our laboratory studies shows that high thermal treatment on an impermeable rock is
302
likely to create new thermal cracks and cause discontinuities opening and slippage. The implication of
303
HDR is that when wells are drilled into high temperature rocks, but with poor flow circulation because of
304
lacking flow path, thermal cracking processes could be a worthwhile pursuit to enhance the permeability.
305
7. Conclusions
306
The study presents the influence of permeability, microstructure and AE response of granite under HTHP
307
condition on HDR system. The law of rock thermal-cracking is indicated clearly, which is very important to
308
understand how thermal stress can be utilized to improve poorly connectivity in a geothermal reservoir.
309
The permeability curve versus temperature up to 500℃ has been divided into two phases based on the
310
critical temperature. And significant increase of permeability can be noticed after rising over the critical
311
temperature value. The test results show that the critical temperature decreases with increasing confining
312
pressure. AE events show that there are multi-periods of thermal-cracking during the heating process, and
313
intensive thermal cracking occurs before 200℃ due to the effect of high pressure. Microscopic investigations
314
show that thermal crack is gradually developed as temperature increasing. It is observed that thermal cracking
315
occurs first at grain boundaries in forms of inter-granular microcracks along apparent weaknesses under 200
316
℃, which develop and connect to form larger cracks under 300℃, and eventually propagate to cutting-through
317
macrocracks under 500℃. Intra-granular cracks and closed inter-granular cracks around crystal grains are
318
observed when heating to 500℃, changing the granite into mylonitic texture. These findings indicate that
319
thermal damage in granite under HTHP is induced by intra-granular thermal stress and inter-granular thermal
320
stress.
321
The experimental results show here illustrate the performances of permeability, thermal cracking and
322
microstructure under HTHP, and show that one of the most important properties of HDR system, permeability
323
can be enhanced by heating to critical temperatures.
324
Acknowledgments
325
This work was funded by National Natural Science Foundation of China (grant number 50534030; grant
326
number 51374257).
327
References:
328 329 330 331 332 333 334 335 336 337 338
[1] Parker, R. H.; Jupe, A. In situ leach mining and hot dry rock (HDR) geothermal energy technology. Miner Eng, 1997, 10, 301-308. [2] Menéndez, B.; David, C.; Darot, M. A study of the crack network in thermally and mechanically cracked granite samples using confocal scanning laser microscopy. Phys Chem Earth (A), 1999, 24, 627-632. [3] Somerton, W. H.; Gupta, V. S. Role of Fluxing Agents in Thermal Alteration of Sandstones. J Petrol Technol, 1965, 17, 585-588. [4] Jones, C.; Keaney, G.; Meredith, P. G.; et al. Acoustic emission and fluid permeability measurements on thermally cracked rocks. Phys Chem Earth, 1997, 22, 13-17. [5] Rutqvist, J.; Freifeld, B.; Min, K. B.; et al. Analysis of thermally induced changes in fractured rock permeability during 8 years of heating and cooling at the Yucca Mountain Drift Scale Test. Int J Rock Mech Min Sci, 2008, 45, 1373-1389. 9
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[6] Keshavarz, M.; Pellet, F. L.; Loret, B. Damage and Changes in Mechanical Properties of a Gabbro Thermally Loaded up to 1000°C. Pure Appl Geophys, 2010, 167, 1511-1523. [7] David, C.; Menéndez, B.; Darot, M. Influence of stress-induced and thermal cracking on physical properties and microstructure of La Peyratte granite. Int J Rock Mech Min Sci, 1999, 36, 433-448. [8] Nasseri, M H B.;, Schubnel, A.; Young, R. P. Coupled evolutions of fracture toughness and elastic wave velocities at high crack density in thermally treated Westerly Granite. Int J Rock Mech Min Sci, 2007, 44, 601616. [9] Heard, H. C. Thermal expansion and inferred permeability of climax quartz monzonite to 300°C and 27.6 MPa. Int J Rock Mech Min Sci Geomech Abstr, 1980, 17, 289-296. [10] Reuschlé, T.; Haore, S. G.; Darot, M. The effect of heating on the microstructural evolution of La Peyratte granite deduced from acoustic velocity measurements. Earth Planet Sci Lett, 2006, 243, 692-700. [11] Yavuz, H.; Demirdag, S.; Caran, S. Thermal effect on the physical properties of carbonate rocks. Int J Rock Mech Min Sci, 2010, 47, 94–103. [12] Chaki, S.; Takarli, M.; Agbodjan, W. P. Influence of thermal damage on physical properties of a granite rock: Porosity, permeability and ultrasonic wave evolutions. Constr Build Mater, 2008, 22, 1456-1461. [13] Ghassemi, A.; Tarasovs, S.; Cheng, H. D. A 3-D study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs. Int J Rock Mech Min Sci, 2007, 44, 1132-1148. [14] Ghassemi, A.; Zhou, X. A three-dimensional thermo-poroelastic model for fracture response to injection/extraction in enhanced geothermal systems. Geothermics, 2011, 40, 39-49. [15] Geraud, Y.; Mazerolle, F.; Raynaud, S.; et al. Crack location in granitic samples submitted to heating, low confining pressure and axial loading. Geophys J Int, 1998, 133, 553-567. [16] Sun, Q.; Zhang, W.; Xue, L.; et al. Thermal damage pattern and thresholds of granite. Environ Earth Sci, 2015, 74, 1-9. [17] Tenzer, H. Development of hot dry rock technology. GHC bulletin; 2001;14-22. [18] Wallroth, T.; Eliasson, T.; Sundquist, U. Hot dry rock research experiments at Fjällbacka, Sweden. Geothermics, 1999, 28, 617-625. [19] Tran, N. H.; Rahman, S. S. Development of hot dry rocks by hydraulic stimulation: Natural fracture network simulation. Theor Appl Fract Mec, 2007, 47, 77-85. [20] Zeng, Y. C.; Wu, N. Y.; Su, Z.; et al. Numerical simulation of heat production potential from hot dry rock by water circulating through a novel single vertical fracture at Desert Peak geothermal field. Energy, 2013, 63, 268-282. [21] Guo, B.; Fu, P.; Hao, Y.; et al. Thermal drawdown-induced flow channeling in a single fracture in EGS. Geothermics, 2016, 61, 46-62. [22] Zhao, Y. S.; Feng, Z.; Feng, Z.; et al. THM (Thermo-hydro-mechanical) coupled mathematical model of fractured media and numerical simulation of a 3D enhanced geothermal system at 573K and buried depth 6000–7000M. Energy, 2015, 82, 193-205. [23] Siratovich, P. A.; Villeneuve, M. C. Cole, J. W.; et al. Saturated heating and quenching of three crustal rocks and implications for thermal stimulation of permeability in geothermal reservoirs. Int J Rock Mech Min Sci, 2015, 80, 265-280. [24] Hadgu, T.; Kalinina, E.; Lowry, T. S. Modeling of heat extraction from variably fractured porous media in Enhanced Geothermal Systems. Geothermics, 2016, 61, 75-85. [25] Zhang, F. Z.; Xu, R. N.; Jiang, P. X. Thermodynamic analysis of enhanced geothermal systems using impure CO2 as the geofluid. Appl Therm Eng, 2016, 37, 1277-1285. [26] Dwivedi, R. D.; Goel, R. K.; Prasad, V. V. R.; et al. Thermo-mechanical properties of Indian and other granites. Int J Rock Mech Min Sci, 2008, 45, 303-315. [27] Chen. S. W.; Yang, C. H.; Wang, G. B. Evolution of thermal damage and permeability of Beishan 10
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granite[J]. Appl Therm Eng, 2017, 110:1533-1542. [28] Battaillé, A.; Genthon, P.; Rabinowicz, M.; et al. Modeling the coupling between free and forced convection in a vertical permeable slot: Implications for the heat production of an Enhanced Geothermal System. Geothermics, 2006, 35, 229-271. [29] Summers, R.; Winkler, K.; Byerlee, J. Permeability changes during the flow of water through westerly granite at temperatures of 100°-400°C. J Geophys Res Solid Earth, 1978, 83:339-344. [30] Morrow, C. A.; Lockner, D. A.; Moore, D. E.; et al. Permeability of granite in a temperature gradient. J Geophys Res Atmos, 1981, 86, 3002-3008. [31] Morrow, C. A.; Moore, D. E.; Lockner, D. A. Permeability reduction in granite under hydrothermal conditions. J Geophys Res, 2001, 106, 30551-30560. [32] Xie, J. L.; Zhao, Y. S. Meso-mechanism of permeability decrease or fluctuation of coal and rock with the temperature increase. Chin J Rock Mech Eng, 2017, 36, 543-551 [in Chinese].
11
ACCEPTED MANUSCRIPT 1 2
List of Table and Figure Captions
3
Table 1. Contents of minerals for granite samples
4
Table 2. Scale statistics of the thermal crack cell of the 500th layer in x-y section
5
Table 3. Scale statistics of the thermal crack cell of the 700th layer in x-y section
6
Table 4. Scale statistics of the thermal crack cell of the 900th layer in x-y section
7
Table 5. Scale statistics of the thermal crack cell of the 512nd layer in x-z section
8
Table 6. Scale statistics of the thermal crack cell of the 1260th layer in y-z section
9 10
Figure 1. 600℃20MN triaxial test equipment with servo controlled loading system
11
Figure 2. Flow chart of permeability experiment
12
Figure 3. Permeability change curves of sample NO.7
13
Figure 4. Evolution of critical temperature versus confining stress
14
Figure 5. Relation curves of AE event and permeability of granite NO.7 under conditions of HTHP
15
Figure 6. Meso-images of granite structure at normal temperature
16
Figure 7. X-ray graph analysis of granite
17
Figure 8. Thermal cracking CT sections of the granite in different temperature
18
Figure 9. Three dimension sections of the granite at 500℃
19
Figure 10. The extraction system of HDR thermal energy
ACCEPTED MANUSCRIPT Table 1 Contents of minerals for granite samples Mineral
Illite
Quartz
Feldspar
Calcite
Siderite
Others
Quality Percent
25%
28%
43%
1%
1%
2%
Table 2 Scale statistics of the thermal fracture cell of the 500th layer in x-y section Area/mm
Equivalent circular radius
2
/mm
0.1958×0.2198
0.0430
0.117043
0.2032×0.2734
0.0555
0.132980
143×200
0.2642×0.3695
0.0976
0.176278
127×116
0.2346×0.2143
0.0503
0.126502
No.
Number of pixels
Absolute size /mm
1
106×119
2
110×148
3 4
Table 3 Scale statistics of the thermal fracture cell of the 700th layer in x-y section Area/mm
Equivalent circular radius
2
/mm
0.1626×0.1572
0.02556
0.09020
60×175
0.1109×0.2117
0.02348
0.08645
3
50×195
0.0923×0.2395
0.22106
0.26526
4
90×190
0.1662×0.3438
0.05714
0.13486
5
66×76
0.1219×0.1404
0.01711
0.07381
No.
Number of pixels
Absolute size /mm
1
88×130
2
Table 4 Scale statistics of the thermal fracture cell of the 900th layer in x-y section Area/mm
Equivalent circular radius
2
/mm
0.2069×0.2346
0.04854
0.12430
109×167
0.2013×0.2020
0.04066
0.11377
103×139
0.1903×0.1681
0.03199
0.10091
4
91×106
0.168×0.1282
0.02154
0.08280
5
140×200
0.2587×0.2149
0.05559
0.13302
No.
Number of pixels
Absolute size /mm
1
112×194
2 3
Table 5 Scale statistics of the thermal fracture cell of the 512nd layer in x-z section Area/mm
Equivalent circular radius
2
/mm
0.3390×0.4740
0.1606
0.22616
113×146
0.2088×0.2697
0.0563
0.13388
3
249×228
0.4506×0.4212
0.1897
0.24579
4
169×227
0.3122×0.4194
0.1309
0.204153
5
346×249
0.6392×0.4601
0.2941
0.30596
No.
Number of pixels
Absolute size /mm
1
104×257
2
Table 6 Scale statistics of the thermal fracture cell of the 1260th layer in y-z section No.
Number of pixels
Absolute size /mm
Area/mm
Equivalent circular radius
2
/mm
ACCEPTED MANUSCRIPT 1
148×256
0.2734×0.473
0.1293
0.2028
2
180×224
0.3326×0.4138
0.1376
0.2093
3
252×184
0.4656×0.3399
0.1582
0.2244
4
120×272
0.2217×0.5026
0.1114
0.1883
5
224×240
0.4139×0.4434
0.1835
0.2416
Figure1 600℃20MN triaxial test equipment with servo controlled loading system Nitrogen jar
Gas charging device
Pressure meter
Specimen
Axial loading shaft
Valve
Tar Collection
Tridirectional valve Exit
Soap flow meter Valve
Flow meter Gas collection
Figure 2 Flow chart of permeability experiment
ACCEPTED MANUSCRIPT 8 Phase ℃
Phase ℃
Permeability ℃ D*10-6℃
6 2 MPa 3 MPa 4 MPa
4
2
0 0
100
200
300
400
500
600
Temperature (℃ )
Figure 3 Permeability change curves of sample NO.7 400
Critical temperature ℃ °C℃
350 300 250 200 150 10
20
30
40
50
60
70
80
Confining pressure (MPa)
Figure 4 Evolution of critical temperature versus confining stress 7# granite 8# lane
6
40000
5
30000
permeability/10-6D
Multi vabration Amp. and time
35000
4
25000
3
20000 15000
2
10000
1
5000
0
0 0
50
100
150
200
250
300
350
temperature/°C
400
450
500
550
Figure 5 Relation curves of AE event and permeability of granite NO.7 under conditions of HTHP
ACCEPTED MANUSCRIPT
Intergranular cement Grain boundary
intergranular Grain interior
interior
Figure 6 Meso-images of granite structure at normal temperature
Figure 7 X-ray graph analysis of granite
ACCEPTED MANUSCRIPT (a)Test 80kv; M=157; T=20°C
(b) 80kv; M=157; T=200°C
(c) 80kv; M=171; T=300°C
(d) 80Kv; M=105; T=500°C
Figure 8 Thermal cracking CT sections of the granite in different temperature
ACCEPTED MANUSCRIPT Test Conditions:80kv
M=105
T=500°C
the 500th layer in x-y section
the 700th layer in x-y section
the 1462nd layer in x-z section
the 1260th layer in y-z section
the 1260th layer in x-z section
the 1462nd layer in y-z section
the 900th layer in x-y section
Figure 9 Three dimension sections of the granite at 500°C
the 300th layer in y-z section
ACCEPTED MANUSCRIPT
Figure 10 The extraction system of HDR thermal energy
Yangsheng Zhao, Zijun Feng, Yu Zhao, Zhijun Wan PII:
S0360-5442(17)30850-2
DOI:
10.1016/j.energy.2017.05.093
Reference:
EGY 10901
To appear in:
Energy
Received Date:
06 August 2016
Revised Date:
28 March 2017
Accepted Date:
14 May 2017
Please cite this article as: Yangsheng Zhao, Zijun Feng, Yu Zhao, Zhijun Wan, Experimental investigation on thermal cracking, permeability under HTHP and application for geothermal mining of HDR, Energy (2017), doi: 10.1016/j.energy.2017.05.093
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT
Highlights 1. Performances of permeability, thermal cracking and microstructure in granite under high temperature and high pressure (HTHP) were investigated. 2. The related feature between thermal-cracking law and permeability under HTHP was revealed. 3. The influence of rock properties under HTHP on the sustainability of extracting Hot Dry Rock (HDR) geothermal energy system was demonstrated.
ACCEPTED MANUSCRIPT
1
Experimental investigation on thermal cracking, permeability
2
under HTHP and application for geothermal mining of HDR
3
Yangsheng Zhao1
Zijun Feng1
Yu Zhao 2,3*
Zhijun Wan4
4
1. Mining Technology Institute, Taiyuan University of Technology, Taiyuan, China 030024;
5
2. School of Civil Engineering, Chongqing University, Chongqing, China 40045;
6
3. Key Laboratory of New Technology for Construction of Cities in Mountain Area( Chongqing University ),
7
Ministry of Education, Chongqing, China 400030;
8 9
4. School of Mining Engineering, China University of Mining & Technology, Xuzhou, China 221008;
10
Abstract: Thermal cracking behavior of granite at high temperature and high pressure (HTHP) is the key
11
to the performance of Hot Dry Rock (HDR) geothermal energy extraction system. In this study,
12
permeability tests accompanying acoustic emission (AE) tests in granites are first conducted under HTHP
13
by 600℃ 20MN servo control rock triaxial testing machine. The test results show that granites, nearly
14
impermeable rocks, can show a striking increase of permeability by heating from the critical temperature.
15
The growth curve of granite permeability shows two phases because of the multi-period of thermal-
16
cracking in the heating process from room temperature to 500℃. The coupled effect of temperature and
17
pressure shows that critical temperature of permeability change decreases with increasing confining
18
pressure. Then, a detailed characterization of the sample microstructure is presented using Micro-CT
19
method. It is discovered that thermal cracking mainly occurs at grain boundaries in forms of inter-
20
granular microcracks along apparent weaknesses, and develops with increasing temperature. Meanwhile
21
intra-granular cracks are observed when heating to 500℃, indicating that thermal cracking in granite
22
under HTHP is induced by both intra-granular and inter-granular thermal stress. At last, experimental
23
stimulation and application for geothermal mining of HDR are discussed.
24 25
Key words: thermal cracking; permeability; AE event; Micro-CT; Hot Dry Rock
26 27 28 29
*Corresponding author: Yu Zhao, Prof, Ph.D. Tel: 86-023-65123363 Email: [email protected]
1
ACCEPTED MANUSCRIPT Nomenclature cw
30
specific heat of the water [L2T−2K−1]
cr
specific heat of the rock matrix
Tr
temperature of rock matrix
[L2T−2K−1]
[K]
[ML-3]
ρr
density of rock matrix
[K]
λr
thermal conductivity of rock-matrix
Trb
fracture surface temperature
W
source sink term of heat
ρw
density of water
Tw
water temperature
λw
thermal conductivity of water
T
temperature
t
time
b
crack aperture
[MLT−3K−1]
[K]
[T]
p [L]
kfi
[ML-3] [MLT−3K−1]
[K]
[ML-1T−3] [ML−1T−2]
water pressure in pore or crack water permeability coefficient
[LT-1]
1. Introduction
31
Hot Dry Rock (HDR) geothermal energy is mainly stored in granite. HDR technology inevitably involves
32
artificial enhancement of the permeability of the rock heat exchanger, colloquially known as geothermal
33
reservoir[1]. Thermal cracking plays an important role in exploitation of geothermal of HDR. Thermal stress
34
can significantly change physical properties through development of thermal cracks[2], which may enhance the
35
permeability, speed up the fracture propagation and finally trigger free convection. Therefore, it is of
36
considerable importance to understand how the geothermal reservoir will be modified by thermal cracking.
37
Focusing on rock’s thermal cracking, many works have been done. Researchers committed to studying
38
the influence of thermal cracking on physical properties in the early times. Laboratory studies showed that
39
thermal cracking could change rock permeability[3-5], strength[6], porosity[7], elastic moduli[8], and other
40
mechanical properties[9-11]. In addition, the transition of quartz crystals from phase
to phase
in granite at
41
high temperature was
42
slip[13]. Ghassemi found that the fracture aperture had a significant increase after the extraction of geothermal
43
energy[14]. Thermal stress can also cause formation of new cracks. Laboratory experiments attempting to
44
induce thermal cracking are presented by many scholars[15-16]. Although numerous experiments about
45
thermally treated rocks have been conducted to investigate the thermal-induced changes in mechanical
46
properties, the evolution of thermal cracks is still not completely understood.
observed[12].
It is generally believed that thermal stresses can cause fracture opening and
47
The reservoir stimulation determines the HDR system’s performance to a great extent[17], which can be
48
regarded as a function of the effective permeability between the wells. To investigate the creation and
49
evolution of the reservoir, abundant numerical simulations and experiments associated with reservoir
50
stimulation of HDR geothermal system have been carried out by various researchers[18-21]. Ghassemi coupled
51
flow and heat transport to thermo-poroelastic deformation in a discretely fractured reservoir and examined the
52
physical phenomena that govern fluid injection/extraction[14]. Zhao established a theoretical model of fractured
53
rock mass deformation, seepage, and heat transfer to simulate the extraction of HDR geothermal energy[22].
54
Siratovich presented a new methodology designed to replicate thermal stressing and subsequent cooling under
55
water saturated conditions[23]. Hadgu studied the influence of fracture orientation on production temperature in
56
low permeability geothermal system through thermal-hydrologic simulations[24].
57
It is more special for experimental investigation on thermal cracking to consider effects of high
58
temperature and high pressure. In particular, previous experiments were conducted under either low pressure
59
or unstressed state conditions. However, large amounts of HDR geothermal energies are located at depth of
60
3000-10000m below the surface[25], with an average temperature of 400℃-500℃. How do changes in high
61
temperature and high pressure influence the physical properties of rocks? And how do changes in physical 2
ACCEPTED MANUSCRIPT 62
properties of rocks influence the sustainability of extracting hot dry rock geothermal energy? This study tries
63
to provide insight to the mechanism of thermal cracking induced damage on permeability at a condition of
64
HTHP and assist in stimulation optimization.
65
2. Experimental investigation on rock permeability
66
2.1 Equipment
67
The permeability tests were carried out by 600℃ 20MN servo control rock triaxial testing machine with
68
HTHP (Figure 1) in China University of Mining and Technology. It composes of three parts including host
69
loading system, auxiliary system for sample assembly and measurement system. The stress, pressure,
70
temperature and other parameters can be controlled by the host loading system during testing. The axial and
71
lateral load of the triaxial test equipment can be loaded on the sample separately. The deformation of the tested
72
sample can be precisely measured by grating sensor with a precision of 0.005 mm. The maximum axial and
73
lateral loads both are 10 MN. The maximum axial pressure on sample is 318 MPa, while the lateral pressure is
74
250 MPa. The size of tested sample is Ф 200×400 mm, which is 64 times that of a standard sample. The
75
maximum heated temperature of testing equipment is not less than 600℃. The whole stiffness of the
76
equipment is greater than 9×1010 N/m.
77
The permeability test and heating process were conducted simultaneously. Nitrogen was used as flow
78
gas, and was controlled in the airintake by valve and barometer. Gas flow in the air outlet was measured by
79
soap-foam flowmeter and glass rotameter. The Figure 2 shows the flow chart of permeability measurement.
80
Due to the large scale of sample, the heating process was slowly preformed (5℃/h) to have an adequate
81 82
heating.
2.2 Sample preparation and test procedure
83
Three granite samples (NO.2, NO.4 and NO.7) for permeability tests were cored from Pingyi, Shandong
84
province in China, named “Luhui granite”. Samples were first produced into cylindrical roughcast by stone
85
processing machine and then polished carefully to the size of Ф 200×400 mm. They were heated from room
86
temperature to 500℃ at confining pressure of 75MPa, 25Mpa and 12.5Mpa, respectively. The goal of this
87
study is to replicate the thermal cracking process that may occur in a geothermal environment at depth of
88
3000m, 1000m and 500m.
89
The permeability test procedures are summarized as follows:
90
1) Measuring the size of sample and then installing it into the pressure chamber in the auxiliary system.
91
2) Loading the axial and confining pressure to predetermined value and then heating the sample manually
92
with the heating rate of 5℃/h.
93
3) Heating the sample to plateau temperature (50℃, 100℃, 150℃, …, 450℃, 500℃) and holding the
94
plateau temperature for 2 hours. Then injecting nitrogen and measuring the permeability of sample by means
95
of monitoring the gas flow rate.
96 97 98
4) Repeating the heating process to reach the next plateau temperature and then conducting the permeability test similarly, until accomplishing all permeability tests.
2.3 Results
99
The evolution of permeability is analyzed for samples successively, with the main results being
100
exemplified by sample NO.7. Permeability in different gas pressures has increased over the originally
101
observed value after thermal treatment, and shows the same varying trend at different gas pressure (Figure 3). 3
ACCEPTED MANUSCRIPT 102
It begins to increase when heating to 65℃~80℃, and gets its first peak value at the temperature of 150℃. For
103
the next heating process of temperature range from 150℃ to 350℃, the permeability changes a little. It slowly
104
decreases to a vale value area and then increases slightly. After 350℃, it increases sharply and reaches its
105
second peak value with the temperature at about 400℃~450℃. Thus, the growth curve of permeability can be
106
divided into two phases. At initial stage of thermal cracking action, the permeability shows a weak variation.
107
When the temperature rising over a certain value, defined as critical temperature, the permeability shows a
108
sharply increase. The variation of critical temperature against confining pressure is plotted in Figure. 4. It can
109
be seen that the critical temperature decreases rapidly when the confining pressure less than 30MPa, while it
110
tends toward steady value for confining pressure over 30 MPa. The most reasonable explanation may be that
111
intra-granular thermal stress is induced by shrinkage and decomposition of minerals from high confining
112
pressure[26]. Apart from the intra-granular thermal stresses, there are also inter-granular thermal stresses due to
113
the anisotropic expansion[27]. When these stresses exceed the local strength, microcracks are generated. As
114
confining pressure increasing, the pore radius decreases and finally reaches its minimum value. Therefore,
115
thermal expansion and deformation are completely restrained, and no more intra-granular thermal stresses are
116
induced. This indicates that inter-granular thermal stresses play a decisive role in thermal cracking, and critical
117
temperature are independent of confining pressure. In an isotropic, homogeneous granite, Chaki indicated that
118
500℃ could be the critical temperature of permeability changes under low confining pressure (0.8MPa)[12].
119
He observed a weak variation in permeability over the temperature range of 105-500 ℃, but a rapid increase
120
above 500℃, further supporting the validity of our experimental results.
121
The sensitivity of the permeability to temperature indicates that the permeable networks in thermal-
122
treated granite are likely to be multi-fracture dominated. Increasing permeability in a zone with a high
123
geothermal gradient will trigger free convection[28]. The test results illustrate that, for HDR geothermal system,
124
permeability can be enhanced through the simple application of thermal cracking.
125
3. AE experiment of rock thermal cracking
126
3.1 AE testing methodology
127
To investigate the effect of temperature on thermal cracking, AE response was detected by AE sensor
128
during the heating process. Four AE sensors were strictly installed on the top and bottom of the compression
129
chamber using magnetic stand of clock gauge. Signals of thermal cracking were recorded, including AE event
130 131
number, AE event energy, AE event duration and AE amplitude.
3.2 AE characteristics of rock thermal-cracking
132
The curve of AE response varying with temperature is plotted in Figure 5. From the figure we can obtain:
133
1) At the initial period of heating process (35℃ to 40℃), AE events occur with AE product
134
(multiplication of vibration amplitude and time) of 7000, indicating the beginning of thermal-cracking. The
135
recorded product of AE response mounts to 16000 when temperatures rise to 65℃, demonstrating that
136
intensive thermal cracking happens in this temperature stage.
137
2) The AE response is relative quiet at about 65℃ to 110℃. For the next heating period of 110℃ to 230
138
℃, the product value reaches 28000 and the average value is 13000. The long duration of AE response
139
manifests that tempestuous thermal cracking occurs during this temperature stage.
140
3) The AE response represents relative faintness with temperature ranging from 230℃ to 270℃.
141
4) The AE response is active with further temperature increasing from 270℃ to 340℃, while its average 4
ACCEPTED MANUSCRIPT 142
product value is lower than that of 110℃-230℃.
143
5) After AE response undergoing a plateau at about 340℃ to 400℃, it turns to active again during the
144
period of 400℃ to 500℃, and the product reaches the maximum value of 37000, representing the most
145
powerful cracking.
146
The above analyses demonstrate that a few intensive periods of thermal cracking occur during the heating
147
process. The simultaneous measurement of permeability shows that rock permeability represents a peak value
148
area in intensive thermal-cracking process (Figure 5). It slowly decreases to a vale value area in inactive period
149
of AE events. When the next thermal-cracking peak area comes out, the permeability increases again. After
150
experienced repeating thermal-cracking accumulation, the permeability is more and more high and thermal-
151
cracking is more intensive synchronously. Note that intensive thermal cracking occurs before 200℃ under
152
HTHP, which is contrasted to the experimental results in ref [4] where very little damage occurs below 300°C.
153
We considered that high confining pressure contributes to making such a great difference. With the increase of
154
temperature, the crystal water and bounded water escape, and porosity increases. Synchronously, the
155
interspace due to the loss of crystal and bounded water is compressed under high pressure. This will cause
156
internal frictions and thus generate AE events. Therefore, intensive thermal cracking is noticed at low
157
temperature.
158
4. Investigations on micro-structure of granitic thermal crack
159
Samples for micro-CT experiments are specially processed under condition of HTHP, and are carried out
160
by the μCT225kVFCB micro-CT experimental system designed by China Taiyuan University and
161
Technology. This investigation is essential to understand the effect of thermal treatment, as any macroscopic
162
property change to the samples should be observable at the microstructural level[23]. Samples are processed to
163
approximate circular cylinders with the size of Ф 2.7 mm × 20 mm. The amplification coefficient can be up to
164
105 times. Flat lattice of scanning is 2048×2048, and the size of scanning cell is 1.847µm. In order to analysis
165
internal meso-structure, we cut the sample into 1000 layers in longitudinal direction with each layer of
166 167
4.1 Main contents and meso-structure of granite
1.847µm to.
168
Prior to the experiment, Luhui granite is epigranular and dense. As shown in Table.1, Luhui granite is a
169
strongly heterogeneous brittle and hard rock, mainly consist of feldspar, quartz, and illite, et.al. Figure 6 shows
170
the meso-structure of granite in the room temperature using high-accuracy micro-CT. Crystal grain, boundary
171
of grain, binding material among grain and grain pore can be clearly differentiated. The component and
172
content of granite mineral are observed through X-ray diffract analysis spectrum, as shown in Figure 7. It can
173
be observed that the main mineral compositions (feldspar, quartz and illite) have almost the same proportion.
174
However, mechanical properties of feldspar, quartz and illite differ greatly, making granites with intensively
175 176
heterogeneous.
4.2 Evolution of thermal crack in the heating process
177
The microstructure of rock controls the macroscopic physical properties. In granite, the crack damage is
178
clearly evident in heated samples through the generation of cracks (Figure 8). The sizes of scanning cell and
179
sample are 1.09 µm×1.09 µm and 1.72 mm×1.447 mm, respectively.
180
Room temperature: As can been seen from Figure 8a, no obvious crack in micro scale is observed.
181
Basically, the cross section of the sample consists of three parts: high density area located in the right side with 5
ACCEPTED MANUSCRIPT 182
the X-ray absorption coefficient of 0.0302749, low density located in the middle with the coefficient of
183
0.0124444 and, sub-low density area located in the left side with the coefficient of 0.0166752. There are some
184
twisty lines formed by some connecting scanning cells with obvious lower density locating in the low density
185
area in the middle, which turn to be the first thermal cracking region.
186
200℃: It can be clearly seen from Figure 8b that the crystal particles are surrounded by a large majority
187
of microcracks in weakening lines when heated to 200℃. But a large-closed polygon crack around granitic
188
particles has not yet been formed. For example, the looked-like macro-long crack in the left upper side actually
189
consists of many disconnect microcracks, with the length of 23 µm, 34 µm, 45 µm, 18 µm, 32 µm, 46 µm, 104
190
µm and 18 µm (bottom to upper), respectively.
191
300℃: Microcracks further develop and propagate with creasing temperature. Large cracks can be
192
observed, and the crack length increased significantly (Figure 8c) when heating to 300℃. For example, the
193
length of crack on the border of the right side is 316 µm, and the upper right side one is 308 µm. Both of them
194
are rapidly developed, much greater than the length value of 200℃. Meanwhile the size of crack around the
195
granite crystal particles is also increased.
196
500℃: When heating to 500℃, microcracks develop to macrocracks cutting through the whole rock
197
sample, and closed polygonal cracks around each crystal grain are completely formed, leading to the formation
198 199
of mylonitic texture in granite (Figure 8d). Meanwhile, some intra-granular cracks begin to initiate.
4.3 Analysis on the characteristics of thermal crack under 500℃
200
Table 2~6 show the size of granite grain in different sections under 500℃. As shown in Table 3, the size
201
of rock grains in the 700th layer of x-y section can be clearly identified in the range of 0.04mm to 0.1mm, and
202
the equivalent radius of circle is in the range of 0.1 mm to 0.2 mm.
203
(1)As shown in Figure 9, CT scanning images of the 300th, 500th, 700th and 900th layers(x-y section)
204
depict that the thermal cracks are almost polygon distribution around weak plane of granite grains boundary.
205
The area of broken cell is 0.1-0.2 mm2, the equivalent radius of circle is 0.15-0.25 mm, which is close to the
206
size of granite grain. Due to thermal damage happening in the weak area of particle joint, the circle equivalent
207
radius of crack cell is 0.05mm larger than the size of granite grain (Table.2, Table.3 and Table.4).
208
(2)The CT scanning images of longitudinal sections (x-z section and y-z section) also demonstrate that
209
thermal cracking happens in the weak area of particle joint. As shown in Table 5 and Table.6, the sizes of
210
crack cell in the 512th and 1260th layer of X-Z section are both 0.1~0.3mm.
211
(3)Intra-granular cracks are observed under 500℃, such as the crack located at the bottom right corner in
212
700th layer (x-y section). In the middle part of 1260th layer (y-z section), a thermal crack unit also penetrated
213
a long and thin rock particle.
214
5. Geothermal mining of HDR mechanism
215
5.1 Concept of geothermal HDR system
216
The concept of HDR is to exploit energy resources from the earth by drilling wells into hot, crystalline
217
rock at great depth. A well is drilled first to inject cold water at high pressure to stimulate or hydraulically
218
fracturing the natural rock joints, thereby creating a geothermal reservoir. Injected cold water picks up heat
219
and returns to the surface via production well (Figure 10). The artificial geothermal reservoir forms the heart
220
of HDR energy extraction. In a low-permeability geothermal reservoir, heat is transported through rock matrix
221
which is slow due to the low thermal conductivity of rock. While in a good connectivity reservoir, heat is 6
ACCEPTED MANUSCRIPT 222 223
transported by a convention system.
5.2 Evolution of reservoir and heat transport
224
The implication of enhanced permeability tests indicates that the mechanism of reservoir stimulation
225
appears to be a complementary mix of high thermal cracking and interplay of regional stress fields. The
226
microscopic investigations clearly demonstrate the evolution of the reservoir under thermal-cracking process.
227
It is observed that thermal cracking occurs first at grain boundaries in forms of microcracks along apparent
228
weaknesses. These microcracks gradually connect and propagate under thermal stress, and eventually develop
229
to macrocracks when heated to high temperature. Microseismic monitoring can be used for detecting reservoir
230
development in suit. We use AE events to exploit the effects that thermal stress places on the reservoir in
231
laboratory environment. On one hand, the initiation of AE event indicates the temperature necessary for
232
fracture propagation and, on the other hand the extent of AE response reveals the rate of growth in fractures
233
and porosity. AE events were first monitored when heated to about 60℃, and after that a few intensive periods
234
were monitored, indicating new thermal cracks formation. Thus we can use these induced events to locate the
235
thermalised crack and follow its growth.
236
Permeability, the main mechanism to be envisaged for the creation of HDR reservoir, has been
237
significantly enhanced in our test. In general, granite has negligible porosity and permeability before heating.
238
With the onset of thermal cracking, the permeability is stimulated, increased by two orders of magnitude from
239
4.7×10-8 to 6.03×10-6 D of sample NO.7. Meanwhile, multiple peak behavior of AE response and
240
permeability are clear indications of the presence of multiple flow paths. The permeability evolves with
241
increased temperature is essential to relating this relationship to thermal stimulation optimization.
242
The heat transmission takes place both in the rock matrix and the fracture, and is transferred by
243
convection and conduction. We have previously reported a thermo-hydro-mechanical coupled model for
244
enhanced geothermal system[22]. In the model, fluid density is a function of the water pressure and temperature,
245
and rock mass is simplified as a fracture media consisting of fractures and matrix rock block of pores and
246
cracks. The conservation of thermal energy for the heat transport fluid flowing in the fractures:
247 248
cw
wTw 2 w2Tw cw w k fi p,i Tw ,i r Trb Tw t b
(1)
The conservation of thermal energy for the heat conduction in HDR (or matrix rock block):
r cr
249
Tr rTr ,ii W t
(2)
250
Where is density, c specific heat, T temperature, thermal conductivity, W source sink term of heat,
251
p water pressure. The subscript w, r and rb refer to the mobile fluid phase, rock matrix and fracture surface,
252
respectively. The conservation of thermal energy of the rock mass fracture describes the heat exchange process
253
with simultaneous conduction and convection, which is generally more accurate than discussing simple
254
conduction or convection.
255
6 Discussions
256
6.1 Thermal cracking
257
Thermal stress will be induced by changes of temperature in the inner of heterogeneous rock, regardless
258
of the mechanical state of granite. When the stresses exceed bearing capacity of inner grain and cement,
259
thermal cracking happens. The thermal expansion coefficients of multi-crystals mineral are completely 7
ACCEPTED MANUSCRIPT 260
different and all of them are a function of temperature. According to thermal elastic theory, thermal stress will
261
first occur in different crystals mineral and gets its maximum value in the grain boundary. Therefore, thermally
262
induced cracks in granite are mainly inter-granular cracks at initial stage of thermal cracking action. However,
263
intra-granular cracks begin to initiate at late stage of thermal cracking action. Intra-granular thermal stresses
264
can be induced by shrinkage and decomposition of minerals under high confining pressure. When these
265
stresses exceed the local strength, intra-granular cracks are generated. It is concluded that under low pressure
266
conditions, differential thermal expansion of adjacent minerals is considered responsible for the thermal
267
damage in granite at elevated temperatures. Under high pressure and high temperature conditions, however,
268
thermal cracking can occur both between adjacent crystalline grains (inter-granular thermal stress) and within
269
grains (intra-granular thermal stress).
270
Owing to the complex composing of granite and great difference in thermal expansion coefficients among
271
crystals grains, granites show multi-period of thermal-cracking under heating processes. Take NO.7 granite
272
sample as example, there exists four main thermal-cracking intensive stages before heating to 500℃, which
273 274
are 55℃ to 65℃, 110℃ to 230℃, 270℃ to 340℃and 400℃ to 500℃.
6.2 Permeability changes.
275
The variation of gas permeability against temperature of heat treatment experienced two phases, a
276
weak variation and an exponential rush. At initial stage of thermal cracking action, microcracks may be
277
generated but with low cracking density, and connected network has not been created. In this phase, the
278
permeability increased first and then it dropped to a vale value area. Numerous studies have observed
279
permeability reduction in granite under heating processes[29-31]. Xie and Zhao find that thermal
280
expansions of rock grains turn out to be outward deformation before heating to 150℃. But some of them
281
change to inward deformation over the temperature range of 150-350℃[32], which gives a reasonable
282
explanation of our varied permeability results. Obviously, outward expansions of rock grains increase the
283
porosity and connectivity of cracks, while inward expansions tend to close cracks thus decreasing the
284
permeability. Although microcracking started in granite at about 60 ℃, most of the mineral grains were
285
microcracked at late stage of thermal cracking action (about 350 ℃). In this stage, microcracks develop
286
to macrocracks, and intra-granular cracks are created which improve the connectivity of the crack
287 288
network significantly. As a result, permeability increased sharply. 6.3 Experimental stimulation and application for geothermal mining of HDR
289
Permeability test results show that granite, nearly impermeable rock, could show a striking increase
290
of permeability by heating from the critical temperature likely to be found in a geothermal environment at
291
great depth. Sun suggested that 400℃ could be a critical threshold of the thermal damage of granite
292
under unstressed state conditions[16]. While our experimental results showed that the value of critical
293
temperature deceases to 150℃ under confining pressure of 75MPa, which makes thermal stimulation
294
happen effortlessly. Although the temperature and pressure increase with the depth, which gives a higher
295
thermal efficiency, drilling costs and equipment costs become proportionately more significant.
296
Therefore, the depth of the reservoir would be significant. It is necessary to carry out extensive feasibility
297
studies before a site is selected for the development of HDR geothermal system. Based on the
298
experimental results that the critical temperature of granite changes little at confining pressure above
299
30MPa, we prefer that confining pressure of 30MPa could be a reliable value when employing thermal 8
ACCEPTED MANUSCRIPT 300
stimulation to enhance the permeability of reservoir. The development of thermal cracks on the
301
microscopic scale in our laboratory studies shows that high thermal treatment on an impermeable rock is
302
likely to create new thermal cracks and cause discontinuities opening and slippage. The implication of
303
HDR is that when wells are drilled into high temperature rocks, but with poor flow circulation because of
304
lacking flow path, thermal cracking processes could be a worthwhile pursuit to enhance the permeability.
305
7. Conclusions
306
The study presents the influence of permeability, microstructure and AE response of granite under HTHP
307
condition on HDR system. The law of rock thermal-cracking is indicated clearly, which is very important to
308
understand how thermal stress can be utilized to improve poorly connectivity in a geothermal reservoir.
309
The permeability curve versus temperature up to 500℃ has been divided into two phases based on the
310
critical temperature. And significant increase of permeability can be noticed after rising over the critical
311
temperature value. The test results show that the critical temperature decreases with increasing confining
312
pressure. AE events show that there are multi-periods of thermal-cracking during the heating process, and
313
intensive thermal cracking occurs before 200℃ due to the effect of high pressure. Microscopic investigations
314
show that thermal crack is gradually developed as temperature increasing. It is observed that thermal cracking
315
occurs first at grain boundaries in forms of inter-granular microcracks along apparent weaknesses under 200
316
℃, which develop and connect to form larger cracks under 300℃, and eventually propagate to cutting-through
317
macrocracks under 500℃. Intra-granular cracks and closed inter-granular cracks around crystal grains are
318
observed when heating to 500℃, changing the granite into mylonitic texture. These findings indicate that
319
thermal damage in granite under HTHP is induced by intra-granular thermal stress and inter-granular thermal
320
stress.
321
The experimental results show here illustrate the performances of permeability, thermal cracking and
322
microstructure under HTHP, and show that one of the most important properties of HDR system, permeability
323
can be enhanced by heating to critical temperatures.
324
Acknowledgments
325
This work was funded by National Natural Science Foundation of China (grant number 50534030; grant
326
number 51374257).
327
References:
328 329 330 331 332 333 334 335 336 337 338
[1] Parker, R. H.; Jupe, A. In situ leach mining and hot dry rock (HDR) geothermal energy technology. Miner Eng, 1997, 10, 301-308. [2] Menéndez, B.; David, C.; Darot, M. A study of the crack network in thermally and mechanically cracked granite samples using confocal scanning laser microscopy. Phys Chem Earth (A), 1999, 24, 627-632. [3] Somerton, W. H.; Gupta, V. S. Role of Fluxing Agents in Thermal Alteration of Sandstones. J Petrol Technol, 1965, 17, 585-588. [4] Jones, C.; Keaney, G.; Meredith, P. G.; et al. Acoustic emission and fluid permeability measurements on thermally cracked rocks. Phys Chem Earth, 1997, 22, 13-17. [5] Rutqvist, J.; Freifeld, B.; Min, K. B.; et al. Analysis of thermally induced changes in fractured rock permeability during 8 years of heating and cooling at the Yucca Mountain Drift Scale Test. Int J Rock Mech Min Sci, 2008, 45, 1373-1389. 9
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[6] Keshavarz, M.; Pellet, F. L.; Loret, B. Damage and Changes in Mechanical Properties of a Gabbro Thermally Loaded up to 1000°C. Pure Appl Geophys, 2010, 167, 1511-1523. [7] David, C.; Menéndez, B.; Darot, M. Influence of stress-induced and thermal cracking on physical properties and microstructure of La Peyratte granite. Int J Rock Mech Min Sci, 1999, 36, 433-448. [8] Nasseri, M H B.;, Schubnel, A.; Young, R. P. Coupled evolutions of fracture toughness and elastic wave velocities at high crack density in thermally treated Westerly Granite. Int J Rock Mech Min Sci, 2007, 44, 601616. [9] Heard, H. C. Thermal expansion and inferred permeability of climax quartz monzonite to 300°C and 27.6 MPa. Int J Rock Mech Min Sci Geomech Abstr, 1980, 17, 289-296. [10] Reuschlé, T.; Haore, S. G.; Darot, M. The effect of heating on the microstructural evolution of La Peyratte granite deduced from acoustic velocity measurements. Earth Planet Sci Lett, 2006, 243, 692-700. [11] Yavuz, H.; Demirdag, S.; Caran, S. Thermal effect on the physical properties of carbonate rocks. Int J Rock Mech Min Sci, 2010, 47, 94–103. [12] Chaki, S.; Takarli, M.; Agbodjan, W. P. Influence of thermal damage on physical properties of a granite rock: Porosity, permeability and ultrasonic wave evolutions. Constr Build Mater, 2008, 22, 1456-1461. [13] Ghassemi, A.; Tarasovs, S.; Cheng, H. D. A 3-D study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs. Int J Rock Mech Min Sci, 2007, 44, 1132-1148. [14] Ghassemi, A.; Zhou, X. A three-dimensional thermo-poroelastic model for fracture response to injection/extraction in enhanced geothermal systems. Geothermics, 2011, 40, 39-49. [15] Geraud, Y.; Mazerolle, F.; Raynaud, S.; et al. Crack location in granitic samples submitted to heating, low confining pressure and axial loading. Geophys J Int, 1998, 133, 553-567. [16] Sun, Q.; Zhang, W.; Xue, L.; et al. Thermal damage pattern and thresholds of granite. Environ Earth Sci, 2015, 74, 1-9. [17] Tenzer, H. Development of hot dry rock technology. GHC bulletin; 2001;14-22. [18] Wallroth, T.; Eliasson, T.; Sundquist, U. Hot dry rock research experiments at Fjällbacka, Sweden. Geothermics, 1999, 28, 617-625. [19] Tran, N. H.; Rahman, S. S. Development of hot dry rocks by hydraulic stimulation: Natural fracture network simulation. Theor Appl Fract Mec, 2007, 47, 77-85. [20] Zeng, Y. C.; Wu, N. Y.; Su, Z.; et al. Numerical simulation of heat production potential from hot dry rock by water circulating through a novel single vertical fracture at Desert Peak geothermal field. Energy, 2013, 63, 268-282. [21] Guo, B.; Fu, P.; Hao, Y.; et al. Thermal drawdown-induced flow channeling in a single fracture in EGS. Geothermics, 2016, 61, 46-62. [22] Zhao, Y. S.; Feng, Z.; Feng, Z.; et al. THM (Thermo-hydro-mechanical) coupled mathematical model of fractured media and numerical simulation of a 3D enhanced geothermal system at 573K and buried depth 6000–7000M. Energy, 2015, 82, 193-205. [23] Siratovich, P. A.; Villeneuve, M. C. Cole, J. W.; et al. Saturated heating and quenching of three crustal rocks and implications for thermal stimulation of permeability in geothermal reservoirs. Int J Rock Mech Min Sci, 2015, 80, 265-280. [24] Hadgu, T.; Kalinina, E.; Lowry, T. S. Modeling of heat extraction from variably fractured porous media in Enhanced Geothermal Systems. Geothermics, 2016, 61, 75-85. [25] Zhang, F. Z.; Xu, R. N.; Jiang, P. X. Thermodynamic analysis of enhanced geothermal systems using impure CO2 as the geofluid. Appl Therm Eng, 2016, 37, 1277-1285. [26] Dwivedi, R. D.; Goel, R. K.; Prasad, V. V. R.; et al. Thermo-mechanical properties of Indian and other granites. Int J Rock Mech Min Sci, 2008, 45, 303-315. [27] Chen. S. W.; Yang, C. H.; Wang, G. B. Evolution of thermal damage and permeability of Beishan 10
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granite[J]. Appl Therm Eng, 2017, 110:1533-1542. [28] Battaillé, A.; Genthon, P.; Rabinowicz, M.; et al. Modeling the coupling between free and forced convection in a vertical permeable slot: Implications for the heat production of an Enhanced Geothermal System. Geothermics, 2006, 35, 229-271. [29] Summers, R.; Winkler, K.; Byerlee, J. Permeability changes during the flow of water through westerly granite at temperatures of 100°-400°C. J Geophys Res Solid Earth, 1978, 83:339-344. [30] Morrow, C. A.; Lockner, D. A.; Moore, D. E.; et al. Permeability of granite in a temperature gradient. J Geophys Res Atmos, 1981, 86, 3002-3008. [31] Morrow, C. A.; Moore, D. E.; Lockner, D. A. Permeability reduction in granite under hydrothermal conditions. J Geophys Res, 2001, 106, 30551-30560. [32] Xie, J. L.; Zhao, Y. S. Meso-mechanism of permeability decrease or fluctuation of coal and rock with the temperature increase. Chin J Rock Mech Eng, 2017, 36, 543-551 [in Chinese].
11
ACCEPTED MANUSCRIPT 1 2
List of Table and Figure Captions
3
Table 1. Contents of minerals for granite samples
4
Table 2. Scale statistics of the thermal crack cell of the 500th layer in x-y section
5
Table 3. Scale statistics of the thermal crack cell of the 700th layer in x-y section
6
Table 4. Scale statistics of the thermal crack cell of the 900th layer in x-y section
7
Table 5. Scale statistics of the thermal crack cell of the 512nd layer in x-z section
8
Table 6. Scale statistics of the thermal crack cell of the 1260th layer in y-z section
9 10
Figure 1. 600℃20MN triaxial test equipment with servo controlled loading system
11
Figure 2. Flow chart of permeability experiment
12
Figure 3. Permeability change curves of sample NO.7
13
Figure 4. Evolution of critical temperature versus confining stress
14
Figure 5. Relation curves of AE event and permeability of granite NO.7 under conditions of HTHP
15
Figure 6. Meso-images of granite structure at normal temperature
16
Figure 7. X-ray graph analysis of granite
17
Figure 8. Thermal cracking CT sections of the granite in different temperature
18
Figure 9. Three dimension sections of the granite at 500℃
19
Figure 10. The extraction system of HDR thermal energy
ACCEPTED MANUSCRIPT Table 1 Contents of minerals for granite samples Mineral
Illite
Quartz
Feldspar
Calcite
Siderite
Others
Quality Percent
25%
28%
43%
1%
1%
2%
Table 2 Scale statistics of the thermal fracture cell of the 500th layer in x-y section Area/mm
Equivalent circular radius
2
/mm
0.1958×0.2198
0.0430
0.117043
0.2032×0.2734
0.0555
0.132980
143×200
0.2642×0.3695
0.0976
0.176278
127×116
0.2346×0.2143
0.0503
0.126502
No.
Number of pixels
Absolute size /mm
1
106×119
2
110×148
3 4
Table 3 Scale statistics of the thermal fracture cell of the 700th layer in x-y section Area/mm
Equivalent circular radius
2
/mm
0.1626×0.1572
0.02556
0.09020
60×175
0.1109×0.2117
0.02348
0.08645
3
50×195
0.0923×0.2395
0.22106
0.26526
4
90×190
0.1662×0.3438
0.05714
0.13486
5
66×76
0.1219×0.1404
0.01711
0.07381
No.
Number of pixels
Absolute size /mm
1
88×130
2
Table 4 Scale statistics of the thermal fracture cell of the 900th layer in x-y section Area/mm
Equivalent circular radius
2
/mm
0.2069×0.2346
0.04854
0.12430
109×167
0.2013×0.2020
0.04066
0.11377
103×139
0.1903×0.1681
0.03199
0.10091
4
91×106
0.168×0.1282
0.02154
0.08280
5
140×200
0.2587×0.2149
0.05559
0.13302
No.
Number of pixels
Absolute size /mm
1
112×194
2 3
Table 5 Scale statistics of the thermal fracture cell of the 512nd layer in x-z section Area/mm
Equivalent circular radius
2
/mm
0.3390×0.4740
0.1606
0.22616
113×146
0.2088×0.2697
0.0563
0.13388
3
249×228
0.4506×0.4212
0.1897
0.24579
4
169×227
0.3122×0.4194
0.1309
0.204153
5
346×249
0.6392×0.4601
0.2941
0.30596
No.
Number of pixels
Absolute size /mm
1
104×257
2
Table 6 Scale statistics of the thermal fracture cell of the 1260th layer in y-z section No.
Number of pixels
Absolute size /mm
Area/mm
Equivalent circular radius
2
/mm
ACCEPTED MANUSCRIPT 1
148×256
0.2734×0.473
0.1293
0.2028
2
180×224
0.3326×0.4138
0.1376
0.2093
3
252×184
0.4656×0.3399
0.1582
0.2244
4
120×272
0.2217×0.5026
0.1114
0.1883
5
224×240
0.4139×0.4434
0.1835
0.2416
Figure1 600℃20MN triaxial test equipment with servo controlled loading system Nitrogen jar
Gas charging device
Pressure meter
Specimen
Axial loading shaft
Valve
Tar Collection
Tridirectional valve Exit
Soap flow meter Valve
Flow meter Gas collection
Figure 2 Flow chart of permeability experiment
ACCEPTED MANUSCRIPT 8 Phase ℃
Phase ℃
Permeability ℃ D*10-6℃
6 2 MPa 3 MPa 4 MPa
4
2
0 0
100
200
300
400
500
600
Temperature (℃ )
Figure 3 Permeability change curves of sample NO.7 400
Critical temperature ℃ °C℃
350 300 250 200 150 10
20
30
40
50
60
70
80
Confining pressure (MPa)
Figure 4 Evolution of critical temperature versus confining stress 7# granite 8# lane
6
40000
5
30000
permeability/10-6D
Multi vabration Amp. and time
35000
4
25000
3
20000 15000
2
10000
1
5000
0
0 0
50
100
150
200
250
300
350
temperature/°C
400
450
500
550
Figure 5 Relation curves of AE event and permeability of granite NO.7 under conditions of HTHP
ACCEPTED MANUSCRIPT
Intergranular cement Grain boundary
intergranular Grain interior
interior
Figure 6 Meso-images of granite structure at normal temperature
Figure 7 X-ray graph analysis of granite
ACCEPTED MANUSCRIPT (a)Test 80kv; M=157; T=20°C
(b) 80kv; M=157; T=200°C
(c) 80kv; M=171; T=300°C
(d) 80Kv; M=105; T=500°C
Figure 8 Thermal cracking CT sections of the granite in different temperature
ACCEPTED MANUSCRIPT Test Conditions:80kv
M=105
T=500°C
the 500th layer in x-y section
the 700th layer in x-y section
the 1462nd layer in x-z section
the 1260th layer in y-z section
the 1260th layer in x-z section
the 1462nd layer in y-z section
the 900th layer in x-y section
Figure 9 Three dimension sections of the granite at 500°C
the 300th layer in y-z section
ACCEPTED MANUSCRIPT
Figure 10 The extraction system of HDR thermal energy