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Investigation on fracture creation in hot dry rock geothermal formations of China during hydraulic fracturing
Journal Pre-proof Investigation on fracture creation in hot dry rock geothermal formations of China during hydraulic fracturing Zhou Zhou, Yan Jin, Yijin Zeng, Xudong Zhang, Jian Zhou, Li Zhuang, Shunyuan Xin PII:
S0960-1481(20)30150-6
DOI:
https://doi.org/10.1016/j.renene.2020.01.128
Reference:
RENE 12991
To appear in:
Renewable Energy
Received Date: 2 July 2019 Revised Date:
24 January 2020
Accepted Date: 26 January 2020
Please cite this article as: Zhou Z, Jin Y, Zeng Y, Zhang X, Zhou J, Zhuang L, Xin S, Investigation on fracture creation in hot dry rock geothermal formations of China during hydraulic fracturing, Renewable Energy (2020), doi: https://doi.org/10.1016/j.renene.2020.01.128. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Zhou Zhou: Conceptualization, Methodology, Writing - Original Draft Yan Jin: Conceptualization, Supervision Yijin Zeng: Supervision, Validation Xudong Zhang: Resources, Validation Jian Zhou: Methodology, Formal analysis, Validation Li Zhuang: Investigation, Data Curation, Writing - Review & Editing Shunyuan Xin: Writing - Original Draft, Data Curation
1
Investigation on fracture creation in hot dry rock geothermal
2
formations of China during hydraulic fracturing
3
Zhou Zhoua*, Yan Jina, Yijin Zengb, Xudong Zhangb, Jian Zhoub, Li Zhuangc,
4
Shunyuan Xina
5
a
6
Petroleum (Beijing), Beijing, China
7
b
Sinopec Research Institute of Petroleum Engineering, Beijing, China
8
c
Korea Institute of Civil Engineering and Building Technology, Goyang, Gyeonggi,
9
Republic of Korea
State Key Laboratory of Petroleum Resources and Prospecting, China University of
10
* Corresponding author: [email protected]
11
Abstract:
12
The abundant geothermal energy in hot dry rock (HDR) formations is an attractive
13
renewable energy resource with great potential. China will develop its first HDR
14
geothermal formation in the Gonghe Basin. HDR is a hard and low-permeability
15
granite containing very few fluids. Development requires fluids to cyclically flow
16
between injection and production wells to extract geothermal energy in the artificial
17
heat transfer zone. Hydraulic fracturing is the main technology for creating flow paths.
18
But few studies have investigated fractures in HDR geothermal formations. This
19
paper investigated fractures as flow paths in HDR geothermal formations during
20
hydraulic fracturing. Hydraulic fractures were simulated using a custom true- triaxial
21
hydraulic fracturing test system in a realistic formation environment, in which a
22
scaled wellbore was used that was built in outcrop granite rock from the Gonghe
23
Basin. Fracture creation in granite was investigated via experiments, as well as
24
influence factors, and what experience could be achieved. This study can be used to
25
design and evaluate hydraulic fracturing projects in potential HDR geothermal
26
formations. 1
27
Keywords: Hot dry rock geothermal energy; Hydraulic fracturing; Fracture creation;
28
High temperature; True triaxial
29
1. Introduction
30
Geothermal energy is a renewable energy resource that is developed for heating and
31
electricity generation. The geothermal heat source is directly applied in heat pumps,
32
room and space heating, aquaculture and agricultural heating, bathing and swimming,
33
and industrial uses (Moya D. et al., 2018). In 2015, these applications were expected
34
to replace 350 million barrels of equivalent oil and avert 148 million tons of
35
greenhouse gas emissions (Lund J. and Boyd T., 2015). In addition, according to
36
statistical data, geothermal power plants for electricity generation already had a total
37
installed capacity of 12.729 MW in 2016, which is expected to double by 2020
38
(Bertani R., 2016). Therefore, geothermal energy is a significant potential sustainable
39
energy alternative to fossil fuel energy, which can reduce greenhouse gas emissions.
40
Geothermal energy can be discovered beneath the surface. However, the economic
41
potential of formations is based on their depth and temperature. These formations are
42
divided into shallow low temperature reservoirs and deep high temperature energy
43
resources. Shallow geothermal energy is located at hundreds of meters underground,
44
and the temperature is below 100
45
shallow geothermal formations is to extract energy to heat buildings in the urban areas
46
and factories and farms in rural areas (Magraner T. et al., 2010). The depth of deep
47
geothermal resources can reach 5 km, and the temperature can reach 400
48
higher (Tomac I. and Sauter M.,2018). The energy in deep geothermal formations is
49
mainly developed for the generation of electricity. This technology requires harvesting
50
energy from geothermal energy resources through fluid transport to power plants
51
(Moya D. et al., 2018). Recently, conventional electric power generation, which
52
requires geothermal formation temperature below 200 , has been successfully
53
implemented in North America, Europe, and East Asia (Moya D. et al., 2018; Tomac I.
(Bayer P. et al., 2019). The main application of
2
and
54
and Sauter M.,2018). The development of geothermal temperatures higher than 200
55
is attractive and should be considered in future applications. These formations are
56
called hot dry rock (HDR) geothermal formations (Feng Y. et al., 2014).
57
HDR geothermal formations usually consist of hard and low- permeability granite.
58
There is very little or no fluid in the formations because of the high temperature.
59
Hence, the development of HDR geothermal formations requires an enhanced
60
geothermal system that can efficiently convert geothermal energy into electricity
61
(Olasolo P. et al., 2016). In an enhanced geothermal system, injection and production
62
wells are completed that allow fluids to extract geothermal energy. Hydraulic
63
fracturing is one of the most significant technologies for achieving sufficient and
64
stable flow paths for geothermal energy extraction (Hanano M., 2004). Therefore, it is
65
necessary to understand how fractures are created as flow paths in HDR formations
66
by hydraulic fracturing.
67
The purpose of this paper is the provision of a good understanding of
68
thermo-hydro-mechanical interaction on hydraulic fracture creation. In the HDR
69
geothermal formation, the interaction is different from other formations because
70
formation temperature is much higher, and rock is harder so that the fracture creation
71
based on the interaction is unclear and worth to study. In addition, the study in this
72
paper is a fundamental research in the geothermal area. As shown in Figure 1, it
73
contributes to further studies of geothermal energy usage, transportation, and
74
distribution.
3
75 76
Figure 1: Flow chart to indicate scientific contribution of this paper
77
Before introducing the purpose and main research contents of this study, it briefly
78
reviews the hydraulic fracturing method for conventional oil and gas exploitation and
79
the difference as well as the challenge of application of hydraulic fracturing in HDR
80
formations.
81
1.1 Review of Hydraulic Fracturing
82
The typical procedure of hydraulic fracturing includes the following three steps.
83
First, fracturing fluid is pumped into the wellbore to increase the pressure at the
84
bottom of the well. The wellbore is sealed through the steel casing. However, at the
85
bottom of the well, there is no casing and only formation rock. Thus, with the increase
86
in pumping volume, the pressure continues to rise until the rock fracturing pressure.
87
When this pressure is exceeded, the rock is broken, and fractures are generated (see
88
Figure 2).
4
89 90
Figure 2: Concept of hydraulic fracture creation.
91
Second, when fluid injection is continued, the newly created fractures, called
92
hydraulic or artificial fractures, grow in the formation. If the formation contains
93
geologically weak planes such as natural fractures or interlayers, they will affect
94
hydraulic fracture propagation. There are three possible interactions when hydraulic
95
fractures contact weak planes. One possible interaction is that hydraulic fractures
96
propagate along the weak planes, which leads to reopening and activation, and the
97
original propagation direction changes. Second, hydraulic fractures propagate across
98
the weak planes, and no direction change occurs. The last possible interaction is
99
fracture propagation both along and across the weak planes, as shown in Figure 3 (a)
100
101
and (b), respectively.
(a)
(b)
5
102
Figure 3: The interaction between artificial and natural fractures. (a) artificial
103
fractures propagating along natural fractures; (b) hydraulic fractures propagating
104
across natural fractures.
105
Third, sand or ceramic proppant material is injected. It is required to distribute the
106
proppants inside the hydraulic fractures. When pumping ceases, the proppants prevent
107
the fractures from closing.
108
Fractures in the formation is mainly controlled by the in- situ stresses. According to
109
the direction, the in- situ stresses are divided into vertical and horizontal stresses.
110
Horizontal stresses are subdivided into the maximum and minimum horizontal
111
stresses, whose directions are perpendicular to each other (see Figure 4). A fracture
112
can be initialed at any point in the formation that is the weakest. However, the
113
direction of fracture propagation is parallel to the maximum horizontal stress direction,
114
and perpendicular to the minimum horizontal stress direction.
115 116
Figure 4: Fracture propagation along the maximum horizontal stress direction
117
Whether hydraulic fractures could reopen natural fractures or geological layers is
118
determined by various factors, but there are two most significant factors: the rock
119
strength and fracture shear strength. When the net pressure in the fractures exceeds
120
the rock strength, hydraulic fractures are more likely to propagate across weak planes; 6
121
and when the net pressure is higher than the shear strength, natural fractures or layers
122
could be reopened. For different formations, the rock strength and fracture shear
123
strength vary, which sometimes results in hydraulic fractures easily activating, and at
124
times, hydraulic fractures preferentially propagate with no direction change.
125
The purpose of hydraulic fracturing in HDR geothermal formations requires a heat
126
transfer area between the injection and production wells. Thus, in the ideal situation,
127
the created fractures, including artificial and active natural fractures should propagate
128
and spread into as large a formation area as possible. In HDR geothermal formations,
129
the formation rock is much harder. Table 1 summarizes the compressive strength
130
among the various rocks from previous experiments. The higher the strength the more
131
difficult to break the rock. However, very few previous papers have investigated the
132
processes of fracture initiation and propagation in HDR formations. Hence, this paper
133
studied fracture creation through hydraulic fracturing simulation experiments with
134
real HDR formation rocks under actual formation conditions.
135
Table 1: Compressive Strength of Formation Rocks
Compressive Strength MPa
Hot dry rock
Shale
Carbonate
Sandstone
150~467
80~180
40~120
33~100
Note: Confining pressures are from 0 MPa to 40 MPa 136
1.2 Laboratory Experimental Studies of Fracture Creation
137
Hydraulic fracture was normally investigated and revealed how to be created under
138
various geological and operation conditions (Daneshy A., 1973; Zoback M.D. et al.,
139
1977; Behrmann L.A. and Elbel J.L., 1991; De Pater C.J. et al., 1994; Mogi K., 2007;
140
Kwaśniewski M. et al., 2013), the interactions between artificial and natural fractures
141
(Zhou, J. et al., 2008; Zhou, J. et al., 2010), and how to maximize the stimulation area
142
through fracture network establishment (Yost II, A.B. et al., 1989; Hossain M. M. et
143
al., 2000; Pater, C.J.d. and Beugelsdijk L.J.L., 2005; Olsen, T.N. et al., 2009; Cipolla, 7
144
C.L. et al., 2009). However, most of the previous experiments were for shales,
145
sandstones, or other oil and gas formations. A few studies have focused on fracture
146
propagation in hard granite rocks at high temperature (Frash, L.P. et al., 2014; Zhou, Z.
147
et al., 2018).
148
Temperature variations could result in changes in the shear strength (Friedman M. et
149
al., 1979), compressive strength (Faoro I. et al, 2013), rock porosity and permeability
150
(Faoro I. et al, 2013), and fracture toughness (Nasseri M. et al., 2007; Wang, X.-Q. et
151
al., 2013). In addition, more preexisting fractures were observed in rock with
152
increasing temperature Nasseri M. et al., 2007). However, investigation of fracture
153
initiation and propagation during hydraulic fracturing test under high-temperature
154
conditions was relatively rare.
155
This paper was for the first time to investigate hydraulic fracture behavior of large
156
granite outcrop samples under a high temperature of 200
157
conditions. Besides the scientific contribution, the work in this paper is a guide in
158
fields for hydraulic fracturing of HDR geothermal formations.
159
2. Experimental Setup
160
2.1 Rock sample
161
Rock samples were obtained from the Gonghe Basin in Northwest China, where the
162
first HDR geothermal formation is to be developed. The rock was an outcrop of the
163
same geological formation with similar rock properties. The rock properties are
164
summarized in Table 2
165
Table 2: Properties of Rock Sample
and true triaxial stress
Test 1
Test 2
Test 3
Compressive Strength, MPa
388.4
386.8
384.7
Young’s Modulus, GPa
50.2
46.8
40.0
8
Brazilian Tensile strength, MPa
12.42
12.26
12.30
Peak Strain, %
0.83
1.2
1.1
Note: 1. Test temperature: 200 ; Confining pressure: 40 MPa
2. Brazilian Tensile strength was tested under 200 pressure
and no confining
166
The tests were according to the standard procedure of international society for rock
167
mechanics. Each result was the average of tens of testing results. According to the
168
properties, the rock is difficult to break. But once fractures are created, the fracture
169
propagation is expected to be very fast because the rock is brittle.
170
Then, the rock was prepared for the hydraulic fracturing simulation experiment. The
171
rock samples were cut into cubes with a size of 300 mm by 300 mm by 300 mm. In
172
the middle of the samples, holes were drilled that represented scaled down wellbores.
173
In these wellbores, fracturing fluid was injected to create hydraulic fractures in the
174
rock samples (see Figure 5).
175
(a)
(b)
176
Figure 5: Experimental samples. (a) The actual rock sample; (b) A schematic diagram
177
of the rock sample interior.
178
2.2 Experiment system
179
A true- triaxial hydraulic fracturing test system as shown in Figure 6 was applied to
180
simulate the hydraulic fracturing of the HDR geothermal formation in the laboratory.
9
181
This system has been widely applied for research in the oil and gas industry to
182
monitor and study hydraulic fracture propagation.
183
This true- triaxial hydraulic fracturing test system was specially designed to simulated
184
hydraulic fracturing in a realistic formation environment at actual reservoir
185
temperatures and pressures. The temperature function includes heating, thermal
186
insulation, temperature control, and thermometry parts. The heating part has nine
187
electrical heating rods with each of 2000 watt power. The thermal insulation part is
188
the ceramic panels which thick is calculated to ensure the heating zone can keep
189
constant temperature up to 250 ℃ (at 20 ℃ ambient temperature). Six temperature
190
sensors around the sample and thermoregulator are combined to control temperature.
191
The external stresses are under servo control. The system can provide the vertical,
192
maximum horizontal, and minimum horizontal in-situ stress, and injection pressure.
193
The maximum injection pressure is up to 100 MPa.
194 195
Figure 6:
Custom designed true- triaxial hydraulic fracturing test system for
196
formation temperatures and pressures.
197
In the simulation system, the wellbore and fracturing equipment were scaled down so
198
that the whole hydraulic fracturing process could be conducted in the laboratory.
199
Table 3 listed the experimental setup for the all samples.
200
Table 3: Operation Parameters during the Experiments Horizontal in-situ stress difference
10 MPa (similarity) 10
20 MPa (similarity)
Temperature
200
/ Control group: room temperature 1 mPa.S
Fracturing fluid viscosity Pumping rate
3.8 m3/min (similarity)
10 m3/min (similarity)
1.9 m3/min (similarity)
Note: there is a similarity calculation from laboratory to field data 201
Heating process for each sample would take 15 to 20 days. Each day, the temperature
202
was only increased 10 degree centigrade and kept the temperature for 24 hours. This
203
was to ensure the heating was smooth and temperature increase could not induce any
204
damage in the rock. More details of this system can be found in Zhou, Z. et al. (2018).
205
3. Experimental Result
206
The goal of hydraulic fracturing in HDR geothermal formations is to establish a heat
207
transfer area to extract as much thermal energy as possible through fluid flow. Thus,
208
the fractures that are created as flow paths are significant during extraction. This
209
paper studied fracture creation under the formation conditions of the Gonghe Basin.
210
3.1 Influence of in- situ stress differences
211
With increasing depth, one of the main changes in the formation occurs for the in- situ
212
stress. The difference between the maximum and minimum horizontal stresses
213
increases. Thus, fracture propagation would be different in shallow and deep
214
formations.
215
Since the potential hydraulic fracturing locations in the Gonghe Basin occur at a depth
216
between 2000 m and 3500 m, the experiments simulated the stress conditions for
217
shallow (2200 m) and deep (3500 m) depths.
218
There were five samples including three shallow and two large depth samples.
219
Fracture propagation in the rock samples was directly observed in the laboratory.
220
Photograph of fractured samples and schematic fracture diagram in 2D and 3D are 11
221
given in the figures for comparing hydraulic fracturing results for different test
222
conditions. Figure 7 and Figure 8 show fracture creation under small and large
223
horizontal stress differences, respectively.
224
(a) Sample #2
225
(b) Sample #7
12
226 227
(c) Sample #8 Figure 7: Fracture creation under small horizontal stress difference.
228
(a) Sample #9
229
(b) Sample #10 13
230
Figure 8: Fracture creation under large horizontal stress difference.
231
When hydraulic fracturing occurred at shallow depths under small stress differences
232
(see Figure 7), fracture propagation was not straight. The direction of propagation
233
changed. At large depths with relatively large horizontal stress difference, hydraulic
234
fractures propagating straight along the direction of the maximum horizontal stress, as
235
shown in Figure 8.
236
3.2 Influence of weak plane
237
Weak planes include geological layers and natural fractures that cause strength
238
reduction in the formation. Weak planes can increase the stimulation area when they
239
are connected with hydraulic fractures. In a traditional oil reservoir, such as shale or
240
sandstone, hydraulic fractures can either propagate across or along weak planes, or
241
both. These interactions are the main mechanisms to create a complicated fracture
242
network in the formation. In HDR formations, it is also expected that the fracture
243
network can make the heat transfer area as large as possible.
244
The experimental results are depicted in Figure 9.
245
(a) Sample #11 14
246
247
(b) Sample #5
(c) Sample #6
248
Figure 9: Hydraulic fracture creation with influence of weak planes. (a) Sample #11:
249
geological dikes have about 45° inclination angle with horizontal stresses (b) Sample
250
#5: natural fracture is parallel to the borehole direction (c) Sample #6: natural fracture
251
is perpendicular to the borehole direction
15
252
Sample #11 contained geological dikes that were connected to layers in the samples.
253
Sample #5 and #6 included natural fractures. It was observed that hydraulic fractures
254
only propagated along the layers and natural fractures. There was no case in which the
255
artificial fractures penetrated across geological layers or natural fractures. Particularly
256
in sample #5, natural fractures occurred at the bottom of the sample and were
257
connected with the wellbore. Thus, during fracturing, the propagation direction was
258
toward the bottom rather than along the direction of the maximum horizontal stress.
259
3.3 Influence of pumping rate
260
The in- situ stresses and weak planes are geological properties that cannot be
261
controlled during the hydraulic fracturing treatment. The pumping rate is the working
262
operation parameter that can be designed. Based on the experiences in shale
263
formations, the pumping rate should be high enough to create a fracture network.
264
Hence, pumping rate was paid more attention in the experiments.
265
Figure 10 shows the hydraulic fracture creation under the high pumping rate of 10
266
m3/min.
267
Sample #1
268
Figure 10: Hydraulic fracturing at the pumping rate of 10 m3/min (similarity
269
calculation) 16
270
In sample #1, hydraulic fractures were created on both sides of the wellbore, but they
271
only propagated on one side.
272
A high pumping rate could create either uni-lateral or bi-lateral fracture growth in
273
HDR geothermal formations. It remains unclear which factors and mechanisms
274
influence uni-lateral or bi-lateral fracture propagation. Figure 11 to Figure 13
275
compared the experimental results under three different pumping rates.
276
Sample #11
277
Figure 11: Hydraulic fracturing at the high pumping rate of 10 m3/min (similarity
278
calculation)
279
Sample #12 17
280
Figure 12: Hydraulic fracturing at the intermediate pumping rate of 3.8 m3/min
281
(similarity calculation)
282
Sample #9
283
Figure 13: Hydraulic fracturing at the small pumping rate of 1.9 m3/min (similarity
284
calculation)
285
Hydraulic fractures under both high and low pumping rates connected with weak
286
planes, which would change the propagation direction (see Figure 11 and Figure 12)
287
or propagate straight across without any further connections (see Figure 10 and Figure
288
13). Hence, it is difficult to determine the effect of pumping rate change on fracture
289
creation.
290
In addition, cyclic injection was studied. During this operation, the pump is started
291
and stopped regularly over time comparing the constant rate the whole time. First, the
292
pump is set to increase the pressure. When the pressure reaches the designed value,
293
which is usually below the fracturing pressure of the rock, pumping is ceased. Once
294
the pressure drops, pumping is started again, and the procedure is repeated. The cyclic
295
injection can result in stress alternations to induce rock vibration so that the weak
296
planes in the formation are reopened and can become more easily connected with
297
hydraulic fractures (Zimmermann, G. et al., 2010; Zang, A. et al., 2016; Zhuang, L. et
298
al., 2017 and 2019). 18
299
Figure 14 shows the experimental result of sample #3, while the result of sample #4 is
300
shown in Figure 15.
301
Sample #3
302
303 304
Figure 14: Cyclic injection with designed step- by- step pressure increases
Sample #4 Figure 15: Cyclic injection with the constant design pressure
19
305
In sample #3, cyclic injection was implemented based on the designed step- by- step
306
pressure increase program. In sample #4 the designed maximum injection pressure in
307
different cycles are approximately the same. In both of the two experiments, natural
308
fractures were connected by hydraulic fractures. The experimental results indicate that
309
the cyclic injection has great potential to activate natural fractures and thus increase
310
the connection possibility. It is worth to have more studies on it in HDR geothermal
311
formation.
312
4. Experimental Discussion
313
According to Figure 7, the fracture propagation with small horizontal stress difference
314
was not a straight direction. This occurs because that when the maximum and
315
minimum horizontal stresses are similar, fractures can propagate in various directions.
316
If the extreme case occurred in which both values are the same, fractures are expected
317
to grow in any direction. Although this extreme case does not occur at all in
318
formations, the maximum and minimum horizontal stresses are more similar at
319
shallow depths, which could still allow fractures to potentially propagate in various
320
directions. The experiment result in the large horizontal stress difference (see Figure 8)
321
indicated that the large stress difference restricted and controlled fracture propagation
322
at large depths.
323
Therefore, hydraulic fracturing under small horizontal stress difference could
324
establish a larger stimulation area because fractures are expected to exhibit a
325
complicated growth path. In addition, there are more chances for the artificial
326
fractures to encounter weak planes.
327
For the impact from the weak planes, unlike traditional oil formations, in HDR
328
formations, hydraulic fractures are more likely to propagate along weak planes rather
329
than propagate across them. The reason is that the rock matrix of HDR formations is
330
much more difficult to break than the rock matrix of shale or carbonate formations.
331
According to Table 1, the strength of rock matrix of HDR formations was two to four 20
332
times higher than that of the rock matrix of shale or carbonate formations. Hence, in
333
HDR formations, hydraulic fractures are more likely to reopen natural fractures or
334
layers, which have lower strength compared to that of the rock matrix.
335
Figure 10 indicated the uni-lateral fracture growth in HDR geothermal formation. The
336
reason was that a high pumping rate in very brittle rock could lead to very fast
337
propagation. In the experiment, it only took five minutes for the hydraulic fracture to
338
propagate from the wellbore to the boundary of the sample, compared with more than
339
30 minutes in a shale sample at the same pumping rate level. Once very fast
340
propagation happened on one side, flow paths were very quickly established on that
341
side so that the fracturing fluid only flowed along those paths, and no propagation
342
occurred on the other side. Such propagation phenomena on only one lateral fracture
343
growth were reported in the field, as shown in Figure 16 (Ziagos J. et al., 2013; Jung
344
R., 2013).
345 346
Figure 16: Uni-lateral hydraulic fracture propagation in RH-11 of the Rosemanowes
347
HDR geothermal formation, Camborne, United Kingdom (Jung R., 2013) 21
348
5. Conclusions
349
This paper investigated fracture creation of flow paths in HDR geothermal formations.
350
Hydraulic fracturing was simulated in the laboratory in a realistic formation
351
environment. The conclusions are as follows:
352
(1) The propagation of hydraulic fractures was controlled by the in-situ stress. A small
353
horizontal stress difference could allow deviation during propagation so that hydraulic
354
fracturing could establish large stimulation area. A large horizontal stress difference
355
resulted only in straight propagation. This phenomenon was the same as was observed
356
during hydraulic fracturing in oil and gas formations.
357
(2) Unlike hydrocarbon formations, hydraulic fractures could only propagate along
358
weak planes, including natural fractures and geological layers, when they became
359
connected in HDR geothermal formations. The propagation direction changed to the
360
same orientation as that of weak planes. The case in which hydraulic fractures
361
propagate across the weak planes in granite rock rarely occurred.
362
(3) The pumping rate during hydraulic fracturing had little influence on fracture
363
creation in HDR geothermal formations. The main factors that impacted fracture
364
creation were geological properties such as the in-situ stress and natural fractures.
365
However, custom designed pumping procedures, like cyclic rate, could increase the
366
possibility of enlarging the heat transfer area.
367
Based on the studies in this paper, there are some suggestions for the proposed
368
hydraulic fracturing operations in HDR geothermal formations in fields. First, the
369
potential depth of hydraulic fracturing should be selected as shallow as possible,
370
because the in-situ stress is lower. Hydraulic fracturing can therefore create a
371
relatively larger heat transfer area. Second, geological information on weak planes
372
must be analyzed for predicting the most likely orientation of hydraulic fractures in
373
the field. 22
374
Acknowledgments
375
The authors would like to acknowledge the support from the National Key R&D
376
Program of China (Grant No. 2018YFB1501802), National Natural Science
377
Foundation of China (Grant No. 51811540403), and Science Foundation of the China
378
University of Petroleum, Beijing (Grant No. 2462016YJRC017).
379
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380
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Appendix
492
During the hydraulic fracturing, the pressure was recorded of time.
27
#1
#2
28
#3
#4
#6
29
#7
#8
#9
30
#10
#11
#12
31
493
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➢ Hydraulic fracturing was simulated in high temperature via experiment ➢ Fracture creations were studied in outcrop granite of HDR geothermal formation. ➢ In-situ stress and weak planes affects fracture propagation ➢ Custom designed pumping procedures can enlarge the heat transfer area
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0960-1481(20)30150-6
DOI:
https://doi.org/10.1016/j.renene.2020.01.128
Reference:
RENE 12991
To appear in:
Renewable Energy
Received Date: 2 July 2019 Revised Date:
24 January 2020
Accepted Date: 26 January 2020
Please cite this article as: Zhou Z, Jin Y, Zeng Y, Zhang X, Zhou J, Zhuang L, Xin S, Investigation on fracture creation in hot dry rock geothermal formations of China during hydraulic fracturing, Renewable Energy (2020), doi: https://doi.org/10.1016/j.renene.2020.01.128. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Zhou Zhou: Conceptualization, Methodology, Writing - Original Draft Yan Jin: Conceptualization, Supervision Yijin Zeng: Supervision, Validation Xudong Zhang: Resources, Validation Jian Zhou: Methodology, Formal analysis, Validation Li Zhuang: Investigation, Data Curation, Writing - Review & Editing Shunyuan Xin: Writing - Original Draft, Data Curation
1
Investigation on fracture creation in hot dry rock geothermal
2
formations of China during hydraulic fracturing
3
Zhou Zhoua*, Yan Jina, Yijin Zengb, Xudong Zhangb, Jian Zhoub, Li Zhuangc,
4
Shunyuan Xina
5
a
6
Petroleum (Beijing), Beijing, China
7
b
Sinopec Research Institute of Petroleum Engineering, Beijing, China
8
c
Korea Institute of Civil Engineering and Building Technology, Goyang, Gyeonggi,
9
Republic of Korea
State Key Laboratory of Petroleum Resources and Prospecting, China University of
10
* Corresponding author: [email protected]
11
Abstract:
12
The abundant geothermal energy in hot dry rock (HDR) formations is an attractive
13
renewable energy resource with great potential. China will develop its first HDR
14
geothermal formation in the Gonghe Basin. HDR is a hard and low-permeability
15
granite containing very few fluids. Development requires fluids to cyclically flow
16
between injection and production wells to extract geothermal energy in the artificial
17
heat transfer zone. Hydraulic fracturing is the main technology for creating flow paths.
18
But few studies have investigated fractures in HDR geothermal formations. This
19
paper investigated fractures as flow paths in HDR geothermal formations during
20
hydraulic fracturing. Hydraulic fractures were simulated using a custom true- triaxial
21
hydraulic fracturing test system in a realistic formation environment, in which a
22
scaled wellbore was used that was built in outcrop granite rock from the Gonghe
23
Basin. Fracture creation in granite was investigated via experiments, as well as
24
influence factors, and what experience could be achieved. This study can be used to
25
design and evaluate hydraulic fracturing projects in potential HDR geothermal
26
formations. 1
27
Keywords: Hot dry rock geothermal energy; Hydraulic fracturing; Fracture creation;
28
High temperature; True triaxial
29
1. Introduction
30
Geothermal energy is a renewable energy resource that is developed for heating and
31
electricity generation. The geothermal heat source is directly applied in heat pumps,
32
room and space heating, aquaculture and agricultural heating, bathing and swimming,
33
and industrial uses (Moya D. et al., 2018). In 2015, these applications were expected
34
to replace 350 million barrels of equivalent oil and avert 148 million tons of
35
greenhouse gas emissions (Lund J. and Boyd T., 2015). In addition, according to
36
statistical data, geothermal power plants for electricity generation already had a total
37
installed capacity of 12.729 MW in 2016, which is expected to double by 2020
38
(Bertani R., 2016). Therefore, geothermal energy is a significant potential sustainable
39
energy alternative to fossil fuel energy, which can reduce greenhouse gas emissions.
40
Geothermal energy can be discovered beneath the surface. However, the economic
41
potential of formations is based on their depth and temperature. These formations are
42
divided into shallow low temperature reservoirs and deep high temperature energy
43
resources. Shallow geothermal energy is located at hundreds of meters underground,
44
and the temperature is below 100
45
shallow geothermal formations is to extract energy to heat buildings in the urban areas
46
and factories and farms in rural areas (Magraner T. et al., 2010). The depth of deep
47
geothermal resources can reach 5 km, and the temperature can reach 400
48
higher (Tomac I. and Sauter M.,2018). The energy in deep geothermal formations is
49
mainly developed for the generation of electricity. This technology requires harvesting
50
energy from geothermal energy resources through fluid transport to power plants
51
(Moya D. et al., 2018). Recently, conventional electric power generation, which
52
requires geothermal formation temperature below 200 , has been successfully
53
implemented in North America, Europe, and East Asia (Moya D. et al., 2018; Tomac I.
(Bayer P. et al., 2019). The main application of
2
and
54
and Sauter M.,2018). The development of geothermal temperatures higher than 200
55
is attractive and should be considered in future applications. These formations are
56
called hot dry rock (HDR) geothermal formations (Feng Y. et al., 2014).
57
HDR geothermal formations usually consist of hard and low- permeability granite.
58
There is very little or no fluid in the formations because of the high temperature.
59
Hence, the development of HDR geothermal formations requires an enhanced
60
geothermal system that can efficiently convert geothermal energy into electricity
61
(Olasolo P. et al., 2016). In an enhanced geothermal system, injection and production
62
wells are completed that allow fluids to extract geothermal energy. Hydraulic
63
fracturing is one of the most significant technologies for achieving sufficient and
64
stable flow paths for geothermal energy extraction (Hanano M., 2004). Therefore, it is
65
necessary to understand how fractures are created as flow paths in HDR formations
66
by hydraulic fracturing.
67
The purpose of this paper is the provision of a good understanding of
68
thermo-hydro-mechanical interaction on hydraulic fracture creation. In the HDR
69
geothermal formation, the interaction is different from other formations because
70
formation temperature is much higher, and rock is harder so that the fracture creation
71
based on the interaction is unclear and worth to study. In addition, the study in this
72
paper is a fundamental research in the geothermal area. As shown in Figure 1, it
73
contributes to further studies of geothermal energy usage, transportation, and
74
distribution.
3
75 76
Figure 1: Flow chart to indicate scientific contribution of this paper
77
Before introducing the purpose and main research contents of this study, it briefly
78
reviews the hydraulic fracturing method for conventional oil and gas exploitation and
79
the difference as well as the challenge of application of hydraulic fracturing in HDR
80
formations.
81
1.1 Review of Hydraulic Fracturing
82
The typical procedure of hydraulic fracturing includes the following three steps.
83
First, fracturing fluid is pumped into the wellbore to increase the pressure at the
84
bottom of the well. The wellbore is sealed through the steel casing. However, at the
85
bottom of the well, there is no casing and only formation rock. Thus, with the increase
86
in pumping volume, the pressure continues to rise until the rock fracturing pressure.
87
When this pressure is exceeded, the rock is broken, and fractures are generated (see
88
Figure 2).
4
89 90
Figure 2: Concept of hydraulic fracture creation.
91
Second, when fluid injection is continued, the newly created fractures, called
92
hydraulic or artificial fractures, grow in the formation. If the formation contains
93
geologically weak planes such as natural fractures or interlayers, they will affect
94
hydraulic fracture propagation. There are three possible interactions when hydraulic
95
fractures contact weak planes. One possible interaction is that hydraulic fractures
96
propagate along the weak planes, which leads to reopening and activation, and the
97
original propagation direction changes. Second, hydraulic fractures propagate across
98
the weak planes, and no direction change occurs. The last possible interaction is
99
fracture propagation both along and across the weak planes, as shown in Figure 3 (a)
100
101
and (b), respectively.
(a)
(b)
5
102
Figure 3: The interaction between artificial and natural fractures. (a) artificial
103
fractures propagating along natural fractures; (b) hydraulic fractures propagating
104
across natural fractures.
105
Third, sand or ceramic proppant material is injected. It is required to distribute the
106
proppants inside the hydraulic fractures. When pumping ceases, the proppants prevent
107
the fractures from closing.
108
Fractures in the formation is mainly controlled by the in- situ stresses. According to
109
the direction, the in- situ stresses are divided into vertical and horizontal stresses.
110
Horizontal stresses are subdivided into the maximum and minimum horizontal
111
stresses, whose directions are perpendicular to each other (see Figure 4). A fracture
112
can be initialed at any point in the formation that is the weakest. However, the
113
direction of fracture propagation is parallel to the maximum horizontal stress direction,
114
and perpendicular to the minimum horizontal stress direction.
115 116
Figure 4: Fracture propagation along the maximum horizontal stress direction
117
Whether hydraulic fractures could reopen natural fractures or geological layers is
118
determined by various factors, but there are two most significant factors: the rock
119
strength and fracture shear strength. When the net pressure in the fractures exceeds
120
the rock strength, hydraulic fractures are more likely to propagate across weak planes; 6
121
and when the net pressure is higher than the shear strength, natural fractures or layers
122
could be reopened. For different formations, the rock strength and fracture shear
123
strength vary, which sometimes results in hydraulic fractures easily activating, and at
124
times, hydraulic fractures preferentially propagate with no direction change.
125
The purpose of hydraulic fracturing in HDR geothermal formations requires a heat
126
transfer area between the injection and production wells. Thus, in the ideal situation,
127
the created fractures, including artificial and active natural fractures should propagate
128
and spread into as large a formation area as possible. In HDR geothermal formations,
129
the formation rock is much harder. Table 1 summarizes the compressive strength
130
among the various rocks from previous experiments. The higher the strength the more
131
difficult to break the rock. However, very few previous papers have investigated the
132
processes of fracture initiation and propagation in HDR formations. Hence, this paper
133
studied fracture creation through hydraulic fracturing simulation experiments with
134
real HDR formation rocks under actual formation conditions.
135
Table 1: Compressive Strength of Formation Rocks
Compressive Strength MPa
Hot dry rock
Shale
Carbonate
Sandstone
150~467
80~180
40~120
33~100
Note: Confining pressures are from 0 MPa to 40 MPa 136
1.2 Laboratory Experimental Studies of Fracture Creation
137
Hydraulic fracture was normally investigated and revealed how to be created under
138
various geological and operation conditions (Daneshy A., 1973; Zoback M.D. et al.,
139
1977; Behrmann L.A. and Elbel J.L., 1991; De Pater C.J. et al., 1994; Mogi K., 2007;
140
Kwaśniewski M. et al., 2013), the interactions between artificial and natural fractures
141
(Zhou, J. et al., 2008; Zhou, J. et al., 2010), and how to maximize the stimulation area
142
through fracture network establishment (Yost II, A.B. et al., 1989; Hossain M. M. et
143
al., 2000; Pater, C.J.d. and Beugelsdijk L.J.L., 2005; Olsen, T.N. et al., 2009; Cipolla, 7
144
C.L. et al., 2009). However, most of the previous experiments were for shales,
145
sandstones, or other oil and gas formations. A few studies have focused on fracture
146
propagation in hard granite rocks at high temperature (Frash, L.P. et al., 2014; Zhou, Z.
147
et al., 2018).
148
Temperature variations could result in changes in the shear strength (Friedman M. et
149
al., 1979), compressive strength (Faoro I. et al, 2013), rock porosity and permeability
150
(Faoro I. et al, 2013), and fracture toughness (Nasseri M. et al., 2007; Wang, X.-Q. et
151
al., 2013). In addition, more preexisting fractures were observed in rock with
152
increasing temperature Nasseri M. et al., 2007). However, investigation of fracture
153
initiation and propagation during hydraulic fracturing test under high-temperature
154
conditions was relatively rare.
155
This paper was for the first time to investigate hydraulic fracture behavior of large
156
granite outcrop samples under a high temperature of 200
157
conditions. Besides the scientific contribution, the work in this paper is a guide in
158
fields for hydraulic fracturing of HDR geothermal formations.
159
2. Experimental Setup
160
2.1 Rock sample
161
Rock samples were obtained from the Gonghe Basin in Northwest China, where the
162
first HDR geothermal formation is to be developed. The rock was an outcrop of the
163
same geological formation with similar rock properties. The rock properties are
164
summarized in Table 2
165
Table 2: Properties of Rock Sample
and true triaxial stress
Test 1
Test 2
Test 3
Compressive Strength, MPa
388.4
386.8
384.7
Young’s Modulus, GPa
50.2
46.8
40.0
8
Brazilian Tensile strength, MPa
12.42
12.26
12.30
Peak Strain, %
0.83
1.2
1.1
Note: 1. Test temperature: 200 ; Confining pressure: 40 MPa
2. Brazilian Tensile strength was tested under 200 pressure
and no confining
166
The tests were according to the standard procedure of international society for rock
167
mechanics. Each result was the average of tens of testing results. According to the
168
properties, the rock is difficult to break. But once fractures are created, the fracture
169
propagation is expected to be very fast because the rock is brittle.
170
Then, the rock was prepared for the hydraulic fracturing simulation experiment. The
171
rock samples were cut into cubes with a size of 300 mm by 300 mm by 300 mm. In
172
the middle of the samples, holes were drilled that represented scaled down wellbores.
173
In these wellbores, fracturing fluid was injected to create hydraulic fractures in the
174
rock samples (see Figure 5).
175
(a)
(b)
176
Figure 5: Experimental samples. (a) The actual rock sample; (b) A schematic diagram
177
of the rock sample interior.
178
2.2 Experiment system
179
A true- triaxial hydraulic fracturing test system as shown in Figure 6 was applied to
180
simulate the hydraulic fracturing of the HDR geothermal formation in the laboratory.
9
181
This system has been widely applied for research in the oil and gas industry to
182
monitor and study hydraulic fracture propagation.
183
This true- triaxial hydraulic fracturing test system was specially designed to simulated
184
hydraulic fracturing in a realistic formation environment at actual reservoir
185
temperatures and pressures. The temperature function includes heating, thermal
186
insulation, temperature control, and thermometry parts. The heating part has nine
187
electrical heating rods with each of 2000 watt power. The thermal insulation part is
188
the ceramic panels which thick is calculated to ensure the heating zone can keep
189
constant temperature up to 250 ℃ (at 20 ℃ ambient temperature). Six temperature
190
sensors around the sample and thermoregulator are combined to control temperature.
191
The external stresses are under servo control. The system can provide the vertical,
192
maximum horizontal, and minimum horizontal in-situ stress, and injection pressure.
193
The maximum injection pressure is up to 100 MPa.
194 195
Figure 6:
Custom designed true- triaxial hydraulic fracturing test system for
196
formation temperatures and pressures.
197
In the simulation system, the wellbore and fracturing equipment were scaled down so
198
that the whole hydraulic fracturing process could be conducted in the laboratory.
199
Table 3 listed the experimental setup for the all samples.
200
Table 3: Operation Parameters during the Experiments Horizontal in-situ stress difference
10 MPa (similarity) 10
20 MPa (similarity)
Temperature
200
/ Control group: room temperature 1 mPa.S
Fracturing fluid viscosity Pumping rate
3.8 m3/min (similarity)
10 m3/min (similarity)
1.9 m3/min (similarity)
Note: there is a similarity calculation from laboratory to field data 201
Heating process for each sample would take 15 to 20 days. Each day, the temperature
202
was only increased 10 degree centigrade and kept the temperature for 24 hours. This
203
was to ensure the heating was smooth and temperature increase could not induce any
204
damage in the rock. More details of this system can be found in Zhou, Z. et al. (2018).
205
3. Experimental Result
206
The goal of hydraulic fracturing in HDR geothermal formations is to establish a heat
207
transfer area to extract as much thermal energy as possible through fluid flow. Thus,
208
the fractures that are created as flow paths are significant during extraction. This
209
paper studied fracture creation under the formation conditions of the Gonghe Basin.
210
3.1 Influence of in- situ stress differences
211
With increasing depth, one of the main changes in the formation occurs for the in- situ
212
stress. The difference between the maximum and minimum horizontal stresses
213
increases. Thus, fracture propagation would be different in shallow and deep
214
formations.
215
Since the potential hydraulic fracturing locations in the Gonghe Basin occur at a depth
216
between 2000 m and 3500 m, the experiments simulated the stress conditions for
217
shallow (2200 m) and deep (3500 m) depths.
218
There were five samples including three shallow and two large depth samples.
219
Fracture propagation in the rock samples was directly observed in the laboratory.
220
Photograph of fractured samples and schematic fracture diagram in 2D and 3D are 11
221
given in the figures for comparing hydraulic fracturing results for different test
222
conditions. Figure 7 and Figure 8 show fracture creation under small and large
223
horizontal stress differences, respectively.
224
(a) Sample #2
225
(b) Sample #7
12
226 227
(c) Sample #8 Figure 7: Fracture creation under small horizontal stress difference.
228
(a) Sample #9
229
(b) Sample #10 13
230
Figure 8: Fracture creation under large horizontal stress difference.
231
When hydraulic fracturing occurred at shallow depths under small stress differences
232
(see Figure 7), fracture propagation was not straight. The direction of propagation
233
changed. At large depths with relatively large horizontal stress difference, hydraulic
234
fractures propagating straight along the direction of the maximum horizontal stress, as
235
shown in Figure 8.
236
3.2 Influence of weak plane
237
Weak planes include geological layers and natural fractures that cause strength
238
reduction in the formation. Weak planes can increase the stimulation area when they
239
are connected with hydraulic fractures. In a traditional oil reservoir, such as shale or
240
sandstone, hydraulic fractures can either propagate across or along weak planes, or
241
both. These interactions are the main mechanisms to create a complicated fracture
242
network in the formation. In HDR formations, it is also expected that the fracture
243
network can make the heat transfer area as large as possible.
244
The experimental results are depicted in Figure 9.
245
(a) Sample #11 14
246
247
(b) Sample #5
(c) Sample #6
248
Figure 9: Hydraulic fracture creation with influence of weak planes. (a) Sample #11:
249
geological dikes have about 45° inclination angle with horizontal stresses (b) Sample
250
#5: natural fracture is parallel to the borehole direction (c) Sample #6: natural fracture
251
is perpendicular to the borehole direction
15
252
Sample #11 contained geological dikes that were connected to layers in the samples.
253
Sample #5 and #6 included natural fractures. It was observed that hydraulic fractures
254
only propagated along the layers and natural fractures. There was no case in which the
255
artificial fractures penetrated across geological layers or natural fractures. Particularly
256
in sample #5, natural fractures occurred at the bottom of the sample and were
257
connected with the wellbore. Thus, during fracturing, the propagation direction was
258
toward the bottom rather than along the direction of the maximum horizontal stress.
259
3.3 Influence of pumping rate
260
The in- situ stresses and weak planes are geological properties that cannot be
261
controlled during the hydraulic fracturing treatment. The pumping rate is the working
262
operation parameter that can be designed. Based on the experiences in shale
263
formations, the pumping rate should be high enough to create a fracture network.
264
Hence, pumping rate was paid more attention in the experiments.
265
Figure 10 shows the hydraulic fracture creation under the high pumping rate of 10
266
m3/min.
267
Sample #1
268
Figure 10: Hydraulic fracturing at the pumping rate of 10 m3/min (similarity
269
calculation) 16
270
In sample #1, hydraulic fractures were created on both sides of the wellbore, but they
271
only propagated on one side.
272
A high pumping rate could create either uni-lateral or bi-lateral fracture growth in
273
HDR geothermal formations. It remains unclear which factors and mechanisms
274
influence uni-lateral or bi-lateral fracture propagation. Figure 11 to Figure 13
275
compared the experimental results under three different pumping rates.
276
Sample #11
277
Figure 11: Hydraulic fracturing at the high pumping rate of 10 m3/min (similarity
278
calculation)
279
Sample #12 17
280
Figure 12: Hydraulic fracturing at the intermediate pumping rate of 3.8 m3/min
281
(similarity calculation)
282
Sample #9
283
Figure 13: Hydraulic fracturing at the small pumping rate of 1.9 m3/min (similarity
284
calculation)
285
Hydraulic fractures under both high and low pumping rates connected with weak
286
planes, which would change the propagation direction (see Figure 11 and Figure 12)
287
or propagate straight across without any further connections (see Figure 10 and Figure
288
13). Hence, it is difficult to determine the effect of pumping rate change on fracture
289
creation.
290
In addition, cyclic injection was studied. During this operation, the pump is started
291
and stopped regularly over time comparing the constant rate the whole time. First, the
292
pump is set to increase the pressure. When the pressure reaches the designed value,
293
which is usually below the fracturing pressure of the rock, pumping is ceased. Once
294
the pressure drops, pumping is started again, and the procedure is repeated. The cyclic
295
injection can result in stress alternations to induce rock vibration so that the weak
296
planes in the formation are reopened and can become more easily connected with
297
hydraulic fractures (Zimmermann, G. et al., 2010; Zang, A. et al., 2016; Zhuang, L. et
298
al., 2017 and 2019). 18
299
Figure 14 shows the experimental result of sample #3, while the result of sample #4 is
300
shown in Figure 15.
301
Sample #3
302
303 304
Figure 14: Cyclic injection with designed step- by- step pressure increases
Sample #4 Figure 15: Cyclic injection with the constant design pressure
19
305
In sample #3, cyclic injection was implemented based on the designed step- by- step
306
pressure increase program. In sample #4 the designed maximum injection pressure in
307
different cycles are approximately the same. In both of the two experiments, natural
308
fractures were connected by hydraulic fractures. The experimental results indicate that
309
the cyclic injection has great potential to activate natural fractures and thus increase
310
the connection possibility. It is worth to have more studies on it in HDR geothermal
311
formation.
312
4. Experimental Discussion
313
According to Figure 7, the fracture propagation with small horizontal stress difference
314
was not a straight direction. This occurs because that when the maximum and
315
minimum horizontal stresses are similar, fractures can propagate in various directions.
316
If the extreme case occurred in which both values are the same, fractures are expected
317
to grow in any direction. Although this extreme case does not occur at all in
318
formations, the maximum and minimum horizontal stresses are more similar at
319
shallow depths, which could still allow fractures to potentially propagate in various
320
directions. The experiment result in the large horizontal stress difference (see Figure 8)
321
indicated that the large stress difference restricted and controlled fracture propagation
322
at large depths.
323
Therefore, hydraulic fracturing under small horizontal stress difference could
324
establish a larger stimulation area because fractures are expected to exhibit a
325
complicated growth path. In addition, there are more chances for the artificial
326
fractures to encounter weak planes.
327
For the impact from the weak planes, unlike traditional oil formations, in HDR
328
formations, hydraulic fractures are more likely to propagate along weak planes rather
329
than propagate across them. The reason is that the rock matrix of HDR formations is
330
much more difficult to break than the rock matrix of shale or carbonate formations.
331
According to Table 1, the strength of rock matrix of HDR formations was two to four 20
332
times higher than that of the rock matrix of shale or carbonate formations. Hence, in
333
HDR formations, hydraulic fractures are more likely to reopen natural fractures or
334
layers, which have lower strength compared to that of the rock matrix.
335
Figure 10 indicated the uni-lateral fracture growth in HDR geothermal formation. The
336
reason was that a high pumping rate in very brittle rock could lead to very fast
337
propagation. In the experiment, it only took five minutes for the hydraulic fracture to
338
propagate from the wellbore to the boundary of the sample, compared with more than
339
30 minutes in a shale sample at the same pumping rate level. Once very fast
340
propagation happened on one side, flow paths were very quickly established on that
341
side so that the fracturing fluid only flowed along those paths, and no propagation
342
occurred on the other side. Such propagation phenomena on only one lateral fracture
343
growth were reported in the field, as shown in Figure 16 (Ziagos J. et al., 2013; Jung
344
R., 2013).
345 346
Figure 16: Uni-lateral hydraulic fracture propagation in RH-11 of the Rosemanowes
347
HDR geothermal formation, Camborne, United Kingdom (Jung R., 2013) 21
348
5. Conclusions
349
This paper investigated fracture creation of flow paths in HDR geothermal formations.
350
Hydraulic fracturing was simulated in the laboratory in a realistic formation
351
environment. The conclusions are as follows:
352
(1) The propagation of hydraulic fractures was controlled by the in-situ stress. A small
353
horizontal stress difference could allow deviation during propagation so that hydraulic
354
fracturing could establish large stimulation area. A large horizontal stress difference
355
resulted only in straight propagation. This phenomenon was the same as was observed
356
during hydraulic fracturing in oil and gas formations.
357
(2) Unlike hydrocarbon formations, hydraulic fractures could only propagate along
358
weak planes, including natural fractures and geological layers, when they became
359
connected in HDR geothermal formations. The propagation direction changed to the
360
same orientation as that of weak planes. The case in which hydraulic fractures
361
propagate across the weak planes in granite rock rarely occurred.
362
(3) The pumping rate during hydraulic fracturing had little influence on fracture
363
creation in HDR geothermal formations. The main factors that impacted fracture
364
creation were geological properties such as the in-situ stress and natural fractures.
365
However, custom designed pumping procedures, like cyclic rate, could increase the
366
possibility of enlarging the heat transfer area.
367
Based on the studies in this paper, there are some suggestions for the proposed
368
hydraulic fracturing operations in HDR geothermal formations in fields. First, the
369
potential depth of hydraulic fracturing should be selected as shallow as possible,
370
because the in-situ stress is lower. Hydraulic fracturing can therefore create a
371
relatively larger heat transfer area. Second, geological information on weak planes
372
must be analyzed for predicting the most likely orientation of hydraulic fractures in
373
the field. 22
374
Acknowledgments
375
The authors would like to acknowledge the support from the National Key R&D
376
Program of China (Grant No. 2018YFB1501802), National Natural Science
377
Foundation of China (Grant No. 51811540403), and Science Foundation of the China
378
University of Petroleum, Beijing (Grant No. 2462016YJRC017).
379
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Appendix
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During the hydraulic fracturing, the pressure was recorded of time.
27
#1
#2
28
#3
#4
#6
29
#7
#8
#9
30
#10
#11
#12
31
493
32
➢ Hydraulic fracturing was simulated in high temperature via experiment ➢ Fracture creations were studied in outcrop granite of HDR geothermal formation. ➢ In-situ stress and weak planes affects fracture propagation ➢ Custom designed pumping procedures can enlarge the heat transfer area
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: