Hydraulic fracturing in high-temperature granite characterized by acoustic emission

Journal of Petroleum Science and Engineering 178 (2019) 475–484

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Hydraulic fracturing in high-temperature granite characterized by acoustic emission

T

Yuekun Xinga, Guangqing Zhanga,b,∗, Tianyu Luoc, Yongwang Jianga, Shiwen Ninga a

Department of Engineering Mechanics, College of Petroleum Engineering, China University of Petroleum, Beijing, 100249, PR China State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 100249, PR China c Guangdong University of Petrochemical Technology, Guangdong, 525000, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: High-temperature Hydraulic fracturing Granite Fracturing characteristics Laboratory simulation Acoustic emission

Given the limited successes of hydraulic fracturing in enhanced geothermal systems (EGS), understanding of the hydraulic fracturing characteristics in high-temperature granites remains challenging and crucial. In this study, four groups of cube granite specimens (dimensions: 300/300/300 mm ) were tested to investigate the characteristics of the hydraulic fracture, for two confining pressures (0.1/0.1/0.1 MPa and 10/25/30 MPa ) and temperatures (20 °C and 120 °C). Acoustic emission (AE) was employed to characterize the hydraulic fracturing processes. The experimental results show that for different temperatures the fracture geometry is almost unchanged, whereas the injection pressure curves, spatial distribution of AE energy, and AE-based source mechanisms are significantly changed. (1) At 120 °C, The fracture pressures are increased by 3.6~4.9 MPa and two remarkable pressure peaks appear at the injection pressure curves, implying the hydraulic fracture propagates intermittently with increasing resistances. (2) The spatial distributions of AE energy delineate a microcrack-band distributing along the hydraulic fracture. At the high temperature (120 °C ), the effective width of the microcrack-band is reduced by 40~56.4%, and the fracture energy is reduced by about 75% adjacent to the wellbore (about 40% of the fracture length) in the microcrack-band. (3) Based on the AE source analysis, the fracture mechanisms in the microcrack-band indicate the high-temperature (120 °C ) reduces the proportion of shear microcracks by 6~12%. The characteristics of high-temperature reducing the effectiveness of EGS hydraulic fracturing are due to the change in granite microstructures from temperature induction and the transient temperature differential (ΔT ) between granite and fracturing fluid. In the EGS hydraulic fracturing, the net pressure should be enhanced in real-time with hydraulic fracture propagation to avoid fracturing arrest, ΔT between high-temperature granite and fracturing fluid should be lowered to enlarge the stimulated reservoir volume, and the proppant is suggested to be appropriately placed to prevent the further reduction of the fracturing effectiveness from fracture closures.

1. Introduction The high-temperature granite has become an important reservoir material for enhanced geothermal systems (EGS). To date, hydraulic fracturing is the major treatment to enhance the geothermal reservoir permeability (Tomac and Sauter, 2018; Wu et al., 2017). Given the limited successes of hydraulic fracturing in hot dry rock (Giardini, 2009; Mccartney et al., 2016; Trugman et al., 2016), understanding of the hydraulic fracturing characteristics in high-temperature granite remains challenging and crucial. So far, the numerical simulation is the major method for discussing hydraulic fracturing at high temperatures. The simulation results reveal temperature variations are fundamental to hydraulic fracturing, where cold water is injected into high-temperature reservoirs. For example, ∗

temperature variations change the growth rate of hydraulic fracture (Zhang et al., 2015; Tomac and Gutierrez, 2017). The thermal stress induced by temperature differential (ΔT ) promotes the activation of natural cracks surrounding hydraulic fracture (Perkins and Gonzalez, 1985; Dahi Taleghani et al., 2014). The temperature-changed pore pressures change both the fracture pressure and the fracturing direction in high-temperature reservoir (Abuaisha et al., 2016). The above studies provide significant references for deepening the understanding of high-temperature hydraulic fracturing. However, the theoretical models employed in the above numerical simulations mainly involve thermo-elasticity, linear elastic fracture and cohesive crack models, in which the numerical simulations pose four limitations. Firstly, given the rock fracturing is not an elastic process (Bažant, 2002), the thermoelectricity (Fjar et al., 2008) currently

Corresponding author. Department of Engineering Mechanics, College of Petroleum Engineering, China University of Petroleum, Beijing, 100249, PR China. E-mail address: [email protected] (G. Zhang).

https://doi.org/10.1016/j.petrol.2019.03.050 Received 22 October 2018; Received in revised form 24 January 2019; Accepted 18 March 2019 Available online 21 March 2019 0920-4105/ © 2019 Published by Elsevier B.V.

Journal of Petroleum Science and Engineering 178 (2019) 475–484

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List of symbols EGS CTE FPZ ΔT AE x , y, z x i , yi , z i t0 ti

v Eiae di

the P-wave velocity of the material (mm/μs ) the AE energy of an AE waveform (mv⋅μs ) the distance between the AE source and the AE sensor (mm ) n the number of synchronous waveforms (unity) E ae the calibrated AE energy (mv⋅μs ) the polarity value of an AE event (unity) pol Api the first-pulse amplitudes of an AE event (mv ) type-C, type-T, type-S implosion/collapse, tensile and shear source (microcrack)

Enhanced Geothermal Systems Coefficients of Thermal Expansion (1/ °C or 1/K ) Fracture Process Zone temperature differential (°C) Acoustic Emission coordinates (mm ) AE sensor locations (mm ) the occurrence time of AE event (μs ) the onset time received by each AE sensor (μs )

size granite specimens under true triaxial stresses. For laboratory simulations of hydraulic fracturing, the measurement approaches are fundamental to characterizing the hydraulic fracture. Traditionally, the injection pressures and fracture geometries are most widely used to delineating hydraulic fracture propagation (Fan and Zhang, 2014; Zhou et al., 2017). In recent years, some optical and acoustical approaches were employed to characterize hydraulic fracturing, such as high-resolution CT imaging (Kumari et al., 2018), fiber Bragger grating (Yang et al., 2017), visual fracturing (Altammar et al., 2018), acoustic emission (AE) (Zhou et al., 2018a,b), etc. Since the rock-like materials exhibit a distinct multiscale-microcrack zone (i.e. FPZ) surrounding the crack tip prior to failure (Hoagland et al., 1973; Otsuka and Date, 2000), the identification accuracy of microcrack zone is limited by the resolution of optical measurements, whereas the AEbased measurements can characterize the rock fracturing process more comprehensively, with the elastic waves (acoustic emission) released by multiscale-microcracks (Ishida, 2001). The AE-based characterizations for hydraulic fracturing includes AE parameters (Liu et al., 2017), AE locations (Zhou et al., 2018a,b), and AE-based fracture mechanisms (Ishida, 2001). Since the hydraulic fracture propagates across the fully developed FPZ, a microcrack-band is generated along the fracture growth path. There are two indicators for hydraulic fracturing effectiveness at different temperatures: microcrack-band volume and fracture mechanisms in microcrack-band (shear microcracks will not close when the net pressure vanishes (Pine and Batchelor, 1984; Rutqvist et al., 2015; Rinaldi et al., 2015)). Based on the AE-based identification of FPZ (Zhang et al., 2018), the integrated characterization of AE energy distribution and AE source mechanisms is supposed to be well suitable for delineating microcrack-band, which has not been reported for high-temperature hydraulic fracturing recently. In this study, the high-temperature hydraulic fracturing is tested on four groups of cube granite specimen (dimensions: 300/300/300 mm ), with AE characterization. The experiments show the geometry of hydraulic fracture surface at 120 °C is similar to that at 20 °C, whereas the injection pressure, the spatial distributions of AE energy and AE-based fracturing mechanisms reveal the prominent characteristics of high-

available is hardly applicable on rock fracturing at high temperatures. Moreover, the assumption of linearly elastic fracture mechanics implies the sequential rupture of atomic bonds leads to fracture propagation (Lawn, 1975), which does not comply with the common observation that real crack surface of rock was generated from the fracture process zone (FPZ) (i.e. microcracking zone) (Xing et al., 2019). Furthermore, the assumption of cohesive crack model indicates the FPZ is simplified as an effective crack (Barenblatt, 1959; Dugdale, 1960). Although the cohesive crack model can characterize the FPZ length (Zhang et al., 2018), it cannot delineate the FPZ width. Thus, the microcrack zone surrounding hydraulic fracture is neglected in the simulations based on cohesive crack model. In addition, both the linearly elastic fracture model and cohesive crack model are actually temperature-independent. Above all, the previous numerical simulations are insufficient for evaluating the effectiveness of EGS hydraulic fracturing. Therefore, the laboratory simulations may be more suitable for characterizing hydraulic fracturing at high temperatures at this stage, by supplying more references for field applications, modifications of theoretical models and improvements of numerical simulation. In recent years, the laboratory simulations and monitoring of truetriaxial hydraulic fracturing have been widely studied (Fan and Zhang, 2014; Zhou et al., 2017, 2018; Zhou et al., 2018a,b) at room temperatures, whereas there are limited experimental studies (Kumari et al., 2018) on high-temperature hydraulic fracturing. Kumari et al. (2018) simulated the high-temperature hydraulic fracturing on the cylindrical granite specimens with a diameter of 22.5 mm . The experiments were performed at 300 °C and pseudo-triaxial pressures, detected with high-resolution CT. At 300 °C, Kumari's experimental results show the fracture pressure is reduced, the intergranular and trans-granular microcracks are generated surrounding the wellbore, and the fracture width decreases adjacent to the wellbore. Kumari's investigation is significant for understanding the geometry of hydraulic fracture in high-temperature granite. Kumari's experiments (2018) with Granite specimens of 22.5 mm in diameter seem to give a reliable fracture initiation pressure, however much larger specimens and true reservoir stresses are needed to investigate the fracture propagation. Therefore, it is necessary to conduct high-temperature hydraulic fracturing on large-

Fig. 1. Specimen preparations: (a) photo of the specimen; (b) arrangements of AE sensor and wellbore. 476

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Finally, the hydraulic fracturing of the high-temperature specimen starts by injecting low-temperature fracturing fluid after the confining pressures are applied.

temperature hydraulic fracture, providing significant references for the optimizations of EGS hydraulic fracturing. 2. Experimental procedure

2.4. Acoustic emission (AE) monitoring 2.1. Specimen preparation 2.4.1. Parameters of the AE system Acoustic emission (AE) is defined as high-frequency elastic waves emitted from microcrack (Ishida et al., 2017), which takes place when the stored strain energy released is over the critical threshold (Zhang et al., 2018). The threshold level was 33 dB . 10 AE sensors with a resonance of 150 kHz were employed to characterize the fracture propagation. All the AE signals were amplified with 40 dB gain by pre-amplifiers. The AE waveforms, with its length of 656 μs , were recorded by AE recording system at 5000 kHz sampling frequency. The P-wave velocities of Shandong granite at 20 °C and 120 °C were 3.8 mm/μs and 3.6 mm/μs .

In this study, the specimens were taken from Shandong China, with the following properties: Young's modulus 38~43 GPa , Poisson's ratio 0.2~0.3, and uniaxial compressive strength 102.9 MPa . Four groups of specimens were cut into cubes (Fig. 1) with the dimensions of 300 mm × 300 mm × 300 mm . A wellbore was drilled with a diameter of 25 mm and a depth of 160 mm . An open hole with a depth of 5 mm was prefabricated at the bottom of the wellbore. The stainlesssteel pipeline was bonded in the center of the wellbore with the heatresistant epoxide-resin glue. A water-swellable rubber ring with a thickness of 20 mm was placed at the pipeline outlet, to improve the wellbore water-proof at high temperatures and injection pressures. Ten circular holes (Fig. 1b) with a diameter of 20 mm and a depth of 25 mm were drilled for placing AE sensors as in Fig. 1b. AE sensors are so distributed in the specimen that all the AE events can be detected during hydraulic fracture propagating.

2.4.2. Determinations of AE sources and AE energy AE sources can be located by a group of synchronous waveforms (AE event) (Fig. 3) received by no less than four channels. The locating process can be described by nonlinear equations (Kurz et al., 2008) as in Eq. (1).

2.2. Experimental programs

ti = The experiments are divided into four groups in terms of temperatures and confining pressures, in Table 1. The CaCl2 solution is employed as the fracturing fluid, of low temperature (− 20 °C) and low viscosity (1 mPa⋅s ), and red dye is added for better observation. The experiments in Table 1 were performed with the large-scale physical simulation system of hydraulic fracturing (Fig. 2a) at high temperatures and under confining stresses. The system is consisted of an external heating furnace, an AE monitoring system, and a hydraulic fracturing simulation platform (machine model: 300R2) (Fig. 2b) with the modules of high temperatures and confining pressures.

(x − x i )2 + (y − yi )2 + (z − z i )2 / v + t0

(1)

where coordinates (x , y, z ) are the points (mm ) where AE events take place, coordinates (x i , yi , z i ) the sensor locations (mm ), t0 the occurrence time (μs ) of AE event, ti the onset time (μs ) received by each sensor and v the P-wave velocity (mm/μs ) of the material. For a single waveform, the area between the amplitude-time curve and the threshold value is identified as the AE energy (Fig. 3), which is related to fracture energy (Landis and Baillon, 2002; Zhang et al., 2018). Given the different attenuations of AE waveforms from the propagation distances of waves, the AE energy should be calibrated as follows: n

2.3. Heating of the high-temperature specimen

E ae =

n

∑ Eiae di/ ∑ di i=1

For high-temperature tests, the specimens were firstly heated in an external heating furnace to improve the heating efficiency. Since the heat transfer coefficient of granites is very low, the final temperature in the external heating furnace should be higher than the designed temperature (120 °C) as in Table 1, to make the internal temperature of the specimen reach 120 °C. The temperature in the external heating furnace was raised to 140 °C with a low heating rate of 1 °C/min to minimize the high temperature gradient. Considering the servo temperature control system and heat losses of the external heating furnace, the actual temperature inside the external heating furnace fluctuated between 125 °C and 140 °C at the setting temperature of 140 °C. All the high-temperature specimens in the external furnace were kept for 16 h (heating time is dependent on the applied equipment) at 140 °C, to make the internal temperature of the specimen reach 123 °C (measured with thermocouple prefabricated at the outlet of injection pipe). Note that heating the granite specimen with the external heating furnace will cause a temperature differential between the surface and interior of the specimen owing to the low heat transfer coefficient of granite. As the heating-induced temperature differential is measured to be about 2 °C, only 1.7% (2/120) of the designed temperature (120 °C), the propagation of hydraulic fracture is supposed to be slightly affected by the heating-induced temperature. Before hydraulic fracturing, the temperature in the heating chamber of 300R2 simulation system was maintained at 120 °C. Then the hightemperature specimen (123 °C ) was placed with AE sensors and transported to 300R2 simulation system, in which the temperature of the specimen is decreased by about 3 °C, while the specimen temperature was also about 120 °C.

(2)

i=1

Eiae

is the AE energy (mv⋅μs ) of a waveform, di the distance (mm ) where between the AE source and the AE sensor, n the number of the synchronous waveforms, and E ae the calibrated AE energy (mv⋅μs ). 2.4.3. AE-based fracture mechanisms In this study, the polarity of the initial P-wave motion was employed to identify the fracture mechanisms of microcracks. Based on the fault plane solution of granite (Lei et al., 1992; Zang et al., 1998; Benson et al., 2008), if most AE sensors receive compressive (negative polarity) or dilatational (positive polarity) first motions of AE waves, the AE events indicate the onsets of tensile microcracks and implosion/collapse microcracks. Otherwise, the AE events are generated from shear microcracks. The polarity value ( pol ) can be employed to identify the fracture mechanisms as expressed in Eq. (3). n

pol = (1/ n)

∑ sign (Api )

(3)

i=1

Table 1 Experimental programs.

477

Specimen

temperature (°C )

confining pressure (MPa)

injection rate (ml/ min)

1 2 3 4

20 120 20 120

0.1/0.1/0.1 0.1/0.1/0.1 10/25/30 10/25/30

2 2 2 2

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Fig. 2. The large-scale physical simulation system of hydraulic fracturing at high temperatures and stresses: (a) schematic diagram of the simulation system; (b) photo of 300R2 simulation platform.

under different confining pressures as in Table 1. Since the red dye is added into the fracturing fluid, the geometries of hydraulic fracture can be identified with the red-dyeing surfaces inside the granite specimens. As shown in Fig. 4, fracture geometries change barely at different temperatures. For specimens 1 (20 °C ) and 2 (120 °C) (confining pressure: 0.1/0.1/0.1 MPa ), the hydraulic fractures propagate along the directions with low strength (Fig. 4a and b). For specimen 3 (20 °C ) and specimen 4 (120 °C) (confining pressure: 10/25/30 MPa ), the hydraulic fractures propagated perpendicular to the minimum principal stress (Fig. 4c and d). The single geometries of hydraulic fracture at 120 °C are similar to that at 20 °C. The above experimental results show the effect of high temperatures on granite cannot be identified with hydraulic fracture geometries, though the previous investigations indicate both the microstructures (Darot et al., 1985; Balme et al., 2004) and combined water (Zhang et al., 2016) of granite are changed at high temperatures, and sudden thermal shock is generated when the cool fracturing fluid touches the high-temperature granite (Kumari et al., 2017), varying both the physical (Tian et al., 2016) and mechanical properties (Dwivedi et al., 2008) of granite. Hydraulic fracture at high temperatures should be studied further, as discussed in the next context.

Fig. 3. The schematic diagram: an AE event consisted by a group of synchronous waveforms.

4. Evolution characteristics of injection pressures and AE parameters

where pol is polarity value (unity) of AE event, Api is the first-pulse amplitudes (mv) of an AE event, n the number of the synchronous waveforms belong to an AE event. pol can be used to identifying the fracture mechanisms of AE events (Zang et al., 1998). − 0.25 ≤ pol ≤ 0.25 indicates shear (type-S) source, − 1 ≤ pol < − 0.25 indicates tensile (type-T) source, and 0.25 < pol < 1 indicates implosion/collapse (type-C) source. Since the hydraulic fracture propagates across the completely developed FPZ (microcrack zone), a microcrack-band is supposed to be generated surrounding the hydraulic fracture. As the shear microcracks do not close completely even though the net pressure vanishes, for microcrack-band, both the width and the proportion of type-S microcracks are correlated to the stimulated reservoir volume. Therefore, the effectiveness of hydraulic fracture at different temperatures involves two indicators (characteristics): the microcrack-band width and the proportion of type-S microcracks. The above two indicators were characterized with the spatial distribution of AE energy and AE source mechanisms, presented in the next context.

As mentioned above, the geometries of hydraulic fracture at 120 °C are similar to that at 20 °C, in which hydraulic fracture at high temperatures should be investigated further in terms of microcrack distributions. In this section, the AE parameters and the injection pressure are combined to characterize the propagation of hydraulic fracture. Injection pressure curves present the global response of hydraulic fracture propagation, whereas AE events indicate the local responses generated from microcracks surrounding the hydraulic fracture. Accumulated AE energy and the frequency of AE events are reported to be related to energy dissipation (Vidya Sagar and Raghu Prasad, 2009; Zhang et al., 2018) and microcracks nucleation/coalescence (Ohnaka and Mogi, 1982), respectively. Therefore, the integrated analysis of the injection pressure, AE energy and AE frequency can reveal the propagation characteristics of hydraulic fracture more comprehensively. 4.1. Responses of injection pressures and AE corresponding to hydraulic fracture propagation

3. Geometries of high-temperature hydraulic fractures

4.1.1. Features of hydraulic fracture propagation at high temperatures At different temperatures, the large fluctuation of the injection pressure curves corresponds with the evolution characteristics of AE

The hydraulic fracturing was tested at different temperatures and 478

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Fig. 4. The hydraulic fracture surfaces affected by temperatures and confining pressures: (a) the fracture surface generated at 20 °C and under 0.1/0.1/0.1 MPa ; (b) the fracture surface generated at 120 °C and under 0.1/0.1/0.1 MPa ; (c) the fracture surface generated at 20 °C and under 10/25/30 MPa; (d) the fracture surface generated at 120 °C and under 10/25/30 MPa .

Fig. 5. Injection pressure curves and AE parameters affected by temperatures and confining pressures: (a) the experiment at 20 °C and under 0.1/0.1/0.1 MPa ; (b) the experiment at 120 °C and under 0.1/0.1/0.1 MPa ; (c) the experiment at 20 °C and under 10/25/30 MPa ; (d) the experiment at 120 °C and under 10/25/30 MPa . 479

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parameters, as in Fig. 5. When the injection pressure curve fluctuates drastically (pressure decreases sharply), the accumulated AE energy increases sharply, and AE events are rapidly generated with a wide bandwidth of average frequency (45 kHz ∼ 240 kHz). The rapid increment of accumulated AE energy indicates the dramatic growth of dissipated energy during granite fracturing. If the dissipated energy per unit length reaches fracture energy, the real fracture surface is generated along the propagation direction (Barenblatt, 1959; Dugdale, 1960; Zhang et al., 2018). The high-frequency AE events indicate the onsets of microcracks, and the low-frequency AE events are generated from the coalescence of microcracks (Ohnaka and Mogi, 1982). Combining the physical mechanisms of AE energy and frequency, we can obtain the features of hydraulic fracture propagation in granite of high temperatures: the remarkable fluctuations of injection pressures, the sharp increment of accumulated AE energy and the wide bandwidth of AE frequency.

4.2. Fracture pressures and resistances of high-temperature hydraulic fracture When the fracture pressure is reached during the injection (hydraulic fracture initiation), the competition between the driving force and the fracture growth resistance determines hydraulic fracture propagation rate, i.e. the stable or unstable propagation of a fracture, which is fundamental to the effective stimulation volume of hydraulic fractures (Zhang et al., 2015). For high-temperature hydraulic fracturing, temperatures affect the intensity between the driving force (injection pressure in Fig. 5) and the fracture growth resistance (such as temperature-dependent fracture energies). In this section, the abrupt fluctuations of the injection pressures and the reasons are discussed. 4.2.1. Characteristics The injection pressure curves at 120 °C are significantly different from that at 20 °C as in Fig. 5. The fracture pressures are larger at 120 °C than at 20 °C by 3.6 ∼ 4.9 MPa , and two peaks are present along the injection pressure curves. The two features of injection pressures reveal two mechanical characteristics of high-temperature hydraulic fracturing. (1) The increment of fracture pressures (at 120 °C) indicate high temperatures increase the tensile strength of granite, which is consistent with the previous investigations (Zhang et al., 2016). (2) The two peaks of injection pressures (at 120 °C) imply the increments of fracture propagation resistances, making the high-temperature hydraulic fracture propagate intermittently. When the second peak appears on the injection pressure curve, the hydraulic fracture has propagated by a certain distance. Since the net pressure of the second peak is almost the same as the first peak, the propagation resistance corresponding to the second pressure peak is enhanced based on the fracture mechanics (Bažant, 2002).

4.1.2. Identification of induced microcrack-band The above features also imply the hydraulic fracture propagates by interconnecting massive microcracks in FPZ ahead of the fracture tip, which provides a basis for identifying the generation of microcrackband surrounding hydraulic fracture. Since the onsets of microcracks are energy-dissipative (Otsuka and Date, 2000), the real hydraulic fracture is generated if the dissipated energy in FPZ per unit length reaches fracture energy. As depicted in the region M of Fig. 5, a sharp drop of injection pressure after the pressure peak corresponds to a large accumulation of AE energy and a wide bandwidth of AE frequency, which indicates the hydraulic fracture rapidly propagates across the FPZ, with the generation of microcrack-band. The spatial distribution of AE energy in region M can be employed to characterize the microcrack-band surrounding hydraulic fracture, which will be discussed in the next context.

Fig. 6. Spatial distributions of AE energy and source mechanisms affected by temperatures and confining pressures: (a) the experiment at 20 °C and under 0.1/0.1/0.1 MPa ; (b) the experiment at 120 °C and under 0.1/0.1/0.1 MPa ; (c) the experiment at 20 °C and under 10/25/30 MPa ; (d) the experiment at 120 °C and under 10/25/30 MPa . 480

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dissipated energy along the length and width of the microcrack-band. Since the hydraulic fracture is generated from an energy-dissipative FPZ (Zhang et al., 2018), the dissipated energy along the length of microcrack-band indirectly characterizes the resistance variations during fracture propagation. Moreover, the dissipated energy along the width of microcrack-band delineates the effective width of microcrackband, i.e. the volume of microcrack-band.

4.2.2. Mechanisms As mentioned above, at high temperatures, fracture pressures are increased and the hydraulic fracture propagates intermittently with increasing resistances, which is derived from two mechanisms. (1) Due to the escapement of weak bound water (Zhang et al., 2016) and the expansion of mineral components at 120 °C (Darot et al., 1985; Balme et al., 2004), both the original and new microcracks are closed leading to the enhancement of frictions on microcrack surfaces (Darot et al., 1985; Balme et al., 2004), which make the microcracks hard to be interconnected. Therefore, the fracture pressure of high-temperature granite is increased. (2) The temperature of fracturing fluid increases with the propagation of hydraulic fracture at high temperature, which decreases the transient-ΔT between fracturing fluid and high-temperature granite. Since the transient-ΔT destroys granite by thermal stresses (Zhang et al., 2015) and the deformation-mismatching of different mineral components (Förster et al., 1996; Krupka et al., 1999), with the gradually diminishing of ΔT during the propagation of hydraulic fracture, hydraulic fracture will be arrested (fracture propagation resistance is increased), if the net pressure is not improved in time.

5.2.2. Effect of high temperatures on fracture energy in microcrack-band The AE energy histograms in Fig. 6 reveal the distributions of dissipated energy during hydraulic fracture propagation. Based on the cohesive crack model (Barenblatt, 1959; Dugdale, 1960), the FPZ of rock-like materials can be regarded as a cohesive crack. The cohesive crack presents a softening behave due to the onsets of massive microcracks in FPZ. If the accumulated dissipated energy per unit length of FPZ (cohesive crack) reaches fracture energy, a real fracture surface is generated. When the hydraulic fracture is completely generated, the accumulated AE energy along the fracture propagation path can be employed to characterize the variations of fracture energy in microcrack-band. As shown in Fig. 6b and d, high-temperature significantly influences the distribution of accumulated AE energy along the hydraulic fracture. There is a low AE energy zone in the microcrack-band adjacent to the wellbore at 120 °C. More specifically, the fracture energy of high-temperature granite adjacent to the wellbore is much lower than that far from the wellbore. For high-temperature specimen-2 (temperature: 120 °C; confining pressure: 0.1/0.1/0.1 MPa ), the length of low AE energy zone is 75 mm . If we regarded the injection outlet as an origin (Fig. 6b), the normalized accumulated AE energy is about 0.13 in the range of 0–75 mm along the length of microcrack-band, whereas the normalized accumulated AE energy is about 0.52 in the range of 75–175 mm. The fracture energy in the low-AE-energy zone (0–75 mm) is about 0.25 of that in microcrack-band (75~175 mm ) away from the wellbore. For high-temperature specimen-4 (temperature: 120 °C; confining pressure: 10/25/30 MPa ), similarly, the fracture energy in the low-AE-energy zone (0–40 mm) is 0.2 times as large as that in the microcrack-band (40–125 mm) away from the wellbore. The low fracture-energy zone is supposed to be generated by the transient-ΔT . Since the temperatures of high-temperature granite and fracturing fluid are 120 °C and − 20 °C, the transient-ΔT (140 °C) is generated when the cold fluid is initially injected into the high-temperature granite. Under transient-ΔT , the granite is destroyed by creating microcracks due to the mismatching of the coefficients of thermal expansion (CTE) (Förster et al., 1996; Krupka et al., 1999). As the onsets of microcracks are energy-dissipative (Le et al., 2011), the total dissipated energy in region M (Fig. 5) is decreased, implying the transient-ΔT reduces the granite fracture energy of generating a real fracture (per unit length). Since the temperature of fracturing fluid is increased with hydraulic fracture propagation, the ΔT between hightemperature granite and the fracturing fluid is gradually diminished, suppressing the reduction of fracture energy along the fracture propagation path. Therefore, at high temperatures (120 °C), the transition from low-fracture-energy zone to high-fracture-energy zone indicates the increased resistance along microcrack band, which is consistent with the two-peaks injection pressures in Fig. 5.

5. Spatial distribution characteristics of AE energy As mentioned above, at different temperatures, hydraulic fractures propagate rapidly when the injection pressures sharply decrease after pressure peak. AE events are generated during the fracturing processes with a remarkable accumulation of AE energy in region M (Fig. 5). Since rock fracturing includes the generation of FPZ consisted of massive microcracks, AE events in region M (Fig. 5) are supposed to be released from the microcrack-band surrounding hydraulic fracture. In this section, the spatial distributions of AE energy are employed to characterize the microcrack-band surrounding hydraulic fractures, further delineating the prominent characteristics of hydraulic fracture at high temperatures. 5.1. Characterization of microcrack-band surrounding hydraulic fracture The coordinates (i.e. AE sources) of all the AE events in Fig. 5 are obtained with Eq. (1). Each AE source in region M (Fig. 5) involves AE energy calibrated with Eq. (2). Since the maximum and minimum AE energy differ by 3~4 orders of magnitude, the spatial distributions of AE energy are presented in Fig. 6, with the point sizes proportional to the logarithm of AE energy. The AE events are projected on the bottom plane orthogonal to the fracture surfaces, delineating the microcrack-band surrounding hydraulic fracture. The microcrack-bands are clearly identified as in Fig. 6. For example, at the temperature of 120 °C and under the confining pressure of 0.1/0.1/0.1 MPa (specimen 2), almost all AE events expressed as dissipated energy in region M of Fig. 5b populate in a narrow band as shown in Fig. 6b. Since AE energy is related to dissipated energy (Vidya Sagar and Raghu Prasad, 2009; Landis and Baillon, 2002; Zhang et al., 2018) generated from the onsets of microcracks during granite fracturing, the energy-based AE narrow band (Fig. 6b) indicates the spatial distribution of dissipated energy (i.e. the microcrack-band surrounding hydraulic fracture). As shown in Fig. 6, the energy-based AE narrow band is symmetrically distributed on both sides of the real hydraulic fracture, which validates the microcrackband characterized by AE energy.

5.2.3. Effect of high temperatures on the width of microcrack-band Since the previous investigations show the density of microcracks is positively correlated to rock permeability (Simpson et al., 2001), for the same hydraulic fracture length, the wider the microcrack-band is, the larger the stimulated volumes of granite formation are. Therefore, the width of microcrack-band can be seemed as an indicator for hydraulic fracturing effectiveness at different temperatures. The effective width of microcrack-band can be identified with the AE energy histograms. As shown in the AE energy histograms along the width of microcrack-band (Fig. 6), AE energy is concentrated at a small

5.2. Characteristics of microcrack-band surrounding high-temperature hydraulic fracture 5.2.1. Characterization of AE energy distributed along the length and width of microcrack-band As in Fig. 6, the AE energy was summed in each 5 mm along the length and width of the microcrack-band. Then the normalized energy histograms can be obtained, which present the distributions of 481

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decreased from 65% to 57%, with a reduction of 12.3%. Above all, the high-temperature (120 °C) reduced the proportion of shear microcracks by 6.3%~12.3% in the microcrack-band surrounding the hydraulic fracture.

zone, although AE events populated in a much larger one. Therefore, the effective width of microcrack-band can be obtained from AE energy concentration zone. Experimental results show the effective width of the high-temperature microcrack-band is remarkably narrower than that at room temperature. When the temperature of granite is 120 °C (Fig. 6b and d), the effective widths of specimen-2 and specimen-4 are 41 mm and 60 mm , respectively. And when the temperature of granite is 20 °C (Fig. 6a and c), the effective widths of specimen-1 and specimen-3 are 94 mm and 100 mm , respectively. The above results reveal high-temperature (120 °C) reduces the effective width of the microcrack-band surrounding hydraulic fracture by 40~56.4%. Transient-ΔT is a prominent characteristic of high-temperature hydraulic fracturing (Dahi Taleghani et al., 2014; Zhang et al., 2015). Based on the experimental programs and results at high temperatures, the width decrement of the microcrack-band is supposed to be changed by transient-ΔT . Based on the AE energy histograms (Fig. 6) along the width direction at 120 °C, the dissipated energy is concentrated more prominently on the fracture propagation path than that at 20 °C. The microcrack-band is generated from the FPZ where hydraulic fracture propagates. At high temperatures, since transient-ΔT distributes predominantly along the hydraulic fracture, the damage adjacent to the symmetry axis of FPZ is extremely high although the FPZ is narrower than that at room temperatures. As long as the accumulated dissipated per unit FPZ-length reaches the fracture energy, the real fracture surface is generated (Barenblatt, 1959; Dugdale, 1960; Zhang et al., 2018). Therefore, the width of microcrack-band decreased at high temperatures. Given the decreased width of microcrack-band surrounding hydraulic fracture at high temperatures, the effective stimulated volume of high-temperature hydraulic fracture is decreased for the same hydraulic fracture length.

6.2. Effect of high temperature on fracture mechanisms of hydraulic fracture Figs. 6 and 7 present high-temperature reduces the proportion of shear microcracks in the microcrack-band, decreasing the hydraulic fracturing effectiveness of granite reservoir. Due to the incoordinate shrinkages of different mineral components under transient-ΔT , tensile and compressive deformations are generated at the boundaries and in the interiors of mineral components (Förster et al., 1996; Krupka et al., 1999), which is supposed to reduce the proportions of shear-type microcracks. A surface deformation test under transient-ΔT was performed on a granite slice to validate the effect of transient-ΔT on fracture mechanisms in microcrack-band, expounded in the next paragraph. The granite slice was firstly heated to 240 °C and suddenly cooled with low-temperature (0 °C) water, to highlight the effect of transientΔT on granite fracture mechanisms. The surface deformations of hightemperature granite were characterized with the digital image correlation (DIC). DIC is a particle tracking method for determining particle displacement in digital images (Zhang et al., 2018). The surface strains (Fig. 8) were extracted by matching a small zone between the searching areas of original (before cooling) and deformed images (cooling lasted 2 s), with the normalized cross correlation method (Blaber et al., 2015). Fig. 8 depicts the horizontal and vertical strain fields of granite slice under transient-ΔT . The uncoordinated strains caused by CTE-mismatching are mainly ranged from −0.01 to 0.01. For any point on the granite surface, if both the horizontal and vertical strains are tensile or compressive, the deformation is regarded as type-T and type-C, respectively. Otherwise, the DIC-based deformation is regarded as type-S (i.e. the signs of horizontal and vertical strains are opposite). By counting the horizontal and vertical strains of all the pixels in the strain fields (Fig. 8), the shear deformations account for 48% whereas the others account for 52%. The proportion of shear sources in Fig. 8 (ΔT = 240 °C) is much lower than that in Fig. 7 (ΔT = 120 °C). Fig. 8 indicates the ratio of shear microcracks is decreased with the increment of transient-ΔT . The tensile and compressive microcracks are more remarkably generated by the incoordinate shrinkages of different mineral components under transient-ΔT , which reduces the proportion of shear-type sources. Above all, the proportion of shear microcracks in the microcrack-

6. Fracture mechanisms of high-temperature hydraulic fracture The above investigations show the microcrack-band width is a significant indicator for hydraulic fracturing effectiveness at different temperatures. The proportion of shear microcracks in microcrack-band is another indicator for hydraulic fracturing effectiveness. In dry hot rock reservoirs, shear cracks hold open without the supports of the fracturing fluid (Pine and Batchelor, 1984; Rutqvist et al., 2015; Rinaldi et al., 2015), whereas the tensile cracks will be closed. Therefore, the proportion of shear microcracks in the microcrack-band is fundamental to the fracturing effectiveness of EGS reservoir, which can be identified with AE-based source mechanisms. 6.1. Effect of high temperatures on fracture mechanisms in microcrack-band The previous studies show AE source mechanisms indicate the microcrack fracture mechanisms (Zang et al., 1998; Ishida, 2001). As mentioned above, the polarity of the initial P-wave motion was employed to identify the source mechanisms (Eq. (3)). The spatial distributions of AE source mechanisms are presented in Fig. 6, with different markers indicating type-S, type-T and type-C sources. As the proportion of shear microcracks in microcrack-band is an indicator for hydraulic fracturing effectiveness, the proportions of shear microcracks at different temperatures are presented as in Fig. 7. As shown in Fig. 7, shear microcracks occupy the highest proportion in the microcrack-band. The proportions of shear-type microcracks range from 57% to 65%, whereas the proportions of shear-type microcracks obviously changed at different temperatures. For example, under the confining pressure of 0.1/0.1/0.1 MPa , when the temperature is increased from 20 °C to 120 °C, the proportion of shear-type microcracks decreased from 64% to 60%, with a reduction of 6.3%. At the confining pressure of 10/25/30 MPa , when the temperature is increased from 20 °C to 120 °C, the proportion of shear-type microcracks

Fig. 7. The statistical diagram of fracture mechanisms in microcrack-band affected by temperatures and confining pressures (CPa: confining pressure). 482

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Fig. 8. The surface strain fields of a high-temperature granite slice under transient-ΔT (240 °C ): (a) surface strain field in horizontal direction; (b) surface strain field in vertical direction.

band. The effective width of the microcrack-band is reduced by 40~56.4% at 120 °C, which indicates high temperatures decrease the effective stimulated volume of hydraulic fracturing. (4) Based on the fracture mechanism in the microcrack-band, the proportion of shear microcracks is decreased by 6~12% at 120 °C, implying high temperatures reduce the effectiveness of hydraulic fracturing in granite. (5) The optimizations of EGS hydraulic fracturing were proposed. The net pressure should be enhanced in real-time with fracture propagation to avoid fracturing arrest. ΔT should be reduced between high-temperature granite and fracturing fluid to improve the effectiveness of the hydraulic fracture. The proppants are suggested to be appropriately added into the fracturing fluid to prevent the further reduction of fracturing effectiveness, avoiding fracture closures.

band is prominently decreased at high temperatures, which indicates the hydraulic fracturing effectiveness of granite reservoir is reduced at high temperatures. 7. Discussion: optimizations of EGS hydraulic fracturing The above investigations show injection pressures, spatial distribution of dissipated energy, the width of microcrack-band, and fracture mechanisms in microcrack-band consisted the prominent characteristics of high-temperature hydraulic fracturing. At high temperatures, the hydraulic fracture propagates intermittently with an increased resistance. The effective width of the microcrack-band is reduced by 40~56.4%, and fracture energy in the microcrack-band is obviously low adjacent to the wellbore. The proportion of shear microcracks is reduced by 6~12% in the microcrack-band. The above characteristics indicate the effectiveness of hydraulic fracturing in granite reservoir is reduced at high temperatures. Given the characteristics of high-temperature hydraulic fracture, optimizations of hydraulic fracturing in EGS reservoir are proposed. (1) The net pressure should be enhanced in real-time with the increment of fracture length, to avoid fracturing arrest. (2) If the injection of fracturing fluid is used to propagate a single hydraulic fracture, the ΔT between high-temperature granite and the fracturing fluid is suggested to be reduced, to improve the stimulated reservoir volume of the hydraulic fracture. (3) The proppant is suggested to be appropriately added into the fracturing fluid to prevent the further reduction of the effective stimulation volumes, such as hydraulic fracture closures.

Acknowledgments

8. Conclusions

References

In this study, four groups of cube granite specimen (dimensions: 300/300/300 mm ) were tested to investigate the characteristics of hydraulic fracture, for confining pressures (0.1/0.1/0.1 MPa and 10/25/30 MPa ) and temperatures (20 °C and 120 °C). Acoustic emission (AE) was employed to characterize the hydraulic fracturing processes. The main findings are summarized as follows:

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This work was financially supported by the National Natural Science Foundation of China (grant number 51774299), and National Science and Technology Major Projects of China (grant numbers 2016ZX05050, 2017ZX05069). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.03.050.

(1) The fracture geometries are almost unchanged at different temperatures, whereas the injection pressure curves and AE characteristics are significantly changed at different temperatures. (2) At 120 °C, the injection pressures present two peaks and fracture pressures are increased by 3.6 ∼ 4.9 MPa , which indicates the hydraulic fracture propagates intermittently with an increased resistance. (3) The spatial distributions of AE energy characterize a microcrackband surrounding the hydraulic fracture. At high-temperature (120 °C), the fracture energy is reduced by about 75% adjacent to the wellbore (about 40% of the fracture length) in the microcrack483

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