Influence of different thermal cycling treatments on the physical, mechanical and transport properties of granite

Geothermics 78 (2019) 118–128

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Influence of different thermal cycling treatments on the physical, mechanical and transport properties of granite

T

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Peihua Jina,b, Yaoqing Hua,b, , Jixi Shaoa,b, Guokai Zhaoa,b, Xiaozhou Zhua,b, Chun Lia,b a b

Institute of Mining Technology, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China Key Laboratory of In-situ Property Improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Granite Enhanced geothermal system High temperature Thermal cycling Thermal gradient cracking

Two different types of thermal cycling treatments (slow heating followed by slow cooling or rapid quenching) were carried out on the granite samples in the present research. The responses of physical, mechanical and transport properties of the granite after various thermal cycling treatments were compared and analyzed. Nondestructive tests (mass, volume, longitudinal wave propagation and permeability) and destructive tests (uniaxial compression and Brazilian splitting) were utilized to characterize these properties. Variations of all the properties demonstrate that thermal cycling (heating and cooling) can induce damage in the samples. Mass, density, P-wave velocity, UCS, Young’s modulus and tensile strength decrease, whilst volume and permeability increase with rising temperature. Additionally, the results also demonstrate the importance of cooling method. Rapid quenching has a more significant effect on granite samples, showing lower density, P-wave velocity and strength, but greater permeability, which is due to the generation of thermal gradient cracks. Moreover, the difference between P-wave velocity and UCS of slow cooling and those of rapid quenching increases with temperature up to 500 °C, which can be attributed to the more significant thermal shock cracking induced by higher temperaturegradient stress in the samples during rapid quenching. The microstructural analyses from thin sections show that microcracking in granite subjected to thermal cycling treatments was induced in different severity depending on peak temperature and cooling method.

1. Introduction Different mechanisms have been suggested to characterize the thermal cracking process in granite subjected to high temperature: (1) the thermal expansion mismatch between adjacent minerals in the rock, (2) thermal expansion anisotropy within individual minerals, and (3) thermal gradient (Richter and Simmons, 1974; Yong and Wang, 1980; Fredrich and Wong, 1986). And thus two different types of microcracks can be generated due to the thermal loading: thermal cycling cracks and thermal gradient cracks (Jansen et al., 1993; Zhao, 2015). The thermal cycling cracks form due to the thermal stress in a homogeneous temperature field, while the thermal gradient cracks in an inhomogeneous temperature field. Initiation of microcracks and propagation of existing cracks occurs when the local thermal stress induced by both temperature field exceed the local strength of rock (Kranz, 1983). The thermal cycling cracks can significantly affect the physical, mechanical and permeability behavior of rock (Darot et al., 1992; Glover et al., 1995; David et al., 1999), which may have crucial implications for the successful characterization of deep geothermal

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reservoirs and for the correct numerical simulation of enhanced geothermal systems (MIT, 2006; Ghassemi, 2012; Shao et al., 2015). However, during the development and operation process of Enhanced Geothermal Systems (EGS), one will often encounter many situations where thermal gradient cracks may occur. For example, while drilling wellbores in high-temperature rocks, the surrounding rocks along the borehole would cool down by drilling-mud circulation leading to temperature and stress redistribution around the borehole which may cause new fractures or even borehole failure (Bérard and Cornet, 2003; Shao et al., 2014; Zhao et al., 2015b; Siratovich et al., 2016). During the long-term heat extraction, the surface of a primary hydraulic fracture is cooled by the cycling fluid and induces thermal contraction, which can in turn result in fracture propagation or secondary cracking, thus increasing the permeability of the geothermal reservoir (Kohl et al., 1995; Ghassemi et al., 2007; Zhao et al., 2008; Safari and Ghassemi, 2015; Ghassemi and Tao, 2016; Kamali-Asl et al., 2018). Consequently, thermal stimulation has been suggested as an effective way to enhance reservoir permeability (Siratovich et al., 2011; Tarasovs and Ghassemi, 2012; Ghassemi and Tarasovs, 2015).

Corresponding author at: Institute of Mining Technology, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China. E-mail address: [email protected] (Y. Hu).

https://doi.org/10.1016/j.geothermics.2018.12.008 Received 29 March 2018; Received in revised form 5 December 2018; Accepted 10 December 2018 0375-6505/ © 2018 Elsevier Ltd. All rights reserved.

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temperature, which varied from 100 to 600 °C, at intervals of 100 °C. The heating rate was controlled at 2 °C/min in order to avoid temperature gradients and thermal shock. After the peak temperature was reached, it was held constant for more than 2 h. Finally, specimens used for rapid cooling were taken out of the furnace and dipped into a water tank (∼20 °C) until the room temperature was reached. The average cooling rate was about 100 °C/min during rapid quenching. The rest of the specimens were left in the furnace to undergo slow cooling at a rate of 1–2 °C/min.

So far, plentiful studies in the literature have been emphasized on the influence of thermal cycling cracks on rock properties as ultrasonic wave velocity, uniaxial compressive strength (UCS), Young’s modulus, tensile strength, fracture toughness, permeability and thermal properties (Heuze, 1983; Zhao et al., 2012; Wang et al., 2013; Becattini et al., 2017; Griffiths et al., 2017; Kumari et al., 2017b; Yang et al., 2017a,b; Zhao et al., 2017; Griffiths et al., 2018). For example, Nasseri et al. (2007) evaluated the effect of thermal damage on the mode I fracture toughness of granite. Griffiths et al. (2017) investigated the influence of thermal cycling microcrack characteristics on the stiffness and strength of rock quantitatively. Kant et al. (2017) reported the thermal diffusivity, heat capacity and thermal conductivity of Central Aare granite subjected to thermal cycling temperature range from 25 to 500 °C. Combined with optical microscopic observation and X-ray micro CT scanning, Yang et al. (2017a) analyzed the thermal damage characteristics of granite after exposure to high temperature from 25 to 800 °C. However, only a few studies attempted to investigate the effect of thermal gradient cracks on the physical and mechanical properties of rocks (Lam dos Santos et al., 2011; Brotóns et al., 2013; Kim et al., 2013; Shao et al., 2014; Siratovich et al., 2015; Zhao, 2015; Kumari et al., 2017a; Heap et al., 2018a). Shao et al. (2014) carried out unconfined compressive tests to examine the effect of different cooling rates on the mechanical behavior of granite with different grain sizes. Kim et al. (2013) conducted a series of experiments to investigate the influence of rapid thermal cooling on the mechanical properties of various rock samples. The results demonstrated that crack growth occurred in some rock types (granite and diabase with ore veins) while crack healing occurred in other rock types (diabase without ore veins, quartzite, and skarn). Zhao (2015) used particle mechanics method to simulate the thermally induced micro- and macrocracks in granite and concluded that macrocracks induced by thermal gradient generally develop from relatively cool regions towards relatively warm regions. A recent study of Heap et al. (2018a) compared changes in physical properties and strength of andesite due to slow- and shock-cooling and found that slow- and shock-cooling did not measurably change strength, connected porosity, or permeability of a microcracked andesite. Furthermore, the effects of the thermal gradient cracks on the transport properties of rock were rarely reported in the literature (Siratovich et al., 2015; Kumari et al., 2018). To investigate the effect of thermal gradient cracks on granite, two different types of thermal cycling treatments (i.e., slow heating followed by slow cooling or rapid quenching) were carried out on the granite samples in this study. Slow cooling leading to homogeneous temperature distribution may cause thermal cycling cracks, while rapid quenching would further induce more thermal gradient cracks in the samples. The paper presents a comparative analysis of the results of physical, mechanical and permeability tests of granite subjected to different thermal cycling treatment. In addition, the development of microcracks within the samples was studied by optical microscope observation and the thermal deterioration mechanisms within the samples after different thermal cycling treatments were revealed.

2.2. Physical property tests Four basic physical properties, including size, mass, density and Pwave velocities of all cylindrical cores were measured before and after thermal cycling treatments. Note that all the thermal-cycling specimens were dried in air for at least 48 h before mass measurement. The P-wave velocity (Vp ) measurements were taken along the sample axis using an NM-4B non-metallic ultrasonic detection analyzer. Some vaseline was daubed between the transducers and sample surface to provide a good acoustic coupling. And a small load of about 5 N was applied on the sample for each measurement to improve the contact and reproducibility. 2.3. Mechanical property tests The uniaxial compression and Brazilian splitting tests were conducted using an electric-hydraulic servo-controlled testing machine. Cylindrical cores of 50 mm in diameter by 100 mm long (resulting in a length-diameter ratio of 2:1) were prepared for uniaxial compression test and discs 50 mm in diameter by 25 mm thick for the Brazilian splitting test, which is in accordance with the ISRM suggested method (ISRM, 2007). The samples subjected to different thermal cycling treatments were deformed to failure at a constant strain rate of 10−5 s−1 for both tests. The Young’s modulus was determined from the slope of the more-or-less straight line portion of the stress-strain curve for each uniaxial compression experiment. 2.4. Permeability tests The pressure pulse decay method is used to conduct the permeability tests. The permeability tests were performed using a Smart Perm III Ultra-Low Permeameter using Nitrogen gas as the pore fluid. In a typical permeability test, the axial and confining pressures were controlled at a low value (i.e., 5 MPa) to avoid mechanical damage to the specimens. The initial pore pressure was 2.5 MPa, and the samples were left to equilibrate 1 h prior to permeability testing. The permeability of the specimens is calculated according to the following equations (Brace et al., 1968; Sander et al., 2017):

Pu (t ) − Pd (t ) = e−αt Pu (0) − Pd (0) k=

2. Experimental material and methods

αμLVu Vd APm (Vu + Vd )

(1)

(2)

where Pu (t ) − Pd (t ) is the pressure difference between the upstream and downstream cylinders at time t , Pu (0) − Pd (0) is the pressure difference between the upstream and downstream cylinders at the initial stage (∼0.25 MPa in this test), α is the slope of − ln (ΔP (t )/ ΔP0) versus time, which is calculated using the recorded upstream and downstream pressure data. Vu and Vd in Eq. (2) are the volumes of upstream and downstream cylinders, respectively. Pm is the mean pore pressure, Pm = (Pu (t ) + Pd (t ))/2 . L and A are the length and cross-sectional area of the specimen, respectively, μ is the dynamic viscosity of Nitrogen gas. For permeability determination, the Klinkenberg effect was evaluated carefully following the method reported in Heap et al. (2018b) and we found that the Klinkenberg effect can be negligible at a mean

2.1. Material characterization and thermal cycling procedure In this study, granite is selected as the studied material, which is a common reservoir rock type in EGS (Vidal and Genter, 2018). The rock material is collected from Shandong province in China. It is identified as a medium-fine grained monzonitic granite through microscopic thin section analysis. The modal composition of the granite is as follows: about 35% plagioclase, 40% k-feldspar, 20% quartz and 5% biotite. Slow heating and different cooling treatments were administered to the specimens according to the following procedure. Firstly, the specimens were placed in the muffle furnace and heated to the designated 119

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calculated by Eqs. (3) and (4) to characterize the mass and volume change degree.

pore pressure of about 2.3 MPa in our permeability tests. 2.5. Microstructural analyses Optical microscope observations (both cross and plane polarized lights) on thin section were conducted to study the development of microcracks within the specimens. Petrographic properties (mineralogical composition, texture and microstructure) can be identified from the optical micrographs (Freire-Lista et al., 2016; Liu et al., 2016; Kant et al., 2017). 30 ×20 mm thin slices with 5 mm thickness were prepared and subjected to the same thermal cycling procedure with cylindrical cores. And then thin sections 30 μm thick were prepared using low-speed diamond blade saw to prevent new cracks. Typically, thin sections subjected to 25, 200, 400 and 600 °C were selected for comparison and analysis. Both cross- and plane-polarized micrographs were obtained.

δm =

mb − ma × 100% mb

(3)

δV =

Va − Vb × 100% Vb

(4)

Where, mb , Vb , ma , Va are the mass and volume of a granite specimen before and after thermal cycling treatment, respectively. Density is an intrinsic property of rocks, therefore the actual value is adopted here to show its variation after thermal treatment. The actual value of bulk density may provide a good basis for the description of rock quality and thermal damage. Fig. 1 shows the relationship between δm and δV and temperature for different thermal cycling treatments. All the test results of each temperature and thermal cycling treatment were plotted in the figure, and lines going through the average value were kept to show the general trends of the data. As shown in Fig. 1(a), with temperature rising, the mass loss rate shows a gradual increasing trend, i.e., the mass of the granite after thermal cycling treatments decreases continuously. This decrease in mass can be attributed to the water loss in the granite samples. Different states of water will escape from the granite sample corresponding to different temperature ranges. Specifically, around 100 °C, the absorbed water would escape; in the range 100–400 °C, the bounded water and crystal water of minerals would escape; in the range

3. Results and analyses of physical tests 3.1. Mass, volume and bulk density The basic physical properties of granite specimens after different thermal cycling treatments are listed in Table 1. To investigate the influence of thermal cycling on the mass and volume of the granite, the mass loss rate (δm ) and volume expansion rate (δV ) were introduced and

Table 1 Physical properties of granite specimens before and after different thermal cycling treatments. Specimen

25-1 25-2 25-3 100-1s 100-2s 100-3s 200-1s 200-2s 200-3s 300-1s 300-2s 300-3s 400-1s 400-2s 400-3s 500-1s 500-2s 500-3s 600-1s 600-2s 600-3s 100-1r 100-2r 100-3r 200-1r 200-2r 200-3r 300-1r 300-2r 300-3r 400-1r 400-2r 400-3r 500-1r 500-2r 500-3r 600-1r 600-2r 600-3r

Before thermal treatment

After thermal treatment

m/g

L/mm

D/mm

ρ/(kg/m3)

VP/(m/s)

m/g

L/mm

D/mm

ρ/(kg/m3)

VP/(m/s)

495.09 505.65 509.04 523.33 529.94 523.12 517.24 528.67 526.14 534.46 534.66 501.67 515.14 493.04 508.30 515.82 521.38 490.03 479.99 508.67 515.49 503.99 514.17 499.16 505.15 511.86 525.66 527.90 522.11 498.77 513.05 516.84 523.45 494.24 525.71 511.54 521.24 516.07 525.69

100.52 101.6 101.88 100.52 101.14 99.86 99.51 100.96 100.42 102.01 102.95 100.21 102.78 102.08 102.12 101.48 100.06 101.36 101.61 99.05 101.58 100.54 102.86 101.92 101.34 100.34 101.72 101.02 102.06 99.70 101.88 101.12 101.75 100.02 101.77 101.12 101.62 102.34 102.22

48.85 48.98 49.05 50.16 50.20 50.18 50.12 50.32 50.24 50.31 50.14 49.25 49.25 48.48 49.11 49.51 50.14 48.30 47.99 49.82 49.55 49.22 49.04 48.58 49.05 49.78 49.98 50.22 49.72 49.32 49.46 49.78 49.99 48.94 49.88 49.45 49.74 49.34 49.90

2628 2641 2644 2635 2647 2649 2635 2633 2643 2636 2630 2628 2631 2617 2628 2640 2639 2639 2612 2634 2632 2635 2646 2642 2638 2621 2634 2638 2635 2619 2621 2626 2621 2627 2644 2634 2640 2637 2630

4137 4064 4059 4245 4159 4114 4013 4196 4184 4250 4168 4057 4144 4100 3958 4076 4118 3929 4023 3939 4022 4129 4164 4200 4140 4230 4156 4073 4032 4004 4049 4061 3944 4168 3997 3965 3939 4067 3962

– – – 523.24 529.77 522.97 517.13 528.53 525.98 534.17 534.43 501.25 514.60 492.58 507.81 515.26 520.87 489.47 478.98 507.85 514.60 503.91 514.07 499.05 504.99 511.77 525.57 527.75 521.97 498.64 512.84 516.61 523.16 493.83 525.24 511.12 520.60 515.38 525.02

– – – 100.56 101.25 99.95 99.68 101.06 100.42 102.08 102.99 100.36 102.9 102.24 102.26 101.76 100.22 101.62 102.08 99.66 102.12 100.54 102.95 101.94 101.40 100.42 101.84 101.18 102.12 99.86 102.02 101.22 101.78 100.39 102.13 101.42 102.44 103.22 103.23

– – – 50.16 50.26 50.22 50.14 50.36 50.34 50.32 50.18 49.28 49.28 48.52 49.12 49.82 50.24 48.50 48.19 50.14 49.88 49.25 49.16 48.70 49.12 49.81 50.04 50.28 49.78 49.38 49.54 49.79 50.06 49.10 50.06 49.64 50.14 49.78 50.46

– – – 2633 2637 2642 2627 2626 2632 2631 2624 2619 2622 2606 2621 2597 2622 2607 2573 2581 2579 2630 2630 2627 2628 2614 2624 2627 2626 2607 2608 2621 2612 2598 2613 2604 2574 2565 2543

– – – 4021 4046 4043 3713 3606 3678 3323 3268 3254 2705 2859 2702 2206 2264 2147 1341 1209 1174 3927 3852 3817 3581 3622 3620 2954 3038 2958 2327 2259 2302 1618 1598 1699 1088 1085 1085

Note: m – mass; D – Diameter; L – Length; ρ – Density; VP – P-wave velocity. The suffix “s” and “r” denote slow cooling and rapid quenching, respectively. 120

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Fig. 1. a) Mass loss rate versus temperature; (b) volume expansion rate versus temperature.

400–600 °C, the constitution water of mineral would escape (Sun et al., 2015; Zhang et al., 2016). The mass decreases by about 0.13% and 0.18% at 600 °C for two different thermal cycling treatments, respectively. It can also be seen from Fig. 1(a) that the mass loss rate of rapid quenching samples is apparently lower than that of slow cooling samples, which can be ascribed to the fact that the samples absorbed a small amount of water in the quenching process. The absorbed moisture in the samples was not absolutely removed during the drying process. The relationship between the volume expansion rate and temperature for two different thermal cycling treatments is depicted in Fig. 1(b). A gradual increase in volume expansion rate was noted for both thermal cycling treatments in the temperature range 100–400 °C. However, when the peak temperature was beyond 400 °C, a remarkable increase was observed. In the case of rapid quenching, a further rapid increase to about 2.8% by 600 °C can be seen. The volume expansion can be owing to the permanent thermal expansion strain (both axial and lateral) in granite samples subjected to thermal cycling, which is caused by microcracking in granite (Thirumalai and Demou, 1974). The samples failed to regain its original length and diameter (Table 1) even at the point where cooling was made to the initial temperature before heating (Lin, 2002). Compared to slow cooling, the volume expansion in rapid quenching samples is larger. It is believed that propagation of existing thermal cycling cracks and initiation of new thermal gradient cracks occurred during rapid quenching process. The variation of density is a comprehensive effect of mass loss and volume expansion. Fig.2 displays the relationship between density and temperature for two different thermal cycling treatments. A similar trend is observed for both cooling methods. Overall, the density

exhibits a downward trend with increasing temperature. A sharper reduction in density is observed after 500 °C. For slow cooling, the density is reduced by 2.3% at 600 °C, and there is a 2.9% reduction in the rapid quenching samples at 600 °C. The sharp reduction can be attributed to increased thermal microcracking across the alpha-beta phase transition of quartz (Glover et al., 1995). The rapid quenching samples are noted to have a lower density than the slow cooling ones, which suggests that more microcracks were introduced to the granite samples when cooling is realized by immersion in water.

Fig. 2. Density versus temperature.

Fig. 3. P-wave velocity versus temperature.

3.2. P-wave velocity P-wave velocity can provide an accurate evaluation of the thermal deterioration in granite exposed to high temperature (Freire-Lista et al., 2016; Inserra et al., 2013). Therefore the ultrasonic test has been widely used to characterize thermal cracking in rocks and quantify the overall damage degree (Chaki et al., 2008; Jansen et al., 1993; Reuschlé et al., 2006). Results of P-wave velocity for each thermal cycling granite sample are also presented in Table 1. The P-wave velocity data were collected prior to and after thermal cycling treatment. Average values and standard deviation of each set of measurement are calculated and plotted in Fig. 3. It can be seen from Fig. 3 that the P-wave velocities of the granite samples before heating are comparable (around 4000 m/s) and the standard deviations are low, indicating that the initial samples are similar in properties and intact. As shown in Fig. 3, in the slow cooling case, the granite samples exhibit a low initial reduction in P-wave velocity, 1.2% at 100 °C, and then a linear decrease until 69.6% by 600 °C. For rapid quenching, the

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Table 2 Mechanical properties and permeability of granite specimens after different thermal cycling treatments. Specimen

25-1 25-2 25-3 100-1 100-2 100-3 200-1 200-2 200-3 300-1 300-2 300-3 400-1 400-2 400-3 500-1 500-2 500-3 600-1 600-2 600-3

Slow cooling

Rapid quenching

σc /MPa

E/GPa

σt /MPa

k/(×10−18m2)

σc /MPa

E/GPa

σt /MPa

k/(×10−18m2)

128.66 130.29 131.38 135.48 126.50 140.33 126.52 115.67 112.61 106.14 131.13 98.29 116.64 95.44 104.55 85.88 96.02 97.43 67.02 43.43 64.19

12.94 12.60 12.89 12.60 12.45 13.84 12.30 11.50 11.97 10.65 12.31 10.81 11.45 10.53 11.15 10.99 10.74 10.62 6.24 5.48 7.65

7.10 8.77 7.93 7.61 7.85 7.41 7.51 7.31 7.35 7.04 6.25 7.25 6.29 6.70 6.38 3.34 4.22 4.63 1.66 2.07 2.78

1.117 1.172 1.097 0.568 0.699 0.439 0.591 0.857 0.725 0.846 1.097 0.961 2.145 2.289 2.353 3.115 2.164 5.892 40.489 47.908 35.488

– – – 120.73 124.68 133.01 109.57 122.73 97.56 112.25 98.32 95.28 85.93 91.68 81.68 39.51 76.82 95.28 64.41 39.40 43.61

– – – 11.32 11.13 13.85 10.35 12.11 11.49 12.04 10.31 9.88 9.54 10.52 10.21 4.57 8.75 8.77 5.76 4.24 3.71

– – – 6.70 7.28 5.88 6.53 6.43 6.38 5.61 4.63 5.22 4.83 4.79 5.14 3.40 3.98 3.14 1.45 1.63 2.18

– – – 0.681 0.810 0.584 0.823 0.695 0.797 1.903 1.110 2.682 4.477 4.501 4.063 18.131 18.241 18.935 91.786 89.419 90.544

Note: σc – uniaxial compressive strength; E – Young’s modulus; σt – tensile strength; k – permeability.

4.1. Stress-strain curves

relationship between P-wave velocity and thermal cycling temperature is approximately linear decreasing. However, at 600 °C, the P-wave velocity for rapid quenching declines by 73.4%, which demonstrates that the decrease is greater in the rapid quenching samples compared to slow cooling. This is a result of additional thermal gradient cracks generated during the quenching process. Additionally, differences in the P-wave velocity values between the two cooling methods gradually increase with temperature rising up to 500 °C. This difference can be attributed to the fact that with increasing temperature, the temperature gradient inside the granite specimen caused by rapid quenching becomes more significant. Consequently greater damage is introduced to the granite sample by the thermal stress induced by a higher thermal gradient in the sample.

Fig. 4 illustrates the representative stress-strain curves of granite after exposure to different thermal cycling treatments and different temperatures. It can be seen that the stress-strain curves of all the granite samples display a similar behavior. The curves exhibit a concave-upward shape from initial loading, which is mainly due to the closure of pre-existing microcracks (Walsh, 1965). For both cooling methods, the initial non-linear deformation stage becomes more and more pronounced as the temperature increases, especially at 600 °C. This can be attributed to more thermal cracking caused by higher temperature treatments (Yang et al., 2017a). Following the stage of microcrack closure, the samples deformed elastically, with the stress increasing approximately linearly with increasing strain. For granite samples slowly cooled from 25 to 600 °C and quenched from 25 to 500 °C, a sudden stress drop is observed following the peak stress. For granite samples quenched from 600 °C, however, the post-peak stress drop is more gradual (Fig. 4b). It should be noted that the stress-strain curves show that the failure modes of all the granite samples are brittle (Siratovich et al., 2016; Violay et al., 2017).

4. Results and analyses of mechanical tests The mechanical properties of granite specimens subjected to different thermal cycling treatments and various temperatures are summarized in Table 2.

Fig. 4. Representative stress-strain curves of granite samples (a) slow cooling; (b) rapid quenching. 122

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Fig. 5. UCS versus temperature for different thermal cycling treatments.

Fig. 6. Young’s modulus versus temperature for different thermal cycling treatments.

4.2. Uniaxial compressive strength The variation of UCS versus temperature for two different thermal cycling treatments is displayed in Fig. 5. In general, UCS decreases with the increasing temperature from 25 to 600 °C for both cycling treatments, which is consistent with the results obtained by Kumari et al. (2017a). A slight increase (3%) is observed up to 100 °C for the slow cooling samples, whereas a decreasing trend is observed for rapid quenching ones. Compared to slow cooling, the decrease of UCS is more remarkable for rapid quenching. For example, when the sample was subjected to 500 °C, the UCS is reduced to 72% for slow cooling and 54% for rapid quenching, compared to room temperature. When the temperature was elevated to 600 °C, UCS of the sample is reduced to 45% for slow cooling and 36% for rapid quenching, compared to room temperature. Such case is a product of far more microcracking in rapid quenching process, as discussed above. Moreover, it appears that the difference between UCS of slow cooling and that of rapid quenching increases with temperature up to 500 °C, which is in good agreement with the trend of P-wave velocity. This proves that the change of P-wave velocity can accurately reflect the thermal decay of UCS of granite. After 500 °C, UCS of slow cooling samples displays a sharp decrease, which is ascribed to the transition of quartz crystals from phase α to phase β at 573 °C (Glover et al., 1995).

Fig. 7. Tensile strength versus temperature for different thermal cycling treatments.

4.4. Tensile strength 4.3. Young’s modulus

Variations in the tensile strength of the granite samples with temperature after different thermal cycling treatments are plotted in Fig. 7. It can be seen that the tensile strength of two treatments both show a trend of gradual decrease with temperature increasing. Over the entire range of thermal cycling temperatures, the tensile strength of slow cooling samples was reduced by about 73% (from 7.93 MPa to 2.17 MPa) and the rapid quenching by about 78% (from 7.93 MPa to about 1.75 MPa). In the case of slow cooling, the tensile strength initially decreases slowly and then decreases sharply after 400 °C A similar trend was reported by Yin et al. (2015), who investigated the relationship between the tensile strength of Laurentian granite and treatment temperature and found this inflection point is 450 °C. This difference is due to the distinction of mineral composition. The tensile strength of rapid quenching samples shows a similar trend as slow cooling. However, from 25 to 600 °C, the rapid quenching samples all show lower tensile strength compared to the slow cooling ones. The tensile strength results also highlight the influence of cooling methods.

Fig. 6 shows Young’s modulus of the granite samples after different thermal cycling treatments at different temperatures. Similar to the results of UCS, the Young’s modulus shows gradual decreasing trends with rising temperature for both cooling regimes. A slight increase (1%) in Young’s modulus is observed at 100 °C for the slow cooling samples, which is in accordance with the trend of UCS. As shown in Fig. 6, reduction of Young’s modulus of the rapid quenching samples is more intense than that of the slow cooling samples at the same temperature. For instance, the Young’s modulus of slow cooling and rapid quenching samples at 600 °C decrease by 50% and 64%, respectively, compared to room temperature. For slow cooling samples, the Young’s modulus initially decreases slowly and then decreases sharply after 500 °C. While for rapid quenching samples, the transition temperature is 400 °C. The results demonstrate that rapid quenching accelerated the thermal deterioration in granite samples. The Young’s modulus results also give evidence of the generation of thermal gradient cracks (Walsh, 1965; David et al., 2012).

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The initial specimen (at room temperature) was included in Fig. 9 for comparison. As shown in Fig. 9(a, b), no visible microcracks can be observed in the granite at room temperature, which indicates that the granite specimen is a dense rock with low permeability. In the case of rapid quenching from 200 °C (Fig. 10a, b), intragranular microcrack through quartz and intergranular microcrack were observed, which was not observed under the same temperature and slow cooling conditions (Fig. 9c, d). Many visible microcracks appeared in the microstructure of slow-cooling specimen from 400 °C. In detail, intergranular microcrack between K-feldspar and plagioclase and intragranular microcrack inside K-feldspar were seen in Fig. 9e, f. In rapid quenching specimen from 400 °C (Fig. 10c, d), an obvious intragranular microcrack inside K-feldspar propagates and coalesce with other intergranular microcracks, forming cracks throughout the entire specimen, which may greatly enhance the permeability (Fig. 8). The amount of microcracks is significantly increased in slow-cooling specimens from 600 °C (Fig. 9g, h). It is due to the quartz mineral transformation at 573 °C (Glover et al., 1995), which leads to a severe damage to the microstructure of granite specimen. As a consequence, the slow cooling samples subjected to 600 °C show a dramatic reduction of stiffness and strength and a significant increase in permeability (Figs. 5–8). Transgranular and intergranular microcracks appeared in rapid quenching specimen from 600 °C (Fig. 10e, f), which are much wider than those of slow-cooling ones.

Fig. 8. Permeability versus temperature for different thermal cycling treatments.

5. Results and analyses of permeability tests The calculated permeability results of each thermal cycling granite sample are listed in Table 2. It is shown that the magnitude order of granite sample’s intrinsic permeability at room temperature is 10−18 m2. The test results and average values of permeability against cycling temperature for different thermal cycling treatments are shown in Fig. 8. The graph displays a slight increase in permeability for slow cooling samples before 500 °C, whereas a significant increase after 500 °C, which is consistent with the previous results of Chaki et al. (2008). To be specific, as temperature increases, the permeability of slow cooling samples increases by 2.3 times at 500 °C whereas 35.6 times up to 600 °C. Zhao et al. (2017) pointed out that when the temperature exceeds a certain value, defined as critical temperature, the permeability of granite shows a sharp increase. The experimental results in our study demonstrated that 500 °C could be a critical temperature of permeability change for slow cooling granite at ambient pressure. However, in the rapid quenching case, the critical temperature is reduced to 400 °C, which is consistent with the variation tendency of Young’s modulus. With temperature rising, the permeability of rapid quenching samples increases by 2.9 times at 400 °C whereas 15.3 times at 500 °C, even 79.3 times at 600 °C. The permeability of rapid quenching samples is always larger than that of slow cooling samples with temperature increasing, which demonstrates that rapid quenching treatment enhances the permeability of granite. It is in accordance with the investigation with respect to the thermal stimulation of (Siratovich et al., 2011, 2015), who confirmed that heating creates damage in the slow-cooled samples, while rapid quenching brings a much higher degree of damage. Many numerical simulation studies (Ghassemi et al., 2005; Koh et al., 2011; Tarasovs and Ghassemi, 2012; Fu et al., 2015; Safari and Ghassemi, 2015; Zhao et al., 2015a) also illustrate the role of thermal gradient induced stress to create new cracks, which may increase the long-term reservoir permeability.

7. Discussion 7.1. Mechanism of thermal cracking in two different thermal cycling treatments According to the calculation method of Wang et al. (2013), assuming a thermal diffusivity of granite of 10−6 m2/s, the time constant for temperature equilibrium in the specimens of radius 25 mm is 625 s (about 10 min). Therefore, in the case of slow cooling, the thermal gradients within the specimen can be ignored during slow heating (2 °C/min) and slow cooling (1–2 °C/min). The thermal cracking in the sample is primarily caused by the differential and incompatible thermal expansion (or contraction) between minerals with different thermoelastic moduli or between similar, but misaligned anisotropic grains (Kranz, 1983). The absolute value of thermal cycling temperature is the only factor that affects the thermal cracking degree. On the other hand, in the case of rapid quenching, the cooling rate can be up to 100 °C/ min. Very high temperature gradients form along the radial direction of the specimen, leading to large tensile tangential stress at the rock surface (Kim et al., 2013; Shao et al., 2014). The maximum tensile tangential stress generated at the rock surface can be evaluated following the calculation of Kim et al. (2013): σt max = EαΔT /(1 − ν ) . For example, when the heated sample at a temperature of 500 °C was immersed in cold water (25 °C), a maximum tensile stress of about 207 MPa could be generated at the rock surface (assuming α = 3 × 10-5/ °C , ν = 0.25 and E equals the average Young’s modulus of samples slow-cooled from 500 °C), which is much larger than the tensile strength of the granite sample (∼3.5 MPa under 500 °C). Therefore, it can be expected that significant thermal cracking would occur, which eventually leads to more serious degradation of the mechanical strength and stiffness and enhancement of permeability (Figs. 5–8). Difference of the two cooling method is the cooling rate. This study highlights the influence of cooling rate on the mechanism of thermal microcracking in granite. The extent of effect varies depending on the cooling rate. The more rapid the cooling rate is, the more serious is the thermal damage. Furthermore, the thermal gradient cracks could also be generated in the rapid heating process. The influence of heating rate on the strength and microstructure of granite has been studied by a number of authors (Todd, 1973; Yong and Wang, 1980; Chen et al., 2017; Rossi et al., 2018). It is found that an increased heating rate resulted in an increase in AE hit rate (Yong and Wang, 1980; Chen et al., 2017), which imply

6. Results of microstructural analyses Microcracks in rock were generally classified as intragranular microcracks (contained with a single grain), intergranular microcracks (along the grain boundaries) and transgranular microcracks (affecting more than one grain) (Kranz, 1983; Freire-Lista et al., 2016). Optical micrographs of granite subjected to different thermal cycling treatments (200, 400 and 600 °C) were shown in Figs. 9 and 10. 124

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Fig. 9. Optical microscopy images of thin sections of the granite specimens after slow cooling. The abbreviations used to indicate minerals are: K-Fsp = potassium feldspar, Pl = plagioclase, Qz = quartz, Bi = biotite. Note: left: cross-polarized light; right: plane-polarized light.

gradient stress is highest at the sample surface and decreases with the crack depth (Shao et al., 2014). Therefore, the microcracking degree is most significant close to the sample surface and decreases to minimum inside the center. This affirmation can be evidenced by the microscopic observation of Rossi et al. (2018), where shows a higher local crack density within a region close to the rock surface. The microcrack distribution pattern during rapid quenching requires more detailed investigations in future studies.

that more thermal microcracking occurs at greater heating rate. Rossi et al. (2018) conducted flame-jet heating treatment on Rorschach sandstone and Central Aare granite and postulated that thermal cracking is dominated by the stress concentrations caused by high thermal gradients at high heating rates. Theoretically, samples subjected to different thermal cycling treatments would show different microcrack distribution patterns. To be specific, slow heating and slow cooling leads to isotropically oriented and homogeneously distributed cracks, while rapid quenching leads to heterogeneous distribution. This difference is due to that the temperature distribution within the sample is homogeneous during slow cooling, while inhomogeneous during rapid quenching. The inhomogeneous temperature field produced during rapid quenching leads to heterogeneously distributed thermal stress. The level of thermal-

7.2. Implications for geothermal engineering 7.2.1. Wellbore stability During drilling or long-term water circulation to extract heat, the hot surrounding rock of the injection well would inevitably experience 125

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Fig. 10. Optical microscopy images of thin sections of the granite specimens after rapid quenching. The abbreviations used to indicate minerals are: KFsp = potassium feldspar, Pl = plagioclase, Qz = quartz, Bi = biotite. Note: left: cross-polarized light; right: plane-polarized light.

controlled by the thermal stress induced by the thermal gradient. It is demonstrated by Wu et al. (2007) by heating or cooling the samples prior to injecting the fracturing fluid. The influence of thermal microcracking on the hydraulic stimulation of geothermal reservoir merits further study.

a quenching process. Our study suggests that further thermal gradient cracks were induced during rapid quenching. As a result, on the one hand, it is beneficial for well drilling. On the other hand, it tends to cause borehole instability. Cooling may induce pervasive tensile microcracks prior to macroscopic failure localization (Bérard and Cornet, 2003), which ultimately results in borehole breakouts (Siratovich et al., 2016). Based on the thermoelastic theory, a number of numerical simulations were conducted for modeling the temperature and stress redistribution around the wellbore during drilling fluid circulation (Yan et al., 2014; Tao and Ghassemi, 2010). It is found that a tensile stress is applied on the borehole when the temperature of the borehole wall decreases (Yan et al., 2014). Our results demonstrated the significant role of the thermal gradient cracks and provided reliable parameters for accurate simulation of wellbore stability.

8. Conclusion The influence of different thermal cycling (slow heating followed by slow cooling or rapid quenching) on the physical, mechanical and transport properties of granite samples were systematically investigated. Non-destructive tests (mass, volume, longitudinal wave propagation and permeability) and destructive tests (uniaxial compression and Brazilian splitting) were utilized to characterize these properties. Furthermore, optical microscopic technique was applied to study the development of microcracks within the samples to understand the thermal deterioration mechanisms in the granites after different thermal cycling treatments. The following conclusions can be obtained based on the experimental results.

7.2.2. Thermal/hydraulic stimulation Thermal stimulation is a reservoir permeability enhancement technique which is achieved by injecting low-temperature fluid into the high-temperature geothermal reservoir (Siratovich et al., 2011). The permeability results in our study demonstrate the feasibility of thermal stimulation. Considering the beneficial effect of thermal gradient cracks during rapid heating or quenching, intermittent water injection/extraction may be suggested for thermal fracturing more efficiently. Actually, hydraulic stimulation of geothermal reservoir is a coupled process of thermal microcracking and hydraulic fracturing (Zhao et al., 2008). The initiation and propagation of hydraulic fractures can be

(1) Mass of the samples after thermal cycling decreases continuously, which is ascribed to the water loss in the samples during heating process. Although the samples absorbed extra moisture in the quenching process, the density of rapid quenching samples is lower than that of slow cooling ones. This is due to the larger volume expansion in the rapid quenching case, which has strong 126

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(2)

(3)

(4)

(5)

implications of propagation of existing microcracks and generation of new microcracks. The P-wave velocity and UCS results exhibit a similar trend. In detail, in the slow cooling case, the samples exhibit a slight variation at 100 °C, and then a linear decrease until 600 °C. For rapid quenching, the relationship between P-wave velocity and UCS and temperature is approximately linear decreasing. In addition, the difference between P-wave velocity and UCS of slow cooling and that of rapid quenching increases with temperature up to 500 °C, which can be attributed to the more significant thermal shock cracking induced by higher temperature-gradient stress in the samples during rapid quenching. The Young’s modulus results exhibit a similar tendency to the tensile strength. An initial steady decrease, followed by a sharp decrease can be observed with the increase of temperature for both thermal cycling treatments. The transition temperature is identified as 400–500 °C. On the other hand, the rapid quenching samples have lower Young’s modulus and tensile strength compared to slow cooling. The Young’s modulus and tensile strength results also give evidence of the generation of thermal gradient cracks. For slow cooling, a thermal treatment of 500 °C is identified as the critical temperature regarding the change of permeability. While in the rapid quenching case, the critical temperature is reduced to 400 °C. Rapid quenching accelerated the thermal deterioration of granite samples, and enhanced the permeability of granite, which has significant implications for thermal stimulation. The optical microscope observations indicate that microcracking damage has occurred as a result of thermal cycling and these cracks are responsible for the decrease in physical and mechanical properties and increase in permeability.

Ghassemi, A., Tarasovs, S., Cheng, A.H.D., 2005. Integral equation solution of heat extraction-induced thermal stress in enhanced geothermal reservoirs. Int. J. Numer. Anal. Methods Geomech. 29, 829–844. Ghassemi, A., Tarasovs, S., Cheng, A.H.D., 2007. A 3-d study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs. Int. J. Rock Mech. Min. Sci. 44, 1132–1148. Glover, P.W.J., Baud, P., Darot, M., Meredith, P.G., Boon, S.A., Leravalec, M., Zoussi, S., Reuschlé, T., 1995. A/β phase transition in quartz monitored using acoustic emissions. Geophys. J. Int. 120, 775–782. Griffiths, L., Heap, M.J., Baud, P., Schmittbuhl, J., 2017. Quantification of microcrack characteristics and implications for stiffness and strength of granite. Int. J. Rock Mech. Min. Sci. 100, 138–150. Griffiths, L., Lengliné, O., Heap, M.J., Baud, P., Schmittbuhl, J., 2018. Thermal cracking in westerly granite monitored using direct wave velocity, coda wave interferometry, and acoustic emissions. J. Geophys. Res. (Sol. Ea.). 123, 2246–2261. Heap, M.J., Coats, R., Chen, C.-f., Varley, N., Lavallée, Y., Kendrick, J., Xu, T., Reuschlé, T., 2018a. Thermal resilience of microcracked andesitic dome rocks. J. Volcanol. Geoth. Res. 367, 20–30. Heap, M.J., Reuschlé, T., Farquharson, J.I., Baud, P., 2018b. Permeability of volcanic rocks to gas and water. J. Volcanol. Geoth. Res. 354, 29–38. Heuze, F.E., 1983. High-temperature mechanical, physical and thermal properties of granitic rocks- a review. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 20, 3–10. Inserra, C., Biwa, S., Chen, Y., 2013. Influence of thermal damage on linear and nonlinear acoustic properties of granite. Int. J. Rock Mech. Min. Sci. 62, 96–104. ISRM, 2007. The Complete ISRM Suggested Methods for Rock Characterisation, Testing and Monitoring: 1974–2006. ISRM Commission on Testing Methods, Ankara, Turkey. Jansen, D.P., Carlson, S.R., Young, R.P., Hutchins, D.A., 1993. Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in lac du bonnet granite. J. Geophys. Res. (Sol. Ea.) 98, 22231–22243. Kamali-Asl, A., Ghazanfari, E., Perdrial, N., Bredice, N., 2018. Experimental study of fracture response in granite specimens subjected to hydrothermal conditions relevant for enhanced geothermal systems. Geothermics 72, 205–224. Kant, M.A., Ammann, J., Rossi, E., Madonna, C., Höser, D., Rudolf von Rohr, P., 2017. Thermal properties of central aare granite for temperatures up to 500°C: irreversible changes due to thermal crack formation. Geophys. Res. Lett. 44, 771–776. Kim, K., Kemeny, J., Nickerson, M., 2013. Effect of rapid thermal cooling on mechanical rock properties. Rock Mech. Rock Eng. 47, 2005–2019. Koh, J., Roshan, H., Rahman, S.S., 2011. A numerical study on the long term thermoporoelastic effects of cold water injection into naturally fractured geothermal reservoirs. Comput. Geotech. 38, 669–682. Kohl, T., Evansi, K.F., Hopkirk, R.J., Rybach, L., 1995. Coupled hydraulic, thermal and mechanical considerations for the simulation of hot dry rock reservoirs. Geothermics 24, 345–359. Kranz, R.L., 1983. Microcracks in rocks: a review. Tectonophysics 100, 449–480. Kumari, W.G.P., Ranjith, P.G., Perera, M.S.A., Chen, B.K., Abdulagatov, I.M., 2017a. Temperature-dependent mechanical behaviour of australian strathbogie granite with different cooling treatments. Eng. Geol. 229, 31–44. Kumari, W.G.P., Ranjith, P.G., Perera, M.S.A., Shao, S., Chen, B.K., Lashin, A., Arifi, N.A., Rathnaweera, T.D., 2017b. Mechanical behaviour of australian strathbogie granite under in-situ stress and temperature conditions: an application to geothermal energy extraction. Geothermics 65, 44–59. Kumari, W.G.P., Ranjith, P.G., Perera, M.S.A., Chen, B.K., 2018. Experimental investigation of quenching effect on mechanical, microstructural and flow characteristics of reservoir rocks: thermal stimulation method for geothermal energy extraction. J. Petrol. Sci. Eng. 162, 419–433. Lam dos Santos, J.P., Rosa, L.G., Amaral, P.M., 2011. Temperature effects on mechanical behaviour of engineered stones. Constr. Build. Mater. 25, 171–174. Lin, W., 2002. Permanent strain of thermal expansion and thermally induced microcracking in inada granite. J. Geophys. Res. (Sol. Ea.) 107 ECV 3-1-ECV 3-16. Liu, X., Yuan, S., Sieffert, Y., Fityus, S., Buzzi, O., 2016. Changes in mineralogy, microstructure, compressive strength and intrinsic permeability of two sedimentary rocks subjected to high-temperature heating. Rock Mech. Rock Eng. 49, 2985–2998. MIT, 2006. The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century. Massachusetts Institute of Technology. Nasseri, M.H.B., Schubnel, A., Young, R.P., 2007. Coupled evolutions of fracture toughness and elastic wave velocities at high crack density in thermally treated westerly granite. Int. J. Rock Mech. Min. Sci. 44, 601–616. Reuschlé, T., Gbaguidi Haore, S., Darot, M., 2006. The effect of heating on the microstructural evolution of la peyratte granite deduced from acoustic velocity measurements. Earth. Planet. Sci. Lett. 243, 692–700. Richter, D., Simmons, G., 1974. Thermal expansion behavior of igneous rocks. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 11, 403–411. Rossi, E., Kant, M.A., Madonna, C., Saar, M.O., Rohr, P.R.V., 2018. The effects of high heating rate and high temperature on the rock strength: feasibility study of a thermally assisted drilling method. Rock Mech. Rock Eng. 51, 2957–2964. Safari, R., Ghassemi, A., 2015. 3d thermo-poroelastic analysis of fracture network deformation and induced micro-seismicity in enhanced geothermal systems. Geothermics 58, 1–14. Sander, R., Pan, Z., Connell, L.D., 2017. Laboratory measurement of low permeability unconventional gas reservoir rocks: a review of experimental methods. J. Nat. Gas Sci. Eng. 37, 248–279. Shao, S., Wasantha, P.L.P., Ranjith, P.G., Chen, B.K., 2014. Effect of cooling rate on the mechanical behavior of heated strathbogie granite with different grain sizes. Int. J. Rock Mech. Min. Sci. 70, 381–387. Shao, S., Ranjith, P.G., Wasantha, P.L.P., Chen, B.K., 2015. Experimental and numerical

Acknowledgments This work was funded by National Natural Science Foundation of China (Grants No. 51574173 and 51225404) and Science and Technology Major Project of Shanxi Province (No. MQ2016-01). References Becattini, V., Motmans, T., Zappone, A., Madonna, C., Haselbacher, A., Steinfeld, A., 2017. Experimental investigation of the thermal and mechanical stability of rocks for high-temperature thermal-energy storage. Appl. Energy 203, 373–389. Bérard, T., Cornet, F.H., 2003. Evidence of thermally induced borehole elongation: a case study at soultz, France. Int. J. Rock Mech. Min. Sci. 40, 1121–1140. Brace, W.F., Walsh, J.B., Frangos, W.T., 1968. Permeability of granite under high pressure. J. Geophys. Res. 73, 2225–2236. Brotóns, V., Tomás, R., Ivorra, S., Alarcón, J.C., 2013. Temperature influence on the physical and mechanical properties of a porous rock: San julian’s calcarenite. Eng. Geol. 167, 117–127. Chaki, S., Takarli, M., Agbodjan, W.P., 2008. Influence of thermal damage on physical properties of a granite rock: porosity, permeability and ultrasonic wave evolutions. Constr. Build. Mater. 22, 1456–1461. Chen, S., Yang, C., Wang, G., 2017. Evolution of thermal damage and permeability of beishan granite. Appl. Therm. Eng. 110, 1533–1542. Darot, M., Gueguen, Y., Baratin, M.L., 1992. Permeability of thermally cracked granite. Geophys. Res. Lett. 19, 869–872. David, C., Menéndez, B., Darot, M., 1999. Influence of stress-induced and thermal cracking on physical properties and microstructure of la peyratte granite. Int. J. Rock Mech. Min. Sci. 36, 433–448. David, E.C., Brantut, N., Schubnel, A., Zimmerman, R.W., 2012. Sliding crack model for nonlinearity and hysteresis in the uniaxial stress–strain curve of rock. Int. J. Rock Mech. Min. Sci. 52, 9–17. Fredrich, J.T., Wong, T.-f., 1986. Micromechanics of thermally induced cracking in three crustal rocks. J. Geophys. Res. (Sol. Ea.) 91, 12743–12764. Freire-Lista, D.M., Fort, R., Varas-Muriel, M.J., 2016. Thermal stress-induced microcracking in building granite. Eng. Geol. 206, 83–93. Fu, P., Hao, Y., Walsh, S.D.C., Carrigan, C.R., 2015. Thermal drawdown-induced flow channeling in fractured geothermal reservoirs. Rock Mech. Rock Eng. 49, 1001–1024. Ghassemi, A., 2012. A review of some rock mechanics issues in geothermal reservoir development. Geotech. Geol. Eng. 30, 647–664. Ghassemi, A., Tao, Q., 2016. Thermo-poroelastic effects on reservoir seismicity and permeability change. Geothermics 63, 210–224. Ghassemi, A., Tarasovs, S., 2015. Analysis of fracture propagation under thermal stress in geothermal reservoirs. World Geothermal Congress 2015.

127

Geothermics 78 (2019) 118–128

P. Jin et al.

controlling hydraulic fractures. J. Geophys. Res. 112, 1–15. Yan, C., Deng, J., Yu, B., Li, W., Chen, Z., Hu, L., Li, Y., 2014. Borehole stability in hightemperature formations. Rock Mech. Rock Eng. 47, 2199–2209. Yang, S.-Q., Ranjith, P.G., Jing, H.-W., Tian, W.-L., Ju, Y., 2017a. An experimental investigation on thermal damage and failure mechanical behavior of granite after exposure to different high temperature treatments. Geothermics 65, 180–197. Yang, S.-Q., Xu, P., Li, Y.-B., Huang, Y.-H., 2017b. Experimental investigation on triaxial mechanical and permeability behavior of sandstone after exposure to different high temperature treatments. Geothermics 69, 93–109. Yin, T., Li, X., Cao, W., Xia, K., 2015. Effects of thermal treatment on tensile strength of laurentian granite using brazilian test. Rock Mech. Rock Eng. 48, 2213–2223. Yong, C., Wang, C., 1980. Thermally induced acoustic emission in westerly granite. Geophys. Res. Lett. 7, 1089–1092. Zhang, W., Sun, Q., Hao, S., Geng, J., Lv, C., 2016. Experimental study on the variation of physical and mechanical properties of rock after high temperature treatment. Appl. Therm. Eng. 98, 1297–1304. Zhao, Z., 2015. Thermal influence on mechanical properties of granite: a microcracking perspective. Rock Mech. Rock Eng. 49, 747–762. Zhao, Y., Meng, Q., Kang, T., Ning, Z., Baoping, X.I., 2008. Micro-CT experimental technology and meso-investigation on thermal fracturing characteristics of granite. Chin. J. Rock Mech. Eng. 27, 28–34 (In Chinese). Zhao, Y., Wan, Z., Feng, Z., Yang, D., Zhang, Y., Qu, F., 2012. Triaxial compression system for rock testing under high temperature and high pressure. Int. J. Rock Mech. Min. Sci. 52, 132–138. Zhao, Y., Feng, Z., Feng, Z., Yang, D., Liang, W., 2015a. THM (thermo-hydro-mechanical) coupled mathematical model of fractured media and numerical simulation of a 3d enhanced geothermal system at 573 K and buried depth 6000–7000 m. Energy 82, 193–205. Zhao, Y., Feng, Z., Xi, B., Wan, Z., Yang, D., Liang, W., 2015b. Deformation and instability failure of borehole at high temperature and high pressure in hot dry rock exploitation. Renew. Energy 77, 159–165. Zhao, Y., Feng, Z., Zhao, Y., Wan, Z., 2017. Experimental investigation on thermal cracking, permeability under HTHP and application for geothermal mining of HDR. Energy 132, 305–314.

studies on the mechanical behaviour of australian strathbogie granite at high temperatures: an application to geothermal energy. Geothermics 54, 96–108. Siratovich, P.A., Sass, I., Homuth, S., Bjornsson, A., 2011. Thermal stimulation of geothermal reservoirs and laboratory investigation of thermally induced fractures. Trans. – Geotherm. Resour. Council 35, 1529–1535. Siratovich, P.A., Villeneuve, M.C., Cole, J.W., Kennedy, B.M., Bégué, F., 2015. Saturated heating and quenching of three crustal rocks and implications for thermal stimulation of permeability in geothermal reservoirs. Int. J. Rock Mech. Min. Sci. 80, 265–280. Siratovich, P.A., Heap, M.J., Villeneuve, M.C., Cole, J.W., Kennedy, B.M., Davidson, J., Reuschlé, T., 2016. Mechanical behaviour of the rotokawa andesites (new zealand): insight into permeability evolution and stress-induced behaviour in an actively utilised geothermal reservoir. Geothermics 64, 163–179. Sun, Q., Zhang, W., Xue, L., Zhang, Z., Su, T., 2015. Thermal damage pattern and thresholds of granite. Environ. Earth Sci. 74, 2341–2349. Tao, Q., Ghassemi, A., 2010. Poro-thermoelastic borehole stress analysis for determination of the in situ stress and rock strength. Geothermics 39 (3), 250–259. Tarasovs, S., Ghassemi, a., 2012. On the role of thermal stress in reservoir stimulation. Thirty-Seventh Workshop on Geothermal Reservoir Engineering. Thirumalai, K., Demou, S.G., 1974. Thermal expansion behavior of intact and thermally fractured mine rocks. AIP Conf. Proc. pp. 60–71. Todd, T.P., 1973. Effect of Cracks on the Elastic Properties of Low Porosity Rocks. PhD thesis. Mass. Inst. of Technol., Cambridge. Vidal, J., Genter, A., 2018. Overview of naturally permeable fractured reservoirs in the central and southern upper rhine graben: insights from geothermal wells. Geothermics 74, 57–73. Violay, M., Heap, M.J., Acosta, M., Madonna, C., 2017. Porosity evolution at the brittleductile transition in the continental crust: implications for deep hydro-geothermal circulation. Sci. Rep. 7, 7705. Walsh, J.B., 1965. The effect of cracks on the compressibility of rock. J. Geophys. Res. 70 (2), 381–389. Wang, X.-Q., Schubnel, A., Fortin, J., Guéguen, Y., Ge, H.-K., 2013. Physical properties and brittle strength of thermally cracked granite under confinement. J. Geophys. Res. (Sol. Ea.) 118, 6099–6112. Wu, R., Germanovich, L.N., van Dyke, P.E., Lowell, R.P., 2007. Thermal technique for

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