An experimental study of fractured sandstone permeability after high-temperature treatment under different confining pressures

Journal of Natural Gas Science and Engineering 34 (2016) 55e63

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An experimental study of fractured sandstone permeability after hightemperature treatment under different confining pressures Qi-Le Ding a, **, Feng Ju b, *, Shuai-Bing Song b, Bang-Yong Yu b, Dan Ma b a School of Mines, State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, 221116, Xuzhou, People’s Republic of China b State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, 221116, Xuzhou, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 March 2016 Received in revised form 10 June 2016 Accepted 13 June 2016 Available online 16 June 2016

A detailed understanding of the effects of temperature and confining pressure on permeability is critical for projects such as underground coal gasification, reconstruction after a gas disaster, and disposal of nuclear waste. In this study, uniaxial compression experiments were conducted on sandstone after exposure to high temperature, and water flow tests were then performed on the fractured sandstone. A non-Darcy method was adopted to calculate the permeability. The mechanical properties were enhanced and the permeability slowly decreased when the temperature was below 400
C. When the temperature exceeded 400
C, the formation of new cracks and the extension of existing cracks were observed using a scanning electron microscope; the volume increased rapidly, and the mechanical properties significantly decreased. The permeability increased rapidly as the temperature increased from 400 to 600
C, while the change in permeability was modest at temperatures above 600
C. An exponential function was used to fit the permeability and temperature data above 400
C. As the confining pressure increased from 2 to 8 MPa, the permeability initially decreased sharply and then decreased at a considerably slower rate. Temperature slightly affected the change in permeability at a higher confining pressure. © 2016 Elsevier B.V. All rights reserved.

Keywords: Experimental study Fractured sandstone Permeability High temperature Confining pressure

1. Introduction Underground mining has long been considered a high-risk activity (Wang et al., 2013). Violent mining disturbances cause stress concentrations and separation along strata planes that can cause the bending and subsequent fracturing of rock layers (Adhikary and Guo, 2015). A number of studies have demonstrated the mechanics of strata deformation induced by underground mining (Ropski and Lama, 1973; Kesserü, 1984; Singh et al., 1986). The permeability of fractured rocks is considerably higher than that of rocks that remain intact. Forster and Enever (1992) conducted field tests in a coal mine in Australia and showed that permeability increased by three orders of magnitude in a fractured zone. Many coal mines are threatened by groundwater during the mining process (Wang and Park, 2003; Wu et al., 2004; Zhang, 2005). One of the most dangerous hazards is water inrushes from

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q.-L. Ding), [email protected] (F. Ju). http://dx.doi.org/10.1016/j.jngse.2016.06.034 1875-5100/© 2016 Elsevier B.V. All rights reserved.

Ordovician limestone under Permo-Carboniferous coal seams (Zhang et al., 2014b) where the resident aquifer contains a large amount of water under high pressure (Bieniawski, 1982; Zhang and Shen, 2004; Zhang and Peng, 2005). Mining activities cause strata failures and form fractured zones (Xiao and Xu, 2000); these fractures can become water flow channels and affect the stability of excavated structures (Chegbeleh et al., 2009). Many studies have indicated that high-pressure groundwater can break through fractured strata and burst into the working face (Li and Zhou, 2006; Wu and Zhou, 2008; Li et al., 2011; Zhu and Wei, 2011; Zhu et al., 2013). These challenges are correlated with fracture density, rock mechanical properties, and enhanced fluid flow (Davoodi et al., 2013; Barani et al., 2014). Zhang et al. (2013) demonstrated that the permeability of brittle rocks was significantly higher at the critical failure point due to fracture formation during compression. Zhu et al. (2002) showed that the water flow properties of integrated and fractured rocks differed significantly and that permeability is closely related to the style of deformation destruction. Previous research has shown that permeability is closely correlated with fracture evolution (Larsen et al., 2010; Larsen and Gudmundsson, 2010; Thararoop et al., 2012; Wang et al., 2015;

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Zou et al., 2015), which is affected by ambient conditions, such as temperature (Cai et al., 2014). Ferrero and Marini (2001) studied the behavior of two types of marble after high-temperature treatments of up to 600
C using microscopic analyses and open porosity tests; they found that new cracks formed and that open porosity increased with increasing temperature. Numerous studies have demonstrated that high temperature affects the physical and meraud et al., 1992; Chopra, 1997; chanical properties of rocks (Ge Zhang et al., 2001; Zhao et al., 2012; Ozguven and Ozcelik, 2014; Zhang et al., 2014a; Shao et al., 2015a; Ding et al., 2016). Many countries, including the USA, China, and India, have reported underground coal fires (Coates and Heffern, 2000; Michalski, 2004; Coates et al., 2005; Chatterjee, 2006; Kuenzer and Stracher, 2012; Zeng et al., 2015). The spontaneous combustion of coal seams is a hazard that causes not only resource loss and environmental pollution but also other hazards, such as gas explosions and water inrushes (Hu et al., 2015; Lu and Qin, 2015; Shao et al., 2015b). Qin et al. (2009) conducted in site tests in a coalfield fire area and showed that the maximum temperature reached 1200
C. In addition, the rapid development of in site gasification techniques (Shackley et al., 2006; Khadse et al., 2007; Stanczyk et al., 2011) provides an excellent opportunity to use coal resources where the temperature of the working face can exceed 1000
C (Niu et al., 2014). Rock masses involved in projects such as post-disaster reconstruction after coal fires and underground coal gasification have experienced high-temperature treatments that caused fractures and changes in seepage behavior in the strata as a result of the combustion process (Otto and Kempka, 2015). Along with the mechanical behavior deterioration caused by excavation effects, thermal stress causes permeability changes. Few studies on water flow properties have considered both stress changes and high-temperature effects. In this paper, we performed uniaxial compression tests on sandstone after high-temperature treatment and water flow tests on fractured sandstone using an MTS815.02 Material Testing System and a self-made water flow device to investigate the effect of temperature and confining pressure on permeability. These experimental results offer an accurate understanding of water inrushes in underground projects related to high temperature. 2. Experimental materials and testing procedures 2.1. Experimental materials The sandstone used in this study was fine-grained and collected using a 165-mm vertical drill core from a depth of 192 m in the Pingshuo coalmine located in Shuozhou City, Shanxi Province, China, as shown in Fig. 1. The Shuozhou sandstone was composed of feldspar, quartz, kaolinite, illite, chlorite, calcite, siderite, and small amounts of other minerals. The sandstone density and porosity were approximately 2.51 g/cm3 and 3.4  102, respectively. All experiments were conducted on cylindrical specimens with diameters of 50 mm and lengths of 100 mm in accordance with the ISRM standard (Fairhurst and Hudson, 1999). 2.2. Experimental equipment and testing procedures 2.2.1. Experimental equipment Compression tests were performed using an MTS815.02 Material Testing System with a maximum loading capacity of 1700 kN. A GWD-02A electric furnace was used to heat the specimens to a maximum temperature of 1100
C. The seepage properties were tested using an MTS815.02 and a self-made water flow device, as shown in Fig. 2. The perforated plate (5) ensured that the liquid flowed regularly, and the felt pad

(6) prevented the fluid from polluting the MTS815.02 system. Epoxy resin was used to separate the specimen and the cylindrical barrel (8) and to inhibit the radial flow of the liquid. The triaxial cell was connected to the cylindrical barrel (8) via a one-way valve that included a valve core (13), a valve chest (14), a spring (11), a plate (10) and a screw (15). The self-made pipes (1 and 12) were used to vary the confining pressure. The fluid starts from the pore water pressure load system, flows through globe valves S12 and S14 and the one-way valve, reaches the specimen, and then flows out via globe valves S1 and S15. The water flow velocity is controlled by the pore water pressure load system. A controller, including a distributor, a personal computer and software, is used to collect and analyze the data.

2.2.2. Testing procedures The experimental methods for testing mechanical behavior and permeability vary according to the research background, including tests conducted during the process of high-temperature treatment and after high-temperature treatment; the latter method was studied in this paper. Both axial stress and confining pressure affect rock in underground projects, and mechanical properties in uniaxial tests and triaxial tests differ. We conducted uniaxial compression experiments to study the effect of high-temperature treatment on the mechanical behavior of sandstone. (1) We performed uniaxial compression tests on the sandstone after high-temperature treatment. Seven temperature levels were set: 20 (room temperature), 200, 400, 500, 600, 700 and 800
C. The temperature settings are closely correlated to the site conditions, and the temperature of a fire in a coalfield is approximately 800
C (Burton et al., 2006; Couch, 2009). Each temperature was tested on three specimens. These samples were numbered, and their mass and dimensions were measured before the compression tests. The specimens were heated to their designated temperature at a rate of 20
C/min, held at that temperature for 2 h to ensure that the materials were fully heated, and subsequently cooled back to room temperature. The mass and dimensions were then measured again. After high-temperature treatment, the specimens underwent uniaxial compression tests that were performed by displacement control at a rate of 0.2 mm/min. The mechanical properties, including the peak strength, Young’s modulus and peak strain, were obtained. (2) We performed water flow tests on the fractured sandstone. The specimens were first heated to the target temperature at a rate of 20
C/min, maintained at that temperature for 2 h, and finally cooled back to room temperature. After hightemperature treatment, the specimens were uniaxially compressed to a given strain value that was 1.3 times greater than the average peak strain obtained in the uniaxial compression test at the same temperature. The deformation values before and after unloading were considered identical because after reaching peak strength, the deformation could not recover. The pre-fractured specimens were circumferentially wrapped in PVC bands and thermo-shrinking plastic covers to prevent fragments from separating from the integrated structure. After the specimens were placed on the testing platform, confining pressures s3 (s3 ¼ 2, 5, and 8 MPa) were applied at a rate of 0.08 MPa/s; then, constant axial loads were applied. The confining pressure settings are correlated to the site conditions. Stress meters were distributed in the 21,105 working face of the Pingshuo coalmine where the sandstone was collected, and the monitoring results indicated that most of the stress values ranged from 2

Q.-L. Ding et al. / Journal of Natural Gas Science and Engineering 34 (2016) 55e63

a

57

b

N 0 100 200m

Legend

21106 working face

21105 working face

21104 working face

Fault

Thickness (m)

Cumulative Thickness (m)

Rock Mass

2.0

192.0

Sandstone

1.4

193.4

Mudstone

4.8

198.2

No.11 Coal Seam

c

Fig. 1. Geological conditions. (a) Layout of working faces in Pingshuo coalmine; (b) borehole column; (c) specimens.

Fig. 2. Seepage behavior testing system. A: pressure transducer, B: supercharger, C: relief valve, D: voltage stabilizer, E: differential pressure transducer, F: drainage, S1eS15: switches, 1, 12: pipes, 2, 4, 9: rubber seal rings, 3: cover plate, 5: perforated plate, 6: felt pad, 7: separation layer of epoxy resin, 8: cylindrical barrel, 10: plate, 11: spring, 13: valve core, 14: valve chest, and 15: screw.

to 8 MPa. Fig. 3 illustrates the procedures of the water flow test. The piston velocities of each seepage test were set at four levels, namely, 1.8, 3, 6 and 9 mm/min; the pressure was maintained for more than 60 s, thus obtaining pressure gradients under different water flow velocities. The permeability was calculated through linear regression and based on one group of flow velocities Vs ji (i ¼ 1, 2, …, n) and a corresponding group of pressure gradients vp=vxji (i ¼ 1, 2, …, n).

mining underwent stresses that exceeded the peak stress. In addition to the increase in the permeability of the fractured rocks of several orders of magnitude, high water pressure also contributed to the damage. Non-Darcy seepage accurately describes the water flow behavior in both fractured rocks and tight formations such as shale (Chen et al., 2004; Cheng et al., 2004; Miao et al., 2011; Ma et al., 2014). The one-dimensional momentum equation of non-Darcy seepage can be expressed as

vVs vp m ¼   Vs þ rbVs2 þ F vx k vt

3. Permeability calculation based on a non-Darcy approach

rca

Numerous experiments and in site tests have indicated that most damaged rocks in a fractured zone induced by underground

where ca is the acceleration coefficient, Vs is the flow velocity, vp=vx is the pressure gradient, F is the body force, m is the dynamic

(1)

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Q.-L. Ding et al. / Journal of Natural Gas Science and Engineering 34 (2016) 55e63

2.0

Rate of change in mass Rate of change in volume

-0.06

1.5 -0.12 1.0 -0.18 0.5 -0.24 0.0 -0.30 0

100

200

300

400

500

600

700

Rate of change in volume /%

Rate of change in mass /%

0.00

800

o

Temperature / C Fig. 4. Rates of change in the mass and volume of a specimen. All of the symbols represent average values.






r0 ps 1 þ 0:5cf ps ¼

hm i ðrVÞs  bðrVÞ2s ðh  xÞ k

(2)

where cf is the compressibility, ps is the pressure difference between the top and bottom of the specimen, h is the height of the specimen, and x is the vertical upward direction. Equation (2) is simplified by ignoring the compressibility of the liquid:

r0 ps ¼

hm k

i ðrVÞs  bðrVÞ2s ðh  xÞ

(3)

A difference exists between the base pressure pb and top pressure pt, and the ratio of this difference to the specimen height h approximately equals the pressure gradient.

Fig. 3. Seepage test procedures of fractured sandstone.

viscosity (the fluid medium in our experiment was tap water and m ¼ 1.01  109 MPa$s), k is the permeability, r is the mass density of the fluid medium (r ¼ 1.0  106 kg/mm3), and b is a factor of non-Darcy seepage. The seepage reaches a stable state when the water flow lasts until vV=vt ¼ 0. Without considering the body force, the pressure is linearly distributed in an equilibrium state such that

 vp p  pt p ¼ b ¼ b vx s h h

(4)

pb m ¼ Vs  brVs2 h k

(5)

Pressure gradients and water flow velocities are required to determine the non-Darcy seepage properties. The flow velocity Vs is controlled by the piston velocity Vp, and there is a relation between them such that

 Vs ¼

dp ds

2 Vp

(6)

where dp and ds are the diameters of the piston and specimen, Table 1 Pressure gradient and flow velocity of specimens without high-temperature treatment under a confining pressure of 2 MPa. Sample no.

Piston velocity Vp (mm/min)

Flow velocity Vs (m/s)

A1

1.8 3 6 9 1.8 3 6 9 1.8 3 6 9

5.63 9.38 1.88 2.81 5.63 9.38 1.88 2.81 5.63 9.38 1.88 2.81

A2

A3

           

1006 1006 1005 1005 1006 1006 1005 1005 1006 1006 1005 1005

Pressure gradient vp=vx (MPa/m)

Permeability k (m2)

3.17 4.64 11.91 18.24 2.79 5.59 10.52 18.01 2.99 5.35 10.62 18.47

1.98  1015

2.08  1015

2.15  1015

Q.-L. Ding et al. / Journal of Natural Gas Science and Engineering 34 (2016) 55e63 Table 2 Effect of temperature on the peak strength ss, peak strain


T ( C)

20

200

400

500

600

700

800

Sample no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

3s

59

and Young’s modulus E in uniaxial compression tests.

ss (MPa)

3s

 102

E(GPa)

Each sample

Average value

Each sample

Average value

Each sample

Average value

70.1 76.0 73.6 79.7 74.7 78.3 86.8 83.0 78.5 70.9 74.7 71.6 67.4 57.2 55.8 41.7 52.6 49.3 34.6 41.6 30.9

73.2

0.59 0.61 0.64 0.65 0.60 0.64 0.74 0.66 0.70 0.69 0.80 0.71 0.85 0.81 0.83 0.87 0.90 1.03 1.04 1.12 1.08

0.61

12.8 13.9 13.5 13.8 14.0 13.8 13.9 14.2 14.9 12.8 11.9 11.4 9.3 7.9 8.9 5.8 7.2 5.4 4.1 4.6 3.1

13.4

77.6

82.8

72.4

60.1

47.9

35.7

0.63

0.70

0.73

0.83

0.93

1.08

13.9

14.3

12.0

8.7

6.1

3.9

Note: T is temperature.

respectively. m/k can be obtained from the quadratic polynomial fitting between the flow velocities and the corresponding pressure

gradients. Table 1 illustrates the flow velocity, the pressure gradient and the calculated permeability of the specimens without hightemperature treatment under a confining pressure of 2 MPa.

Fig. 5. SEM observations of sandstone after different high-temperature treatments.

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Q.-L. Ding et al. / Journal of Natural Gas Science and Engineering 34 (2016) 55e63

70

σ3=2 MPa

Rate of change in permeability /%

60

Stage 2 Rapid increase

σ3=5 MPa

50

Stage 3 Slow increase

σ3=8 MPa

40 Stage 1 Slow decrease

30 20 10 0 -10 -20

0

100

200

300

400

500

600

700

800

o

Temperature / C Fig. 6. Three different stages for the permeability of fractured sandstone after hightemperature treatment. All of the symbols represent average values.

m k

¼

 

vp i¼1 vx

Vi ðiÞ

Pn

k¼1

Vk4 ðkÞ 

Pn

 

P Vi2 ðiÞ nk¼1 Vk3 ðkÞ i Pn Pn Pn i 2 Pn 4 3 3 i¼1 Vi ðiÞ k¼1 Vk ðkÞ  i¼1 Vi ðiÞ k¼1 Vk ðkÞ

Pn

vp i¼1 vx 

(7)

4. Effect of temperature on the physical and mechanical properties The volume of the sandstone specimen increased after hightemperature treatment. The rate of change was slight when the temperature was below 400
C at an increasing rate of 0.109% compared with the room temperature value. The increase was more significant when the temperature exceeded 400
C. The mass decreased rapidly (at a rate of 0.197%) after exposure to 200
C. However, the rate was modest when the temperature exceeded 400
C. Fig. 4 illustrates the rates of change in volume and mass.

The mechanical properties were enhanced after treatment at 400
C; compared with the room temperature values, the peak strength and Young’s modulus increased by 13.1% and 6.7%, respectively. Treatments at 400
C or above negatively affected the mechanical behavior; compared to the room temperature values, the peak strength and Young’s modulus after the treatment at 800
C decreased by 51.2% and 70.9%, respectively. Table 2 illustrates the effect of temperature on mechanical behavior in uniaxial compression tests. The changes in the mechanical properties resulted from changes to the microstructure that were caused by high-temperature treatment and observed using a scanning electron microscope, as shown in Fig. 5. As the temperature increased, the thermal stress increased, and new microcracks formed along with the coalescence of cracks. Clay minerals, which are susceptible to heat, are often altered after hightemperature treatment (McCabe et al., 2010). Additionally, the loss of hydroxyl structural water (Sun et al., 2013) and the a/b transition of quartz (Glover et al., 1995) also contributed to fracture propagation. The sandstone used in the tests was a typical sedimentary rock with some original pores and cracks. When the temperature increased to 400
C, mineral components expanded, and thermal stress prevented the extension of internal cracks and caused the closure of cracks, thus leading to increased compactness. As a result, the 400
C treatment improved the physical and mechanical properties of the sandstone, including the peak strength and Young’s modulus. When the temperature increased from 400 to 600
C, the thermal stress, including the tensile and compressive stresses, was further increased because of the increasing constraints induced by the different expansion rates of the components. New cracks were generated when the thermal stress reached the tensile strength or shear strength of the sandstone materials, thus decreasing the carrying capacity. As the temperature increased, more new cracks formed, and the original cracks extended, widened and connected; the mechanical characteristics were further negatively affected. The 600 and 800
C treatments caused a rapid decrease in the peak strength and Young’s modulus, and a higher temperature led to a more

Table 3 Permeability of all fractured specimens after high-temperature treatment under different confining pressures. T/
C

20 20 20 200 200 200 400 400 400 500 500 500 600 600 600 700 700 700 800 800 800

s3 ¼ 2 MPa

s3 ¼ 5 MPa

No

k /  1015 m2

A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21

1.98 2.08 2.15 1.98 1.83 1.69 1.59 1.84 1.71 2.34 2.48 2.80 3.13 3.10 2.92 3.15 3.14 3.36 3.19 3.24 3.31

Note: AM is the average value.

AM /  1015 m2 2.07

1.84

1.71

2.54

3.05

3.21

3.24

s3 ¼ 8 MPa

No

k /  1016 m2

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21

5.49 5.80 6.08 5.67 5.26 4.98 4.95 5.29 5.21 6.20 6.23 6.97 7.06 7.48 7.11 7.16 7.77 7.21 7.37 7.77 7.43

AM /  1016 m2 5.79

5.30

5.15

6.47

7.22

7.38

7.52

No

k /  1016 m2

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21

4.33 4.30 4.81 4.39 4.32 4.14 3.90 3.95 4.33 4.55 5.18 4.81 5.02 5.37 5.21 5.18 5.37 5.10 5.34 5.23 5.40

AM /  1016 m2 4.48

4.28

4.06

4.85

5.20

5.22

5.33

Q.-L. Ding et al. / Journal of Natural Gas Science and Engineering 34 (2016) 55e63

significant rate of decrease. After the 600 and 800
C treatments, the peak strain was 36.1% and 77.0% lower than that at room temperature, respectively, which was the result of crack evolution.

3.4 Average value Fitting curve

-15

Permeability k /×10 m

2

3.1

61

2.8

5. Permeability of fractured sandstone after hightemperature treatment under different confining pressures

k=-36.02 exp(-T/129.79)+3.35 2 R =0.992

2.5 2.2 1.9 1.6 400

500

600

700

800

o

Temperature / C

(a) Confining pressure of 2 MPa

7.6

Average value Fitting curve

-16

Permeability k /×10 m

2

7.1 6.6

k=-63.84 exp(-T/123.27)+7.63 2 R =0.994

6.1 5.6 5.1 400

500

600

700

800

  T þ y0 k ¼ A*exp  b

o

Temperature / C

(b) Confining pressure of 5 MPa 5.4 Average value Fitting curve

-16

Permeability k /×10 m

2

5.1 4.8

k=-69.62 exp(-T/99.98)+5.33 2 R =0.991

4.5 4.2 3.9 400

500

600

The evolution of cracks played an important role in the changing of seepage properties and was also correlated with mechanical properties, such as the peak strength, Young’s modulus and peak strain. Fig. 6 illustrates the rates of change in permeability for the fractured sandstone after exposure to different high temperatures under different confining pressures. The permeability rate of change curves can be divided into three stages: slow decrease, rapid increase and slow increase. The permeability decreased slowly when the temperature was lower than 400
C. As the temperature increased, the expansion of mineral components led to the closure of cracks, which narrowed the channels through which water flowed. Table 3 illustrates the permeability of all of the tested specimens. When the confining pressure was 2 MPa and the temperature increased from 20 to 400
C, the permeability decreased by 17.4% to 1.71  1015 m2. When the temperature increased from 400 to 600
C, the permeability increased rapidly. The formation of new cracks and the extension of the original cracks coalesced and widened the channels through which water flowed, thus improving the seepage properties. When the confining pressure was 2 MPa and the temperature increased from 20 to 600
C, the permeability increased by 47.3% to 3.05  1015 m2. During this stage, the mechanical behavior deteriorated significantly. When the temperature exceeded 600
C, the permeability increased slowly. The hightemperature treatment detached some particles from the sandstone materials and discomposed some clay minerals; the separated components blocked the seepage channels, thus decreasing permeability. Overall, the rate at which the permeability increased in this stage was more modest than that of the stage from 400 to 600
C. When the temperature exceeded 400
C, an exponential function was used to fit the permeability and temperature data:

700

800

(8)

When the confining pressures were 2, 5 and 8 MPa, the regression coefficients (R2) of the exponential functions were 0.992, 0.994 and 0.991, respectively, indicating that the exponential function fit the data well, as shown in Fig. 7. As the confining pressure increased, the permeability decreased. The confining pressure closed the axial cracks and prevented the development of macro fracture planes; the pressure caused deformation and blocked the seepage channels, resulting in a decreased permeability. As the confining pressure increased, the permeability initially showed a sharp decrease that later slowed down significantly, as shown in Fig. 8. When the temperature was 20
C and the confining pressure increased from 2 to 5 MPa, the permeability decreased from 2.07  1015 to 5.79  1016 m2, a decrease of 72.0%. When the confining pressure increased from 5 to 8 MPa, the permeability decreased from 5.79  1016 to 4.48  1016 m2, a decrease of 22.6%. A higher confining pressure led to a smaller effect of temperature on permeability, as shown in Fig. 6. When the

o

Temperature / C

(c) Confining pressure of 8MPa

Fig. 7. Relationship between permeability and temperature above 400
C in the water flow tests. All of the symbols represent average values of the experimental data, and the lines represent the theoretical value according to the exponential function (R2 is the regression coefficient of the fitting function).

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Q.-L. Ding et al. / Journal of Natural Gas Science and Engineering 34 (2016) 55e63

temperature ranged from 20 to 400
C, the rate of change varied within 9.5% with a confining pressure of 8 MPa, whereas the value reached 17.4% with a confining pressure of 2 MPa. When the temperature ranged from 500 to 800
C, the rate of change was less than 19.0% with a confining pressure of 8 MPa, whereas the value reached 56.5% with a confining pressure of 2 MPa. 6. Conclusions

Acknowledgments

Uniaxial compression experiments were performed on sandstone after high-temperature treatment. We then conducted water flow tests on fractured specimens using MTS815.02 and a self-made water flow apparatus to investigate the effect of temperature and confining pressure on permeability. The following conclusions can be drawn: (1) The mechanical behavior improved when the temperature was below 400
C; the peak strength and Young’s modulus increased by 13.1% and 6.7%, respectively, compared to 73.2 MPa and 13.4 GPa at room temperature. When the temperature exceeded 400
C, the volume increased rapidly; scanning electron microscopy showed that new cracks had formed, and the original cracks were extended. In this temperature range, the mechanical properties decreased significantly; the peak strength and Young’s modulus at 800
C were 35.7 MPa and 3.9 GPa, respectively, showing decreases of 51.2% and 70.9% compared to the room temperature values. (2) The permeability decreased slowly when the temperature was less than 400
C. The permeability increased rapidly as the temperature increased from 400 to 600
C. When the confining pressure was 2 MPa and the temperature increased from 20 to 600
C, the permeability increased to 3.05  1015 m2, which was an increase of 47.3%. When the temperature exceeded 600
C, the permeability increased slowly. An exponential function fit the permeability and temperature data well when the temperature was above 400
C. (3) As the confining pressure increased, the permeability initially decreased rapidly and then decreased at a more modest rate. When the temperature was 20
C, the permeability was 5.79  1016 and 4.48  1016 m2 with a confining

33

o

20 C o 200 C o 400 C o 500 C o 600 C o 700 C o 800 C

-16

Permeability k /×10 m

2

28 23 18 13 8 3

2

pressure of 5 and 8 MPa, respectively, indicating a decrease of 72.0% and 78.4% compared to the value at a confining pressure of 2 MPa. A smaller effect of temperature on permeability was observed at a higher confining pressure; the rate at which the permeability changed with a confining pressure of 8 MPa at each temperature was significantly lower than that with a confining pressure of 2 MPa.

5

8

Confining pressure σ3 /MPa Fig. 8. Relationship between permeability and confining pressure in the water flow tests. All of the symbols represent average values of the experimental data.

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