ENERGY EXPLORATION EXPLOITATION

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1 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals by Guiqiang Zheng, Zhejun Pan, Zhongwei Chen, Shuheng Tang, Luke D. Connell, Songhang Zhang and Bo Wang reprinted from ENERGY EXPLORATION & EXPLOITATION Volume Number MULTI-SCIENCE PUBLISHING CO. LTD. 5 Wates Way, Brentwood, Essex CM15 9TB, United Kingdom

2 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number pp Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals Guiqiang Zheng 1,2, Zhejun Pan 2,*, Zhongwei Chen 3, Shuheng Tang 1, Luke D. Connell 2, Songhang Zhang 1 and Bo Wang 4 1 School of Energy and Resources, China University of Geosciences, Beijing 10083, P.R. China 2 CSIRO Earth Science and Resource Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton,Victoria 3168, Australia 3 School of Mechanical Engineering, The University of Western Australia, WA, 6009, Australia 4 School of Resource and Geosciences, China University of Mining and Technology, Xuzhou, , P.R. China *Author for corresponding. Zhejun.Pan@csiro.au (Received 8 August 2011; accepted 15 March 2012) Abstract Coal permeability is regarded as one of the most critical parameters for the success of coalbed methane recovery. It is also a key parameter for enhanced coalbed methane recovery via CO 2 and/or N 2 injection. Coal permeability is sensitive to stress and cleat compressibility is often used to describe how sensitive the permeability change to stress change for coal reservoirs. Coalbed methane exploration and production activities and interest of enhanced coalbed methane recovery increased dramatically in China in recent years, however, how permeability and cleat compressibility change with respect to gas species, effective stress and pore pressure have not been well understood for Chinese coals, despite that they are the key parameters for primary and enhanced coalbed methane production. In this work, two dry Chinese bituminous coal samples from Qinshui Basin and Junggar Basin are studied. Four gases, including H e, N 2, CH 4 and CO 2 are used to study permeability behaviour with respect to different effective stresses, pore pressures, and temperatures. The effective stress is up to 5 MPa and pore pressure is up to 7 MPa. Permeability measurements are also carried out at highest pore pressures for each adsorbing gas, at three temperatures, 35, 40 and 45 C. The experimental results show that gas species, effective stress and pore pressure all have significant impact on permeability change for both coal samples. Moreover, the results demonstrate that cleat compressibility is strongly dependent on effective stress. More importantly, the results show that cleat compressibility is also strongly dependent on pore pressure. Cleat compressibility initially decreases with pore pressure increase then it increases slightly at higher pore pressures. However, temperature only has marginal impact on permeability and cleat compressibility change. Keywords: Coalbed methane, Enhanced coalbed methane recovery, Effective stress, Adsorption, CO 2

3 452 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals NOMENCLATURE Gibbs n ads Adsorbed amount of injected gas (mmol/g); C f Cleat compressibility (MPa 1 ); Z Compressibility factor of gas (MPa 1 ); Pump Conditions in the pump; Cell Conditions in the cell; R Gas constant (8.31J/mol 1 /K 1 ). β Gas compressibility (MPa 1 ); k o Initial permeability (md); σ 0 Initial stress (MPa); n inj Injected amount of gas (mmol); k Permeability (md); ν Poisson s ratio; P p Pore pressure (MPa); P Pressure (MPa); P d Pressure of the downstream cylinder (MPa); P d,0 Pressure of downsteam cylinder at initial stage (MPa); P u Pressure of the upstream cylinder (MPa); P u,0 Pressure of upstream cylinder at initial stage (MPa); V R Sample volume (m 3 ); L Sample length (m); σ Stress (MPa); t Time(s); T Temperature ( C); Gibbs n unads Unadsorbed amount of injected gas (mmol/g); V d Volume of the downstream cylinders (m 3 ); V u Volume of the upstream cylinders (m 3 ); V Volume injected from the gas injection pump (m 3 ); µ Viscosity (Pa/s); V void Void volume (m 3 ); E Young s modulus (MPa); 1. INTRODUCTION According to the new evaluation by the Chinese National Ministry of Land and Resources, the coalbed methane (CBM) resource is estimated at trillion m 3 at depth less than 2000 m in the 42 major basins in China (Liu et al., 2009; MLR, 2009). Despite the huge potential in CBM, production is still low in China even after dramatic increase of CBM exploration and production activities in the past few years. This is mainly due to the three-low characteristics of the Chinese coal seams: low reservoir pressure, low permeability and low gas saturation (Luo et al., 2009; Zhang et al., 2001; Zheng, 2005).

4 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number Coal reservoir permeability is regarded as one of the most critical parameters for the success of CBM recovery (Gu and Chalaturnyk, 2005; Palmer, 2009; Shi and Durucan, 2010; Ding et al., 2011). It is directly controlled by the key fracture (cleat) attributes including size, spacing, connectedness, aperture and degree of mineral fill, and patterns of preferred orientation on local and regional scales (Laubach et al., 1998; Yao et al., 2009). During primary and enhanced CBM processes, gases flow through the cleat system to/from the wells due to pressure difference. Coal reservoir permeability is sensitive to the reservoir stress conditions and the gas sorption induced swelling/shrinkage behaviour (Palmer, 2010; Zhou et al., 2011). This can be demonstrated through one of the widely used permeability models, the Shi and Durucan permeability model (S&D model): ν Eε σ σ0 = ( P P ) + V 0 1 ν 3( 1 ν) (1) where σ is the effective horizontal stress, σ 0 is the effective horizontal stress at the initial reservoir pressure, ε V is the volumetric swelling/shrinkage strain, ν is the Poisson s ratio, E is the Young modulus, P is pore pressure, P 0 is the initial pore pressure (Shi and Durucan, 2004; 2005). Pore pressure here means the fluid pressure in the cleat. To relate the permeability with effective stress, the equation below is used (Seidle et al., 1992; Shi and Durucan, 2004): 3 k k e c f ( σ σ0 ) = 0 (2) where k is the permeability, k 0 is the initial permeability, c f is the cleat compressibility, σ is the stress, σ 0 is the initial stress. Cleat compressibility, analogous to pore volume compressibility of conventional reservoirs, is a measure of the cleat volume change ratio with respect to pore pressure change as defined by (Seidle et al., 1992): c f 1 = φ f φ f P p (3) where φ f is cleat porosity, and P p is pore pressure. As can be seen from Eq. (2), cleat compressibility, c f, is the key parameter linking the stress change to permeability change. Obtaining the cleat compressibility from field test has been difficult due to equipment constrains. It is also expensive to measure and the results of such measurements are often ambiguous (Seidle et al., 1992). Few field cleat compressibility measurements have been reported, for instance, McKee et al. (1988) obtained the cleat compressibility of psi 1 (or MPa 1 ) for a San Juan Basin coal seam. Nevertheless, estimations of cleat compressibility can be easily carried out in the laboratory through a series of permeability measurements with respect to effective stress. Based on the bundled matchstick geometry, Seidle et al. (1992) derived the relationship between the permeability and effective stress as shown

5 454 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals in Eq. (2). Thus, cleat compressibility, c f, can be obtained by fitting experimental data using Eq. (2). However, most of the pervious measurements on stress-permeability relationship have been using water and/or air (Dabbous et al., 1974; Durucan and Edwards, 1986; Seidel et al., 1992; Rose and Foh, 1984) and sometimes CH 4 (Somerton et al., 1975). Thus they do not represent the impact from other gases. The impact from different gases is of interest because during enhanced coalbed methane recovery (ECBM), gases such as CO 2 are injected. Recently, Pan et al. (2010) studied cleat compressibility for an Australia coal from Sydney Basin using three different gases, H e, CH 4 and CO 2. Their results showed that cleat compressibility vary significantly with gas species and pore pressure. Nevertheless, their results do not show the relationship between cleat compressibility and effective stress. In permeability calculation and reservoir simulation, cleat compressibility was often treated as constant (McKee et al., 1988; Pekot and Reeves, 2003; Pomeroy and Robinson, 1967; Puri and Seidle, 1991). However, other studies have shown that cleat compressibility is not constant with respective to pore pressure and effective stress (Palmer and Mansoori, 1996; Palmer and Mansoori, 1998; Rushing, 2008; Palmer, 2009; Shi and Durucan, 2010) and cleat compressibility may change exponentially with respect to effective stress change for some coals (McKee et al., 1988; Zhou et al., 2011). Thus, better understanding of how permeability and cleat compressibility change with respect to gas species, pore pressure and effective stress is of great interest, since they are key parameters for CBM/ECBM processes. Furthermore, in ECBM process, N 2 and CO 2, are often injected at temperatures different to the seam temperature (eg., Fujioka et al., 2010; Wong et al., 2006; van Bergen et al., 2006). Other studies (Cui and Bustin, 2005; Elsworth, 1989) have shown that permeability may be sensitive to temperature. However, the impact of temperature on permeability and cleat compressibility is not yet well understood. Study of permeability and cleat compressibility change with respective to gas species, pore pressure and effective stress is rare for Chinese coals, although it would aid the fast developing CBM industry in China. In this work, two bituminous coal samples from Qinshui Basin and Junggar basin of China have been studied. Gases, including H e, N 2, CH 4 and CO 2 are used to study the permeability and cleat compressibility behaviour. H e is considered as non-adsorbing to coal and used as comparison to adsorbing gases. Measurements using different gases at different temperatures are also carried out for ECBM applications since CO 2 and N 2 are often injected at temperatures different to the seam temperature. 2. EXPERIMENTAL METHODS 2.1. Experimental apparatus Experimental apparatus used in this study is a triaxial permeability cell for measurements of gas adsorption and permeability under hydrostatic conditions. The sample cell and other parts of the apparatus are in a temperature-controlled cabinet to maintain constant temperature during the experiment. The schematic plot of the apparatus is shown in Figure 1. Gas is first filled to the Injection Pump A and then injected from Pump A to the sample. After the coal sample reaches adsorption equilibrium, which usually takes a few days to a few weeks depending on the coal, the transient method is applied to measure permeability. Permeability can be calculated

6 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number PRV Coal sample NV PT-5 BV-9 BV-10 PT-6 Upstream cylinder PG PG Downstream cylinder To vacuum machine PG BV-5 BV-3 BV-2 BV-6 BV-8 Confining pump (B) BV-1 Pump box Injection pump (A) Gases BV: Ball valve NV: Needle valve PG: Pressure gauge DP: Differential pressure PT: Pressure transducer 3V: Three way valve PRV: Pressure relief valve Figure1. Schematic plot of the experimental apparatus. from the pressure decay curve measured by a differential pressure transducer installed between the upstream and downstream cylinders. The detailed calculation methods for adsorption and permeability are described in the later sections.

7 456 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals In this work, four gases, H e, N 2, CH 4 and CO 2, are used in sequence to measure the adsorption and permeability at four pore pressure and five confining pressure steps. Measurements are also carried out at three different temperatures when pore pressure is at maximum for each gas. After all experiments for a gas are completed, the coal sample and the system are vacuumed for several days to remove the residual gas before switching to a new gas Sample origin and core preparation Two bituminous coal samples, from Changzhi city of Qinshui Basin and Tiechanggou coal field of Junggar Basin, are studied in this work. The sites for the samples are shown in Figure 2. Qinshui Basin is one of the focal areas for CBM exploration and production in China. Besides the increasing CBM activities in this basin, China s first CO 2 injection in coal to enhance coalbed methane recovery project was carried out in Qinshui Basin (Wong et al., 2006). The CBM resource estimate of Jungaar Basin is 3.83 trillion m 3, which is more than 10% of the national CBM resource estimate (Liu et al., 2007; Tao et al., 2009). Jungaar Basin is also an area with increasing CBM exploration and development activities. Therefore, studying coal samples from these two selected areas will be of great interest for better understanding the CBM/ECBM processes for Chinese coals. The coal samples are collected from the depth of 480 m from Changzi city of Qinshui Basin and 210 m for Tiechanggou coal field of Jungaar Basin. The proximate analysis data for the two samples are summarised in Table 1. The coal samples are Sampling sites Junggar Basin Qinshui Basin Figure 2. Sampling site in Qinshui and Junggar Basins of China.

8 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number Table1. Summary of proximate analysis data. Sample Moisture Volatiles Fixed carbon Ash R o % CZ TCG cored in parallel to the bedding plane in the face cleat direction to a cylindrical shape each with 50 mm in diameter and 100 mm in length. parallel to the plane in the face cleat direction to a cylindrical shape each with 50 mm in diameter and about 100 mm in length. The core sample from Changzi city of Qinshui Basin is named as CZ-1, while the core sample from Tiechanggou coal field of Jungaar Basin is named as TCG-1. Their cylindrical surfaces are smoothed out with plaster to prevent rough surface from damaging the thin lead foil, which is to prevent gas (especially CO 2 ) from diffusing to the sleeve and confining fluid. Plaster is only applied on the cylindrical surfaces so that the cleat structure is not affected. The core sample is then put into a heated vacuum oven at 100 C for several days to remove the moisture. Weight is measured twice a day till it is totally dry. The sample is then wrapped with a thin lead foil then a rubber sleeve before it is installed in the cell. After the core sample is installed, it is vacuumed for three days to remove the inherent gas and be prepared for the experiments Experimental procedures and data analyses Adsorption For the adsorption experiment, the gas of interest is first filled into pump A, an ISCO- 260 syringe pump which can accurately track the volume change. Then the valve (valve NV) between pump A and the core sample is opened to allow gas to flow from pump A to the sample. After the gas adsorption reaches equilibrium, valve NV is closed to be ready for the permeability measurements. Gas adsorption and permeability measurement are repeated to higher pore pressures to obtain adsorption isotherm and permeability behaviour with respect to pore pressure. The excess adsorption amount is calculated through the mass balance equation (Pan et al., 2010): Gibbs Gibbs P V nads = ninj nunads = ZRT pump PVvoid ZRT cell (4) Where void volume, V void, is predetermined using a series of helium injection, since helium is considered to be non-adsorptive to coal. Eq. (5) is used to calculate the absolute adsorption from the measured excess adsorption: n Abs ads = n Gibbs ads ρads ρ ρ ads gas (5) where ρ ads is the adsorbed phase density, ρ gas is the gas phase density.

9 458 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals In this study, all gas compressibility factors and densities of H e, N 2, CH 4 and CO 2 are calculated from the NIST webbook: webbook.nist.gov/chemistry/fluid/ Permeability The permeability measurements follow the adsorption measurement after reaching adsorption equilibrium. For a permeability measurement, the upstream cylinder is charged to the pressure about 30 kpa above the pore pressure or gas pressure in the core sample, while the downstream cylinder pressure is charged to about 30 kpa below the pore pressure, with the same gas as in the core sample. Then the valves between the up and downstream cylinders and the core sample (valves BV-9 and BV-10) are opened to allow gas to flow through the core sample from the upstream cylinder to the downstream cylinder. The diffirential pressure change between the cylinders is monitored by the diffirential pressure transducer and is used in permeability calculation. Confining pressure is provided by another ISCO syringe pump, pump B. In this study, five effective steps, 1 MPa, 2 MPa, 3 MPa, 4 MPa and 5 MPa, are used to obtain the permeability stress relationship at each pore pressure. Effective stress in this work means the difference between confining pressure and pore pressure. The transient method of Brace et al. (1968) is used to measure the permeability. The upstream and downstream cylinders are charged with gas of interest to pressures slightly above and below the cell pressure after reaching adsorption equilibrium, respectively. The Brace method involves observing the decay of the differential pressure between upstream and downstream cylinders across the sample. This pressure decay is combined with the cylinder volumes in the analysis to relate the flow through the sample and thus determine the permeability. The pressure decay curve can be modeled as (Brace et al., 1968): ( Pu Pd) ( P P ) u, 0 d, 0 = e αt (6) Permeability k is linked to the time constant, α, by: k α = + µβ L V R V V u d (7) The four gases, H e, N 2, CH 4 and CO 2, are used in sequence to measure the permeability at different pore and confining pressures and three different temperatures Cleat compressibility From the permeability-stress relationship as shown in Eq. (2), cleat compressibility can be obtained by fitting permeability-stress curves (Seidle et al., 1992; Pan et al., 2010).

10 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Adsorption The adsorption isotherms measured using N 2, CH 4 and CO 2 for CZ-1 and TCG-1 are plotted in Figures 3 and 4, respectively. It can be seen from these two figures that CH 4 and N 2 adsorption isotherms for both coals are similar. However, the adsorbed amount of CO 2 is about 1.5 times that of CH 4 adsorbed over the pressure range for CZ-1 and about 2 times for TCG-1. Furthermore, the adsorbed amount of CO 2 is about 3 times that of N 2 adsorbed for CZ-1 and about 4 times for TCG-1. This shows that coal sample from the Jungaar basin is more preferential to CO 2 adsorption. The differences in adsorption capability for different gases is likely caused by the difference in coal composition, mineral matters and pore structures for the coals (Li et al., 2010). Langmuir volume, V L, and Langmuir pressure, P L, are summarised in Table 2. From this table, it can be seen that Langmuir volume increases and Langmuir pressure decreases in the sequence of using N 2, CH 4 and CO 2 for both samples. The Langmuir volume is 19.53, 32.15, m 3 /t for CZ-1 and 25.25, 27.78, m 3 /t for TCG-1, while the Langmuir pressure is 3.17, 2.34, 2.00 MPa for CZ-1 and 6.94, 2.25, 2.32 MPa for TCG-1, for N 2, CH 4 and CO 2, respectively. The Langmuir volume and Langmuir pressure for TGC-1 measured using N 2 and CO 2 are larger than that for CZ-1; however, they are slightly smaller for TCG-1 than CZ-1 when using CH Permeability Effective stress-permeability relationship Permeability with respect to different pore pressures, effective stresses and temperatures using H e, N 2, CH 4 and CO 2 are measured. The four gases are used in sequence to measure the permeability with four pore pressure steps for each gas. An Absolute adsorption (cm 3 /g) Langmuir model fit N 2 adsorption CH 4 adsorption CO 2 adsorption Pore pressure (MPa) Figure 3. Adsorption isotherms for CZ-1 using N 2, CH 4, CO 2 at 35 C.

11 460 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals Langmuir model fit N 2 adsorption CH 4 adsorption CO 2 adsorption 40 Absolute adsorption (cm 3 /g) Pore pressure (MPa) Figure 4. Adsorption isotherms for TCG-1 using N 2, CH 4, CO 2 at 35 C. Table 2. Langmuir constants for N 2, CH 4 and CO 2. CZ-1 TCG-1 Gas species V L (m 3 /t) P L (MPa) V L (m 3 /t) P L (MPa) N CH CO exception is for sample CZ-1 using CH 4 with only three pore pressure steps performed. For each gas species, all the permeability measurements are performed at 35 C first. When the pore pressure reached maximum pore pressure at around 6 to 7 MPa, the system temperature is increased to 40 C and then 45 C to measure permeability. At each pore pressure, permeability is measured at five confining pressure steps to study the permeability stress relationship. All the permeability measurement results are summarized in Tables 3 and 4 for coal samples CZ-1 and TCG-1, respectively. It can be seen from both Tables 3 and 4 that with increasing of pore pressure, permeability decreases gradually at the same pressure difference between confining and pore pressures. Pressure difference between confining and pore pressures is a special case for effective stress with effective stress

12 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number Table 3. Summary of permeability of CZ-1. H e N 2 CH 4 CO 2 Pore Effective Pore Effective Pore Effective Pore Effective pressure stress Perm pressure stress Perm pressure stress Perm pressure stress Perm T ( C) (MPa) (MPa) (md) (MPa) (MPa) (md) (MPa) (MPa) (md) (MPa) (MPa) (md)

13 462 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals Table 4. Summary of permeability of TCG-1. H e N 2 CH 4 CO 2 Pore Effective Pore Effective Pore Effective Pore Effective pressure stress Perm pressure stress Perm pressure stress Perm pressure stress Perm T ( C) (MPa) (MPa) (md) (MPa) (MPa) (md) (MPa) (MPa) (md) (MPa) (MPa) (md)

14 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number coefficient to be unity. In this work, we use effective stress to describe pressure difference as a convenience. Although effective stress coefficient is often considered as unity for coal in the literature (eg., Seidle et al., 1992), this is not to mean that effective stress coefficient is unity for the coals in this work. The permeability behaviour is consistent with the findings from previous research (Pan et al., 2010; Pini et al., 2009). The causes of permeability decrease with respect to pore pressure may be coal swelling to partially close cleat aperture during the experimental conditions (Connell et al., 2010; Han et al., 2010; Liu and Rutqvist, 2010; Huang et al., 2010) and/or effect from effective stress coefficient (Chen et al., 2011). Moreover, permeability decreases with gas species from H e, N 2, CH 4 to CO 2 in sequence at the same pore pressure and same effective stress, meaning that at the same conditions, permeability measured using CO 2 is smaller than using CH 4, which is smaller than using N 2 and then H e. The causes may also be the impact of swelling on cleat aperture since at the same pore pressure the swelling strain, although not measured in this work, is expected to increase from N 2, to CH 4 and then CO 2 due to its relationship with adsorption capacity (Pan and Connell, 2007). To investigate the permeability change with respect to effective stress, permeability is measured at different confining pressures for each pore pressure step. Confining pressure change is equal to the effective stress change when the pore pressure is kept at constant. Figures 5 and 6 show the permeability results using CO 2 for CZ-1 and TCG-1, respectively, as an illustration. Other gases show similar trend thus not plotted. Permeability is plotted with respective to effective stress at constant pore pressures. It can be seen from Figure 5 that permeability decreases dramatically with increasing effective stress for CZ-1. Permeability decreases from md to md with effective stress from 1 MPa to 5 MPa at pore pressure of 1.7 MPa, which is about 80% change in permeability. Meantime, it can also be seen from the figure that permeability decreases with Permeability (md) Pore pres = 1.7 Mpa Pore pres = 3.2 Mpa Pore pres = 4.6 Mpa Pore pres = 6.1 Mpa Effective stress (MPa) Figure 5. Permeability of CZ-1 measured using CO 2 at different pore pressures.

15 464 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals Figure 6. Permeability of TCG-1 measured using CO 2 at different pore pressures. increasing pore pressures at the same effective stress, for instance, the permeability decreases from md to md when pore pressure changes from 1.7 MPa to 6.1 MPa at the same effective stress of 1 MPa. This is about 57% reduction in permeability. Figure 6 shows that permeability also decreases dramatically with increasing effective stress for TCG-1. Permeability decreases from 1.52 md to 0.27 md with effective stress from 1 MPa to 5 MPa at pore pressure of 1.84 MPa, which is almost 82% reduction in permeability. Furthermore, permeability decreases with increasing pore pressure at the same effective stress, for instance, it decreases from 1.52 md to 0.51 md when pore pressure changes from 1.75 MPa to 6.1 MPa at effective stress of 1 MPa. This is almost 66% reduction in permeability. To compare the permeability between the two samples, it can be seen from Tables 3 and 4 and Figures 5 and 6 that permeability of TCG-1 is higher than that of TC-1when measured using the same species of gas at the same conditions. For example, the CO 2 permeability of TCG-1 at 1.0 MPa effective stress and 1.84 MPa pore pressure is almost 9 times as high as that of CZ-1 at similar conditions. Furthermore, the impact of effective stress on the permeability change is different. The CO 2 permeability of TCG-1 at 5.0 MPa effective stress and 1.84 MPa pore pressure is about 7 times as high as that of CZ-1 at similar conditions. These differences demonstrate that the cleat structures for the two core samples are different and their responses to stress are different and also suggest that their interactions with gases are different.

16 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number Temperature effect on permeability The permeability results of CZ-1 and TCG-1 measured using N 2, CH 4 and CO 2 with different temperature steps, which are 35 C, 40 C and 45 C, are also summarised in Tables 3 and 4, respectively. During each temperature step, it normally takes more than a day to reach the sorption equilibrium before permeability measurements are taken. Permeability is only measured at the highest pore pressure for each gas to study the temperature effect. At each temperature step, permeability is measured at different confining pressures as well. As can be seen from the tables, permeability measured at lower temperature tends to be marginally higher than that measured at higher temperatures for N 2 and CH 4, but slightly lower for CO 2. The effect of increased temperature on permeability is speculated on the effect of temperature on coal strain change from a combined effect of reduced adsorption induced coal swelling and increased thermal expansion due to elevated temperature. However, swelling strain is not measured in this work and it is difficult to separate the adsorption induced coal swelling and thermal expansion even if it is measured. Thus, the cause for this temperature effect on permeability is unclear. Further study will be required to investigate the impact with a larger temperature range to be considered since in field CO 2 injection to enhance coalbed methane recovery experiments, liquid CO 2 has been injected at a temperature much lower than the seam temperature (Wong et al., 2006). However, this will require significant modifications to the current experimental apparatus. To illustrate the effect of temperature on permeability, Figures 7 and 8 are plotted to show the CH 4 permeability results with respect to effective stress for CZ-1 and TCG-1, respectively. It shows that impact of temperature on permeability is only marginal with a temperature change of 10 degrees, although it shows a trend of T = 35, Pore pres = 6.6 MPa T = 40, Pore pres = 7.0 MPa T = 45, Pore pres = 7.3 MPa Permeability (md) Effective stress (MPa) Figure 7. Permeability of CZ-1 measured using CH 4 at different temperatures.

17 466 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals T = 35, pore pres = 6.1 MPa T = 40, pore pres = 6.1 MPa T = 45, pore pres = 6.1 MPa Permeability (md) Effective stress (MPa) Figure 8. Permeability of TCG-1 measured using CH 4 at different temperatures. permeability decrease with temperature increase. A possible explanation is the combined adsorption induced coal swelling and thermal expansion effect on cleat opening, which then affect the permeability. However, since the strain is not measured in this work, it is not possible to examine the relationship between permeability and strain at different temperatures Cleat compressibility The method to calculate the cleat compressibility has been described in the previous section by fitting permeability curves using Eq. (2) (Seidle et al., 1992) Effective stress-cleat compressibility relationship In this study, cleat compressibility is calculated by fitting permeability curves with maximum effective stress up to 5 MPa. The cleat compressibility with a smaller effective stress range (up to 3 MPa) is also calculated to show the cleat compressibility relationship with effective stress. However, during the fitting process, the phenomenon that cleat compressibility value varies largely with selected effective stress span was observed. For example, cleat compressibility of CZ-1 measured using N 2 at pore pressure of 1.1 MPa is MPa 1 when the effective stress changes from 1 to 3 MPa; while it decreases to MPa 1 when the effective stress changes from 3 to 5 MPa. Therefore, cleat compressibilities with every 3 MPa effective stress difference, which is 1 to 3 MPa, 2 to 4 MPa and 3 to 5 MPa, are calculated separately in this study. The cleat compressibility results with different effective stress spans are summarised in Tables 5 and 6 for CZ-1 and TCG-1, respectively. It can be seen from these two tables that cleat compressibility has a strong dependence on selected

18 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number Table 5. Summary of cleat compressibility of CZ-1. H e N 2 CH 4 CO 2 Pore Effective Pore Effective Pore Effective Pore Effective pressure stress c f pressure stress c f pressure stress c f pressure stress c f T ( C) (MPa) (MPa) (MPa 1 ) (MPa) (MPa) (MPa 1 ) (MPa) (MPa) (MPa 1 ) (MPa) (MPa) (MPa 1 ) Average

19 468 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals Table 6. Summary of cleat compressibility of TCG-1. H e N 2 CH 4 CO 2 Pore Effective Pore Effective Pore Effective Pore Effective pressure stress c f pressure stress c f pressure stress c f pressure stress c f T ( C) (MPa) (MPa) (MPa 1 ) (MPa) (MPa) (MPa 1 ) (MPa) (MPa) (MPa 1 ) (MPa) (MPa) (MPa 1 ) Average

20 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number effective stress span. This stress dependent cleat compressibility was also observed from previous research (Durucan and Edwards, 1986). The decrease in cleat compressibility may be because there is an irreducible cleat volume to make cleat less compressible at higher stress (Liu and Rutqvist, 2010). Cleat compressibility values calculated with effectives stress change from 1 to 5 MPa are also included in Tables 5 and 6 for CZ-1 and TCG-1, respectively, as comparison to the cleat compressibility values calculated using different stress ranges Pore pressure-cleat compressibility relationship The cleat compressibility calculated at various pore pressures for CZ-1 and TCG-1 are plotted in Figures 9 and 10, respectively. Figure 9 shows the cleat compressibility at various pore pressures at the effective stress of 1 3 MPa for CZ-1; while Figure 10 shows the cleat compressibility at various pore pressures at effective stress of 2 4 MPa for TCG-1. Cleat compressibility at other effective stress spans has the similar trend as those shown in Figures 9 and 10, thus, they are not plotted. All the results from the four gases are included in the figures. Trend lines using polynomial equations are included to help visually identifying the trend of cleat compressibility with regards to pore pressure for each gas. It can be seen from Figures 9 and 10 that cleat compressibility using different gases differ quite significantly. Cleat compressibility measured using CO 2 is the largest and followed by that measured using CH 4, N 2, and H e at the same pore pressure. One Cleat compressibility (MPa 1 ) CO 2 CH 4 N 2 H e Pore pressure (MPa) Figure 9. Cleat compressibility of CZ-1 by H e, CH 4 and CO 2 with respect to pore pressure at 35 C (effective stress span = 1~3 MPa).

21 470 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals CO 2 Cleat compressibility (MPa 1 ) CH 4 N H e Pore pressure (MPa) Figure 10. Cleat compressibility of TCG-1 by H e, CH 4 and CO 2 with respect to pore pressure at 35 C (effective stress span = 2~4 MPa). possible explanation for this gas species dependence is that it is related to adsorption induced coal swelling. The adsorption of gas causes the matrix swelling, part of which decreases the cleat porosity at the experimental conditions. As can be seen from Eq. (3), cleat compressibility increases with porosity decrease if the porosity change with respect to stress change is constant. More importantly, it can be seen from Figures. 9 and 10 that cleat compressibility tends to decrease initially and then increase slightly with pore pressure increases. Roughly, when the pore pressure is lower than 5 MPa for CZ-1 and 4 MPa for TCG-1, the cleat compressibility decreases with pore pressure increases, however, it increases slightly when pore pressure is higher than 5 MPa for CZ-1 and 4 MPa for TCG-1 for all adsorbing gases. This means that at low pore pressure stage, the cleat compressibility decreases with increasing pore pressure, thus, the impact of effective stress on permeability decreases with pore pressure increase. While at high pore pressure stage, the cleat compressibility increases with increase of pore pressure, which means that the impact of effective stress on permeability increases with increasing pore pressure. The reason for these behaviours may be because that the resistance of coal matrix with increasing pore pressure decreases gradually at low pressure stage, while it increases again at high pressure stage. It also can be seen from the Figures 9 and 10 that when pore pressure is close to 0 MPa, the cleat compressibility tends to reach a common value, which is about MPa 1 for CZ-1 and about MPa 1 for TCG-1. This behaviour is expected

22 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number since the cleat compressibility should be a fixed value using any gas at infinitely low pressure. Based on the cleat compressibility values in Tables 5 and 6, the averaged cleat compressibility measured using N 2, CH 4, and CO 2 at 35 C is calculated and summarised as the average in Tables 5 and 6 to further illustrate the impact of gas species on cleat compressibility. From the tables it can be seen that for CZ-1 the averaged cleat compressibility value is , , , and MPa 1 when using H e, N 2, CH 4 and CO 2 at 35 C, respectively. This represents about 17.3% change from using H e to CO 2. For TCG-1, the averaged cleat compressibility is , , and MPa 1 for H e, N 2, CH 4 and CO 2 at 35 C, respectively. This also represents about 17.6% change from using H e to CO 2. It should be noted that comparison using averaged cleat compressibility is only to illustrate the impact from gas species. When using cleat compressibility to calculate permeability, the impact from gas species, effective stress, pore pressure and temperature should be all considered Temperatures impact on cleat compressibility It can be seen from Table 6 that when temperature rises from 35 to 45 C, cleat compressibility changes only slightly. In order to further study the impact of temperature on cleat compressibility, cleat compressibility changes with temperature are plotted in Figures 11 and 12 for CZ-1 and TCG-1, respectively. Figure 11 shows that cleat compressibility varies with temperature at the same pore pressure at about 7.1 MPa for CZ-1; while Figure 12 shows that cleat compressibility varies with temperature at the same pore pressure of 6.1 MPa for TCG-1. It can be seen from Figures 11 and 12 that the cleat compressibility measured using each species of gas are almost the same with respect to temperature increase, for instance, the cleat compressibility increases only from to about when temperature changes Cleat compressibility (MPa 1 ) CO 2 CH 4 N Temperature ( C) Figure 11. Cleat compressibility of CZ-1 at different temperatures (pore pressure 7.0 MPa).

23 472 Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals 0.15 Cleat compressibility (MPa 1 ) CO 2 CH 4 N Temperature ( C) Figure 12. Cleat compressibility of TCG-1 at different temperatures (pore pressure = 6.1 MPa). from 35 C to 45 C using CH 4 for TCG-1. The trend lines in Figures 11 and 12 show that cleat compressibility only change slightly with respect to temperature. Therefore, temperature has little impact on cleat compressibility. 4. CONCLUSION In this study, gas adsorption and permeability of two bituminous coals from Changzhi city of Qinshui Basin and Tiechanggou coal field of Junggar Basin are measured using four gases, H e, N 2, CH 4 and CO 2 at three temperature steps, 35, 40 and 45 C. Permeability with respect to gas species, pore pressure, effective stress and temperature are studied. Moreover, cleat compressibility with respect to gas species, pore pressure, effective stress and temperature are calculated. The results show that adsorption behaviour for CZ-1 and TCG-1 using N 2 and CH 4 are similar, however, CO 2 adsorption on two coals are quite different suggesting that different gas may adsorb to different sites in the coal. The results also show that effective stress and pore pressure have significant impact on permeability. Permeability decreases dramatically with increasing effective stress at the same pore pressure. Permeability also decreases significantly with increasing pore pressures at the same effective stress. Moreover, gas species also has important impact on permeability. However, temperature has only slight impact on permeability change. Large temperature change may be required to observe its impact on permeability change. More importantly, cleat compressibility is strongly dependent on effective stress. It is also dependent on pore pressure and gas species. Cleat compressibility decreases firstly and then increase slightly with pore pressure increases. However, temperature impact on cleat compressibility is not obvious.

24 ENERGY EXPLORATION & EXPLOITATION Volume 30 Number ACKNOWLEDGEMENTS The financial support from the Australian and Chinese Government Joint Coordination Group on Clean Coal Technology Research & Development Scheme is greatly acknowledged. This research is also funded by the Fundamental Research Funds for the Central Universities, National Basic Research Program of China (Grant No. 2009CB219604), National Natural Science Foundation of China (Grant Nos , ), National Major Research Program for Science and Technology of China (Grant No. 2011ZX ) and Program for Changjiang Scholars and Innovative Research Team in University (IRT0864). REFERENCES Brace W.F., Walsh J.B. and Frangos W.T., Permeability of granite under high pressure. Journal of Geophysical Research 73(6), Chen Z., Pan Z., Liu J., Connell L.D. and Elsworth D., Effect of the effective stress coefficient and sorption-induced strain on the evolution of coal permeability: experimental observations. International Journal of Greenhouse Gas Control 5, Connell L.D., Lu M. and Pan Z., An analytical coal permeability model for triaxial strain and stress conditions. International Journal of Coal Geology 84(3), Cui X.J. and Bustin R.M., Volumetric strain associated with methane desorption and its impact on coalbed gas production from deep coal seams. AAPG Bulletin 89(9), Dabbous M.K., Reznik A.A., Table J.J. and Fulton P.F., The permeability of coal to gas and water. SPE J Dec, Ding S.L., Zhu J.G., Zhen B.P., Liu Z.Y. and Liu Q.F., Characteristics of high rank coalbed methane reservoir from the XiangningMining Area, Eastern Ordos Basin, China. Energy Exploration and Exploitation 29(1), Durucan S. and Edwards J.S., The effects of stress and fracturing on permeability of coal. Mining Science and Technology 3(3), Elsworth D., Thermal permeability enhancement of blocky rocks: Onedimensional flows. International Journal of Rock Mechanics and Mining Sciences 26(3 4), Fujioka M. and Yamaguchi S. and Nako M., CO 2 -ECBM field tests in the Ishikari coal basin of Japan. International Journal of Coal Geology 82(3 4), Gu F. and Chalaturnyk R., Analysis of coalbed methane production by reservoir and geomechanical coupling simulation. Journal of Canadian Petroleum Technology 44(10), Han F., Busch A., Krooss B,M., Liu Z., Van W.N. and Yang J., Experimenal study on fluid transport processes in the cleat and matrix systems of coal. Energy & Fuels 24(12),

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