TY - JOUR
AU - Kang,, Yili
AB - Abstract The combined effects of gas sorption, stress and temperature play a significant role in the changing behavior of gas permeability in coal seams. The effect of temperature on nitrogen and methane permeability of naturally fractured coal is investigated. Coal permeability, P-wave velocity and axial strain were simultaneously measured under two effective stresses and six different temperatures. The results showed that the behavior of nitrogen and methane permeability presented nonmonotonic changes with increasing temperature. The variation in the P-wave velocity and axial strain showed a good correspondence with coal permeability. A higher effective stress limited the bigger deformation and caused the small change in permeability. Methane adsorption and desorption significantly influence the mechanical properties of coal and play an important role in the variations in coal permeability. The result of coal permeability during a complete stress–strain process showed that the variation in permeability is determined by the evolution of the internal structure. The increase in the temperature of the gas saturated coal causes the complex interaction between matrix swelling, matrix shrinkage and micro-fracture generation, which leads to the complex changes in coal structure and permeability. These results are helpful to understand the gas transport mechanism for exploiting coal methane by heat injection. coalbed methane, temperature, permeability, deformation, gas sorption, effective stress Introduction Permeability is one of the most important parameters for primary and enhanced coal seam gas production. The change in permeability is more complex and crucial for gas production from coal reservoirs (Shi and Durucan 2004, Palmer 2009). The actual change in permeability in coal reservoirs is dominated by the combined effects of effective stress and volumetric strain (Harpalani and Zhao 1989). The effective stress changes with the decrease in pore pressure in the depletion in production and increase in the gas injection process. Volumetric strain is caused by gas adsorption/desorption and temperature variations. Several researchers have studied and reported that permeability decreases exponentially with increasing stress (Durucan and Edwards 1986, McKee et al1987, Seidle et al1992). A decrease in permeability caused by increasing the effective stress using non-adsorbing gas and swelling using adsorbing gas have been observed (Pini et al2009, Pan et al2010). Investigation of the permeability of fractured coal using adsorbing and non-adsorbing gas has shown that the permeability measured by methane was lower than that by nitrogen (Somerton et al1975). Lin and Kovscek (2014) experimentally measured adsorption, volumetric strain and permeability on the same piece of core with the same apparatus. It is clear that permeability decreases with an increase in effective stress. Furthermore, gas adsorption and desorption causes coal matrix swelling and shrinkage, which changes the permeability. Methane sorption in coal has the adsorptive–absorptive character (Ceglarska-Stefaríska 1994). Significant changes in coal structure are found after successive cycles of methane adsorption–desorption processes. It indicates that gas sorption causes the rearrangement of the coal structure (Majewska and Ziętek 2007). Matrix shrinkage and damage induced by gas extraction are significant factors in the evolution and distribution of coal permeability. It is found that permeability considering the matrix damage will be enhanced as the coal matrix shrinks to some extent (Xie et al2015). The strength parameters of coal saturated with water, nitrogen and CO2 have been investigated. The result shows that saturation medium and pressure cause different behaviors in coal strength and permeability (Perera et al2011a). The permeability and mechanical properties of coal are influenced by the phase state for CO2 sequestration. It is found that super-critical CO2 presents a greater reduction in permeability and strength (Perera et al2011b, 2013, Perera and Ranjith 2012). A lot of research has shown that temperature also has a significant influence on coal permeability as well as stress and gas adsorption/desorption. A series of experiments on sandstones heated from 400 °C–800 °C have been carried out, and permeability increased by at least 50% (Somerton et al1965). The thermal expansion of heated rock is accompanied by new micro-fracture generation and increased permeability (Heard 1980). The effect of temperature on coal permeability has been investigated by many researchers from various perspectives. Experimental studies on the combined effect of stress, pore pressure and temperature on methane permeability have proposed that ultimate long-term permeability is dependent on the variation stage of stress–strain and temperature (Yin et al2013). Coal permeability for nitrogen changes slightly with temperatures ranging from 20 °C–300 °C (Zhao et al2010). The permeability curve versus temperature of up to 650 °C can be divided into four stages, which are controlled by different factors (Niu et al2014). The effect of temperature on nitrogen permeability was small due to its lower adsorption in coal matrix (Perera et al2012). Due to the different experimental conditions, a unified conclusion of the effect of temperature on coal permeability has not been obtained. But agreement on gas adsorption/desorption inducing variations in permeability has been reached. Nitrogen, methane and carbon dioxide are released from the coal surface with increasing temperature (Levy et al1997, Sakurovs et al2007, Crosdale et al2008). The coupling influence of coal deformation, gas flow and thermal transport makes the prediction of coal permeability complicated. A fully coupled model has been developed to investigate the evolution of coal permeability. The results show that local swelling and macro-swelling of coal matrix play a controlling role in the evolution of permeability (Qu et al2012). In this work, a series of experiments was conducted using both nitrogen and methane under different effective stresses and different temperatures. Gas permeability under triaxial stress and different temperatures was obtained from nitrogen and methane flow experiments. P-wave velocity and axial strain were also simultaneously observed in these experiments. The effect of gas adsorption/desorption, mechanical strength weakening and dynamic gas permeability is elaborated and discussed. Based on these results, the controlling effect of coal structure on the evolution of permeability is presented. 2. Experimental methods 2.1. Experiment sample The research area is located in the Ningwu Basin Shanxi province, China (figure 1). Coal samples were collected from No. 9 in the Taiyuan Formation, and the average thickness of it was 12 ∼ 20 m, the content of vitrinite in maceral was approximately 60% and the inertinite was approximately 40%, with the reflectance in vitrinite at 0.92% ∼1.16%. Figure 1. View largeDownload slide Location of Ningwu Basin, China. Figure 1. View largeDownload slide Location of Ningwu Basin, China. The results of proximate, petrographic analyses and mineral composition are summarized in tables 1 and 2. To mitigate the influence of the composition and structure, all experimental samples for permeability and mechanical testing were collected from the same block. Cylindrical samples were cored from the coal block along the bedding direction using a bit with an inner diameter of 1 inch. The top and bottom faces of the samples were polished flat. The drilled samples were added in a vacuum drying oven at 60 °C for 48 h. Micro-fractures were clearly observed in all of the samples (figure 2(a)). In order to prevent gas from leaking from the radial direction, the coal samples were wrapped with a thin silica gel before being installed into the cell to measure the permeability (figure 2(b)). To reduce experimental error, the samples with no visible difference were selected to perform the acoustic measurement. The samples with the similar results of P-wave and S-wave were chosen to conduct the same kind of permeability and mechanics experiment. Table 1. Proximate and petrographic analyses of the coal samples. Proximate analysis (%) Maceral composition (%) Sample FCad Vad Aad Mad Romax Vitrinite Inertinite Liptinite NW-9# 55.52 34.32 8.88 1.28 1.02 61.32 33.51 5.17 Proximate analysis (%) Maceral composition (%) Sample FCad Vad Aad Mad Romax Vitrinite Inertinite Liptinite NW-9# 55.52 34.32 8.88 1.28 1.02 61.32 33.51 5.17 View Large Table 1. Proximate and petrographic analyses of the coal samples. Proximate analysis (%) Maceral composition (%) Sample FCad Vad Aad Mad Romax Vitrinite Inertinite Liptinite NW-9# 55.52 34.32 8.88 1.28 1.02 61.32 33.51 5.17 Proximate analysis (%) Maceral composition (%) Sample FCad Vad Aad Mad Romax Vitrinite Inertinite Liptinite NW-9# 55.52 34.32 8.88 1.28 1.02 61.32 33.51 5.17 View Large Table 2. Mineral composition of the coal samples. Sample Quartz (%) Calcite (%) Iron Pyrite (%) Siderite (%) Clay (%) Organic matter (%) NW-9# 5 6 5 1 12 71 Sample Quartz (%) Calcite (%) Iron Pyrite (%) Siderite (%) Clay (%) Organic matter (%) NW-9# 5 6 5 1 12 71 View Large Table 2. Mineral composition of the coal samples. Sample Quartz (%) Calcite (%) Iron Pyrite (%) Siderite (%) Clay (%) Organic matter (%) NW-9# 5 6 5 1 12 71 Sample Quartz (%) Calcite (%) Iron Pyrite (%) Siderite (%) Clay (%) Organic matter (%) NW-9# 5 6 5 1 12 71 View Large Figure 2. View largeDownload slide Coal samples used in the experiments included (a) a piece of plug (b) wrapped with a silica gel. Figure 2. View largeDownload slide Coal samples used in the experiments included (a) a piece of plug (b) wrapped with a silica gel. 2.2. Experiment apparatus The permeability of coal was determined using a self-developed high temperature/high pressure triaxial gas adsorption-seepage equipment. This equipment consists of a loading system, triaxial compression chamber, temperature control system, pressure control system, data collection system and a supporting system. Axial displacement, axial stress, the inlet/outlet pressure and flow rate are monitored continuously. The P-wave velocity is measured at every steady state. The schematics of the equipment are illustrated in figure 3. The coal sample is wrapped in a thin silica gel before being installed into the cell. The cell is engineered to sustain a maximum axial pressure of 100 MPa, a maximum confining pressure of 90 MPa with the highest temperature reaching 150 °C. During the experiment, the equipment is designed to warm up automatically and maintain a constant temperature. Bearing acoustic transducers are installed in both of the indenters in the triaxial pressure cell. This kind of acoustic transducer, with a 960 KHz P-wave excitation frequency and 0.1 μs measurement accuracy, can bear an axial stress of 400 MPa. Interval transit time and waveform were collected synchronously during the experimental process. In the process of testing the P-wave velocity, a piezoelectric transducer produced an extremely short lasting-time high-voltage mechanical pulse on the coal sample. With this system, we can calculate the velocity of the acoustic wave by the received pulse time from the end to end of the test coal. The mechanical properties of the coal with gas adsorption were measured with a static (dynamic) state triaxial rock mechanics testing system (RTR-1000, GCTS). Figure 3. View largeDownload slide Triaxial cell with heating and acoustic collection system for the measurement of permeability. Figure 3. View largeDownload slide Triaxial cell with heating and acoustic collection system for the measurement of permeability. 2.3. Measurement of permeability under triaxial stress It is well known that methane has a stronger affinity for coal than nitrogen. In order to determine the effect of gas adsorption on permeability, nitrogen and methane were used to determine the permeability, P-wave velocity and axial strain of naturally fractured coal, respectively. The effective stress in this experiment was taken as 2 and 5 MPa. The test temperature was chosen to range from 30 °C–85 °C. The axial and confining pressures were maintained to be greater than the pore pressure to prevent gas leakage during the test process. In order to eliminate the flow pressure fluctuation, the inlet and outlet pressure should keep constant during the the entire process of the test. The inlet pressure is controlled by constant pressure injecting pump. The outlet pressure is controlled by installing a backpressure valve, which can ensure pressure fluctuation below 0.01 MPa. According to the Terzaghi effective stress formula, the effective stress applied on the coal sample can be calculated based on equation (1). begin{eqnarray}\left\{\begin{array}{l}{{\sigma }_{a}}^{^{\prime} }={\sigma }_{a}-{P}_{1}\\ {{\sigma }_{c}}^{^{\prime} }={\sigma }_{c}-\displaystyle \frac{1}{2}({P}_{1}+{P}_{2})\\ {\sigma }_{e}=\displaystyle \frac{1}{3}({\sigma }_{a}+2{\sigma }_{c})-\displaystyle \frac{1}{2}({P}_{1}+{P}_{2})\end{array}\right.,\end{equation} σa′=σa−P1σc′=σc−12(P1+P2)σe=13(σa+2σc)−12(P1+P2), 1 where σe is average effective stress, MPa; σa is the axial stress, MPa; | ${\sigma }_{a}^{\prime} $ | σa′ the is effective axial stress, MPa; σc is the radial stress, MPa; | ${\sigma }_{{\rm{c}}}^{\prime} $ | σc′ is the effective radial stress, MPa; P1 is the inlet pressure, MPa; and P2 is the outlet pressure, MPa. The dimensions and experimental parameters of sample are listed in table 3. Given that the purpose of this paper is to investigate the effect of gas sorption and temperature on permeability, the stress level was only set to realize the triaxial stress state and gain on the impact of effective stress. The test procedures are as follows: (1) The evacuated experimental coal sample was put into the triaxial pressure cell and other auxiliary apparatus was installed; (2) The experimental system was evacuated for about 2 h; (3) The confining pressure, axial pressure and backpressure were set to a set-point and maintained the pressure stabilization; (4) The gas continuously was displaced to promote gas adsorption on the coal sample; (5) The sample was heated at 30, 40, 50, 60, 70 and 85 °C, respectively, when the core reached adsorption equilibrium (setting 24 h as the equilibrium time in this experiment); (6) The gas flow was monitored by a gas flowmeter, and the gas flow, P-wave velocity and axial strain data were recorded when the gas flow stabilized. Table 3. Sample dimension and experimental parameters. Coal sample Length (cm) Diameter (cm) Gas Effective stress (MPa) Axial pressure (MPa) Confining pressure (MPa) Inlet pressure (MPa) Outlet pressure (MPa) NW-1 3.89 2.52 N2 2 2.12 3.50 1.05 1.03 NW-2 3.75 2.53 5 5.15 6.50 0.95 0.93 NW-3 3.98 2.50 CH4 2 2.12 3.50 1.05 1.03 NW-4 3.54 2.53 5 5.15 6.50 0.95 0.93 Coal sample Length (cm) Diameter (cm) Gas Effective stress (MPa) Axial pressure (MPa) Confining pressure (MPa) Inlet pressure (MPa) Outlet pressure (MPa) NW-1 3.89 2.52 N2 2 2.12 3.50 1.05 1.03 NW-2 3.75 2.53 5 5.15 6.50 0.95 0.93 NW-3 3.98 2.50 CH4 2 2.12 3.50 1.05 1.03 NW-4 3.54 2.53 5 5.15 6.50 0.95 0.93 View Large Table 3. Sample dimension and experimental parameters. Coal sample Length (cm) Diameter (cm) Gas Effective stress (MPa) Axial pressure (MPa) Confining pressure (MPa) Inlet pressure (MPa) Outlet pressure (MPa) NW-1 3.89 2.52 N2 2 2.12 3.50 1.05 1.03 NW-2 3.75 2.53 5 5.15 6.50 0.95 0.93 NW-3 3.98 2.50 CH4 2 2.12 3.50 1.05 1.03 NW-4 3.54 2.53 5 5.15 6.50 0.95 0.93 Coal sample Length (cm) Diameter (cm) Gas Effective stress (MPa) Axial pressure (MPa) Confining pressure (MPa) Inlet pressure (MPa) Outlet pressure (MPa) NW-1 3.89 2.52 N2 2 2.12 3.50 1.05 1.03 NW-2 3.75 2.53 5 5.15 6.50 0.95 0.93 NW-3 3.98 2.50 CH4 2 2.12 3.50 1.05 1.03 NW-4 3.54 2.53 5 5.15 6.50 0.95 0.93 View Large 2.4. Measurement of permeability during complete stress–strain process In order to investigate the effect of the coal structure on permeability, the experiments of nitrogen and methane permeability during complete stress–strain process were conducted using two coal samples with similar P-wave velocity and initial permeability. The test procedures are as follows: (1) The treated coal sample was put into the triaxial pressure cell and experimental apparatus was prepared according to the experimental scheme; (2) The confining pressure was set to 15 MPa and the system temperature was increased to 40 °C after a certain axial pressure was applied to withhold the sample; (3) A vacuum pump was used to evacuate the coal sample for about 2 h; (4) A gas flow pressure of 2 MPa at the inlet and a backflow pressure of 1 MPa at the outlet were applied, and then the gas was displaced continuously for 48 h to make the gas fully adsorbing on the coal sample; (5) The gas flowmeter was open to record and the permeability was calculated when the core reached adsorption equilibrium; (6) Axial pressure was gradually increased under a stress loading mode of 0.01 MPa s−1 at constant confining pressure and the measurement of the strain–stress and the permeability of the saturated coal sample until the coal sample fully failed; (7) The measurements of N2 and CH4 were conducted according to the above steps. 2.5. Data analysis The coal permeability was calculated according to Darcy law with the assumption of a linear relationship between the gas flow and pressure gradient. The gas permeability of the coal can be calculated based on equation (2). |$ begin{eqnarray}K=\displaystyle \frac{2Q\mu L{P}_{0}}{A({P}_{1}^{2}-{P}_{2}^{2})},\end{equation} $| K=2QμLP0A(P12−P22), 2 where Q is the rate of the gas flow, m3/s; μ is the gas dynamic viscosity, Pa · s; L is the length of the sample, m; P0 is the atmospheric pressure, 0.1 MPa; and A is the cross-sectional area of the sample, m2; P1 and P2 are inlet pressure and outlet pressure, respectively, MPa. The P-wave velocity was determined with a computer based on the interval transit time, which was obtained from the waveform of each data point, and the deformation and stress data in the process of loading. The P-wave velocity in the process of loading can be calculated based on equation (3). |$ begin{eqnarray}v=\displaystyle \frac{L(1-\varepsilon )}{T-{T}_{0}}\times {10}^{3},\end{equation} $| v=L(1−ɛ)T−T0×103, 3 where v is the P-wave velocity, km/s; T is the sum of the delay time of both the detector and the test material, μs; T0 is the delay time in the apparatus loop, μs; L is the length of the sample, mm; and ɛ is the axial strain of the sample during the loading process, mm. 3. Results 3.1. Permeability with increasing temperature Nitrogen and methane permeabilities of naturally fractured coal were measured with the increasing of the temperature at the effective stress of 2 and 5 MPa. The permeability–temperature curves under different effective stresses are presented in figure 4. There was a nonlinear variation in permeability with the increase of the temperature. The curves of nitrogen and methane permeability were divided into three stages with the temperature of 30 °C ∼ 85 °C. The first stage showed that permeability decreased with the increase of the temperature varied from 30 to 60 °C. The increasing temperature caused the thermal expansion stress by coal matrix and mineral expansion, thus swelling in all directions. Thermal expansion was limited by triaxial stress and unable to expand to the outer space, which only occupied the space of the pores and fractures. Higher temperature made the coal matrix soften and increased plasticity. Pores in the matrix tend to close under the stress condition. The decline in permeability was attributed to the flow channel narrowing or closing under the function of effective stress. The increasing temperature was believed to accelerate gas desorption, opening up the cleats and thus increasing the permeability. The change in permeability is determined by the coupling effect of the effective stress and matrix shrinkage (Lin et al2008). The degree of declined permeability before 50 °C was larger than that after 50 °C. This phenomenon can be possibly attributed by two factors: (1) the decrease in matrix swelling; and (2) the increase in matrix shrinkage. The degree of the decline in permeability is controlled by the combined effect of these two factors. The second stage presented that permeability increases slightly for nitrogen and significantly for methane when the temperature was raised to 70 °C. Methane permeability at this temperature went beyond the initial value at 30 °C under the effective stress of 2 MPa. Thermal stress gradually increased with the elevating temperature. Due to anisotropic expansion and the difference between the mineral thermal expansion coefficient, the internal structure was changed and micro-fractures were generated. The third stage reported that the permeability declined at a temperature up to 85 °C. Expansion stress dominated the variation in permeability by the closing of the micro-fractures under the condition of triaxial stress. Figure 4. View largeDownload slide Relationship of permeability–temperature under different effective stresses. Figure 4. View largeDownload slide Relationship of permeability–temperature under different effective stresses. 3.2. P-wave velocity and axial strain In order to observe the relationship of coal structure and permeability, the P-wave velocity was obtained at the corresponding state of the permeability testing. The variation trend of the P-wave velocity is basically consistent with that of permeability through the relation of P-wave velocity to temperature (figure 5). P-wave velocity increases with the decreasing porosity and fracture density (Wang et al2015). The P-wave reflects the degree of compaction and the continuity of the coal samples. The variation in the P-wave velocity appears as characteristics of the three stages with increasing temperature.Due to the enhancement of compaction and continuity, velocity of the P-wave increases in the first stage (the temperature of 30 °C–60 °C). The closure of pores and fractures and no new fracture formation lead to the decrease in permeability. The increasing degree of the P-wave velocity for the nitrogen saturated coal samples is greater than that of the methane saturated coal samples. It shows that the nitrogen saturated coal samples have a higher degree of compaction than that of the methane saturated coal samples. In other words, thermal expansion and matrix shrinkage of the methane saturated samples counteract each other with the increase in temperature in this stage. The P-wave velocity presents an obvious decline in the second stage (the temperature rising up to 70 °C). The generation of fractures within the coal lead to the decrease in the P-wave velocity (Cai et al2014). As a result of the generation of fractures, the propagation resistance of the P-wave increases in the coal sample. The P-wave velocity has a dramatic increase in the third stage (the temperature to 85 °C). It indicates that the closure of fractures occurs and makes the samples tight. Figure 5. View largeDownload slide Changing curves of the P-wave velocity with the increasing temperature. Figure 5. View largeDownload slide Changing curves of the P-wave velocity with the increasing temperature. In order to observe the relationship of coal deformation and permeability, the axial strain was also obtained at the corresponding state of the permeability testing. The increasing trend of axial strain is obtained with the increase of temperature (figure 6). The variation of axial strain also has the stage characteristics with increasing temperature. The slight change in axial strain shows that internal expansion dominates the coal deformation when the temperature is below 60 °C. When the temperature is higher than 60 °C, the axial strain has a significant increase. Based on the result of coal structure variation by wave velocity analysis, the new fractures are generated and cause the bigger volumetric deformation. Due to the effect of restriction by confining pressure, coal expansion is realized in the form of axial strain, thus increasing the axial strain. The increasing range of axial strain is smaller at higher effective stress under the condition of triaxial stress. The deformation of coal is affected by the restriction of the higher stress. Figure 6. View largeDownload slide Changing curves of the axial strain with the increasing temperature. Figure 6. View largeDownload slide Changing curves of the axial strain with the increasing temperature. 4. Discussion 4.1. Gas adsorption and desorption Adsorption and desorption experiments of N2 and CH4 were conducted at 30 °C and the ultimate pressure of 12 MPa using a volumetric method. Measured adsorption isotherms can be well fitted by the Langmuir function (figure 7). The Langmuir parameters are obtained from the experimental adsorption isothermal curves. The Langmuir volume and Langmuir pressure of N2 is 6.31 cm3 g−1 and 3.19 MPa. The Langmuir volume and Langmuir pressure of CH4 is 15.46 cm3 g−1 and 3.01 MPa. The adsorption ability of methane is stronger than that of nitrogen. That means that more of the adsorption/retention volume of the methane is taken up by coal at the same pressure during the adsorption and desorption process. Figure 7. View largeDownload slide Adsorption/desorption isotherms of coal. Figure 7. View largeDownload slide Adsorption/desorption isotherms of coal. In order to determine the difference between nitrogen and methane on gas permeability under the conditions of this study, gas permeability at the temperature of 30 °C is defined as the initial permeability. This initial permeability is used to calibrate the permeability under different effective stresses and temperatures by dimensionless treatment. The results show that the permeability is not monotonically decreasing and the lower change in permeability appears with the increase of the effective stress in the three stages for nitrogen and methane (figure 8). Coal permeability decreases with the increase in effective stress under the triaxial compression condition (Aziz et al2013, Feng et al2016). The reason for permeability decrease with the increase of effective stress is attributed to the compression of pores and closure or disconnection of micro-fractures (Durucan and Edwards 1986, Li et al2009, Zhang et al2016). However, the change in gas permeability presents an obvious difference with increasing temperature for nitrogen and methane. The degree of the decline in nitrogen permeability is greater than that of methane. In particular, the permeability dramatically increases when the temperature was increased up to 70 °C, especially methane permeability goes beyond the initial value at 2 MPa. This means that the thermal stress is greater than the effective stress and generates new fractures as a gas flow path. The value of methane permeability fluctuates widely and is inconsistent with the sole effect of stress and temperature. It shows that the gas desorption is beneficial in mitigating permeability damage. Figure 8. View largeDownload slide Dimensionless permeability of naturally fractured coal. Figure 8. View largeDownload slide Dimensionless permeability of naturally fractured coal. The changing rate of permeability at different stages is calculated in table 4, and a negative/positive value means the decrease/increase in permeability. Both the permeability of nitrogen and methane decrease in the first stage and the third stage, and mitigate or even increase in the second stage. Increasing temperature causes the yield deformation of coal at the initial compaction and the elastic deformation stages (Yin et al2013). Gas desorption can significantly affect the volumetric change in micro- and macropores in coal, leading to coal shrinkage (Wang et al2010). Volumetric swelling of methane adsorption is greater than that of nitrogen adsorption (Connell et al2016). Matrix shrinkage with gas desorption make fractures open and lead to an increase in coal permeability (Gray 1987). The higher temperature accelerates the release of adsorbed gas from the coal surface and causes matrix shrinkage (Charrière et al2009). The bigger the gas desorption, the bigger the coal matrix shrinkage. The change in permeability is determined by the competition of thermal expansion, matrix shrinkage and fracture propagation. Thermal expansion dominates the change in permeability in the first stage and the third stage. Matrix shrinkage and fracture propagation control the variation of permeability in the second stage. Table 4. Changing rate of permeability for nitrogen and methane. Change rate of permeability (%) Gas Effective stress (MPa) 30 °C ∼ 60 °C 60 °C ∼ 70 °C 70 °C ∼ 85 °C N2 2 −46.71 1.63 −3.08 5 −37.97 7.17 −10.34 CH4 2 −23.45 28.16 −36.09 5 −12.86 3.11 −5.83 Change rate of permeability (%) Gas Effective stress (MPa) 30 °C ∼ 60 °C 60 °C ∼ 70 °C 70 °C ∼ 85 °C N2 2 −46.71 1.63 −3.08 5 −37.97 7.17 −10.34 CH4 2 −23.45 28.16 −36.09 5 −12.86 3.11 −5.83 View Large Table 4. Changing rate of permeability for nitrogen and methane. Change rate of permeability (%) Gas Effective stress (MPa) 30 °C ∼ 60 °C 60 °C ∼ 70 °C 70 °C ∼ 85 °C N2 2 −46.71 1.63 −3.08 5 −37.97 7.17 −10.34 CH4 2 −23.45 28.16 −36.09 5 −12.86 3.11 −5.83 Change rate of permeability (%) Gas Effective stress (MPa) 30 °C ∼ 60 °C 60 °C ∼ 70 °C 70 °C ∼ 85 °C N2 2 −46.71 1.63 −3.08 5 −37.97 7.17 −10.34 CH4 2 −23.45 28.16 −36.09 5 −12.86 3.11 −5.83 View Large 4.2. The change in mechanical properties In order to determine the weakening of the mechanical property of coal after gas adsorption, the triaxial mechanics experiments of nitrogen and methane saturated coal were conducted. The stress–strain curves of the coal samples saturated with nitrogen and methane are shown in figure 9. The ultimate strength of the gas saturated coal increases with the increase of confining pressure. Due to the stronger affinity for methane than that for nitrogen, the mechanical strength of coal reduced under the same confining stress. It is worth noting that stress–strain curves appear slightly serrated before approaching the peak stress. The coupling effect of fractures initiation, propagation and closure with the increase in axial stress as the major reason for this phenomenon. The structure of coal changes gradually and leads to tensile failure during the axial loading process. The stress drops when a new fracture is generated in the coal sample, and the extent of the change in the stress depends on the fracture size generated. The integral part of the coal could withstand the load, which led to the stress increases again until the emergence of another fracture. During the deformation process, a certain amount of elastic strain energy is accumulated. The elastic energy is suddenly released, which leads to the shear failure of the coal sample. Figure 9. View largeDownload slide Stress–strain curves of the coal samples under the triaxial test. Figure 9. View largeDownload slide Stress–strain curves of the coal samples under the triaxial test. Based on the results of the triaxial test, the parameters of the rock mechanics were calculated (table 5). The compressive strength of the gas saturated samples is 63.71 MPa and 60.58 MPa for nitrogen and methane, respectively, under the confining pressure of 15 MPa. The compressive strength of the methane saturated sample is reduced by 4.91%. The same result of the saturated sample is obtained under the confining pressure of 10 MPa. These results indicate that the adsorption of gas weakens the mechanical properties of the coal samples. The weakening of the mechanical properties of the saturated coal samples are dependent on ability of the gas adsorption. The cohesive force and the angle of the internal friction are obtained by data fitting according to the Mohr–Coulomb law. The results show that both the cohesive force and the angle of the internal friction have a bigger decrease for the methane saturated coal samples. Table 5. Triaxial test and parameters of the rock mechanics. Coal sample Gas Length (cm) Diameter (cm) Confining pressure (MPa) E (GPa) υ σ (MPa) C (MPa) φ(°) NW-7 N2 5.09 2.48 15 6.92 0.38 63.71 5.79 35.39 NW-8 5.14 2.50 10 4.31 0.32 49.95 NW-9 CH4 4.95 2.51 15 5.69 0.45 60.58 5.22 35.06 NW-10 5.09 2.48 10 3.98 0.44 47.08 Coal sample Gas Length (cm) Diameter (cm) Confining pressure (MPa) E (GPa) υ σ (MPa) C (MPa) φ(°) NW-7 N2 5.09 2.48 15 6.92 0.38 63.71 5.79 35.39 NW-8 5.14 2.50 10 4.31 0.32 49.95 NW-9 CH4 4.95 2.51 15 5.69 0.45 60.58 5.22 35.06 NW-10 5.09 2.48 10 3.98 0.44 47.08 View Large Table 5. Triaxial test and parameters of the rock mechanics. Coal sample Gas Length (cm) Diameter (cm) Confining pressure (MPa) E (GPa) υ σ (MPa) C (MPa) φ(°) NW-7 N2 5.09 2.48 15 6.92 0.38 63.71 5.79 35.39 NW-8 5.14 2.50 10 4.31 0.32 49.95 NW-9 CH4 4.95 2.51 15 5.69 0.45 60.58 5.22 35.06 NW-10 5.09 2.48 10 3.98 0.44 47.08 Coal sample Gas Length (cm) Diameter (cm) Confining pressure (MPa) E (GPa) υ σ (MPa) C (MPa) φ(°) NW-7 N2 5.09 2.48 15 6.92 0.38 63.71 5.79 35.39 NW-8 5.14 2.50 10 4.31 0.32 49.95 NW-9 CH4 4.95 2.51 15 5.69 0.45 60.58 5.22 35.06 NW-10 5.09 2.48 10 3.98 0.44 47.08 View Large 4.3. Dynamic permeability of gas flow The dynamic change in permeability is dependent on the change in the coal structure. A significant change in axial load is induced by the external force disturbance during the process of primary and enhanced methane production. In order to determine the effect of the coal structure on permeability, the experiments of the measurement of the permeability during the complete stress–strain process were conducted. The permeability–strain curves have a similar change tendency as the stress–strain curves, indicating that the change in permeability is closely related to the evolution of the internal structure (figure 10). The permeability of the coal sample fluctuates slightly during the initial axial loading, and decreases induced by compaction and nonlinear deformation with the further increase in stress. The stress–strain curves of the samples almost change linearly with the increase in axial stress. The permeability decreases with the increase in axial strain. The pores and micro-fractures are compressed and closed, thus decreasing the flow channel. The change in the volume of the coal varies from compression to expansion between the yield point and peak stress. Relative displacement by the shearing force is generated within the coal. The permeability of nitrogen and methane test begins to increase after a period of decrease. This behavior of the methane test is more obvious than that of the nitrogen test, indicating that the permeability–strain curve lags behind the stress–strain curve. The seepage channels in coal are connected by a number of pores and fractures. During the stress–strain process, the flow channels are constantly changing under the effect of an external force. Gas needs to continually find the new connective channel, leading to gas flow lagging behind the channel formation. The internal fractures appear to be an unstable extension and form the vertical transfixion pattern after the peak stress, thus increasing the permeability drastically. In the residual strength stage, the fractures are filled with the coal powder generated by the breaking of the coal and closed under the effect of confining pressure. The permeability tends to decrease with the increase in axial strain. Figure 10. View largeDownload slide Relation curves of permeability during the complete stress–strain process. Figure 10. View largeDownload slide Relation curves of permeability during the complete stress–strain process. The evolution of permeability is controlled by the coal structure from the changes in the permeability of the complete stress–strain process. It is clear that an increase in effective stress will close the flow channel and lead to the decrease in coal permeability. Gas adsorption and desorption cause matrix swelling and shrinkage, respectively. The variation in temperature leads to both coal matrix deformation and the change in gas adsorption capacity, which usually counteract each other. The temperature has become a crucial factor for the exploiting coal methane by heat injection and CO2 sequestration in deep coal seams. However, the changing behavior of coal structure is a coupled thermal-hydraulic-mechanical-chemical process and its impacts on the evolution of coal permeability still need to be further researched. 5. Conclusions In this work, the experiments of permeability for nitrogen and methane were conducted under different effective stresses and temperatures. The purpose was to investigate the effect of gas sorption and temperature on the permeability of naturally fractured coal under triaxial stress. The P-wave velocity and axial strain were concurrently obtained during the process of measuring the permeability. The measurement of the permeability during the complete stress–strain process was conducted to determine the effect of structural changes on permeability. Based on the results, the following understanding and conclusions were drawn: The fluctuation characteristics of nitrogen and methane permeability were found with the increase in temperature: first decrease, then increase and decrease again. The changing trend of gas permeability presents an obvious difference between nitrogen and methane. This implies that permeability is determined by the combined effects of stress, gas sorption and temperature. The rate of change rate in the permeability of nitrogen and methane showed that the change in permeability is determined by the competition of thermal expansion, matrix shrinkage and fracture propagation. Thermal expansion dominates the change in permeability in the first stage and the third stage. Matrix shrinkage and fracture propagation control the variation in the permeability in the second stage. The variation trend of the P-wave velocity is basically consistent with that of permeability. The results indicate that the change in the internal structure of coal occurs to change during the process of increasing temperature. Due to the effect of restriction by confining pressure, coal expansion is realized in the form of axial strain. The results of axial strain show that matrix swelling is induced by the increase of temperature and affected by stress and gas sorption. 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TI - Effect of temperature on the permeability of gas adsorbed coal under triaxial stress conditions
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2140/aa9a98
DA - 2018-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/effect-of-temperature-on-the-permeability-of-gas-adsorbed-coal-under-H0RxIfrU6P
SP - 386
VL - 15
IS - 2
DP - DeepDyve
ER -