Evolution Of Coal Permeability Research Articles

Influence of hygroscopic expansion of clay minerals on the dynamic evolution of coal permeability

Exploring the evolution characteristics of hygroscopic expansion of clay minerals in coal on its permeability has a positive effect on improving coal seam permeability. Firstly, the dynamic evolution model of coal permeability affected by hygroscopic expansion was established. Secondly, numerical simulation is carried out after an adaptive multi-field multiphase coupling tester verifies the model. Finally, the evolution law of coal permeability under different confining pressures and different moisture absorption expansion coefficients is analyzed. Research shows that the permeability of coal samples decreases first and then increases with the stress in the shape of a deep “V”. Due to the hydration and softening of clay minerals caused by hygroscopic expansion, the peak stress and permeability decreased by 3.8% and 1.69%, respectively, and the plastic strain increased by 6%. In this process, the change of permeability of axially compressed coal samples plays a leading role. Before reaching the yield stress, the influence of hygroscopic expansion on the change of permeability of the coal sample is more obvious. The research results enrich the theory of permeability dynamic evolution and have great significance in guiding coalbed methane exploitation.

Thermo-Hydro-Mechanical Coupling Numerical Simulation on Mechanical Heterogeneity of Coal Rock

Coal rock is a porous medium composed of organic matter and inorganic minerals, and it is very complex and highly heterogeneous. Coal bed methane (CBM) production is a thermo-hydro-mechanical (THM) coupling process in heterogeneous coal rock. THM coupling numerical simulation on the coal rock by considering the effect of mechanical heterogeneity is rarely reported. We use Weibull’s probability density distribution function to characterize the heterogeneity in elastic modulus of the coal rock, establish a THM coupling 3D finite element model of the coal rock by considering the variation in pore pressure caused by methane desorption, the linear thermal expansion effect, and coal rock skeleton shrinkage and deformation, and analyze variation in permeability, porosity, stress, temperature, and pore pressure within the coal rock representative elementary volume (REV) of variable mechanical heterogeneity with the cross-coupling correlation between permeability and porosity, and thermal field, stress field, and pressure field. The results show that the evolution of porosity and permeability in the coal rock is a THM coupling process related to mechanical heterogeneity, thermal expansion effect, pore pressure change caused by CBM desorption, and stressed deformation in the coal rock skeleton. The permeability and porosity fluctuate within the heterogeneous coal rock. The permeability and porosity fluctuate more frequently in the coal rock with stronger mechanical heterogeneity. The mechanical heterogeneity promotes local stress concentration. The time for variation in the stress through the whole the coal rock REV and the value of the first principal stress increase when the coal rock heterogeneity is enhanced. Under the THM coupling effect, the strong heterogeneity of the coal rock causes fluctuation in the thermal field. The evolution of coal porosity and permeability is a THM coupling process. This study provides theoretical guidance for CBM exploitation.

Coal permeability evolution triggered by variable injection parameters during gas mixture enhanced methane recovery

Injection of gas mixture (CO2/N2) into coal seam can both enhance methane recovery and reduce carbon dioxide emission. Coal permeability determines the mass transport speed, which acts an important index in evaluating gas injectability and methane recovery rate. To reveal effects of injected pressure, temperature and CO2 component in mixture on coal permeability evolution, the heat-fluid-solid coupling model in coal seam was proposed and series of simulations on gas mixture enhanced methane recovery were carried out. Results show that the continuous injected gas mixture replaces and drives methane toward the extraction borehole. The coal permeability is related to the heat or gas sorption induced coal swelling/shrinkage, gas pressure and geostress induced deformation, and coal mechanical properties. The coal permeability increases around the extraction borehole as CH4 pressure drops. There appears declining permeability zone as the arrive of CO2, resulting by the greater adsorption affinity than that of N2 and CH4. The higher the injected pressure, the greater increase of permeability. Coal permeability decreases slightly with the injected temperature, however changes dramatically with the varying of injected CO2 component. The effects of injection parameters on permeability evolution are CO2 component, injection pressure, and injection temperature successively.

Modeling and experimental study on methane diffusivity in coal mass under in-situ high stress conditions: A better understanding of gas extraction

Gas desorption and diffusion in pore is not only an important behavior to control the law of gas migration, but also an important theoretical basis for the proposal and optimization of extraction technology. At present, researches on gas diffusion in coal were mainly conducted on granular coal under the stress-free condition. However, on-site gas desorption and diffusion in coal seams occur in coal mass under the in-situ stress condition. In particular, high stress exerts a notable effect on gas diffusion in the deep mining stage. Therefore, experimental study and theoretical model establishment concerning the diffusion characteristics of coal mass under high stress require further investigation. In this work, first, with respect to the desorption and diffusion characteristics of coal mass under high stress, desorption and diffusion experiments were performed on coal mass under different high stress conditions with the aid of a self-developed experimental device, and the corresponding gas desorption laws were obtained. In addition, the law of coal mass permeability evolution under different stresses was obtained through the transient measurement method. Moreover, a coal mass diffusion model which conformed to the characteristics of high stress was established based on reasonable assumptions, a new physical structure model of coal under high stress and the relationship between diffusion length and stress. Moreover, this model, according to the laws of gas desorption under different conditions, it is found that the diffusion coefficient increases with the rise of adsorption equilibrium pressure, and stress is significantly proportional to the diffusion coefficient. Besides, with reference to the relationship between diffusion coefficient and stress value, the fundamental parameter that affects the desorption and diffusion amount, i.e., effective diffusion coefficient, was analyzed. Based on the variations of the effective diffusion coefficient with stress, the mechanism of how the classical pressure relief mining technology to improve gas extraction efficiency was further elaborated. The research results provide a more comprehensive theoretical support for the application of the pressure relief mining technology in deep high-stress coal seams.

On Permeability Evolution of Coal Induced by Temperature, Creep, and Matrix–Fracture Interaction

Research on the permeability evolution of coal with the influence of temperature, matrix–fracture interaction, and creep deformation has theoretical guiding significance in gas control of deep coal mines. An investigation of creep–seepage experiments and theoretical models is carried out at different temperatures. The experimental results show the decrease of permeability induced by the increase of matrix deformation during the creep process. A positive relation between temperature and matrix deformation caused by creep and thermal expansion leads to a lower permeability in the creep–seepage process. Combined with the experimental results of creep deformation and permeability evolution, a new permeability model is developed with the effect of temperature during the matrix–fracture interaction and creep process. A coupling internal swelling coefficient is adopted to characterize the matrix–fracture interaction induced by gas adsorption and thermal expansion. Moreover, the fractional derivative transient pulse method is used to calculate the experimental permeability data, and the validation of the permeability model is performed using experimental data. The proposed model can describe the experimental permeability evolution, which is consistent with the creep–seepage process.

Long-term effect of desorption-induced matrix shrinkage on the evolution of coal permeability during coalbed methane production

In a coal reservoir, matrix shrinkage caused by gas desorption is regarded as a significant factor that can influence natural fracture permeability. It is considered to be a constant and lasting effect throughout coalbed methane (CBM) production based on previous investigations. However, experimental measurements of long-term permeability change have demonstrated that adsorption-induced strain has a time-dependent effect on permeability evolution. This study presents long-term investigations on permeability evolution of coal reservoir during CBM production. A fully coupled two-phase flow model for gas-water is specially proposed. Furthermore, the independent impacts of the matrix swelling during gas adsorption are characterized by a strain-rate-based permeability model. The competing effect between pressure depletion and desorption shrinkage on the permeability change is evaluated during the process of long-term production. The calculations of the proposed model are respectively consistent with experimental and field data and the findings show that matrix shrinkage is the most important factor in permeability evolution towards the wellbore. The effect of matrix shrinkage on permeability enhancement is significant in the primary stage and vanishes with uniform depressurization in the matrix. As a result, matrix shrinkage dominates permeability first, followed by effective stress. A sensitivity analysis of mechanical properties and flow properties on permeability evolution was performed and has demonstrated that the larger the matrix shrinkage-induced strain is, the higher the permeability enhancement is, which can be also enhanced with greater initial permeability and a lower diffusion coefficient of the matrix and the impact of the negative strain induced by decreasing pore pressure is solely related to the bulk modulus. The range of permeability variation is narrowed when porosity is high, which implies that the influence of matrix shrinkage and effective stress on permeability is suppressed. This research will advance the understanding of the permeability change during long-term CBM production.

Influence of Hygroscopic Expansion of Clay Minerals on Dynamic Evolution of Coal Permeability

Influence of Hygroscopic Expansion of Clay Minerals on Dynamic Evolution of Coal Permeability

Studies on Gas Seepage Characteristics in Different Stress Zones of Bottom Coal in Steeply Inclined and Extra-Thick Coal Seams under Mining Action.

The gas released from the bottom coal of the horizontal slicing mining face in steeply inclined and extra-thick coal seams seriously threatens the safety of the upper slicing mining face. To explore the seepage characteristics of bottom coal gas, the coal deformation and gas permeability evolution law of four coal samples in different stress zones of bottom coal in the working face were analyzed through true triaxial fluid–solid coupling seepage experiments. At the same time, the seepage capacity of bottom coal gas was partitioned according to the field test. The results show the following: (1) The gas permeability of the bottom coal stress concentration zone first decreased and then increased with axial pressure loading and confining pressure unloading. The gas permeability of the bottom coal stress relief zone increased rapidly with decreasing axial pressure and confining pressure. The gas permeability of the bottom coal stress recovery zone gradually decreased with the cyclic loading and unloading of axial pressure and tended to be stabilized. (2) The evolution law of gas permeability in the bottom coal was closely related to the damage and deformation of coal. (3) From the original stress zone to the stress recovery zone, the gas seepage capacity of bottom coal can be divided into four zones, namely, the original seepage zone, the seepage reduction zone, the seepage sharp increase zone, and the seepage reduction zone. The gas seepage capacity in the stress concentration zone was more substantial than that of the stress recovery zone. The results of this study are of great significance for strengthening the dynamic disaster prevention and control of bottom coal gas in the horizontal slicing mining face of steeply inclined and extra-thick coal seams.

Coal strain and permeability evolution in a 2D fractured model based on computerized tomography image reconstruction

Coal heterogeneity, especially fracture distribution, affects the mechanical response and fluid seepage in the coal reservoir under complex stress conditions, which is important for exploiting coalbed methane and preventing gas outburst. The internal pores and fractures of coal samples were scanned using an X-ray industrial CT detection system in the State Key Laboratory of Coal Resources and Safe Mining of China University of Mining and Technology (Beijing). Based on CT images, 3D models of coal samples were reconstructed using MIMICS reconstruction and digital terrain model threshold segmentation. The number, size, and fractal dimension of pores and fractures were calculated basing from the vertical section of the 3D models to evaluate the heterogeneity of coal samples, and 2D numerical models of fractured coal samples were established using the Image Import module of COMSOL Multiphysics. Coal strain and permeability evolution during the reduction of gas pressure under constant confining stress were calculated in the numerical models. Results show that the distribution of pores and fractures is dominant to coal permeability. With gas pressure decreasing, the distribution of strain and gas flow rate in the coal samples is inhomogeneous, the maximum of which is observed at the fractures. Furthermore, the shrinkage and gas flow rate of the samples reduce with decreasing gas pressure, thereby alleviating the inhomogeneous distribution of coal strain and gas flow rate. As the gas pressure is decreased, the porosity of the coal samples decreases, the initial width of the fractures increases, and the ratio of current flow to initial flow decreases. When the gas pressure is changed from 2.0 MPa to 1.5 MPa, the ratio of current permeability to initial permeability of the coal samples initially decreases and then increases. Therefore, a dominant switch of effective stress and gas desorption on the permeability and mechanical response occurs. When the gas pressure is decreased, the effect of coal matrix shrinkage due to gas desorption on the permeability of the coal samples exceeds the effect of effective stress.

Evolution of Coal Permeability under Constant Effective Stresses: Direct Measurements and Numerical Modeling

Evolution of Coal Permeability under Constant Effective Stresses: Direct Measurements and Numerical Modeling

Impact of Sorption-Induced Strain and Effective Stress on the Evolution of Coal Permeability under Different Boundary Conditions

Coalbed methane (CBM) is a relatively common unconventional natural gas, which has great exploitation value. Coal permeability is an important parameter that affects the production and production efficiency of CBM, which is mainly controlled by the sorption expansion/contraction strain and effective stress. To study the seepage characteristics of coal in the process of CBM production, we have used CH4 and CO2 as test gases separately and conducted comparative seepage tests of different gases under constant pore pressure conditions. At the same time, the elastic modulus reduction coefficient Rm has been introduced to characterize the sorption strain of coal, following which the permeability models suitable for different boundary conditions were derived according to the stress–strain relationship. Under the two gases, the new model could not only better reflect the law of coal sorption strain but also better reflect the relationship among effective stress, pore pressure, and coal permeability. Under the conditions of constant pore pressure, coal permeability was mainly controlled by effective stress; with the increase of effective stress, permeability decreased sharply initially and then gradually. Under the conditions of uniaxial strain and constant external stress, with an increase of pore pressure and Rm, the matrix sorption expansion strain increased, resulting in a narrowing of the seepage channel, and Rm indirectly inhibited permeability. At this point, coal permeability was mainly controlled by sorption expansion/contraction strain and effective stress. In addition, compared with other permeability models, the new permeability model possesses higher applicability both in theoretical mechanism and in data matching. The general change trend concerning coal permeability, determined by rebound pressure prb, was consistent with the test results, which further verified the applicability of the model. It is believed that the results of this study could provide a basis for subsequent research on the stress–strain–permeability relationship and for the study of efficient development of CBM.

Study on the Permeability Evolution Model of Mining-Disturbed Coal

Coal permeability plays an important role in the simultaneous exploitation of coal and coal-bed methane (CBM). The stress of mining-disturbed coal changes significantly during coal mining activities, causing damage and destruction of the coal mass, ultimately resulting in a sharp increase in permeability. Conventional triaxial compression and permeability tests were conducted on a triaxial creep-seepage-adsorption and desorption experimental device to investigate the permeability evolution of mining-disturbed coal. The permeability evolution models considering the influence of the stress state and stress path on the fracture propagation characteristics were established based on the permeability difference in the deformation stages of the coal mass. The stress-strain curve of the coal was divided into an elastic stage, yield stage, and plastic flow stage. As the axial stress increased, the permeability decreased and then increased, and the curve’s inflection point corresponded to the yield point. The permeability models exhibited a good agreement with the experimental data and accurately reflected the overall trends of the test results. The results of this study provide a theoretical basis for coal mine disaster prevention and the simultaneous exploitation of coal and CBM.

The Role of the Reverse Fault on the Permeability Evolution in Mining Coal

To prevent and control the coal seam gas disaster affected by the reverse fault, we performed gas seepage tests, which consider stress-loading and unloading schemes, to investigate the stress change and coal permeability of the mining coal with reverse fault. The experimental results show that the mechanical behavior and permeability change of the mining coal are related to the distance between the coal and the reverse fault. The stress concentration coefficient of the coal body gradually increases. The closer is the distance between the coal and the reverse fault, the larger are the deviatoric stress peak and strain. In comparison with the coal sample M1 that is 5 m away from the reverse fault, the deviatoric stress peak and axial strain of the coal sample M3, 35 m away from the reverse fault, increase by 40.74% and 26.73%, respectively. In this stage, the permeability of M1, M2, and M3 coal samples increases by 22.1%, 28.0%, and 36.7%, respectively. In another stage, the stress concentration coefficient of coal increases to the peak and then decreases, causing the deviatoric stress peak and strain of coal to rise first and then fall. In comparison with the coal sample M4 that is 65 m away from the reverse fault, the deviatoric stress peak and axial strain of coal sample M6, 5 m away from the reverse fault, decrease by 29.48% and 5.55%, respectively. The permeability of coal samples M4, M5, and M6 increases by 23.6%, 37.2%, and 20.8%, respectively. Based on the gas seepage test results, we established the permeability model of mining-induced coal under the influence of a reverse fault, with consideration of the volume changes of coal fractures induced by adsorption and desorption. In the model, the variations of permeability in both stages of the prepeak and postpeak were deduced, which was verified with the experimental data. The verification results demonstrate that the proposed model has the capacity to predict the permeability evolution of mining coal under the influence of a reverse fault.

Effect of adsorption-induced matrix swelling on coal permeability evolution of micro-fracture with the real geometry

Gas transport in coal induces effective stress variation, matrix swelling/shrinkage, and significantly affects matrix and fracture deformation, resulting in porosity and permeability evolution. However, the heterogeneity and anisotropy of coal are neglected in dual porosity models, which can lead to the deviation from the real physical mechanisms. To uncover the permeability evolution, especially the influence of dynamic matrix-fracture interaction for real fracture distribution, advanced virtual simulation is proposed. In this study, real fracture geometry is taken into account in the physical model based on the CT-scan image, while the mathematical models for coal deformation and gas flow are established. Our calculations are verified against a long-term experimental data under the same boundary condition. Accordingly, the real matrix-fracture interaction caused by adsorption-induced matrix deformation has been visually exhibited, and some new insight into the behavior of fracture permeability in real materials is offered. The results indicate the non-uniform distribution of fracture geometry is responsible for the nonmonotonic change of permeability. It also found that injection pressure, Langmuir strain constant and initial matrix permeability have important influences on the fracture permeability evolution. This research provides valuable insight into the understanding of the permeability change for the real fracture spatial distribution in coal.

Permeability evolution in tectonic coal: The roles of moisture and pressurized water‐injection

AbstractWater often influences the exploitation of Coalbed methane (CBM) as well as the hydraulically enhanced permeability; it is therefore useful to objectively quantify the permeation properties of a CBM reservoir with different moisture contents. Based on the occurrence of CBM and the laws governing gas flow in CBM reservoirs, gas seepage experiments were carried out on the coal collected from the Baijiao mine in Sichuan, China. Various moisture contents were evaluated using natural coal samples, dry coal samples heated in an oven for various periods, and coal samples wet by soaking them in different pressure water under a constant effective confining stress. Results showed that: (1) The coal moisture decreased gradually with the drying time and increased with the soaking time. When coal is soaked in high pressure water (5–20 MPa), the moisture content in coal will increase from 2.60 to 3.85% with the increasing of water pressure. (2) The permeability of soaked coal decreased with the increasing of gas pressure, while the permeability of natural coal firstly decreased and then increased with the pore pressure decrease. (3) For the coal with higher moisture contents, gas permeability is more sensitive to the reduction of pore pressure, which may be due to the alteration of fracture structure induced by high‐pressure water injection. (4) Moisture content of coal affects the slippage effect in coal pores, and a higher moisture in coal will result in the decrease effective pore throat, incurring a stronger slippage flow and a higher apparent permeability of coal. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

Constraints of Pore-Bulk Strain Ratio and Interference Time on the Evolution of Coal Permeability during CO2 Injection

CO2 injection into coal seam triggers a series of processes that are coupled all together through a permeability model. Previous studies have shown that current permeability models cannot explain experimental data as reported in the literature. This knowledge gap defines the goal of this study. We hypothesize that this failure originates from the assumption that the pore strain is the same as the bulk strain in order to satisfy the Betti-Maxwell reciprocal theorem. This assumption is valid only for the initial state without gas sorption and deformation and for the ultimate state with uniform gas sorption and uniform deformation within the REV (representative elementary volume). In this study, we introduce the pore-bulk strain ratio and interference time to characterize the process of gas sorption and its associated nonuniform deformation from the initial state to the ultimate state. This leads to a new nonequilibrium permeability model. We use the model to fully couple the coal deformation and gas flow. This new coupled model captures the impact of coal local transient behaviors on gas flow. Results of this study clearly show that coal permeability is constrained by the magnitudes of initial and ultimate pore-bulk strain ratios and interference time, that current permeability data in the literature are within these bounds, and that the evolutions of coal permeability all experience similar stages from the initial value to the ultimate one.

Grouting Fractured Coal Permeability Evolution Based on Industrial CT Scanning

Gas extraction from coal seams in China faces various middle-term and long-term problems, such as the poor sealing quality and low extraction rate. The mean gas extraction concentration is only 30%. Studying the flowing laws of the grout and fracture plugging mechanism is of important significance to improve the sealing quality and increase the gas extraction efficiency. For this reason, a new coal-based grouting material was prepared in this study, and its parameters such as viscosity were tested. Moreover, a grouting theoretical model with considerations of the flowing of the grout and coal fracture plugging by migration and deposition of slurry particles was constructed. The crack distribution before and after the grouting of fractured coal samples was scanned with an industrial CT, and the fracture distribution of coal samples was reconstructed using an independently compiled MATLAB program. Meanwhile, the variations of the coal permeability before and after the grouting were tested. On this basis, this study built a numerical calculation model of the grouting in fractured coal samples to simulate the leakage stoppage and permeability reduction mechanism of the coal-based sealing material grouting. Results demonstrate that (1) according to the experimental test results, the new coal-based grouting materials achieve a good fracture plugging effect, and the fractures in coal samples after the grouting are filled densely by the grouting particles. (2) According to the simulation results, the permeability of grouting coal samples declines quickly and then tends to be stable. The overall permeability of coal samples and the fracture permeability are decreased by 93.5% and more than 99.9% in average, respectively. (3) Influences of the grouting pressure on the permeability variation of coal samples were investigated through a numerical simulation. It was found that a reasonable grouting pressure for coal samples is about 0.3 MPa. (4) The numerical simulation reproduces the whole process of the grout flowing and the fracture filling. The variation laws of the diffusion and the permeability of the grout which are calculated through a numerical simulation agree well with the experimental results, which verifies the reasonability of the model. Research conclusions can provide important significance in theory and practice to disclose the leakage stoppage and the permeability reduction mechanism of the borehole grouting during the gas extraction and strengthen the sealing effects of extraction boreholes.

Mining‐disturbed coal damage and permeability evolution: Model and validation

AbstractCoalbed methane (CBM) is not only the material cause of gas explosion and coal‐gas outburst disasters during underground coal mining, but also a kind of clean energy. Coal permeability is an important parameter for CBM drainage. Although many coal permeability models have been developed in the past decades to describe the permeability evolution characteristics under elastic state, few of them could explain the permeability behavior of the mining‐disturbed coal which is often the situation of CBM drainage during underground coal mining. The paper analyzed the mechanical factors affecting the damage‐permeability characteristics of mining‐disturbed coal, proposed the concept of the deviatoric stress ratio (DSR), and then established the statistical damage and permeability evolution models based on DSR. Results show that the deformation and damage of coal is controlled by the deviatoric stress and minimum principal stress. The plastic damage degree of coal mass becomes more serious with DSR increase, which leads to the improvement in permeability. The damage constitutive model was established based on the Weibull distribution function and DSR, and then the permeability model of mining‐disturbed coal was built combining the Kozeny–Carman equation. The acoustic emission (AE) and permeability experiments of the loading and unloading coal sample were conducted. After normalizing and fitting the accumulative AE counts, the damage and permeability evolutions with respect to DSR were found out. The model‐predicted permeability could match with the experiment results, verifying the reliability of the theoretical permeability model. The purpose of this paper is to provide a new approach for modeling damage and permeability of mining‐disturbed coal based on DSR. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

A Coal Permeability Model with Variable Fracture Compressibility Considering Triaxial Strain Condition

Fracture compressibility and strain boundary conditions are key factors in the evolution of coal permeability. Previous research has shown that fracture compressibility of coal is variable. Generally, the uniaxial strain condition is adopted for field coalbed methane (CBM) recovery. In this study, we assumed that the coal samples were under triaxial strain conditions combined with McKee’s stress-dependent fracture compressibility function and the improved Shi-Durucan permeability model. A new permeability model, with variable fracture compressibility, was derived. Considering variable fracture compressibility, the calculated results of the model were closer to the experimental values than the model with constant fracture compressibility. Based on previous experimental data, the responses of fracture compressibility to effective stress, gas pressure, gas type, and coal rank were analyzed. Coal fracture compressibility generally decreased with increase in effective stress. With increasing pore pressure, coal fracture compressibility first decreased and then increased when the experimental gas was CO2, CH4, or N2; it first increased and then decreased when the experimental gas was He. The stronger the adsorption capacity of the gas, the greater the coal fracture compressibility under uniform pore pressure. Numerical results of CBM production can be more accurate when the uniaxial and triaxial strain conditions are assumed for a coal seam far away from and near a wellhead, respectively.

Understanding competing effect between sorption swelling and mechanical compression on coal matrix deformation and its permeability

Coal deformation during gas depletion primarily involves the effective stress controlled bulk compression and gas sorption/desorption-induced coal matrix swelling/shrinkage. The two concurrent deformation behaviors can significantly impact coal permeability. Majority coal permeability models are defined as a cubic function of coal porosity, in which the porosity is a linear superposition of bulk strain and sorption strain. Nevertheless, laboratory measurements have observed that besides the sorption-induced swelling the injected gas pressure can also hydrostatically compress coal matrix. The observations reflect the complexity of fracture-matrix interaction on the coal porosity, which brings significant challenges to the reliable prediction of the evolution of coal permeability. In this study, we developed an experimental approach to separating the sorption-induced swelling strain of coal matrices and hydrostatic compression. We also modified a coal-gas interaction model by introducing an internal swelling coefficient and implemented into a coupled numerical model. Our experimental results based on nitrogen injection show that gas pressure rise can increase both the swelling strain and hydrostatic compressive strain of the coal specimen. If the measured coal strain is uncorrected, the corresponding isothermal sorption-strain parameters will be underestimated, resulting in the overestimation of the coal permeability. Numerical modeling results show that when the confining stress remains unchanged, the matrix swelling contribution to narrow the fracture width can attenuate with the increasing pore pressure. When the effective stress remains unchanged, the matrix swelling contribution can increase. Comparison of the published permeability data and these modeling results suggests that the actual contribution of the matrix deformation to fracture is significantly related to geomechanical state of coal reservoir, indicating that the internal swelling coefficient should be considered as a key variable when selecting a coal permeability model.