Coal Permeability Model Research Articles

Characterization of energy-driven damage mechanism and gas seepage in coal under mining-induced stress conditions

Gas seepage and progressive failure of coal are common energy-driven mining phenomena. A comprehensive understanding of the energy-driving mechanism behind the catastrophic behavior of mining-induced coal is fundamental to innovating the technology of coal and gas co-mining. Thus, this study simulated three typical mining stress evolution process in protective coal-seam mining (PCM), top-coal caving mining (TCM), and non-pillar mining (NM) to investigate the energy evolution and distribution patterns of coal. The results indicate a strong correlation between energy dissipation and gas seepage. By transitioning from PCM and TCM to NM, the peak elastic strain energy of gas-bearing coal increased by 155.92 %, and the ratio of peak dissipative energy decreased from 51 % to 41 %. Under the PCM stress path, gas seepage decreased the energy storage by 13.52 %, whereas the pre-mining pressure relief and enhanced permeability simulation increased in peak dissipation energy by 49.66 %. Using the cumulative dissipative energy as a damage variable reveals that the degree of coal damage evolution under PCM is higher than other mining methods. Based on the energy-driven damage mechanism, a new coal permeability model was established, and its comparison with classical permeability model demonstrated its excellent fitting effectiveness.

Simulation Study of Coal Seam Gas Extraction Characteristics Based on Coal Permeability Evolution Model under Thermal Effect.

The permeability evolution law of high temperature and high stress coal seam is determined by the influence of multiphase coexistence and multifield coupling. In an environment greatly affected by disturbance and high temperature, the coal permeability model under the coupling of thermal and mechanical creep is not only a vital framework from which to examine gas migration law in multiphase and multifield coal seams but also an important theoretical foundation for gas control in coal seams. The influence of high-temperature environment on creep deformation and permeability is analyzed by several creep seepage tests under different temperature conditions.A mathematical model for the evolution of coal permeability considering the influence of temperature is established through the theory of matrix-crack interaction based on gas adsorption and desorption and thermal expansion deformation. Based on the permeability model under the coupling of thermal and mechanical creep, the numerical model of gas migration, seepage field, diffusion field, stress field, and temperature field is constructed, and the law of gas migration in coal seam under multifield coupling is explored. The influence law of thermal effect on gas extraction characteristics is analyzed, in which the time-varying mechanism of temperature field, the relationship between creep deformation and temperature and pressure, the influence of creep deformation on permeability, the dynamic distribution of gas pressure, and the change of gas extraction quantity are described in detail. It is concluded that the influence of temperature on permeability is much greater than that of creep deformation and that a high initial coal seam temperature is beneficial to gas extraction. It provides theoretical basis and technical guidance for the study of multifield coupled gas migration and coal seam gas treatment.

An analytical transient coal permeability model: Sorption non-equilibrium index-based swelling switch

Long-term permeability experiments have indicated that sorption-induced swelling can switch from internal to bulk depending on the evolutive sorption status. However, this sorption swelling switch mechanism has not been considered in current analytical permeability models. This study introduces a normalized sorption non-equilibrium index (SNEI) to characterize the sorption status, quantify the dynamical variations of matrix swelling accumulation and internal swelling partition, and formulate the sorption swelling switch model. The incorporation of this index into the extended total effective stress concept leads to an analytical transient coal permeability model. Model results show that the sorption swelling switch itself results in the permeability switch under stress-constrained conditions, while the confined bulk swelling suppresses the permeability recovery to the continuous reduction under displacement-constrained conditions. Model verifications show that current experimental observations correspond to the early stages of the transient process, and they could be extended to the whole process with these models. This study demonstrates the importance of the sorption swelling switch in determining permeability evolution using simple boundary conditions. It provides new insights into experimentally revealing the sorption swelling switch in the future, and underscores the requirement of a rigorous model for complex coupled processes in large-scale coal seams.

Evolution of coal permeability during gas/energy storage

Both carbon dioxide and hydrogen can be stored in coal seams as two enabling components of energy transition from fossil-based systems to renewable sources. In both cases, understanding the evolution of coal permeability under the influence of gas adsorption is extremely important. The gas sorption-induced deformation is commonly treated by analogous calculation of thermal expansion. This assumption has long been proved to be inconsistent with observations as reported in the literature. In this study, we hypothesize that the difference between the assumption and the reality is due to self-constrained/facilitated swelling phenomena during gas injection. Under this new hypothesis, coal could be constrained or facilitated depending on coal internal structures and processes. A concept of fictitious stress is introduced to quantify coal self-constrained or facilitated degree and converted into the equivalent effective stress. This conversion has transformed the conventional effective stress principle to unconventional one. This has led to new generic coal permeability model, which has been validated by experimental data. An analysis of stress state evolution during gas storage process is conducted. Our results suggest that our coal permeability model is a valuable tool for evaluation of gas storage in coal seams.

Experimental study on the influence of ultrasonic excitation duration on the pore and fracture structure and permeability of coal body

The study aims to explore the effect of ultrasonic duration on the pore structure and seepage characteristics of coal. Herein, we adopted a self-developed ultrasonic excitation experiment system using low-field nuclear magnetic resonance (NMR) technology and NM-4B non-metallic ultrasonic wave velocity detector. The study investigated NMR relaxation time T2 patterns and the number of pores of each size, internal structural damage, and seepage characteristics of the coal body before and after ultrasonic fracturing. Based on the Warren-Root model, the permeability model of coal under ultrasonic fracturing was established, revealing the changing law of ultrasonic fracturing at different times on the change of pore scale, connectivity, and nuclear magnetic permeability of the coal body. The mechanism of ultrasonic fractured coal body evolution at different times was clarified. The results showed that the ultrasonic fracturing at different times improved the number and volume of all size pores in the coal body, expanded primary fractures, and developed newborn fractures. The rate of change in pore size, porosity, wave velocity, and coal permeability had a linear relationship with the fracturing time. The growth rate of seepage pore volume proportion was positively correlated to the decreased rate of adsorption pore volume proportion. The fracturing duration leads to the interconnection of micropores within the coal body, forming medium and large pore fractures. This transformation between pores and fractures enhances the pore structure and seepage characteristics of the coal body. The coal rock permeability model can be used to calculate the permeability of a coal body, which follows the same change law as nuclear magnetic permeability. When evaluating the index using absolute and relative errors, the range of relative error is 0.97%–7.91%. The error variation range is small, indicating that the established model can effectively reflect the change law of porosity and permeability of the coal body responding to ultrasonic fracturing. The ultrasonic fracturing rendered the ultrasonic cavitation takes micropores as fracturing units, developed pores of all sizes, connected closed pores, increased free fluid space proportion, and improved the coal permeability.

Research on a permeability model of coal damaged under triaxial loading and unloading

It is important to research permeability models to clarify the evolution of methane seepage from coal seams and to reduce the occurrence of gas accidents. However, existing permeability models do not fully account for the influences of effective stress and adsorption/desorption on the permeability of coal rocks damaged during the three-dimensional stress loading and unloading process in coal mining. This study proposed a novel coal permeability model by more robustly considering the effects of stress and gas adsorption/desorption based on the generalized Hooke's law and Gibbs' equation. Then, a series of triaxial stress loading–unloading seepage experiments for coal rock were performed to validate the proposed model. The experimental research shows that triaxial stress loading and unloading have a significant effect on coal rock permeability. Before coal rock failure, the gas permeability shows a “V”-shaped development trend. After the deviatoric stress exceeds the compressive strength of the coal, the coal permeability increases sharply. Theoretical verification shows that the established exponential permeability model can effectively reflect not only the dynamic increase in the permeability of damaged coal under triaxial stress loading–unloading conditions but also the decrease in coal rock permeability during the increase in gas pressure with a constant effective stress. Moreover, the influence factors introduced in the permeability model can effectively reflect the influences of effective stress and gas adsorption/desorption on coal permeability under triaxial stress loading–unloading with different gas pressure conditions. These findings elucidate the variation in methane permeability of damaged coal rock under triaxial stress loading and unloading. The established permeability model can be used to carry out other numerical simulation studies to provide theoretical support for the prediction of gas permeability and efficient recovery of coalbed methane for coal mining working faces.

A multiphysics model for biogenic gas extraction from coal seams

Approximately 20% of global natural gas resources, including coalbed methane (CBM), could be microbial in origin. This discovery has attracted great interests in extracting biogenic gas in coal seams. Previous studies have demonstrated that this could be achieved through injecting nutrients solution, either with or without microbes, into a coal reservoir. The injected nutrients transport in coal, stimulate the growth of microbes, and enhance their metabolic activities. Through these complex processes, the organic components in coal are biodegraded into methane gas. The generated biogenic gas may be either in free phase or adsorbed on coal grains and can be extracted as an integral part of the gas-in-place. Although this bio-stimulation technique has been proposed for decades, generating and extracting additional biogenic methane in coal seams is still in a stage of conceptual development. In this study, we develop a modelling tool to validate this concept under the laboratory conditions and upscale to reservoir conditions. The model consists of a complete set of partial differential equations (PDEs) to define: (1) coal deformation, (2) water and gas flow, (3) multispecies reactive transport, and (4) microbial growth/decay and adsorption/desorption. All these processes are coupled through coal porosity and permeability model that links hydrological, mechanical, chemical and biological processes together. The multiphysics model is verified against laboratory coal bioconversion data. The verified model is applied to simulate practical operation in which nutrients solution is continuously delivered into coal seams. Simulation results capture all important processes involved and validate the effectiveness of coal-to-methane bioconversion and its extraction.

Pore Permeability Model Based on Fractal Geometry Theory and Effective Stress

AbstractA reasonable coal seam permeability model should be established to accurately estimate the extraction effectiveness of coalbed methane (CBM). Existing permeability models typically ignore the influence of pore structure parameters on the permeability, leading to an overestimation of the measured permeability, and consequently, the CBM production cannot be effectively predicted. This paper presents a novel permeability model based on discrete pore structures at the micro–nano scale. The model considers the interaction between the pore fractal geometry parameters, coal deformation, and CBM transport inside these pores. The contributions of key pore geometry parameters, including the maximum pore diameter, minimum pore diameter, porosity, and fractal dimensions, to the initial permeability were investigated. A numerical analysis showed that the influence of fractal dimension on the permeability is finally reflected in the influence of pore structure parameters. The initial permeability is exponential to the minimum pore diameter and proportional to the maximum pore diameter and porosity. In addition, the macroscopic permeability of the coal is positively correlated with the maximum pore diameter, minimum pore diameter, and porosity, with the minimum pore diameter having the most significant influence on the permeability evolution process. This research provides a theoretical foundation for revealing the gas flow mechanism within coal seams and enhancing the extraction effectiveness of CBM.

The effect of stress, pressure and temperature on CBM migration with elastic–plastic deformation

The laws relating to changes in permeability in coal reservoirs is central to the simultaneous exploitation of coal and gas. Under the condition of multi–field influences caused by stress, pressure, and temperature, elastic–plastic deformation in the matrix–fracture is complex, which has made it difficult to analyze the coalbed methane (CBM) migration law. The coal matrix is deformed by gas adsorption and thermal expansion. However, deformation in the coal matrix is not equal to fracture deformation, meanwhile, the coal mining process causes obvious plastic deformation. Therefore, in this work, the sensitivity coefficient (Sf) was used to quantify fracture deformation, and the sensitivity coefficient of fracture number to the damage value (ζ) was specified to quantify matrix deformation. Further, a coal reservoir permeability model under elastic–plastic deformation was established. The proposed permeability model was verified by laboratory test data under different boundary conditions. On this premise, the contribution of effective stress and adsorption to permeability was discussed, with the influencing factors of Sf being analyzed, and a sensitivity analysis of Sf and ζ was studied. The results revealed that the permeability model proposed in this work can predict the change law of permeability with elastic–plastic deformation effected by stress, pressure and temperature. The theoretical results from the proposed permeability model were helpful in understanding the law relating to changes in permeability in a complex geological environment during the simultaneous exploitation of coal and gas.

Characterization of anisotropic coal permeability with the effect of sorption-induced deformation and stress

Changes in permeability are related to coalbed methane (CBM) production activities, and to coal mining safety. However, inherent properties, i.g. anisotropy and heterogeneity make the gas flow mechanism more complex. In this work, an anisotropic coal permeability model with synergistic stress compression and gas adsorption was constructed to simulate gas flow behavior in all directions in coal. Changes in coal anisotropic permeability are restricted by the fracture deformation in coal in all directions. Wherein, the fracture deformation in coal is controlled by the effective volume strain coordinated by gas adsorption and stress. The influence of stress can be expressed by employing the generalized Hooke’s law. Then, based on the coal matrix bridge structure, an internal swelling coefficient has been proposed to assess the control of the matrix swelling on fracture width during the gas adsorption process to further obtain the fracture deformation caused by gas adsorption. The anisotropic coal permeability model was verified by laboratory test data under different effective stresses, gas pressures, with respect to their combined influences. Subsequently, based on the experimental conditions of uniaxial strain, the permeability model of anisotropic coal during gas depletion was further deduced, and changes in anisotropic coal permeability under uniaxial strain were also calculated. In addition, the sensitivity of the reduction rate of the elastic modulus, and the internal swelling coefficient was also studied, to reveal the matrix-fracture interaction mechanism.

Differential Strain Index-Based Multiphysics Model for Coal Seam Gas Production

Conventional multiphysics for coal seam gas production is generally coupled with poroelasticity-based coal permeability models, which are derived under the assumption that gas pressure is distributed uniformly within the representative elementary volume (REV). Under this assumption, the pore swelling/shrinking strain is equal to the bulk one. However, this assumption is considered as the primary reason for the inconsistency between experimental data, field data, and model results. In this study, a new concept of differential strain index (DSI) is proposed to theoretically define the relation among desorption-induced strains of the coal bulk, pores and solid grains. DSI is a function of the gas pressure, and its magnitudes are regulated by the Langmuir constants of both the solid grains and the coal bulk. Furthermore, a DSI-based coal permeability model is incorporated into the coupled multiphysics model for coal seam gas production. The model results of both laboratory-scale and field-scale show that coal permeability changes over a wide range during gas production. These changes are controlled by the DSI, and the magnitudes of change are defined by the Langmuir strain ratio of solid grains to coal bulk, while the trends of change are defined by the Langmuir pressure ratio of solid grains to bulk coal.

A novel workflow based on physics-informed machine learning to determine the permeability profile of fractured coal seams using downhole geophysical logs

Permeability is arguably the most important parameter in gas pre-drainage and production practices in coal mining and coal seam gas industries. In fractured coal seams, permeability can significantly vary both laterally and vertically. Knowing the vertical variation of permeability in relatively thick coal seams is important in optimizing inseam pre-drainage design in underground coal mining and improving reservoir management and completion design efficiency in the coal seam gas industry. Existing empirical/statistical coal permeability models often yield poor results in estimating permeability profiles where there is significant variation in the characteristics of coal fracture systems. This study presents a novel workflow for evaluating the permeability profile of naturally fractured thick coal seams using downhole geophysical logging data. The workflow combines physics-based simulation, laboratory experiments, and a data-driven machine learning approach for estimating the permeability profile. As part of this workflow, several coal specimens from the study coal seam are first tested under different stresses to measure their permeability, density, and ultrasonic responses. Numerical simulation of the Navier-Stokes fluid flow and elastic wave propagation was then performed on a range of constructed coal block geometries with various fracture densities. In the next step, a physics-informed, neural network-based model (PINN) was trained using the data obtained from both laboratory experiments and numerical simulation. The model inputs included density, as well as compressional and shear wave velocities of the coal seam and its output is the trend of permeability variation across the coal seam interval. In-situ permeability measurement at a certain depth (e.g., from well pressure testing) was then used to convert the permeability trend to the entire permeability profile of the coal seam along its interval. The performance of the proposed workflow was finally evaluated using a suite of downhole geophysical logging data from a borehole intersecting two coal seams in an eastern Australian basin. This study demonstrates that the proposed workflow is relatively simple to implement and yet accurate for evaluating the permeability profile of a coal seam across its interval.

Modeling of anisotropic coal permeability under the effects of matrix-fracture interaction

Research relating to changes in coal's permeability has a positive significant effect for coal mining safety. The construction of an accurate permeability model through appropriate methods would help to understand the flow characteristics of coalbed methane (CBM). However, it is worth noting that it is necessary to include the influence of anisotropy and heterogeneity when analyzing changes in coal's permeability, otherwise the results would distort the actual conditions. In this work, we considered, fully, the interaction of coal matrix-fractures, and established a coal permeability model. In addition, the effects of stress compaction, sorption-induced deformation, including the slippage effect have been incorporated into the permeability model. With respect to this proposition, the anisotropic strain law for coal has been analyzed based on the generalized Hooke constitutive model. And the anisotropic sorption-induced deformation mechanism in coal has been analyzed through an improved sorption-induced deformation equation. Following experimental results, an anisotropic coal permeability model was constructed. Through the constructed permeability model, we described further the change characteristics relating to anisotropic permeability in coal. In addition, the permeability model was verified through an anisotropic seepage laboratory test. The influence of the elastic modulus reduction coefficient, and the ratio of the elastic modulus to adsorption expansion modulus on coal's permeability have also been analyzed. Further, the elastic modulus reduction coefficient that determines the fracture width and porosity, thereby affecting coal's permeability, has also been considered. Moreover, the ratio of the elastic modulus during adsorption expansion that determines sorption-induced deformation, and indirectly affects permeability, has also been calculated. This work aims to further understand changes in gas' permeability under the combination of multiple fields during coal mining through the theoretical results from the proposed anisotropic permeability model.

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 new coal reservoir permeability model considering the influence of pulverized coal blockage and its application

In order to accurately predict the production performance of coalbed methane (CBM) wells and to formulate a reasonable production system, this paper established a coal reservoir permeability model considering the influence of pulverized coal blockage. Then, on the basis of this model, the flow velocity sensitivity (FVS) experimental data of 15 groups of coal samples taken from the Baode Block, Qinshui Basin, Liulin Block, Hancheng Block, and the Huanglong Coalfield were fitted to determine the permeability models for different coal samples. On this basis, this newly established permeability model was incorporated into a previously developed CBM well performance analysis software, and production history matching was carried out on two CBM wells. Finally, the effects of the parameters of pulverized coal blockage on the permeability of coal reservoirs and the production performance of CBM wells were studied by taking the fitting parameters of CBM Well W1 as the reference. And the following research results are obtained. First, this new model considering the influence of pulverized coal blockage can quantitatively describe the variation of coal reservoir permeability with fluid velocity. In addition, this model can be incorporated into a CBM numerical simulation software or a CBM well performance analysis software to apply it in a wide range. Second, the coal reservoir permeability is less affected by pulverized coal blockage in the Baode Block, but this effect shall not be ignored in the Qinshui Basin and the Huanglong Coalfield. Third, the greater the theoretical maximum permeability damage degree (Dmax) and the permeability damage degree index (n) are, the lower the relative flow velocity (v0.5) corresponding to the critical flow velocity of pulverized coal blockage is and the more obvious the effect of pulverized coal blockage on coal reservoir permeability is. Fourth, in order to reduce the adverse effect of pulverized coal blockage on coal reservoir permeability, it is suggested to reduce the production pressure difference appropriately in the process of production, especially in the initial stage of gas production, so as to avoid severe damage to coal reservoir permeability.

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.

Impact of equilibration time lag between matrix and fractures on the evolution of coal permeability

In conventional dual porosity models, the interactions between matrix and fractures are normally characterized through two equilibrium systems within the same REV (representative elementary volume). This pseudo-steady approach cannot capture the true impact of the non-equilibrium period within each system since it ignores the true transient nature of the fracture-matrix interaction. In this study, a conventional dual porosity model is extended to include the impact of equilibration time lag between matrix and fractures caused by their contrasting properties. To incorporate this important mechanism, the matrix REV is divided into two sub-REVs by using the MINC concept (Multiple Interacting Continua). The time lag effect is defined as a function of the difference between the strain in the matrix REV and that in the sub-matrix REV, and incorporated into a coal permeability model. Consequently, the coal permeability evolves also from initial to final equilibrium. Conventional dual porosity/permeability models represent two end points (initial and final equilibrium) while this new permeability model represents the evolution of coal permeability between these two end points. The model is verified against experimental observations of the evolution of coal permeability under a constant effective stress that extend for more than 80 days. If effective stress remains unchanged, conventional permeability models predict no permeability changes while our new model predicts that coal permeability evolves as a function of time from initial to ultimate equilibrium. Our results suggest that the impact of matrix strain variations on the evolution of coal permeability is significant and should not be ignored.

Coal permeability evolution during damage process under different mining layouts

ABSTRACT Coal permeability is akey component that affects gas migration. Considering seepage behavior of gas during mining is critical for improving the production profile of coal-bed methane (CBM) wells, and preventing coal and gas outburst. Aseries of seepage tests have been carried out to simulate the coal permeability evolution law during mining-induced actions. Coal damage has been assessed by using atri-axial servo seepage testing system. The results show that the coal permeability under PCM, Top-coal Caving Mining (TCM) and NM are all kept constant before the deviatoric stress reaches the peak strength, then increases mildly and finally increases rapidly. The effective stress decreases during confining pressure relief, and the permeability increases exponentially with the decrease of effective stress, which is quite different from the traditional triaxial compression test. According to continuum damage mechanics, adamage constitutive model of coal was proposed to characterize coal mechanical characteristics by mining-induced activities. Furthermore, according to the matchstick model, anew coal permeability model is proposed, which could accurately reflect seepage behavior under different mining layouts.

A new anisotropic coal permeability model under the influence of stress, gas sorption and temperature: Development and verification

Modeling approach to permeability especially anisotropic permeability of coal in front of working face is of significance for gas extraction in coal mining. Considering three-dimensional stress condition and multi-field influence, in this study, an anisotropic permeability model was suggested for coal in front of working face. The anisotropic permeability was assumed to be controlled by the directional fracture strain under the condition of multi-field influence caused by effective stress, adsorption-desorption and thermal stress. The directional fracture strain induced by effective stress was governed by the generalized Hooke constitutive model. Based on the matrix-fracture interaction, the constitutive model of directional fracture strain induced by adsorption-desorption was established from the improved Langmuir equation. Assuming that the coal matrix was connected by the matrix bridge, the constitutive model of directional fracture strain induced by thermal stress was constructed by proposing the thermal internal swelling coefficient. Consequently, a new anisotropic coal permeability model was developed based on the directional fracture strain under the influence factor of stress, gas sorption and temperature. Moreover, the new permeability model was verified by laboratory tests under different effective stresses, gas pressures, temperatures and their combined influence. The theoretical results of proposed anisotropic permeability model may provide a better understanding of permeability evolution in multi-field coupling geological environment during coal mining.