The pore size distribution, pore shape and connectivity, and fractal characteristics are investigated to determine the pore characteristics of three different samples of middle−high rank coal. Pores of more than and less than 10 nm were measured using mercury intrusion porosimetry (MIP) and gas adsorption, respectively. The pore size distribution was verified with the initial methane diffusion rate and CH 4 desorption. Fractal dimensions of seepage pores and adsorption pores were counted using the results from MIP and gas adsorption, respectively. First, the results show that micropores and transition pores occupy the most volume and specific surface area. Micropores and transition pores, as well as porosity, gradually increase as coal rank increases. Second, the fractal dimensions of seepage pores and adsorption pores gradually increase with increasing coal rank, which shows that coalification makes pore structure more complex and pore surface rougher. Additionally, the fractal dimensions of bigger pores are greater than those of smaller pores, implying that the surface and structure of bigger pores is rougher and more complex than those of smaller pores, respectively. Finally, the connectivity of coal has a close relationship with macropores rather than coal rank.

Coal permeability is an important parameter for coalbed methane (CBM) production and CO 2-enhanced coalbed methane (ECBM) recovery. Coal permeability is mainly controlled by the structure of the coal. In this study, based on the pore-elasticity principle and the coal matrix-fracture interactions, we developed an analytical coal permeability model and its five forms under the corresponding specific boundary conditions, and four of them are validated by matching the corresponding field data or experimental data. The study presents a discussion of the permeability evolution for the different forms. The results show that the coal permeability under constant volume and constant effective stress conditions are controlled by the matrix sorption deformation and that the coal permeability under uniaxial strain and constant external stress conditions is governed by both the effective stress and the matrix sorption deformation. The constant volume model may be a special form of the constant effective stress model. For the same pressure drawdown, the permeability that is predicted under constant external stress conditions decreases more than that the permeability that is predicted under uniaxial strain conditions, and the permeability change induced by the matrix sorption deformation depended on the values of the internal swelling partition. The boundary conditions are important to the model, and different boundary conditions will lead to different evolutions of coal permeability. The permeability model can only be used to predict the evolution of permeability under the corresponding boundary conditions.

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Coal permeability is a key parameter that affects the drainage of coal bed methane (CBM). Owing to the lower original permeability of coal seams in China, more attention was paid to the study of damage‐induced permeability laws. However, the theory of damage‐induced permeability still needs to be improved. In this paper, the permeability evolution experiments during the complete stress–strain process of coal were carried out firstly. The results are shown that with the increase in axial strain, the coal permeability decreases slowly at first, then increases rapidly, and finally increases slowly or even remains unchanged, and the peak stress point can be regarded as the extreme point of permeability. Then, the equivalent plastic strain was used to describe the damage of coal. Based on the above results, a new damage‐induced permeability model was developed, and the new model can match the experimental data of permeability very well. In addition, solid–gas coupling models of coal were developed based on the new permeability model, and the numerical results show that there is a region of sudden increase in permeability around the borehole where plastic failure occurs, which indicates that the damage‐induced permeability model is reasonable. Finally, the results of numerical simulation of gas predrainage in combination of soft coal and hard coal under different cases were used to optimize the borehole layout of gas predrainage in the field.

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