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ANSYS was used in this study to simulate the change in permeability of loaded coal under negative pressure. The ANSYS transient structural module was employed to simulate the coal loading conditions and to calculate the coal volumetric strain and coal porosity change. In the ANSYS Fluent module, the porosity of the porous medium was changed to replace the coal deformation, and the outlet pressure was changed to simulate the change of negative pressure. By using 1stOpt software, the relationships among the negative pressure, effective stress, and permeability of coal were fitted, and the binary nonlinear function of permeability varying with effective stress and negative pressure was obtained, and the correlation coefficient was excellent. Results showed that the coal permeability had a negative exponential relationship with effective stress. Hence, the permeability decreased with the increase of the effective stress. The change in stress loading mode caused a sudden change in porosity and permeability. The coal permeability had an approximately linear relationship with negative pressure. Hence, permeability increased with an increase in negative pressure. However, with the increase of effective stress, the influence of negative pressure on coal permeability was gradually weakened.

Based on gas adsorption theory, high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen gas adsorption (LT-N2GA), CO2 adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS) techniques were used to analyze the pore structures of six coal samples with different metamorphisms in terms of pore volume, specific surface area (SSA), pore size distribution (PSD) and pore shape. Combined with the gas adsorption constant a, the influence and mechanism of the pore structure of different coal ranks on gas adsorption capacity were analyzed. The results show that there are obvious differences in the pore structure of coals with different ranks, which leads to different adsorption capacities. To a large extent, the pore shapes observed by SEM are consistent with the LT-N2GA isotherm analysis. The pore morphology of coal samples with different ranks is very different, indicating the heterogeneity among the coal surfaces. Adsorption analysis revealed that mesopore size distributions are multimodal and that the pore volume is mainly composed of mesopores of 2–15 nm. The adsorption capacity of the coal body micropores depends on the 0.6–0.9 nm and 1.5–2.0 nm aperture sections. The influence of coal rank on gas desorption and diffusion is mainly related to the difference in pore structure. The medium metamorphic coal sample spectra show that the number of peaks in the high-wavenumber segment is small and that it is greater in the high metamorphic coal. The absorption intensity of the C–H stretching vibration peak of naphthenic or aliphatic hydrocarbons varies significantly among the coal samples. Over a small range of angles, as the scattering angle increases, the scattering intensity of each coal sample gradually decreases, and as the degree of metamorphism increases, the scattering intensity gradually increases. That is, the degree of metamorphism of coal samples is directly proportional to the scattering intensity. The influence of coal rank on gas adsorption capacity is mainly related to the difference in pore structure. The gas adsorption capacity shows an asymmetric U-shaped relationship with coal rank. For higher rank coals (Vdaf < 15%), the gas adsorption consistently decreases significantly with increasing Vdaf. In the middle and low rank coal stages (Vdaf > 15%), it increases slowly with the increase of Vdaf. We believe that the results of this study will provide a theoretical basis and practical reference value for effectively evaluating coal-rock gas storage capacity, revealing the law of CBM enrichment and the development and utilization of CBM resources.

Methane gas is mainly present in coal in two forms: free and adsorbed. There are a large number of closed pores inside the coal, which makes it difficult to measure the gas content of the coal. Therefore, studying the nanoscale closed pores of coal is of great importance for gas control. To study the pore structure characteristics of coal with different deformation degrees and to analyze the volume fraction of closed pores in coal, various coal samples were analyzed by the low-temperature liquid nitrogen adsorption method (LT-N2GA), the carbon dioxide adsorption method, and small-angle X-ray scattering (SAXS). The variation of parameters such as the pore size, pore volume, specific surface area, and degree of metamorphism was compared by using different methods to obtain the proportion of the closed pore volume of different coal samples. The results show that with the increase of the degree of coal metamorphism, the total pore volume and specific surface area of coal samples show a decreasing trend first and then an increasing trend, while the average pore diameter of coal samples gradually increases first and then decreases sharply. When the degree of deterioration of coal is low (volatile content > 20%), the closed pores of coal account for more than 48% of the open pores. When the degree of deterioration of coal samples is relatively high (volatile content

A self-developed gas desorption and diffusion experimental system was used to conduct isothermal methane gas desorption and diffusion experiments, and the pore structure of coal samples was analyzed. A new mathematical model for gas diffusion in coal particles was established, and the diffusion coefficient was calculated using the new model. The influences of particle size and the adsorption equilibrium pressure on the methane gas diffusion rate, gas diffusion quantity, and diffusion coefficient were analyzed and discussed. The results show that under different adsorption equilibrium pressures and different particle sizes, the diffusion rate and quantity vary substantially. The larger the particle size, the slower the initial gas diffusion rate and the longer the time required to reach desorption equilibrium. The larger the adsorption pressure, the greater the initial gas diffusion rate and the larger the quantity of diffusion gas accumulated. However, the coal particle size has little influence on the amount of gas diffusion. It was found that as the particle size decreases, the diffusion coefficient in the first diffusion phase is significantly reduced, which decays with a negative exponential function over time. However, the adsorption equilibrium pressure has no obvious influence on the diffusion coefficient. In addition, the methane gas diffusion process was divided into three phases, namely, phase I, rapid diffusion phase (0–10 min); phase II, slow diffusion phase (10–100 min); and phase III, smooth diffusion phase (100 min); and the diffusion mechanism corresponding to each phase was analyzed. These research results provide a theoretical and practical reference value for gas prevention and control in coal mines.