Coal is a natural and complex porous medium, which contains pore-fracture structures of different scales. The pore-fracture structures of coal directly affect the characteristics of gas storage and migration. In this paper, high-resolution X-ray CT scanning technology is used to quantitatively characterize the pore-fracture structure of medium and high-rank coals. First, the porosity inversion method is used to determine the optimal threshold segmentation of pores and fracture in medium and high-rank coals. Then, the pore and fracture model of representative element volume (REV) was extracted and established to characterize the area porosity, pore surface area, and shape factor of the two coals, and the relationship between pore surface area and shape factor was investigated. Finally, an equivalent pore network model (PNM) of connected pore topology was established, and pore-throat parameters are statistically analyzed, including pore volume, pore radius, throat radius, throat length, and coordination number. The results show that the average pore diameter and average surface area of macropores in medium-rank coals are larger than those of high-rank coals, and high-rank coals have fewer isolated pores. With the increase of coal rank, the length and radius of the throat gradually decrease, which indicates that the throat of the medium-rank coal has a higher degree of development, and the gas seepage ability in the throat is stronger. The outcome of this study is of great significance for comprehensively understanding the pore-fracture structure of coal in micro-scale.

Although hydraulic fracturing technology has been comprehensively investigated, few scholars have studied the influence of hydraulic fracturing on the effect of coalbed methane (CBM) extraction, and few considered the interaction between water and CBM in the research process, which is not conducive to guiding the engineering design of hydraulic fracturing wells. In this work, a hydraulic‐mechanical‐thermal coupled model for CBM extraction in hydraulic fracturing well is established; it combines gas‐liquid two‐phase infiltration, where nonisothermal adsorption is also considered. The COMSOL Multiphysics software is used to carry out the numerical simulation study of the CBM extraction process in hydraulic fracturing well and analyze the influence of coalbed permeability, initial methane pressure, and fracture length on CBM extraction in hydraulic fracturing well, and the results show that the hydraulic‐mechanical‐thermal coupled model for CBM extraction can be used for CBM extraction research in hydraulic fracturing well. The initial coalbed permeability, initial gas pressure, and fracture length all affect the migration speed of CBM to surface well in different ways and have a greater impact on the CBM production rate of hydraulic fracturing well. The greater the initial coalbed permeability and methane pressure are, the longer the fracture length is and the greater the CMB production rate of hydraulic fracturing well is. The change trend of coalbed permeability during the extraction process of surface fracturing well is directly related to the state of the reservoir. The factors of stress, temperature, and CBM desorption jointly determine the increase or decrease of coal seam permeability.

Practice shows that CO₂/N₂-ECBM is an effective technology to enhance coalbed methane. However, there are few field tests in which the technology is applied to enhance the gas drainage in underground coal mines, and the effect is uncertain. In this study, firstly, the reasons for the decrease of gas drainage efficiency in the exhaustion period were analyzed based on the theory of fluid mechanics. Secondly, the mechanism of N₂ injection to enhance coal seam gas drainage (N₂-ECGD) was discussed: with the gradual decrease of gas pressure in the drainage process, coal seam gas enters a low-pressure state, the driving force of flow is insufficient, and the drainage enters the exhaustion period. The nitrogen injection technology has triple effects of “promoting flow”, “increasing permeability” and “replacing”. Thirdly, the numerical simulations of the nitrogen pressure on drainage effect were carried out based on the fully coupled model. The results show that the higher the nitrogen pressure, the greater the displacement effect between injection and drainage boreholes, the larger the effective range. Finally, a field test of N₂-ECGD was carried out in the Liu Zhuang coal mine in Huainan Coalfield, China. The results show that N₂ injection can significantly enhance the gas flow rate and CH₄ flow rate in the drainage boreholes, and the coal seam gas content decreased 39.73% during N₂ injection, which is about 2.6–3.3 times that of the conventional drainage. The research results provide an important guidance for promoting the application of N₂-ECGD in underground coal mines.