In order to achieve high-efficiency extraction of coal seams with large-diameter borehole and large flow rates, and to rapidly reduce coal seam gas content and pressure, a large-diameter borehole coal seam pressure relief and permeability enhancement technology is proposed. In this paper, numerical simulation is used to study the mechanism of pressure relief and permeability enhancement in coal seams with large-diameter boreholes, and the evolution of stress, cracks, gas, and permeability of coal bodies around boreholes with different diameters is discussed. The research results show that the stress changes in the coal body around the borehole are symmetrically distributed around the borehole. The stress evolution controls the evolution of the coal bodys cracks. Firstly, damage occurs around the borehole and then extends to the upper left and right corners, like a butterfly spreading its wings. The crushing zone increases with the increase of the hole diameter, and the impact radius of the 100-mm, 200-mm, 300-mm, and 350-mm diameter borehole rupture is 0.375m, 0.65m, 1.0m, and 1.25m, respectively. The rupture radius of the double 350-mm diameter borehole can reach 2.4m. The larger the diameter of the borehole, the more fissures will be produced, and the air permeability of the coal body in the affected coal seam area will obviously increase. The research results provide theoretical support for gas drainage in soft, low-permeability, high-gas coal seams.

Coalbed methane (CBM) is an unconventional natural gas primarily stored in coal seams. The efficient recovery of CBM mainly depends on the desorption and diffusion process. In this study, a stepwise wetting–corrosion method employing a combination of surfactant (cocamidopropyl betaine) and acid (hydrochloric acid) was proposed to promote the desorption and diffusion of CBM. The microstructure and CH₄ desorption–diffusion characteristics of the coal samples treated with the stepwise wetting–corrosion method were evaluated at varying concentrations of cocamidopropyl betaine (CAB) and hydrochloric acid (HCl). The relationship between wettability, specific surface area, and CH₄ adsorption–desorption was identified, and the effect of pore connectivity on CH₄ diffusion was investigated. The results indicate that the stepwise wetting–corrosion treatment eliminated mineral blockages within the coal matrix, thereby clearing the microporous pathways and improving the overall pore connectivity for methane transport enhancement. By preventing the contact between the surfactant and the acid, the breakdown of surfactant molecules was inhibited. This enabled homogeneous acidizing throughout the coal matrix, which reduced the specific surface area and increased the methane desorption rate by 13.99%. In addition, a significant reduction in the mass transfer Biot number and a notable enhancement in methane diffusivity were obtained. Therefore, the stepwise wetting–corrosion method combining CAB and HCl shows a potential to increase gas production and will provide an alternative to traditional high-energy fracturing techniques, contributing to efficient and sustainable CBM extraction.