Microscopic pore structure and gas desorption kinetics characteristics of coal in the overthrust fault zone

The coal body in the overthrust fault zone is subjected to tectonic stress for a long time, and its pore structure, mechanical properties, and gas desorption behavior are different from those of a normal coal body, which directly affects the efficient mining of coalbed methane and the prevention and control of coal and gas outburst disasters. In this paper, the coal samples with displacement faults (DF1 and DF2) in the overthrust fault zone and the normal coal samples with xenolithic lithotypes (XL1 and XL2) without structural disturbance are selected to carry out high-pressure gas desorption kinetics, low-temperature liquid nitrogen adsorption experiments, and mechanical index tests to systematically reveal the influence of the overthrust fault on coal gas desorption kinetics. The results show that coal from the fault zone exhibits 35%–42% higher cumulative methane desorption and faster desorption rates at 0.85 MPa compared to intact coal. Pore analysis reveals that fault zone coal has 2.5–3.8 times greater total pore volume and higher pore complexity, as quantified by fractal dimensions. Mechanical tests confirm tectonic stress induces fracture development and structural loosening. Based on these experimental findings, an improved desorption model is proposed, combining pore fractal dimension and a structural damage factor η, to enhance the dynamic prediction accuracy of gas desorption in the overthrust fault zone. This study provides theoretical support and engineering guidance for optimizing coalbed methane mining and gas disaster prevention and control in complex structural areas.