Coal permeability evolution under coupled mining disturbance and gas extraction conditions: An experimental study

Abstract

This study investigates the evolution of coal permeability under mining-induced stress and gas extraction, a key factor for safe and efficient coal seam gas recovery. A simplified triaxial stress path incorporating axial stress, confining stress, and pore pressure was applied to simulate stress relief and gas drainage in protective seam mining. Laboratory experiments on intact and fractured coal samples from the Huainan mining area were conducted using a gas-flow and stress-controlled loading system. Results indicate that coal permeability is strongly dependent on loading history and fracture morphology. The first unloading cycle produced the largest fracture volume reduction (29 % in vertically fractured coal, 23 % in horizontally fractured coal, and 20 %–21 % in intact coal). Irreversible permeability loss correlated with initial porosity, while the third cycle yielded the greatest permeability increase due to cumulative damage and new fracture formation. Decreasing pore pressure amplified its influence on permeability, with intact coal showing higher stress sensitivity and fractured coal exhibiting a declining trend under cyclic loading. Unlike previous studies that focused primarily on monotonic loading or single fracture orientations, this work systematically reveals the fracture-dependent, cyclic nature of permeability evolution under coupled mining stress and gas drainage conditions. These findings highlight the nonlinear, fracture-dependent nature of coal permeability evolution and provide guidance for optimizing gas drainage, enhancing methane recovery, and improving safety in deep coal mining.