Mechanism of permeability enhancement in low-permeability coal seams under the combined effects of mining-induced stress and gas slippage

• Synergistic effects of mining-induced stress and gas pressure on permeability • Correlation between coal deformation characteristics and permeability • Mechanisms of effective stress and slip effect on gas seepage • Validation of a porous-media permeability model based on capillary characteristics The development and utilization of deep coal resources represent an emerging trend. Understanding the seepage mechanisms of deep, low-permeability coal seams under the coupled effects of effective stress and gas pressure is essential for achieving the safe and efficient co-extraction of coal and gas. Using the independently developed ZST-1500 microcomputer-controlled electro-hydraulic servo testing system for multi-field static–dynamic coupling in coal and rock, gas seepage experiments were conducted on low-permeability coal seams under two stress paths, ICVES and ICCES. In addition, a capillary-based porous media seepage model was optimized. The results indicate that upon completing the ICVES loading–unloading cycle, the axial strain increased by 1.62%, the radial strain decreased by 12.86%, and the permeability increased by 46.96%. In contrast, at the end of the ICCES cycle, the axial strain decreased by 1.62%, the radial strain increased by 20.47%, and the permeability decreased by 0.8%. Under both stress paths, the fitting errors between the measured permeability values and those predicted by the capillary-based porous media model were less than 2%. The sensitivity coefficient of permeability to stress increased with rising effective stress, whereas the sensitivity coefficient to gas pressure decreased with increasing gas pressure. The results elucidate the seepage mechanism of deep, low-permeability coal seams under the coupled effects of effective stress and pore pressure, quantify the sensitivities of coal permeability to effective stress and gas pressure, and provide theoretical support for investigating gas seepage behavior in coal seams under multi-field coupling.