Experimental Study on Gas Flow Characteristics in Coal under Different Mining-Induced Stress Paths

To investigate the impact of stress variations on coal permeability during mining, this study combines theoretical analysis with experimental research to reveal the permeability variation patterns under three mining-induced stress conditions. The results indicate that coal permeability exhibits a three-stage variation characteristic under different mining-induced stresses: slow increase, significant increase, and rapid increase. As axial stress increases, permeability initially decreases and then stabilizes. After reaching the strain peak and coal sample failure, through-cracks form on the sample surface, leading to a significant permeability increase. During the quasi-hydrostatic pressure loading stage, permeability decreases with increasing stress, while it gradually recovers during the confining pressure unloading stage, with a notable increase after reaching peak stress. Gas extraction efficiency is particularly ideal after the stress peak, with the most significant effect observed in the pressure relief zone. Based on the Forchheimer equation and the Klinkenberg effect, the calculated results indicate a parabolic variation trend between gas flow rate and pressure gradient, as well as between pore pressure and permeability.