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

A simplified stress path was designed based on decompression mining to explore coal seam permeability variation during stress relief gas extraction in protective layers. This involved axial, confining, and pore stress to simulate coal seamstress during mining and pore pressure during gas extraction. Experiments were conducted on intact and fractured coal samples from Huainan using gas flow and displacement testing equipment. Key findings include: The first unloading cycle significantly impacted fracture volume, with vertically fractured coal showing the greatest variation, followed by horizontally fractured and intact samples, with reductions of 29, 23, 21, and 20%, respectively. Irreversible permeability loss rate is directly proportional to porosity. Both intact and fractured samples showed a porosity decrease followed by an increase, with the third loading/unloading cycle having the greatest impact. As pore pressure decreases, its influence on permeability is greater in fractured samples than in intact samples. The correlation between pore pressure and permeability is stronger in fractured samples. With the increased number of cycles, intact samples' stress sensitivity to pore pressure increases, while that of fractured samples decreases. After three cycles, new fractures develop in both intact and fractured samples, enhancing gas extraction efficiency.