Permeability evolution and modeling analysis of anisotropic coal under different stress paths

Understanding the permeability characteristics of anisotropic coal under various complex stress environments is the key to improving gas extraction efficiency. The true-triaxial permeability tests under four stress paths and three flow directions are carried out using a multifunctional true triaxial geophysical device. The results show that unloading the minimum principal stress while increasing the maximum and intermediate principal stress induces significant fluctuations in coal permeability except along butt cleat direction. The permeability along the face cleat direction (Cf) is 9.9%–182.9% higher than in other directions, indicating greater compressibility of the face cleats. The Biot’s coefficient in the cleat direction is 0.728 ~ 0.841, which is higher than that in the other two flow directions, indicating that natural fractures are more abundant along the face cleats. Under the true triaxial stress path, the Cf value along the end cleat direction is 53.1% to 171.6% higher than that along the face cleat direction, and 9.9% to 182.9% higher than that perpendicular to the bedding direction, indicating that the face cleat has stronger compressibility. Under reduced minimum principal stress, increasing the intermediate principal stress suppresses fracture formation. The permeability model comparison results show that the minimum fitting value 0.9190 of Yu’s permeability model is higher than the minimum fitting value 0.8945 of Chen’s permeability model, and it better fits the fluctuation of permeability evolution, indicating that the permeability model considering cleat characteristics is better in gas drainage prediction. This study provides insights into the seepage characteristics of coal under diverse mining conditions and its permeability anisotropy.