Directional permeability evolution in intact and fractured coal subjected to true-triaxial stresses under dry and water-saturated conditions

Investigations of the directional permeability evolution of intact and fractured coal are conducted under different simulated geological conditions. The effects of fracture geometry, water adsorption and stress conditions on the permeability evolution of coal as a function of stress are systematically studied. The results indicate that permeability anisotropy is more pronounced in fractured coal than in intact coal. The permeability order, i.e., the k(fa) > k(bu) > k(be) relationship, is maintained following the introduction of macrofractures into coal. The fracture compressibility in the butt cleat plane flow direction is higher than that in the other two flow directions for both intact and fractured coal. The presence of water in coal can reduce the permeability by up to one order of magnitude, and a more significant permeability decrease is observed in coal specimens containing rough macrofractures. Permeability hysteresis for both intact and fractured coal is somewhat dependent on the stress condition. The hysteresis effect of coal is more significant under triaxial stress conditions and less pronounced under true-triaxial stress conditions.