Experimental study on the permeability characteristics of laminated shale under cyclic loading

It is of great significance for the safe construction of shale gas tunnels to deeply understand the evolution law of permeability of laminated shale with macro fractures under cyclic disturbance. The permeability evolution of shale specimens containing axial beddings and axial macro fractures under cyclic axial stress and cyclic confining pressure was experimentally studied using the GCTS rock mechanics testing system. The results show that the permeability of shale specimens containing axial beddings did not change significantly during the loading and unloading of axial stress, but it decreased in a negatively exponential form with the increasing confining pressure and increased exponentially with the decreasing confining pressure. The permeability of shale samples containing macroscopic fractures decreased linearly with the increasing axial stress and increased linearly with the decreasing axial stress, while it decreased in a negatively exponential form with increasing confining pressure and increased exponentially with the increasing confining pressure. The permeability of shale specimens containing axial fractures is significantly higher than that of shale specimens with axial beddings, about 9 times the later one. The sensitivity of shale specimens to confining pressure is about 26 times that to axial stress. The permeability of the specimen almost keeps constant regardless of the number of axial stress loadings and unloadings, but decreases in a negatively exponential form with the increasing confining pressure cycles, and the major reduction occurs in the first cycle. The findings can somewhat provide theoretical support for the safe construction of shale gas tunnels.