Response characteristics of shale permeability to cyclic loading under different gas pressures.

The permeability evolution characteristics of low-permeability shale layers directly influence shale gas development. However, the evolution law of shale reservoir permeability in complex environments remains unclear. Therefore, understanding the permeability characteristics of shale layers is of great significance. To address this issue, permeability testing was conducted on shale under conventional triaxial compression and cyclic loading-unloading conditions. The study focused on investigating the permeability evolution of shale under different gas pressures during cyclic loading and unloading processes. The experimental results show that in the conventional triaxial compression experiment, the permeability of shale exhibits distinct stage characteristics with the change of deviatoric stress. In the elastic deformation stage, the permeability decreases slightly with the increase of deviatoric stress; In the plastic deformation stage, the permeability increases rapidly with deviatoric stress, and in the post-failure stage, the permeability continues to increase as deviatoric stress decreases, but the rate of increase slows. During the loading and unloading stages, the permeability of shale varies exponentially with confining pressure under different gas pressure conditions. Additionally, with the increase of gas pressure, the initial permeability of shale also increases. Furthermore, it was found that whether during the loading or unloading stage, for the same number of cycles, the absolute permeability change in shale rose with higher gas pressure. When the gas pressure remains constant, during the loading stage, except for the first loading, the absolute value of the permeability change in shale decreases with increasing loading cycles. during the unloading stage, the absolute value of the permeability change in shale decreases with increasing unloading cycles. Finally, the dimensionless permeability of shale was investigated, and it was found that during the loading stage, the value decreased with increasing confining pressure, while during the unloading stage, the value gradually increased with decreasing confining pressure.