Numerical study of hydraulic characteristics of shale fractures during unloading-induced slipping: Effects of inclination angle and unloading rate

This study investigates the roles of inclination angle of fracture (θ) and unloading rate of confining pressure (Ur) on the hydraulic characteristics of shale fractures during the unloading-induced slipping process. A series of fluid flow simulations in the directions both parallel (x-directional) and perpendicular (y-directional) to the shear direction at different shear displacements (ds) were performed. The results show that the channeling effect is more pronounced in the y-direction than that in the x-direction. As ds increases, the fluid flow becomes less anisotropic, and the ratio of permeability in the y-direction (Ky) to that in the x-direction (Kx) gradually decreases first and then remains almost constant. Both Kx and Ky greatly increase with increasing in θ. The Ur significantly affects the permeability when ds increases to a certain value (i.e., 0.8 mm in this study). The efficiency of permeability enhancement represented by ΔK/Δds is evaluated. The mean ΔK/Δds during the slipping process for samples with θ = 50° is 4.73 times and 5.25 times larger than that for samples with θ = 30° in the x-direction and y-direction, respectively. With increasing Ur, the mean ΔK/Δds slightly increases first and then decreases. The occurrence of stick–slip events may control the ΔK/Δds.