Velocity dependence of permeability evolution and the effect of fluid pressure heterogeneity on frictional stability of longmaxi shale

The permeability evolution of faults and fractures is closely related to variations in slip velocity, while changes in high-pressure fluid distribution also affect the frictional stability. Here we conducted friction experiments on Longmaxi shale sawcut fractures at constant and step velocities to analyze the friction-permeability evolution and the velocity dependence of permeability. We then used COMSOL multiphysics software to recover the fluid pressure distribution along shale fractures under different conditions, focusing on the effects of fluid pressure heterogeneity on the frictional stability of Longmaxi shale. The results show that the permeability of Longmaxi shale sawcut fractures exhibits an overall enhancement with slip velocity switching. However, the cumulative effect of switching slip velocity on permeability evolution is relatively weak, and permeability still decreases with shear displacement. The permeability response parameter lambda increases overall as the friction state parameter (a-b) decreases, exhibiting regular changes with injection pressure, normal stress, and effective normal stress. Additionally, fluid pressure heterogeneity is influenced by both injection pressure and normal stress. Increasing injection pressure amplifies the heterogeneity of fluid pressure distribution, leading to a more significant heterogeneous friction phenomenon, forcing the Longmaxi shale to exhibit velocity-weakening behavior. When studying the frictional stability of various types of rocks under fluid injection, it is essential to analyze the actual distribution of fluid pressure to reveal the influence of fluid injection comprehensively.