Frictional Stability and Permeability Evolution of 3D Carved Longmaxi Shale Fractures and Its Implications for Shale Fault Stability in Sichuan Basin

Fluid injection associated with unconventional resource extraction activities can alter the friction-permeability relationship of fractures, thereby significantly affecting the stability of fractures/faults. In this study, we utilized the three-dimensional (3D) carving technique to reconstruct natural shale fractures recovered from the Longmaxi shale reservoir at a depth of 3700 m in the southeastern Sichuan Basin. Subsequently, we conducted friction-permeability coupling experiments to elucidate the relationship between friction, permeability, and stability for natural shale fractures. In the experiments, confining pressure was maintained at 3 MPa, temperature was maintained at 25 °C, the constant shearing velocity was set to 1 μm/s, and the velocity step was set to 10–1–10–1–10 μm/s. Our results show that the frictional coefficient of natural Longmaxi shale fractures ranges from 0.67 to 0.71, which is higher than that of saw-cut fractures of 0.61–0.62. Furthermore, frictional stability parameter ( a–b ) of the natural Longmaxi shale fracture exhibits velocity strengthening behavior under all conditions, indicating aseismic slip possibly caused by the abundant phyllosilicate content in shale fractures. However, the decrease of roughness and the effect of fluid injection reduce the frictional coefficient of natural fractures during the shearing process. At the same time, fluid injection reduces frictional stability, leading to a transition from velocity strengthening to velocity neutral. In addition, the increase in permeability of the natural fracture is more pronounced due to the velocity step shear compared to constant velocity shear. We confirm that fracture stability is influenced by a combination of factors including fracture surface roughness and fluid injection. Highlights Three-dimensional (3D) scanning and carving techniques were employed to reproduce natural Longmaxi shale fractures. Fluid injection leads to a transition of frictional stability from velocity strengthening to velocity neutral. Frictional coefficient and stability of shale fractures are affected by both roughness and fluid flow. Compared with constant velocity shear, velocity step shear has more obvious effect on permeability enhancement.