Impact of reservoir pressure fluctuations on fracture shear slip behavior in shale gas hydraulic fracturing

To elucidate the effect of fluid pressurization rate on the shear slip behavior of shale fractures during reservoir hydraulic pressure fluctuations, shale slip experiments with constant axial stress were conducted at 0.5, 2, 8, and 30 MPa/min pressurization rates based on the pressure fluctuation characteristics of hydraulically fractured reservoirs of deep shale gas in the Sichuan Basin. The results indicated that quasi-static slip exhibited higher average velocities than creep slip by 2–3 orders of magnitude at equivalent pressurization rates. Additionally, at 30 MPa/min pressurization rates, the slip type transitioned directly from creep to dynamic slip. The slip velocity increases with increasing pressurization rate, and the average velocities of quasi-static slip are 7.10 × 10−4, 2.20 × 10−3, and 5.40 × 10−3 mm/s, respectively. Dynamic slip exhibited the highest critical slip pressure of 7.14 MPa and the largest friction coefficient increased by 55% at a pressurization rate of 2 MPa/min. Friction strength primarily increased by 42% during creep slip at a pressurization rate of 0.5 MPa/min, while the friction coefficient minimally decreased by 0.03% during dynamic slip. Notably, dynamic slip exhibited a significant increasing trend in the percentage of friction coefficient increment. The critical dynamic slip pressure and energy released during slip exhibited an initially increasing and then decreasing pattern over the range of pressurization rates in this experiment, which is a result of the degree of inhomogeneity of the fluid increasing and then decreasing with the pressurization rate. Consequently, this led to maximum accumulation and energy release at the fracture surface, resulting in minimal quasi-static slip displacement, and is not conducive to improved reservoir permeability characteristics and safe shale gas recovery.