TRIAXIAL EXPERIMENTAL STUDY ON FRICTIONAL INSTABILITY OF FAULT ZONES INDUCED BY FLUID PRESSURE ACCUMULATION

Under the background of energy transition, the exploitation of clean energies, such as shale gas and geothermal energy, has become a focus of attention. It is highly important for energy security and the achievement of carbon neutrality. The development of deep oil, gas, and geothermal resources requires large-scale fracturing and remodeling of reservoirs. High-pressure fluid injection often alters the stress on faults, leading to fracture destabilization and increasing the risk of casing deformation. In this paper, friction instability experiments were conducted under triaxial stress conditions in the laboratory. By controlling the pressurization rate of fluid and comparing the evolution of shear stress, slip displacement, and friction coefficient, the influence of fluid pressure on the friction instability of faults is summarized. The results indicate that:(1)The frictional slip of faults induced by high-pressure fluid injection in sandstone shows distinctive stages, including the measurement and preparation stage of the critical slip state, the accumulation stage of fluid pressure, and the friction-slip instability stage of the fault zone. Data recorded during the tests show that unstable slip occurs mainly in the third stage. (2)The occurrence of unstable fast slip requires a certain process of fluid pressure accumulation. When the parameter(a-b)<0, velocity weakening and frictional slip instability occur. Moreover, a higher rate of fluid pressure change induces a greater fault slip displacement. (3)Stress variation, slip displacement, and friction coefficient are time-dependent. The faster the fluid pressure changes, the closer the fluid pressure at slip instability is to the confining pressure.