Three-dimensional coupled simulation study on the reactivation mechanism and stability of faults under different fluid injection rate schemes

The injection rate of fluid is a key engineering factor affecting the stability and reactivation potential of faults, however, the mechanism of injection-induced fault reactivation under the effect of multiple injection rates remains unclear. In this study, we present a 3D coupled fluid-geomechanical conceptual model that includes a mixed-permeability structural fault in a critical strike-slip fault stress state to systematically investigate the effects of different injection rate schemes on the hydro-mechanical (HM) coupling behavior and reactivation potential of the fault, including three constant-rate injection schemes with different durations, four variable-rate injection schemes with different rate shapes and eight cyclic variable-rate injection schemes with different periods. The numerical simulation results show that compared with constant-rate injection schemes, the variable-rate injection schemes can complete the safe injection of fluids without prolonging the construction period, in which the variable-rate injection scheme with monotonous incrementality at the beginning of injection is more conducive to maintaining the stability of fault. The rate shape and cyclic period in cyclic variable-rate injection schemes have a large impact on the HM coupling behaviors and stability of the fault, and appropriately increasing the cyclic period and decreasing the peak rate can help reduce the potential risk of inducing fault reactivation and ensure geo-energy engineering safety. • A site-scale 3D coupled fluid-geomechanical conceptual model is established to describe the hydro-mechanical and slip behavior of fault under critical strike-slip fault stress state. • Three constant-rate injection schemes with different durations, four variable-rate injection schemes with different rate shapes, and eight cyclic variable-rate injection schemes with different periods are considered and designed. • The hydro-mechanical behaviors of the fault under 15 different injection rate schemes are systematically calculated and compared. • An optimized injection rate scheme is proposed to reduce the potential risk of injection-induced fault reactivation in engineering.