Fluid‐induced slip on a rate‐and‐state fault during co‐ and post‐injection: The interplay of fluid pressure diffusion and poroelastic stress

Abstract Subsurface fluid injection, widely employed in enhanced geothermal systems, CO₂ geological storage, and unconventional hydrocarbon extraction, can reactivate pre‐existing faults and induce seismicity. We present a physics‐based numerical framework that integrates fluid pressure diffusion, poroelastic stressing, and rate‐and‐state friction to evaluate fault stability in homogeneous poroelastic reservoirs. Compared with traditional Coulomb failure stress ( CFS ) approaches, which rely on quasi‐static stress changes, the model captures a broad spectrum of slip behaviors, ranging from aseismic creep to dynamic rupture, and quantifies systematic controls exerted by injection rate, tectonic loading rate, fault orientation, and reservoir permeability. Our numerical simulations reveal that poroelastic stressing plays a dominant role in triggering fault slip, particularly at large distances from injection sources and during the early stage of injection. We demonstrate that elevated injection rates and tectonic loading rates produce similar effects on induced seismicity patterns. Favorably oriented faults experience early slip events that precede the arrival of the fluid pressure diffusion front, whereas unfavorably oriented faults exhibit delayed slip that occurs only after the pressure front passage. In low‐permeability formations, fault slip onset is delayed but characterized by higher slip velocities. We additionally examine the post‐injection (shut‐in) stage, during which pore pressure continues to accumulate before eventual dissipation. The results indicate that shut‐in can either mitigate or accelerate slip depending on fault orientation, stress regime, and pre‐shut‐in injection history. Importantly, once slip nucleation has initiated, shut‐in may fail to prevent induced seismicity, underscoring the critical importance of timing in operational mitigation. image