Reactivation of rate-and-state faults induced by CO2 injection: Effects of pore pressure diffusion and fluid pressurization

While injection-induced seismicity has been widely studied, its implications for CO 2 sequestration require reevaluation due to distinct fluid-rock interactions. This study develops a coupled hydromechanical model incorporating rate-and-state friction laws to investigate fault reactivation mechanisms during early-stage CO 2 injection. The competing effects of pore pressure diffusion and fluid pressurization are systematically investigated, considering three key factors: permeability variations within fault damage zones, normal stress variation coefficients, and injection parameters. Numerical simulations reveal that slower CO 2 migration causes limited pressure perturbation (< 0.3 MPa over 15 d) compared to single-phase fluid injection. Fluid pressurization enhances fault strength and delays reactivation, though this stabilizing effect diminishes in low-permeability damage zones. Highly permeable damage zones promote larger rupture areas despite strengthening from pressurization, as reduced effective stress accelerates failure. Paradoxically, while fluid pressurization increases fault strength, it simultaneously elevates seismic risk through amplified stress drops during slip events. Temporal analysis shows that fluid pressurization dominates initial fault response, while sustained pore pressure diffusion ultimately drives reactivation. Increased normal stress variation coefficients and injection rates accelerate localized rupture initiation but restrict propagation due to non-critically stressed states. This discrepancy demonstrates that regions with positive Coulomb failure stress changes do not correlate well with actual slip zones. These findings highlight the critical interplay between transient pressurization effects and progressive pressure diffusion during early CO 2 injection phases, providing crucial insights for seismic risk management in carbon sequestration projects.