Role of Stress Perturbations on Fault Stability During CO₂ Storage in Fractured Shale Reservoirs

Abstract: Storing captured CO₂ in fractured shale reservoirs is a promising and feasible approach to ensure large-scale carbon reduction and realize the dual goals of carbon neutrality and environmental protection. However, increased fluid pressures and decreased effective stress may promote fault/fracture reactivation and the potential to trigger seismicity. As a typical example, we use fractured Longmaxi shale reservoirs in the southeastern Sichuan Basin to explore fluid pressure perturbations on the potential for hazardous seismicity. We conduct double-direct shear experiments on simulated Longmaxi shale gouges to explore the effects of over pressurization. Specifically, we isolate the impacts of fluid pressure reduction rates, magnitudes of initial confined stress and shear velocity, and shale mineralogy on fault peak shear velocity and durations to nucleation. A larger fault peak shear velocity and a shorter nucleation duration are proxies to indicate that the fault may be more readily reactivated. Results identify the pressure reduction rate as one of the most important external factor influencing the fault reactivation style. Elevating the pressure reduction rate apparently increases peak shear velocity to approach the dynamic fault slip rate (mm/s) for earthquake triggering and reduces the duration of nucleation. Lowering the initial confining stress and shear velocity produces similar effects. For Longmaxi shales, elevating the tectosilicate content significantly increases the peak shear velocity and nucleation duration, while elevating carbonate content shows the opposite effect. Results imply that the peak shear velocity of most shale faults should be below a threshold for earthquake triggering and highlight the importance of fault aseismic fault slip in triggering the potential for seismicity during CO₂ storage in fractured shale reservoirs. Highlights: Storing CO₂ in fractured shale reservoirs in Longmaxi shale is viable but may promote fault instability. Increasing normal stress reduction rates and lowering the confining stresses promote fault nucleation. Mineralogy is a single most important intrinsic factor controlling shale fault stability, especially tectosilicates and carbonates.