A Unified Path-Dependent Risk Index for CO$$_2$$ Leakage via Fault Slip and Caprock Fracture

Abstract Safe long-term geological storage of CO $$_2$$ 2 requires managing two coupled geomechanical hazards: caprock fracturing and fault reactivation. These processes are commonly evaluated independently using static failure thresholds, despite field evidence that they interact dynamically during injection. Here, we show that both fault slip tendency and caprock fracture response are governed by a shared poroelastic coupling mechanism that organizes leakage behavior across diverse rock properties. Using an ensemble of 175 fully coupled flow–geomechanics simulations, we demonstrate that variations in caprock and fault stiffness, together with a dimensionless ratio of mechanical coupling strengths, control how injection-induced stresses partition between shear deformation on faults and volumetric deformation in the surrounding matrix. This interaction generates a structured joint slip–fracture state space: stronger coupling stabilizes faults while simultaneously driving the caprock closer to tensile failure. Based on this structure, we introduce a physics-based Leakage Risk Index (LRI) that integrates slip- and fracture-related hazards into a single dimensionless metric. Within the calibrated parameter space, the LRI enables rapid screening and ranking of CO $$_2$$ 2 storage configurations using measurable poroelastic parameters. By projecting time-dependent simulation trajectories onto the LRI surface, we show that leakage risk evolves along sequence-dependent paths. Validation against observations from the In Salah (Algeria) and Castor (Spain) projects supports the framework. The initial mechanical response—slip or fracture—reshapes the stress field and governs subsequent behavior, demonstrating that leakage risk is inherently path-dependent rather than a simple threshold exceedance problem.