Rapid Assessment of CO2 Leakage Risk through Naturally Fractured Caprocks Via a Coupled Hydro-Mechanical-Chemical Model

Summary Numerous numerical modeling studies have been conducted to investigate carbon dioxide (CO2) leakage risk through various mechanisms, such as deep saline aquifer sequestration. As one of the major leakage pathways, the presence of natural fractures and their potential reactivation or opening induced by injection pose a considerable challenge to long-term storage safety. Explicitly representing fracture networks in simulation grids can substantially increase computational cost, particularly when hydrodynamic, mechanical, and geochemical processes are fully coupled. In this work, we propose a novel geomechanics-coupled embedded discrete fracture model (GeoEDFM) that couples mechanical and chemical dynamics and analyzes the CO2 migration in brine formation containing natural fractures. The fracture opening processes are captured using tensile and shear failure criteria. Benchmark tests demonstrate that the proposed approach reduces computational cost by more than 30 times compared with COMSOL® without compromising solution accuracy. Further numerical experiments are carried out to investigate the fracture opening processes and CO2 leakage through both the brine formation and the caprock. Results indicate that fracture permeability evolves nonmonotonically due to coupled flow-stress effects, characterized by pressure-induced fracture opening during injection and progressive closure after CO2 enters the fracture system and pressure dissipates, eventually stabilizing at a lower residual level. Large-scale CO2 leakage mainly occurs when fractures penetrate both the storage aquifer and the barrier layer. Leakage risk increases with fracture dip angle, fracture conductivity (i.e., zero-stress permeability), and fracture density but decreases with higher initial in-situ stress. Furthermore, our simulations demonstrate that beyond certain site-specific critical limits of fracture aperture and density, the coupled hydromechanical opening of fractures significantly accelerates CO2 migration, causing cumulative leakage to rapidly exceed commonly accepted safety margins.