Coupled hydro-mechanical damage and dilatancy modelling of gas-induced fracture reactivation in crystalline host rock for deep geological repositories

Gas generation in deep geological repositories (DGRs) may increase pore pressure and reactivate fractures in crystalline host rock, altering its permeability and gas transport behaviour. This study develops a coupled hydro-mechanical-damage framework that integrates two-phase gas-water flow, effective stress, tensile and shear damage, dilatancy, and anisotropic permeability evolution. Permeability evolves with the local damage state, allowing stress-dependent gas pathways to develop without explicitly prescribing a discrete fracture network. The framework was calibrated and evaluated against laboratory gas injection experiments and field-scale observations. It reproduced the main trends in gas-pressure evolution, breakthrough, and permeability enhancement. Long-term scenario simulations were then performed for the Canadian crystalline DGR concept. The results showed that gas-induced changes were concentrated mainly near the excavation damage zone (EDZ) while the intact granite was less affected. Increased gas loading enhanced gas saturation and damage intensity but did not substantially change the spatial gas migration pattern. Under the investigated scenarios, permeability enhancement promoted localised pressure redistribution without widespread damage propagation. These results demonstrate the importance of fracture reactivation, dilatancy, EDZ properties, and anisotropic permeability evolution in modelling gas migration through crystalline repository systems.