Modeling in-situ gas flow and coupled hydro-mechanical processes in the Opalinus Clay

Deep geological repositories rely on low-permeability host rocks to ensure long-term containment of radioactive waste. The Opalinus Clay is considered a suitable host rock, acting as the primary geological barrier. Corrosion of waste containers under anoxic conditions produces hydrogen gas, which can accumulate due to the low permeability of the formation. Elevated gas pressures may exceed the minimum principal stress, potentially causing fracturing and compromising repository integrity. Since hydrogen generation will persist for more than 100,000 years, understanding gas transport mechanisms in low-permeability rocks is critical. Gas migration may occur through advective–diffusive flow, visco-capillary two-phase flow, dilatancy-controlled gas flow, or advective transport along macroscopic fractures. The dominant mechanism depends on gas pressure and rock microstructure, though controlling variables remain poorly constrained. The GT (Gas Transport) experiment at the Mont Terri Rock Laboratory was conducted to study these processes by direct injection of helium into the Opalinus Clay in stepwise pressure increments until breakthrough occurred. Here, we present results of a coupled hydro-mechanical model using TOUGH-FLAC, linking multiphase flow simulations (TOUGH3) with geomechanics (FLAC3D). We aim to distinguish between gas transport mechanisms through different model configurations by fitting them to the observed pressure response, with emphasis on capturing the onset of dilatant pathways through permeability models. Our simulations reproduce observed injection pressures closely. The models allow investigation of conditions under which different gas transport mechanisms occur, particularly the onset of dilatant pathways. These results provide new insights into gas migration in the Opalinus Clay and its implications for repository safety.