Far-field modelling of THM processes in rock salt formations

Glaciation cycles are one aspect to be considered in assessing the safety of deep geological repository sites for long-term radioactive waste storage. This study examines the impact of time-dependent boundary conditions and thermo-hydro-mechanical (THM) couplings on geological formations under glaciation-induced stresses, pressures and temperature changes. Using OpenGeoSys, an open-source finite element simulator, we analyzed various process couplings to understand the underlying physical processes and numerical instabilities. We simulated vertical cross-sections of geological models relevant to nuclear waste repository sites, incorporating comprehensive geological data to capture the formations’ heterogeneity and structural features. A viscoelastic material model was used for rock salt strata to account for dislocation creep and pressure-solution creep. The study benefited from rigorous automation of the entire simulation workflow, making the setup suitable for evaluating actual repository sites regarding integrity criteria. Although the modeled rock salt strata were hydraulically deactivated, results were highly dependent on hydraulic boundary conditions. Groundwater flow significantly altered the geological temperature profile via advective heat transport and influenced the temperature-dependent creep behavior. The rock salt creep law, applied over the extensive timescales at hand, approached the limits of the Finite Element Method (FEM) with small-strain assumptions. Throughout the modeled glacial cycle, the salt strata exhibit low deviatoric stresses. Fluid pressure and dilatancy criteria are not violated in the repository during the modeled period.