A coupled hydro-mechanical model for simulation of gas migration in host sedimentary rocks for nuclear waste repositories

In a deep geological repository (DGR) for nuclear wastes, several mechanisms such as waste form degradation and corrosion could lead to gas generation. The produced gas can potentially overpressurize the repository, alter the hydraulic and mechanical properties of the host rock and affect the long term containment function of the natural (host rock) and engineered barriers. Thus, the understanding of the gas migration within the host rock and engineered barriers and the associated potential impacts on their integrity is important for the safety assessment of a DGR. In this paper, a coupled hydro-mechanical model for predicting and simulating the gas migration in sedimentary host rock is presented. A detailed formulation coupling moisture (liquid water and water vapor) and gas transfer in a deformable porous medium is given. The model takes into account the damage-controlled fluid (gas, water) flow as well as the coupling of hydraulic and mechanical processes (e.g., stress, deformation). The model also considers the coupling of the diffusion coefficient with mechanical deformation as well as considers the modification of capillary pressure due to the variation of permeability and porosity. The prediction capability of the developed model is tested against laboratory scale and in situ experiments conducted on potential host sedimentary rocks for nuclear waste disposal. The model predictions are in good agreement with the experimental results. The numerical simulations of the laboratory and field gas injection tests provide a better understanding of the mechanisms of gas migration and the potential effects of excessive gas pressure on the host sedimentary rocks. This research work has allowed us to identify key features related to gas generation and migration that are considered important in the long term safety assessment of a DGR in sedimentary host formations.