Hydromechanical Modelling of Gas Migration in Host Rocks for Nuclear Waste Repositories
Deep geological repositories (DGRs) are currently being proposed in several countries to deal with the nuclear waste. The safe long-term disposal of the waste is guaranteed by a multi-barrier system, which includes a natural barrier (host rocks) and an engineered barrier system. Large quantities of gas can be generated during the lifespan of the DGRs due to several processes, such as the corrosion of metal, water radiolysis or microbial degradation. The generated gas could potentially overpressurize the DGR, deteriorate the hydraulic and mechanical properties of the host rock and impact the long term safety of the DGR. Thus, the understanding and prediction of gas migration within the host rock and associated potential impacts on its integrity is critical for the long term safety of a DGR.
Numerous laboratory tests have shown that gas migration in host rock involves complex hydro-mechanical processes, and the classical concepts of two-phase flow in porous medium is inappropriate to predict this gas migration as well as capture the key HM processes. In the study, a coupled HM model, based on double-porosity approach, is developed and implemented into COMSOL Multiphysics® simulation software to assess and predict gas migration in sedimentary host rocks (claystone). The model takes into account the HM behavior of both the porous medium (representing the matrix) and the fractured medium (representing the fractures). The prediction capability of the model is tested against laboratory gas migration tests conducted on potential host sedimentary rocks (claystone). The simulated results are in good agreement with the experimental data, which shows the robustness of the developed model. The validation results have also shown that the developed model, can well capture the main experimental observations, such as the development of gas preferential pathways, volume dilation and gas induced fracturing.
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