Coupled multiphase flow and viscoelastic mechanics modeling of gas injection in a compacted bentonite buffer

Bentonite is chosen as a suitable buffer material for a deep geologic repository for radioactive waste. Thus, understanding the behavior of gas migration in the buffer layer is key to the safety assessment and functioning of such a repository. Based on the gas-injection experiments performed by the British Geological Survey (BGS), the modeling of gas migration in a compacted bentonite is carried out by the multi-phase-flow module (H) coupled with the viscous-elastic geomechanics module (M) of a fully coupled model, called THMC 7.1. Two laboratory scenarios for gas injection into compacted and saturated bentonite confined in a pressure vessel are considered in this simulation study. Injected gas (Helium) accumulates, entering the saturated bentonite after reaching a critical pressure to be detected by the gas filters to show the timing of the “breakthrough”. It is found in the experiments that the total stress reaches the maximum value right after breakthrough but does not exceed the gas injection pressure. It is found that our simulation results can capture the peak lab-test values of total stress and porewater pressure as well as the lab-test timing of breakthrough. Moreover, the decay patterns of both total stress and porewater pressure are well described in the simulations. A comparison of the simulation results with the experimental data shows that our HM coupled modeling can qualitatively and quantitatively model the gas migration behavior and its mechanical contribution to the buffer response. According to the presented simulations, the further improvement of the viscous-elastic geomechanical modelling is also discussed.