A model for describing advective and diffusive gas transport through initially saturated bentonite with consideration of temperature

Abstract

Understanding the gas migration mechanism in an initially saturated bentonite is of great importance for evaluation of the long-term performance of a deep geological repository for disposal of high-level radioactive wastes. Previous researches usually neglect the gas diffusion process, but adopted the advective gas flow theory (visco-capillary flow or dilatant gas flow) to describe the gas transport in the initially water-saturated bentonite clays. In this study, gas migration process was regarded as results of a combination of diffusive transport of dissolved gas through pore water and advective flow along the gas pathways formed in the bentonite matrix. Meanwhile, dilation of the gas flow pathways with variation of the external stress and gas pressure during the advective flow was also considered. Based on this concept, a new model was proposed to describe the advective and diffusive gas transports with consideration of temperature. Validation was conducted using the gas injection tests performed under different boundary conditions in literature. Comparisons show that the diffusion and advection-controlled process provided by the proposed model can effectively simulate gas transport in the compacted GMZ bentonite. For gas migration under lower injection pressures, the contributions of advective gas flow to the gas flux are comparable to that of diffusion. With the increase of the injection pressure, the advective gas fluxes would gradually dominate over the diffusive transport. Meanwhile, predicted degree of saturation of the compacted bentonite is maintained at above 99% during the whole gas migration process, indicating that advective movement of gas was highly discrete in initially saturated bentonite.