Insights into gas migration behavior in saturated GMZ bentonite under flexible constraint conditions

Abstract Gas migration behavior of bentonite-based buffer/backfill materials is one of the most important issues for safety assessment of deep geological repository for disposal of high-level radioactive wastes. In this paper, a series of gas injection tests were conducted on water-saturated bentonite specimens under stepwise increasing gas injection pressure and a flexible boundary condition. Prior to the gas injection tests, the steady state water permeability (stst kw) was determined by the water injection tests with deionized water and the stst kw values obtained were in an order of magnitude of 10-19-10-20 m2, which were slightly lower than the non-steady state ones calculated from the results obtained at the initial stage of gas injection tests. The effective gas permeability measured before and after gas breakthrough on the initially water-saturated bentonite specimens with different dry densities ranges between 5.45 × 10-25 m2 to 2.76 × 10-22 m2. The Peclet number (Pe) was adopted to determine the relative importance of advective gas transport (pressure-driven gas flow) to diffusive gas transport through saturated bentonite specimens. Calculation shows that the Pe values are significantly lower than 1, indicating the dominant role of gas diffusion across the specimens before and after gas breakthrough. As dry density of the specimen increases from 1.3 g/cm3 to 1.7 g/cm3, the gas breakthrough pressure increases from 4.92 MPa to 8.68 MPa. Meanwhile, the residual pressure differences measured after gas breakthrough ranges from 4.04 MPa to 7.88 MPa for specimens with different dry densities. A slight decrease of water content of the specimens experienced gas injection tests and a stable increase of specimen volume at the breakthrough indicate that both the displacement of water from interconnected pathways (capillary effect) and the pathway dilation (mechanical effect) play important roles in gas breakthrough for specimen tested under flexible conditions.