Insights into gas migration in saturated GMZ bentonite using the RCP technique

In a deep geological repository for disposal of high-level radioactive waste, understanding of gas migration behavior in bentonite materials is one of the most important issues for safety assessment of the repository. This study tries to provide insights into gas transport properties in saturated bentonite tested under a constant-volume condition using the residual capillary pressure (RCP) technique. In this regard, a test setup was developed and gas injection tests were conducted on saturated GaoMiaoZi (GMZ) bentonite specimens using the RCP technique. Among which, some specimens that experienced initial gas breakthrough processes were re-saturated and re-submitted to conduct the subsequent gas injection tests. During the tests, gas permeability and gas breakthrough pressures were measured. Gas migration mechanism at lower gas injection pressures was analyzed and the influence of repetitive breakthroughs on sealing efficiency of saturated GMZ bentonite was discussed. Results show that as dry density increases from 1.3 to 1.7 g/cm(3), gas breakthrough pressure increases from 1.37 to 8.56 MPa. Explanations to this conclusion could be that with increasing dry density, the proportion of large pores decreases and the strongly adsorbed water on the surface of minerals increases, leading to harder capillary displacement of pore water and thus a higher gas breakthrough pressure. Meanwhile, both diffusion and advection could play significant roles during the gas migration process at lower injection pressures, while advection could gradually become the dominant transport mechanism as gas injection pressures increase to higher values. Finally, after experienced a gas breakthrough process, the breakthrough pressure of compacted bentonite specimen might not recover to its initial value by re-saturating the specimen, indicating that degradation of sealing capacity of the bentonite materials induced by gas breakthrough process is irreversible.