Gas breakthrough in saturated compacted GaoMiaoZi (GMZ) bentonite under rigid boundary conditions

Abstract

In a geological repository for disposal of high-level radioactive waste, gas breakthrough is an important phenomenon during a gas migration process in the saturated engineered barrier. In this paper, gas injection, swelling pressure, water permeability, and water retention tests were conducted on saturated compacted GaoMiaoZi (GMZ) bentonite to investigate the gas breakthrough mechanism. Results show that, for saturated GMZ bentonite tested under rigid boundary conditions, the gas breakthrough pressure is significantly larger than the swelling pressure and slightly lower than the gas entry pressure obtained from the water retention characteristic and the van Genuchten model. Gas breakthrough pressure deviates from the swelling pressure and approaches the calculated gas entry pressure as the dry density increases. Mechanical and capillary effects are both important to the gas migration process for specimens with lower dry densities, and the capillary effect becomes more important with the increase of dry density. The desaturation and shrinkage of the specimen will result in unexpectedly high and disordered interfacial gas flux. For specimens with higher dry densities, gas will only flow through interconnected larger pores, then result in minor desaturation–shrinkage of the specimen. Finally, a new model with consideration of both mechanical and capillary effects is proposed, which can accurately predict gas breakthrough pressure for a GMZ bentonite specimen.