A new approach for determination of gas breakthrough in saturated materials with low permeability

Gas breakthrough is one of the key characteristics for saturated geological materials with low permeability encountered in deep geological disposal of high-level radioactive waste, shale gas extraction and CO2 capture-geological storage, etc. Although gas breakthrough phenomenon is widely investigated in previous researches, the essential mechanisms are still not clearly explained. In the present work, a new approach for determination of gas breakthrough mechanism was proposed and a series of gas injection tests were conducted for verification. According to the theoretical analysis, the capillary effect, mechanical effect, as well as the interfacial leakage can induce gas breakthrough. The capillary breakthrough indicates the displacement of liquid phase driven by the capillary effect to form continuous flow pathway across the specimen. The mechanical breakthrough indicates the mechanically induced dilatancy of flow pathways followed by a significant decrease of the capillary flow resistance in the specimen. The interfacial breakthrough indicates the failure of the sealing capacity of the lateral boundary during the gas injection test. Based on the inherent characteristics, deformation properties of the specimen, as well as the boundary conditions, relationships between the capillary/mechanical/interfacial breakthrough pressure and the confining pressure were established, which can be used to determine the triggering mechanism of the gas breakthrough. Constant-volume and constant-isotropic stress gas injection tests were subsequently conducted for verification of the proposed approach. Results showed that the mechanical breakthrough is the most likely mechanism for the low permeability specimens tested in the present work.