Investigation on gas migration in saturated materials with low permeability

Investigation of the hydro-mechanical effects on gas migration in saturated materials with low permeability is of great theoretical and practical significances in many engineering fields. The conventional two-phase flow (visco-capillary flow) theory, which regards the capillary pressure as the only controlling factor in gas migration processes, is commonly adopted to describe the gas flow in geo-materials. However, for materials with low permeability, the conventional two-phase flow theory cannot properly describe the gas migration. In this work, hydro-mechanical coupled gas injection tests were conducted. The volumetric variation of the liquid for applying the confining pressure in the specimen cell and the gas flow rate were monitored. Test results indicate that gas migration is influenced by the capillary pressure and the mechanical stress simultaneously. The two key parameters of the gas entry pressure Pentry and the gas induced-dilatancy pressure Pdilatancy are introduced for description of gas migration with respect to the capillary pressure and the mechanical stress effects, respectively. When the gas injection pressure is smaller than the Pentry and the Pdilatancy, the balance between the gas injection pressure and the confining pressure will lead to an intermittent gas flow. Sudden increase of gas flow rate could be observed once the gas injection pressure approaches the Pentry or the Pdilatancy. For higher gas injection pressures, the mechanical stress effects on gas migration could not be neglected. The sudden increase of gas flux under high gas injection pressures could be caused by the mechanical induced-dilatancy of channels, capillary pressure induced-continuous flow pathways, as well as the failure of sealing-efficiency. The failure of sealing-efficiency is closely related to the difference between the gas injection pressure and the confining pressure rather than the properties of the material tested. Monitoring the volume of liquid for applying confining pressure is helpful for detecting the failure of sealing efficiency and the mechanism of gas breakthrough.