As an important unconventional and alternative resource, shale gas has attracted worldwide attention. The breakthrough pressure is a major factor in the generation and migration of shale gas as well as in the evaluation of the caprock sealing capacity. Carboniferous shales are considered to have great potential for the exploitation of shale gas; thus, investigations of the breakthrough pressure and gas effective permeability are significant. Two shale samples taken from the Carboniferous Hurleg Formation in the eastern Qaidam Basin, China, were chosen to conduct breakthrough experiments to investigate the effects of water saturation and CO2–CH4 mixed mole fractions on the breakthrough pressure and gas effective permeability. Prior to the experiments, various relevant parameters (e.g., the porosity, mineral composition, and organic geochemistry; the total organic content, thermal maturity and kerogen type; and microstructure) of these samples were also measured.
The results of our breakthrough experiments show that the breakthrough pressure increases with the water saturation and decreases with the CO2 mole fraction in the gas mixture. The situation for the gas effective permeability is just the opposite. Pore-size distribution measurements indicate that there are many nanoscale micropores that can easily be blocked by water molecules. This results in the reduced connectivity of gas pathways; thus, the breakthrough pressure increases and the gas effective permeability decreases with increasing water saturation. The breakthrough pressure decreases with the CO2 mole fraction because the interfacial tension of the CO2–water system is smaller than that of the CH4–water system. The viscosity of the CO2–CH4 mixture was found to increase with the CO2 mole fraction by fitting a series of values under the same temperature and pressure conditions, leading to an increase in the gas effective permeability. Furthermore, CO2 molecules are smaller than CH4 molecules, making it easier for CO2 to move across pathways. After each breakthrough experiment, the CO2 mole fraction in the effluent was less than that in the injected gas, and it increased over time until reaching the initial injected gas composition. This is because the adsorption and solubility of CO2 in water are greater than those of CH4. This study provides practical information for further investigations of shale gas migration and extraction and the sealing capacities of caprocks.