Molecular Insights into Gas-Water-Shale Interactions for Flue Gas Injection Strategies to Enhance Shale Gas Recovery

Summary The injection of flue gas—a combustion exhaust mixture primarily composed of CO2 and N2—for enhanced shale gas recovery (CO2-N2-ESGR) offers a dual-benefit solution. It combines cost-effective flue gas usage with geological carbon sequestration. Nevertheless, the microscopic mechanisms controlling CO2-N2-ESGR in multicomponent shale mineral nanopores remain unclear, particularly regarding the influence of water on the competitive adsorption and displacement processes. In this work, molecular simulation methods were used to construct a composite shale nanopore model that integrates organic matter, clay mineral, and detrital mineral. The occurrence of water as well as the single-component adsorption and competitive adsorption processes of CH4, CO2, and N2 were simulated. The microscopic characteristics of water occurrence and its influence on gas adsorption were analyzed, and the mechanisms of flue gas for replacing shale gas were explored. The results show that water occurs in the shale composite model as isolated water, water clusters, water films, and corner water, with the corner water being the predominant form. Under moderate water content conditions, water enhances gas adsorption by providing adsorption sites and promoting micropore filling. CO2 exhibits a significantly greater adsorption capacity than CH4, while N2 shows a slightly higher preferential adsorption capacity compared with CH4. In flue gas, N2 synergistically cooperates with CO2 to displace adsorbed CH4. In the shale composite model, CO2 primarily displaces adsorbed CH4 on quartz and illite surfaces through strong electrostatic interactions.