Molecular insights into dual competitive modes of CH4/CO2 in shale nanocomposites: Implications for CO2 sequestration and enhanced gas recovery in deep shale reservoirs

CO₂ enhanced shale gas recovery has significant potential for improving the recovery rate of adsorbed gas and facilitating the geological sequestration of CO₂. Nevertheless, the feasibility and microscopic mechanisms of CO₂ enhanced gas recovery in deep shale gas reservoirs remain to be elucidated. In this study, a quartz-kerogen slit model was constructed to represent the shale composite nanopores of deep shale reservoirs. Utilizing the grand canonical Monte Carlo and molecular dynamics methods, the adsorption and diffusion behavior of CH₄, CO₂ and their mixtures was investigated under high-temperature and high-pressure conditions representative of deep shale reservoirs. The influences of temperature, pressure and aperture size were analyzed. The microscopic mechanisms governing the CH₄/CO₂ competitive adsorption in deep shale nanopores were uncovered by considering the competitive interactions between different rock constituents and quantifying the various gas storage states. The results show that the adsorption capacity of CO₂ is greater than that of CH₄ under the deep shale reservoir conditions. The replacement efficiency of CH₄ by CO₂ is higher in small shale pores. High temperature has a more significant negative impact on the adsorption of CO₂ compared to that of CH₄. Compared to mid-deep shale reservoirs, the nanopores of deep shale reservoirs have a higher proportion of free CH₄/CO₂ states. Quartz exhibits greater affinity for CH₄/CO₂ than kerogen, but kerogen possesses a larger specific surface area, resulting in a higher adsorption capacity per unit volume. The hierarchy of CH₄/CO₂ diffusion capacity in various storage states is as follows: dissolved gas in the kerogen matrix < adsorbed gas on the quartz surface < adsorbed gas on the kerogen surface < free gas in the pore center. This study improves the understanding of CO₂ enhanced gas recovery in deep shale gas reservoirs.