Molecular Insights into Differential Deformation and Gas–Water Interaction of Shale Kerogen in Response to CO₂ Injection

Injecting carbon dioxide (CO2) into shale gas reservoirs has been recognized as a promising strategy for enhancing gas recovery and CO2 sequestration. Despite substantial research efforts dedicated to exploring the impacts of CO2 injection on methane adsorption, the differential deformation of methane and its diffusion behavior in response to CO2 injection under moist conditions have yet to be comprehensively elucidated. In this study, methane adsorption experiments and reconstruction of the macromolecular model were conducted on Longmaxi and Marcellus Formations shale kerogens. Then, using Monte Carlo and molecular dynamics simulations, the gas–water interaction process was analyzed, and the differential deformation and methane adsorption/diffusion in response to CO2 injection were discussed. Results show that when moisture content (MC) is greater than 1.6 wt %, water aggregates from small nanopores to large nanopores and forms water clusters. Water reduces the proportion of accessible nanopores of methane by occupying and cutting the nanopores, resulting in a decrease of methane adsorption amount. During CH4/CO2 binary mixture adsorption, CO2 displaces CH4 from the sulfur-containing structure to the nitrogen-containing structure of the secondary adsorption site. The negative effect of water on methane adsorption/diffusion will be compensated by a gain in the swelling strain induced by the CO2 injection. The efficiency of CO2 replacing methane reaches a maximum at 4 MPa, and both the adsorption selectivity and methane diffusion increase when the MC is between 1.6 and 2.4 wt %. Consequently, the injection of CO2 under optimal pressure and humidity conditions has the potential to significantly enhance the efficiency of methane displacement and CO2 storage significantly. The results of this study offer novel perspectives on shale gas fracturing techniques and contribute to a more comprehensive evaluation of the gas/water production process.