Injection of CO2 into shale plays has been considered as one potential scenario for CO2 sequestration with enhanced CH4 recovery (CS-EGR). In this work, a realistic molecular model of bulk kerogen was constructed by molecular dynamics simulations (MD). Grand canonical Monte Carlo simulations (GCMC) were performed to investigate the replacement of CH4 by CO2 under different reservoir pressures, CO2 mole ratios and kerogen moisture contents. The radial distribution functions (RDF) were computed to discuss the affinity between CH4/CO2 and atoms in the kerogen. Also, the CO2/CH4 adsorption selectivity was computed to evaluate the preferential adsorption of CO2 and CH4. Results show that the CO2/CH4 selectivity increases at the beginning, and then decreases with further increasing pressure, which indicates that low reservoir pressure is beneficial to the efficiency improvement of CS-EGR. The amount of CO2 sequestration and CH4 desorption increases, while the CO2/CH4 selectivity decreases as the CO2 mole ratio increases, indicating the adsorption stability of sequestrated CO2 weakens with increasing CO2 mole ratio. The CO2/CH4 adsorption selectivity decreases initially, and then increases with the kerogen moisture content. Increasing the reservoir moisture content can improve the performance of CS-EGR. The preferential adsorption sites for CH4 are the sulfur-containing groups, while these for CO2 are the nitrogen- and oxygen-containing groups. One adsorption layer is observed for CH4, while two adsorption layers are identified for CO2. This study gains enhanced insights on the replacement exploitation of shale gas by CO2 at molecular scale, and it can provide applicable guidelines for the optimization of CS-EGR in shale gas reservoir.

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