Effect of organic type and moisture on CO2/CH4 competitive adsorption in kerogen with implications for CO2 sequestration and enhanced CH4 recovery

Although research attentions for CO2 injection in gas-bearing reservoirs have been drawn to CO2 sequestration with enhanced gas recovery (CS-EGR), the microscopic competitive adsorption mechanism of methane (CH4) and carbon dioxide (CO2) considering the effect of organic type and moisture remains to be determined. In this work, we focus on the competitive adsorption behaviors of CH4 and CO2 on dry and moist realistic kerogen models of different organic types by performing combined molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The effects of organic type and moisture content on kerogen pore structures, moisture distribution and interaction between CH4/CO2 and kerogen surfaces are discussed in details. Simulation results show that CO2/CH4 adsorption capacity and adsorption selectivity are in the order of kerogen IA < IIA < IIIA, consistent with the sequence of enterable pore volume fraction (IA, 9.38%; IIA, 13.59%; IIIA, 28.88%). H2O molecules are preferentially adsorbed on the sulfur- and oxygen-containing groups at low moisture, and then migrate and aggregate into clusters in the middle of enterable pores at high moisture. The CO2/CH4 adsorption capacity decreases with increasing moisture content, while the CO2/CH4 adsorption selectivity, specific adsorption energy and CO2 isosteric heat decrease at the beginning, and then increase with the moisture content. Moisture has a bigger effect on the adsorption of CO2 than that of CH4. This study indicates that kerogen IIIA is the optimized organic type for CS-EGS due to its large and stable CO2 storage capacity. Despite its negative effect on gas adsorption capacity, moisture can potentially boost the displacement of CH4 by CO2 at certain moisture conditions. Results of this study lay the foundation for future optimization design of CS-EGR projects with application to coal and shale systems.