Insights into the Molecular Competitive Adsorption Mechanism of CH4/CO2 in a Kerogen Matrix in the Presence of Moisture, Salinity, and Ethane

Carbon dioxide (CO2) injection in shale and coal seam gas reservoirs has become one of the most popular ways to promote methane (CH4) production. However, geological factors affecting the CO2 enhanced gas recovery (CO2-EGR) projects have not been studied in great depth, including underground moisture, subsurface water salinity, and other gases accompanying CH4. Thus, a hybrid methodology of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulation is employed to reveal the gas adsorption and displacement mechanisms at a fundamental molecular level. This study generates a type II-D kerogen matrix as the adsorbent. The simulation environment includes 0-5 wt % moisture content, 0-6 mol/L NaCl saline, and 0-5 wt % C2H6 for up to 30 MPa at 308, 338, and 368 K. The impressions of moisture, C2H6, and salinity on gas adsorption and competitive adsorption characteristics are analyzed and discussed. On the basis of the simulation results, the preloaded H2O molecules negatively influence CH4 adsorption, leading to a 44.9% reduction at 5 wt % moisture content. Additionally, 6 mol/L NaCl within 5 wt % moisture content exhibits a further 9.8% reduction on the basis of the moisture effect. C2H6 presents a more noticeable negative impact, of which 5 wt % results in a 73.2% reduction in CH4 adsorption. Moreover, the competitive process indicator, preferential selectivity S-CO2/CH4, is analyzed and discussed in the presence of the mentioned factors. Moisture positively influences S-CO2/CH4, salinity promotes S-CO2/CH4, and C2H6 develops S-CO2/CH4. These factors would encourage the displacement processes of CH4 by CO2 injection. This study provides essential information for better gas resource estimation and gas recovery improvement in unconventional systems.