Molecular Simulation of CO2/CH4 Competitive Adsorption on Shale Kerogen for CO2 Sequestration and Enhanced Gas Recovery

The adsorption behavior and underlying mechanism of CO2 and CH4 binary mixture in shale kerogen significantly affect the CO2 sequestration with enhanced gas recovery project (CS-EGR). In this study, we investigated the competitive adsorption behaviors of CO2 and CH4 in shale kerogen nanopores using grand canonical Monte Carlo (GCMC) method. Kerogen model takes into effect of matrix and slit nanopores and moisture content based on Ungerer's molecular model and scanning electron microscope (SEM) analysis, and is successfully validated against experimental data. The effects of temperature, CO2 and CH4 distribution, moisture content, adsorption selectivity, and optimal formation for injection were discussed. The results show that adsorption amount of CH4 on the kerogen increases with increasing pressure and decreases with increasing temperature. The adsorption selectivity of CO2 over CH4 is 2.53-7.25, which indicates that CO2 is preferentially adsorbed over CH4 under different temperatures. H2O prefers to adsorb inside the kerogen matrix and decrease the volumes of matrix pores with increasing moisture content and even divide some of them into ineffective pores. Compared with the kerogen matrix, H2O molecules have a slight effect on CO2 and CH4 adsorption capacity on the slit surface. Moist content has a negative effect on the desorption amount of CH4. The optimal injection formation for the CS-EGR project is in the shallow stratum. The study will reveal the micromechanism of competitive adsorption of CO2 and CH4 on kerogen and provide some theoretical support for the CS-EGR project.