Impacts of rock components on the competitive adsorption of carbon dioxide and methane in organic shales

Abstract

The competitive adsorption of CO₂ and CH₄ on kerogen and clay surfaces significantly affects CO₂ sequestration and enhanced gas recovery (EGR) in organic shale-gas reservoirs. Conventional laboratory methods struggle to quantify individual gas adsorption in CO₂:CH₄ mixtures. To address this challenge, we aim to quantify (a) the effects of kerogen type, pore structure, and thermal maturity on CO₂:CH₄ competitive adsorption, (b) the impact of clay surface chemistry on the adsorption capacity of organic shale formations, and (c) the influence of different moisture and oil contents on the adsorption capacity of kerogen and clay structures. We used Grand Canonical Monte Carlo (GCMC) simulations (verified against previously documented experimental measurements) to investigate how kerogen composition, pore structure, and thermal maturity, water/oil saturation, and clay surface chemistry influence CO₂ adsorption under reservoir conditions.

Results suggest that changing kerogen from type I to III increases CO₂ adsorption from 1.42 mmol/g to 5.56 mmol/g at 330 K and 20 MPa. Increasing thermal maturity significantly affected CO₂ adsorption, though raising reservoir pressure from 1 MPa to 20 MPa reduces CO₂/CH₄ selectivity. Moreover, the presence of moisture and oil decrease maximum CO₂ adsorption. For clay minerals, the positively charged K-illite enhances CO₂ adsorption by 133 % compared to negatively charged illite and exhibits a CO₂/CH₄ selectivity of 17.2 versus 1.48 in kaolinite. These findings emphasize that reservoir conditions as well as composition critically affect adsorption capacity and selectivity. The introduced molecular simulation framework enabled extensive sensitivity analyses of factors influencing CO₂ storage at conditions that extend beyond the reach of conventional laboratory experiments, which potentially enables the optimization of CO₂ storage strategies in organic shale-gas reservoirs.