Effect of Kerogen Maturity, Water Content for Carbon Dioxide, Methane, and Their Mixture Adsorption and Diffusion in Kerogen: A Computational Investigation

The adsorption behavior of CO2, CH4, and CO2/CH4 mixtures in four different mature kerogens in the absence/presence of water was studied using grand canonical Monte Carlo and classical molecular dynamics methods. The results exhibit that the adsorption isotherms of single-component CO2 or CH4 in kerogen present similar trends and show type I Langmuir adsorption behavior according to the IUPAC classification; the total adsorbed amount of both gases follows the order of type II-A < type II-B < type II-C < type II-D kerogen under the same conditions. The changing behavior of isosteric heat decreases first and then increases, which can explain the heterogeneous characteristic of the kerogen pore surface. The Coulombic and van der Waals interactions between CO2 and kerogens play an important role on adsorption, while for CH4 adsorption, the electrostatic effect is very small, even negligible. The N-, S-, and O-containing groups in kerogen have more remarkable influence on adsorption of CO2 than CH4 because of their strong adsorption energy, therefore notably improving the selectivity of CO2 over CH4 and following the order of type II-A > type II-B > type II-C > type II-D, which is beneficial to carbon capture and storage. Both pressure and temperature have an obvious impact on gas molecule diffusion, and low pressure and high temperature correspond to a large diffusion coefficient. In addition, preabsorbed water has a negative effect on the adsorption of CO2/CH4, and for the same amount of water molecules, the effect follows the order of type II-A > type II-B > type II-C > type II-D kerogens. The binding energy of water-kerogen is stronger than that of pure CH4 or CO2-kerogen. The selectivity of CO2 over CH4 on kerogen increases with an increasing water content.