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Abstract:The present work aims to study the adsorption behavior and dynamical properties of CH4 in clay slit pore with or without cation exchange structures at sizes of 1.0 nm-4.0 nm using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods. The adsorption isotherms of CH4 have been investigated by GCMC simulations at different temperatures and various pore sizes. In the montmorillonite (MMT) clays without a cation exchange structure, from the density profile, we find the molecules preferentially adsorb onto the surface, and only an obvious single layer was observed. The general trend within slit pores is that with increasing pore width, the adsorbed amount will increase. However, the larger pores exhibit lower excess density and the smaller pores exhibit higher excess density. The preloaded water will reduce CH4 sorption. The in plane self-diffusion coefficient of CH4 which is investigated by MD simulations combined with Einstein fluid equation increases rapidly with the pore size increasing at low pressure. Under these given conditions, the effect of temperature has little influence on the in-plane self-diffusion coefficient. In the MMT clays with cation exchange structure, cation exchange has little effect on CH4 adsorption and self-diffusion.

The adsorption behavior of CO 2 , CH 4 , and CO 2 /CH 4 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 CO 2 or CH 4 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 CO 2 and kerogens play an important role on adsorption, while for CH 4 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 CO 2 than CH 4 because of their strong adsorption energy, therefore notably improving the selectivity of CO 2 over CH 4 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 CO 2 /CH 4 , 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 CH 4 or CO 2 −kerogen. The selectivity of CO 2 over CH 4 on kerogen increases with an increasing water content.