Monte Carlo simulations of methane adsorption on kaolinite as a function of pore size

The adsorption characteristics of clay minerals with micro, meso, and macro-scale pore sizes have significant effects on the adsorption properties of porous media such as shale and coal. In this study, the adsorption isotherms, adsorption capacity, weight density distributions, and interaction energies of methane (CH4) on the kaolinite (001) surface were calculated and discussed in detail at 293.15 K and pressures up to 20 MPa using a series of grand canonical Monte Carlo (GCMC) simulations considering the influence of pore size (0.72, 1.44, 2.88, 5.76, and 11.52 nm). The results of the simulations indicate that as the pore size is increased, the absolute adsorption of methane decreases exponentially, especially at lower pressures, eventually reaching a stable value after the pore size of 6-8 nm. The VL values decreased linearly with increasing layer distance, indicating that kaolinite with large layer distances has lower methane adsorption capacities. With increasing spacing, the methane molecules remained tightly adsorbed on the surfaces, and the amount of methane gradually increased in the middle of the micropores. The total interaction energy also decreased exponentially with increasing pore size. Of the total interaction energy, the van der Waals energy contributed more than 98%, and the electrostatic energy contributed less than 2%. Moreover, the van der Waals energy decreased exponentially with increasing pore size, and the electrostatic energy decreased linearly with increasing pore size. Larger artificially produced micropore sizes may have significant effects on shale gas and coalbed methane production. We hope that our research can provide a foundation for further exploration of coalbed gas and shale gas.