Methane Adsorption and Self-Diffusion in Shale Kerogen and Slit Nanopores by Molecular Simulations

Shale gas has redefined the energy landscape in the world. The scientific community makes efforts to exploit shale gas resources with minimum environmental impact. Shale gas is mainly located in micropores and mesopores of shale rocks. Molecular simulations provide insights into the kerogen structure frameworks and hydrocarbon and nonhydrocarbon adsorption on the kerogen-accessible surface. In this work, a new method is introduced to create kerogen slit nanopores from the molecular scale with different surface roughness. The hybrid molecular dynamics-grand canonical Monte Carlo (MD-GCMC) simulations are used to investigate methane adsorption in a rigid and flexible kerogen matrix and slit nanopores at three different temperatures and various pressures. Molecular dynamics simulations are used to study CH4 mobility in the flexible kerogen matrix and slit nanopores. Our simulation results show that the kerogen matrix is a dynamic system in which a coupling may exist between gas adsorption and kerogen matrix structure deformation. The chemical composition and flexibility of kerogen molecules affect adsorption and self-diffusion in the kerogen matrix. Surface roughness and chemical composition have a significant effect on adsorption in the kerogen slit nanopores. The effect of kerogen molecular flexibility on self-diffusion in slit nanopores may not be significant. Our work is based on type II-A kerogen macromolecules, but our methodology for creating kerogen slit nanopores can be used for other types of kerogen molecules.