Effect of surface chemistry for CH4/CO2 adsorption in kerogen: A molecular simulation study

Molecular dynamics simulation and Grand canonical Monte Carlo (GCMC) simulation are used to investigate methane adsorption and diffusion in kerogen. The adsorption isotherms of CH4/CO2 have been investigate by GCMC simulations at different temperatures, Adsorption behavior of both methane and carbon dioxide exhibit type-I Langmuir adsorption behavior, Langmuir equation can be used to fit for these isotherms. The temperature has a negative effect on gas adsorption, the adsorption amounts will decrease with increasing temperature at a given pressure. From the excess adsorption isotherms, there is a maximum adsorption for each temperature, suggesting there exists an optimum pressure for maximum methane storage at specific temperature. Furthermore, van der Waals interaction between adsorbate and kerogen plays a dominant role compared with Coulomb interaction in the process of adsorption, especially for CH4. The N- and S-containing function groups in kerogen have more positive effect on the single-component adsorption of CH4/CO2. The self-diffusion coefficient of CH4/CO2 which is investigated by MD simulations combined with Einstein fluid equation increase with the temperature increase at same pressure. The self-diffusion coefficient decrease with the pressure increase at same temperature. The radial distribution function is used to study structural information of CH4/CO2 adsorbed in kerogen. The volumetric strain has a direct correlation with the injected gas composition, at 340 K, the largest volumetric stain of 4.45% for CH4 and 6.52% for CO2.