Molecular dynamics and energy distribution of methane gas adsorption in shales

Abstract

This study uses simulations to explore the energy distributions involved in the adsorption of methane gas in shales. Molecular mechanics calculations were carried out using the Forcite module in BIOVIA material studio software. The critical challenge in molecular-scale simulations remains the need to improve the description of the gas adsorption prior to up-scaling to a realistic scenario. Resolving this challenge requires the implementation of multi-scale techniques that employ atomistic/molecular-level results as input. Thus, it is pertinent that the appropriate molecular data on CH₄ gas interaction with shale is obtained. This study provides empirical data on CH₄ sorption/adsorption in shale at the molecular level to confirm the CH₄ storage potential of shales. The effect of pressure on the CH₄ sorption/adsorption was also investigated. A vital aspect of this study is elucidating the energy distribution and dominant energy that controls CH₄ sorption/adsorption to serve as a basis for the exploitation of CH₄ in productive shales. Following the intensive simulation exercise, the average total energy of CH₄ sorption varied from approximately −30 to −120 kcal/mol with increase in pressure from 500 to 2500 psi, suggesting increasing thermodynamic stability. The results indicated that van der Waals energy is the major sorption energy with values ranging from 60 to −250 kcal/mol as the sorption pressure increased, while electrostatic energy recorded the least contribution. The total adsorption energy increased from −5 to −16 kcal/mol for reservoir pressure range of 1–15 MPa. This energy distribution data confirmed the possibility of CH₄ adsorption on shale under reservoir pressure conditions.