Investigation of the Driving Force of Replacing Adsorbed Hydrocarbons by CO2 in Organic Matter from an Energy Perspective.

Abstract

Carbon dioxide (CO₂) has been widely used to enhance the recovery of adsorbed hydrocarbons from the organic matter (OM) in shale formations. To reveal the driving force of replacing adsorbed hydrocarbons from OM by CO₂, we performed molecular dynamics (MD) simulations of the replacement process and calculated the interaction forces between CO₂ and hydrocarbons. In addition, based on the umbrella sampling method, steered MD simulations were performed, and the free energy profiles of hydrocarbons were obtained using the weighted histogram analysis method. Results show that the condition of the hydrocarbon replacement by CO₂ requires the hydrocarbon to have sufficient kinetic energy or to have a sufficiently large attractive force exerted to ensure that the hydrocarbon escapes the potential well of the OM. The attractive forces exerted on hydrocarbon molecules by CO₂ can significantly decrease the energy barrier associated with hydrocarbon movement away from the OM surface. Furthermore, both CO₂ and supercritical CO₂ can effectively displace adsorbed hydrocarbon gas (methane) on the OM, while supercritical CO₂ is required to enhance the recovery of adsorbed hydrocarbon oil (n-dodecane). The results obtained in this study provide guidance for enhancing the recovery of adsorbed hydrocarbons by CO₂ in shale formations.