Investigation of the Driving Force of Replacing Adsorbed Hydrocarbons by CO 2 in Organic Matter from an Energy Perspective.

Carbon dioxide (CO 2 ) 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 CO 2 , we performed molecular dynamics (MD) simulations of the replacement process and calculated the interaction forces between CO 2 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 CO 2 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 CO 2 can significantly decrease the energy barrier associated with hydrocarbon movement away from the OM surface. Furthermore, both CO 2 and supercritical CO 2 can effectively displace adsorbed hydrocarbon gas (methane) on the OM, while supercritical CO 2 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 CO 2 in shale formations.