Viscosity Reduction Mechanism of Heavy Oil Using Supercritical CO2: A Molecular Dynamics Simulation

Heavy oil accounts for approximately 70% of global remaining oil reserves, and supercritical CO2 (scCO2) injection is a promising technology for heavy oil viscosity reduction, enhanced oil recovery, and carbon sequestration. In this paper, molecular dynamics (MD) simulation was employed to construct an scCO2-heavy oil system based on the saturates-aromatics-reins-asphaltenes (SARA) components of ZD crude oil (a representative heavy oil used in this study). The radial distribution function (RDF), mean square displacement (MSD), diffusion coefficient, cohesive energy density (CED), and solubility parameter were used to analyze the swelling, extraction, and miscibility processes of scCO2 in heavy oil, as well as the effects of temperature and pressure on system phase behavior. Results show that scCO2 molecules preferentially aggregate at the oil-gas interface, penetrate into heavy oil droplets, induce volume swelling and phase separation, and selectively extract saturated and aromatic hydrocarbons. Temperature and pressure significantly affect scCO2 solubility, intermolecular interaction, and diffusion behavior, thereby dominating the viscosity reduction effect. This paper reveals the microscopic viscosity reduction mechanism of scCO2 on heavy oil and provides theoretical support for field applications of scCO2 flooding in heavy oil reservoirs.