Molecular Insights into CO2 Diffusion Behavior in Crude Oil

CO2 flooding plays a significant part in enhancing oil recovery and is essential to achieving CCUS (Carbon Capture, Utilization, and Storage). This study aims to understand the fundamental theory of CO2 dissolving and diffusing into crude oil and how these processes vary under reasonable reservoir conditions. In this paper, we primarily use molecular dynamics simulation to construct a multi-component crude oil model with 17 hydrocarbons, which is on the basis of a component analysis of oil samples through laboratory experiments. Then, the CO2 dissolving capacity of the multi-component crude was quantitatively characterized and the impacts of external conditions—including temperature and pressure—on the motion of the CO2 dissolution and diffusion coefficients were systematically investigated. Finally, the swelling behavior of mixed CO2–crude oil was analyzed and the diffusion coefficients were predicted; furthermore, the levels of CO2 impacting the oil’s mobility were analyzed. Results showed that temperature stimulation intensified molecular thermal motion and increased the voids between the alkane molecules, promoting the rapid dissolution and diffusion of CO2. This caused the crude oil to swell and reduced its viscosity, further improving the mobility of the crude oil. As the pressure increased, the voids between the internal and external potential energy of the crude oil models became wider, facilitating the dissolution of CO2. However, when subjected to external compression, the CO2 molecules’ diffusing progress within the oil samples was significantly limited, even diverging to zero, which inhabited the improvement in oil mobility. This study provides some meaningful insights into the effect of CO2 on improving molecular-scale mobility, providing theoretical guidance for subsequent investigations into CO2–crude oil mixtures’ complicated and detailed behavior.