Diffusion coefficient and the volume swelling of CO₂/light oil systems: Insights from dynamic volume analysis and molecular dynamics simulation

The oil volume expansion and CO2 diffusion are one of the main mechanism of CO2-enhanced oil recovery (CO2- EOR). This paper established a series of experiments, numerical simulations, and molecular dynamics (MD) simulations to describe mass transfer behaviors between oil-gas phases. Expressly, the CO2 diffusion coefficient and light oil swelling factor are signification parameters to quantify and analyze these behaviors. In detail, the CO2 diffusion coefficient and the oil swelling factor were obtained from the traditional pressure decay method and the advanced MD simulation. Synthetically, the pressure decay method and MD simulation results were mutually verified. The results showed that the equilibrium pressure was proportional to the mole fraction of CO2 and inversely proportional to the mole fraction of light oil. The equilibrium time was proportional to the mole fraction of CO2 and light oil. At the temperature of 333.15 K and the initial pressure of 7.5 MPa, the CO2 diffusion coefficient in light oil was positively correlated with the relative molar proportion of CO2, while the light oil swelling factor was vice versa. In addition, the closer to the CO2-light oil interface, the greater the light oil's potential energy and self-diffusion coefficient, and the stronger the transport ability.