Molecular dynamics study on the diffusion and dissolution of heavy oil in supercritical CO₂ under high-temperature and high-pressure conditions

CO₂-assisted steam thermal recovery is an efficient CO₂-enhanced oil recovery (CO₂-EOR) technique, increases heavy oil production while storing CO₂. To elucidate heavy oil and supercritical CO₂ interactions under high-temperature and high-pressure conditions, PVT experiments were conducted, and molecular dynamics (MD) simulations were used to examine the effects of temperature and pressure on diffusion and dissolution in CO₂. The analysis considered oil and gas density distributions, interaction energies between SARA fractions and CO₂, radial distribution functions (RDFs), and diffusion coefficients obtained from mean square displacement (MSD). The results show that increasing temperature reduces the dissolved gas–oil ratio but enhances the volume coefficient. Light components such as saturates and aromatics interact most strongly with CO₂ and dissolve fastest, whereas heavy components such as resins and asphaltenes have lower interaction energies and dissolve more slowly because of strong π–π association. The interaction energy between CO₂ and light components is 2–4 times stronger than that with heavy components in the temperature range 373–523 K and pressure range 5–20 MPa. The CO₂–SARA interaction energy decreases by about 30%–40% with increasing temperature, but increases by about 50%–60% with increasing pressure. The diffusion rate of SARA molecules is fastest at 523 K and 20 MPa, with diffusion coefficients of 34.8 × 10^{− 9}, 20.4 × 10^{− 9}, 6.2 × 10^{− 9}, and 4.7 × 10^{− 9} m²/s, respectively. Overall, an injection temperature close to 250 ℃ combined with pressures exceeding 10 MPa provides optimal conditions for CO₂ diffusion and dissolution, thereby facilitating the extraction of light fractions and maximizing viscosity reduction and thermal recovery performance.