Abstract:

Numerous oil displacing mechanisms of CO₂ have been widely recognized, but due to reservoir factors, the effectiveness of CO₂ flooding varies significantly under different reservoir conditions. It is necessary to further deepen the research on the micro-interaction mechanisms between CO₂ and crude oil, clarify the CO₂ flooding mode under different reservoir conditions, and maximize the potential of CO₂ flooding. Molecular dynamics simulation methods have been used to study the effects of components, temperature, and pressure on the interaction between oil droplets and CO₂. The kinetic parameters were obtained to quantitatively characterize the oil droplets-CO₂ interaction, clarifying the micro-interaction patterns under different conditions. The simulation results show that the dispersion force is the the main driving force of the interaction between CO₂ and alkane molecules, which mainly includes two aspects: one is the dissolution and diffusion of CO₂ molecules into the oil droplets by overcoming the steric hindrance between alkane molecules, and the other is the extraction attraction of CO₂ molecules to the outer layer molecules of the oil droplets. As the chain length of alkane molecules decreases, the temperature decreases and the pressure increases, the solubility parameter of the oil droplets and the coordination number of CO₂ increase, the curvature of the molecules in the outer layer of the oil droplets decreases, and the interaction between the two is enhanced. It is concluded that CO₂ miscible and near-miscible flooding should be realised as much as possible in light and medium-light reservoirs with lower temperatures and higher pressures, while in medium and heavy reservoirs with higher temperatures and lower pressures, the advantages of CO₂ non-miscible flooding in terms of dissolution viscosity reduction, crude oil volume expansion and energy replenishment should be fully exploited. The study results can provide theoretical guidance for laboratorial research and field application of CO₂ flooding.

Key words: CO₂ flooding, microscopic interaction mechanism, molecular dynamics simulation, dispersion force, dissolution and diffusion