Molecular dynamics simulation and microscopic mechanism of CO₂ composite flooding

A CO₂ composite flooding method was proposed for remaining oil exploitation and achieved significant effects in pilot well tests in order to improve residual oil recovery efficiency in complex fault block oilfields during high water cut period. There are few related studies on the microscopic mechanism of CO₂ composite flooding, and the basic research in this field is urgently demanded. Based on the CT scanning results, combined with the actual development of reservoir, it was clear that the drop-like and film-like residual oil are two types of remain-ing oil that were difficult to recover. A dissolved oil droplet model and a stripped oil film model were constructed and simulated using a molecular dynamics method. The simulation results of the dissolved oil droplet method showed that CO₂ first diffused into oil droplets and increased their volume, and then the oil droplet molecules gradually dissolved into the oil displacement system. The molecular dynamics simulation results of the stripped oil film showed that CO₂ first formed diffusion channels in the oil phase, and then preferentially passed through the diffusion channels to the rock surface. CO₂ formed hydrogen bonds on the surface and generated adsorption.