Reduction in interfacial tension of water–oil interface by supercritical CO2 in enhanced oil recovery processes studied with molecular dynamics simulation

•Supercritical CO2 molecules prefer to accumulate at water–oil interface.•CO2 accumulation leads to changes in the orientation of oil molecules.•Driving force of CO2 accumulation is the water–oil interfacial tension difference.•Comparable interactions of water–CO2 and oil–CO2 cause interfacial tension reduction.

The interfacial tension (IFT) has a significant influence on fluid flow in the supercritical CO2 (scCO2) enhanced oil recovery process. However, it is still challenging to demonstrate the effect of scCO2 on water–oil interface. Therefore, a molecular dynamics simulation is performed to investigate the influence of scCO2 on the water–decane interface. It is observed that CO2 prefers to accumulate and display surface-active at the water–decane interface. The driving force of CO2 accumulation is the IFT difference between water–decane and water–CO2. The IFT of water–CO2–decane decreases linearly with the increase of CO2. The interactions of water–CO2 and decane–CO2 are strong, and the difference between them is small, providing the mechanism for lowering the IFT of water–decane. The increase in the diffusivity for all fluids toward the interface suggests an increase in the mobility at the water–decane interface due to the addition of CO2, which is in favor of enhancing oil recovery.

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