Mechanism study on the influence of wettability on CO2 displacement of shale oil in nanopores

Abstract

A four-phase model of an oil–gas-water-wall system was constructed using the molecular dynamics simulation method. On this basis, the effect of shale reservoir wettability on the mechanism of CO2 flooding of oil will be investigated. The study found that in hydrophilic and mixed wetting systems, the front of the CO2 molecules had a semilunar shape. In lipophilic systems, however, it advanced as a whole. In addition, water molecules in the mixed wet system will form a water bridge, preventing the forward movement of CO2 molecules. CO2 molecules can easily displace oil molecules in pores of different wettability, but cannot remove the water molecules adsorbed on hydrophilic and mixed wetting systems. Oil molecules interact weakly with the hydroxyl group on the wall, but strongly with the methyl group on the wall. CO2 molecules interact strongly with both the methyl and hydroxyl groups on the wall. The difficulty of CO2 in displacing shale oil in different wetting systems is ranked in the following order: mixed wetting system (41.90 %), lipophilic system (91.51 %), hydrophilic system (98.77 %). Additionally, the storage efficiency of the three nanopores for CO2 was 50.53 %, 77.53 %, and 74.24 %, respectively. The research results can provide theoretical support for the development of CO2 in shale reservoirs.