Characterization of the effects of wettability and pore pressure on the interfacial behavior of CO2 interacting with oil-water two-phase on pore walls

Abstract

To date, research studies on film-like residual oil have mainly focused on reservoirs in the late stage of water flooding development. However, there have been relatively few studies on film-like residual oil on pore walls after gas flooding. In this study, a molecular dynamics simulation method was used to investigate the behavior characteristics of fluid-fluid and fluid-solid interfaces and the exploitation potential of film-like remaining oil in water-wet, mixed-wet and oil-wet systems under different pore pressures after CO2 flooding. The results indicated that in the oil-in-water system, the cohesive energy density (CED) of the water films was much greater than that of the CO2 and decane components, and the CED of water films in the water-wet systems was slightly greater than the CED of water films in the mixed-wet systems. Regardless of the water-wet or mixed-wet systems, Einter(CO2−decane) was the strongest between the components. Therefore, CO2 molecules could easily enter the oil films and divide the oil phase into clusters of different sizes. The interfacial tension at the CO2 + decane/H2O and H2O/SiO2 interfaces in both the water-wet and mixed-wet systems decreased with increasing pressure. Under the same pressure, the degree of decrease in interfacial tension at the CO2 + decane/H2O interface in the mixed-wet systems was smaller than that in the water-wet systems. The degree of decrease in interfacial tension at the H2O/SiO2 interfaces in the mixed-wet systems was generally greater than that in the water-wet systems. In the water-in-oil system, because the water films isolated the oil films from the CO2 molecules, the interaction energy between the CO2 molecules and oil phase was weak, so the oil films remained on the quartz surfaces. Due to the existence of the hydrogen bond network inside the water phase, the morphological changes in water films were also small when the CO2 molecules interacted with the water phase. At the CO2/H2O interfaces, under the same pressure, the resistance of CO2 molecules to pass through the water films in the oil-wet systems was greater than that in the mixed-wet systems, and the CO2 molecules in the mixed-wet systems were more likely to enter into the water films. The interfacial tension reduction at the interfaces in the mixed-wet systems was smaller than that in the oil-wet systems. At the H2O/decane interfaces, the power of CO2 molecules breaking through the oil-water interfaces and entering into the oil phase in both the mixed-wet and oil-wet systems was basically equal under the same pressure. The interfacial tension reduction at the interfaces in the mixed-wet systems was almost equal to that of the oil-wet systems. At the decane/SiO2 interfaces, the CED of the oil films in the oil-wet systems was greater than that in the oil-wet systems under the same pressures, and the Einter(CO2−SiO2) in the mixed-wet systems was greater than the Einter(CO2−SiO2) in the oil-wet systems, indicating that CO2 molecules were more easily adsorbed on the quartz surfaces in the mixed-wet systems, and the interfacial tension reduction at the interfaces in the mixed-wet systems was greater than that of the oil-wet systems. The interfacial tension at the CO2/H2O, H2O/decane and decane/SiO2 interfaces in both the mixed-wet and oil-wet systems decreased with increasing pressure. The research results provide a reference for understanding the fluid-fluid and fluid-solid interface behaviors and the exploitation potential of film-like residual oil during CO2 gas flooding.