Microscopic mechanism of CO2 imbibition on mixed-wetting surface of shale oil reservoir

In order to explore the imbibition mechanism of shale surface under mixed wetting condition, a multiphase system under the condition of shale mixed-wetting surface is constructed by MD method, elucidates the CO₂ imbibition mechanism under shale mixed-wetting surface from the aspects of density distribution, radial distribution function and interaction energy of each component. The result shows that: The mechanism of CO₂ imbibition can be divided into two aspects, one is the mutual solubility of oil and gas, the other is the competitive adsorption of oil and gas. From the molecular scale perspective, the imbibition process can be divided into three stages, in the early stage of rapid imbibition, the mixed-wetting surface has a weaker ability to attract CO₂, CO₂ is mainly miscible with the oil phase, resulting in the volume expansion of crude oil, which reduces the viscosity of crude oil, increases the fluidity of crude oil. In the intermediate transition stage, the interaction energy between CO₂ and the surface is 1.88 times that between the oil phase and the surface, CO₂ has an advantage in the competitive adsorption process with the oil phase, and has the ability to imbibe and replace more oil phase. In the later stable stage, the imbibition process roughly ends and the imbibition efficiency no longer increases. The imbibition efficiency of CO₂ first increases and then decreases with the increase of pressure and temperature. It is further found that the water-wetting part of the mixed-wetting surface can inhibit the attraction of the oil molecules in the oil-wetting part, improve the stripping ability of CO₂ to the residual oil adsorbed on the oil-wetting surface, so as to improve the imbibition efficiency.