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

• Based on the molecular dynamics simulation, the CO 2 imbibition mechanism of shale mixed-wetting surface was revealed. • Imbibition process is divided into three stages based on microscopic process and mechanisms. • Quantitatively analyzed the influence of different factors on the imbibition efficiency. • Elucidated the reasons for the difference of CO 2 imbibition effect between mixed-wetting surface and single-wetting surface. 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 CO 2 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 CO 2 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 CO 2 , CO 2 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 CO 2 and the surface is 1.88 times that between the oil phase and the surface, CO 2 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 CO 2 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 CO 2 to the residual oil adsorbed on the oil-wetting surface, so as to improve the imbibition efficiency.