Molecular dynamics simulation of coupled effects of CO2 injection on wettability changes in crude oil-adsorbed sandstone

This research uses molecular dynamics simulations to analyze the coupled influences of CO₂ saturation, oil phase composition (N-decane, mixed oil with asphaltenes, and pure asphaltenes), and ion types and concentrations (NaCl, MgCl₂, CaCl₂) on the water wettability of SiO₂ surfaces in carbonated water–oil-sandstone systems. The results demonstrate that: under vacuum conditions, increasing CO₂ saturation in carbonated water leads to competition between CO₂ and H₂O molecules for adsorption on SiO₂ surfaces, hindering the formation of hydrogen bonds between water and hydroxyl groups on the SiO₂ surface and reducing water wettability. In contrast, under carbonated water–oil-sandstone environments, higher asphaltene content diminishes water adsorption capacity on the rock surface, leading to a reduced number of adsorbed H₂O molecules, fewer hydrogen bonds, and thus weakened water wettability. However, with increasing CO₂ saturation, dissolved CO₂ uniformly distributes along the oil–water interface, where it perturbs H₂O molecules, enhances their diffusion, and reduces the aggregation of oil molecules on the SiO₂ surface, making H₂O penetrate the oil film and directly contact the SiO₂ surface, thereby increasing hydrogen bond formation and water wettability. Under carbonated saline water–oil environments, divalent cations Ca²⁺ and Mg²⁺ exert a stronger inhibitory effect on wettability than monovalent cations Na⁺, particularly in high-concentration Ca²⁺ solutions. Although Mg²⁺ reduces H₂O mobility, its strong hydration promotes hydrogen bonding on the SiO₂ surface, resulting in better wettability than Ca²⁺ systems. Consequently, when injecting CO₂ into saline aquifers, targeting reservoirs with lower salt and asphaltene contents is recommended to maximize enhanced oil recovery.