Molecular scale understanding on the oil–water–calcite wettability: role of acid component and effect of CO₂

Carbon capture, utilization, and storage (CCUS) is a critical strategy for climate change mitigation, often involving CO₂ injection into subsurface reservoirs for enhanced oil recovery (EOR) and geological storage. Reservoir wettability significantly impacts these processes, yet carbonate formations frequently become oil-wet due to organic acid adsorption from crude oil, hindering recovery. While CO₂ injection can alter wettability towards a more favorable water-wet state, the underlying molecular mechanisms involving CO₂, adsorbed organic acids, and the calcite surface are not fully understood. This study employs molecular dynamics (MD) simulations to elucidate these mechanisms, using decane as the main oil component and butyric acid as a model acidic component. Results demonstrate that, even though no chemical reaction is considered in this study, CO₂ significantly enhances the hydrophilicity of the calcite surface originally rendered partially lipophilic by butyric acid adsorption. Furthermore, the influence of CO₂ concentration on the water contact angle was quantitatively evaluated. The water droplet contact angle was 0° for the water-oil-calcite system in the absence of acid molecule but increased to ∼29° by including the butyric acid in the oil phase. The addition of CO₂ reduced the contact angle back to ∼8°, restoring hydrophilicity. This wettability shift is attributed to a competitive adsorption mechanism where sufficient concentrations of CO₂ displace butyric acid from the calcite surface. This displacement is facilitated by the formation of hydrogen bonds between CO₂ molecules and the carboxyl groups of the butyric acid, disrupting the acid's interaction with the calcite surface. By clarifying this interplay among CO₂, acidic components, and the calcite surface, this work provides fundamental, molecular-scale insights into interfacial phenomena. These findings can guide the optimization of CO₂-EOR efficiency and advance CCUS applications in carbonate reservoirs.