Molecular Scale Understanding on the Oil-Water-Calcite Wettability: Role of Acid Component and Effect of CO2

Abstract

Carbon Capture, Utilization, and Storage (CCUS) is a critical strategy for climate change mitigation, often involving CO2 injection into carbonate 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 butyric acid as a model acidic component. Results demonstrate that CO2 significantly enhances the hydrophilicity of the calcite surface initially rendered partially lipophilic by butyric acid adsorption (water droplet contact angle increased from 0° to 29°). 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. 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.