Atomistic Insights into Ion-Driven Interactions of Calcite/Carbonated Brine/Polar Model Oil: Implications for Carbonated Smart Waterflooding

This study investigates the fundamental ion-specific (Na⁺, Cl^-, Mg²⁺, and SO₄^2-) interactions governing a polar model oil (decane + benzoic acid) at the calcite/carbonated brine interface by adopting a fully atomistic molecular dynamics (MD) simulation. By bridging molecular-scale interactions with macroscopic mechanisms, such as interfacial tension (IFT) reduction, oil viscosity, and wettability changes, this work provides the first direct mechanistic validation of phenomena that have previously been inferred only from experimental observations in carbonated smart water flooding systems. The results demonstrate that enhanced interactions between carboxylic acids and anions at the oil/brine interface significantly influence CO₂ diffusion and distribution within the oleic phase, which affects the apparent oil viscosity. While variations in brine ionic composition cause only modest changes in IFT, a pronounced reduction is observed with increased concentrations of polar molecules in the oil phase. Structural analysis reveals that divalent ions (Mg²⁺, SO₄^2-) are excluded from the hydration layers near the calcite surface but alter the arrangement of Na⁺ and Cl^- ions in the hydration layer covering the calcite surface, thereby influencing wettability. Notably, SO₄^2- neutralizes the calcite surface positive charge and facilitates Mg²⁺ access to the interface, promoting desorption of benzoic acid (BA) from the surface through the Mg-BA association. This highlights the cooperative role of SO₄^2- and Mg²⁺ in releasing polar species from the calcite surface. The findings underscore the dominant influence of IFT over contact angle in capillary-driven recovery and show that apparent viscosity is more sensitive to CO₂ content and overall salinity than specific ions. Therefore, from an industrial perspective, maintaining seawater-like salinity enriched with divalent ions offers a practical strategy to enhance the mobilization of polar acidic components during carbonated water flooding in carbonate reservoirs, supporting the design of more efficient Enhanced Oil Recovery (EOR) formulations.