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.