Integrated Equilibrium-Transport Modeling for Optimizing Carbonated Low-Salinity Waterflooding in Carbonate Reservoirs

Low-salinity waterflooding (LSWF) enhances oil recovery at low cost in carbonate reservoirs, but its effectiveness requires the precise control of injected water chemistry and interaction with reservoir minerals. This study specifically investigates carbonated low-salinity waterflooding (CLSWF), where dissolved CO2 modulates geochemical processes. This study develops an integrated transport model coupling geochemical surface complexation modeling (SCM) with multiphase compositional dynamics to quantify wettability alteration during CLSWF. The framework combines PHREEQC-based equilibrium calculations of the Total Bond Product (TBP)—a wettability indicator derived from oil–calcite ionic bridging—with Corey-type relative permeability interpolation, resolved via COMSOL Multiphysics. Core flooding simulations, compared with experimental data from calcite systems at 100∘C and 220, reveal that magnesium ([Mg2+]) and sulfate ([SO2−4]) concentrations modulate the TBP, reducing oil–rock adhesion under controlled low-salinity conditions. Parametric analysis demonstrates that acidic crude oils (TAN higher than 1mg KOH/g) exhibit TBP values approximately 2.5𝑡𝑖𝑚𝑒𝑠 higher than those of sweet crudes, due to carboxylate–calcite bridging, while pH elevation (higher than 7.5) amplifies wettability shifts by promoting deprotonated -COO− interactions. The model further identifies synergistic effects between ([Mg2+]) (ranging from 50 to 200 mmol/kgw) and ([SO2−4]) (higher than 500 mmol/kgw), which reduce (Ca2+)-mediated oil adhesion through competitive mineral surface binding. By correlating TBP with fractional flow dynamics, this framework could support the optimization of injection strategies in carbonate reservoirs, suggesting that ion-specific adjustments are more effective than bulk salinity reduction.