Reservoir Brine, Pressure, and CO₂ Effects on Brine-Oil Interfacial Tension and Rock Wettability: Implications for EOR in Carbonate Rocks

This study investigates the influence of brine composition, ionic species, pressure, and CO₂ injection on wettability and oil-brine interfacial tension in carbonate rocks from the Kurdistan Region of Iraq under simulated reservoir conditions. Contact angle and interfacial tension measurements were conducted at different salinity levels, pressures, and with CO₂-saturated brines to evaluate fluid–rock interactions. Wettability shifted markedly from oil-wet (132.64°) to water-wet (78.61°) as brine salinity decreased to 12 × dilution, with interfacial tension reducing from 17.46 to 12.90 mN/m. CO₂ injection amplified these effects, further lowering contact angle to 102.38° for formation brine and 59.26° at 12 × dilution, while interfacial tension decreased to 13.31 and 9.12 mN/m, respectively. Monovalent ions (Na⁺, K⁺) and sulfate (SO₄²⁻) promoted stronger wettability reversal compared with divalent cations (Ca²⁺, Mg²⁺), highlighting the role of ionic specificity in surface charge alteration. Elevated pressure reinforced oil-wet behavior, with contact angle and interfacial tension increasing beyond 2,500 psi, but CO₂ injection mitigated this trend, sustaining favorable displacement conditions. As one of the first detailed laboratory studies on Kurdistan carbonate rocks, this research provides region-specific insights into wettability alteration and interfacial tension reduction under realistic conditions. While the mechanisms identified are broadly applicable, the thresholds for dilution, CO₂ injection, and brine design remain reservoir-specific and depend strongly on crude oil composition, ionic chemistry, pressure, and temperature. These findings offer a methodological framework to optimize smart water and carbonated water injection strategies for enhanced oil recovery in carbonate reservoirs.