Exploring the effects of salinity and ionic composition on wettability alteration and interfacial properties in carbonate oil reservoirs

This study investigates the effects of salinity and ionic composition of brines on water-rock and water-oil interactions in carbonate reservoirs. The research evaluates equilibrium and dynamic interfacial tension (IFT) at the brine-oil interface, zeta potential, wettability alteration, and calcite dissolution mechanisms using a range of aqueous solutions with varying salinities and divalent ion concentrations. By increasing salinity, from deionized water to a salinity of 39,841 ppm, corresponding to seawater (SW) with a 25 % increase in divalent ions, the salting-in effect and the partitioning of polar oil components, equilibrium IFT decreases to approximately 11.65 mN/m. Dynamic IFT behavior was analyzed using a coupled diffusion and electrical double-layer (EDL) expansion model, showing that surface-active molecules migrate to the oil-water interface, driven by concentration gradients and ionic interactions. This resulted in IFT reduction over time. FT-IR and UV spectra of the interfacial oil and brines were used to study the potential formation of micro-dispersions. Wettability alteration experiments demonstrated that low-salinity brines, particularly 50 %i2v, effectively transformed carbonate and dolomite rocks from oil-wet to water-wet conditions, with contact angles of 47 ± 1° and 62 ± 2°, respectively, attributed to surface complexation. Scanning Electron Microscope (SEM) images and pH variations confirmed greater calcite dissolution with 50dSW, altering the rock's surface morphology. Zeta potential measurements highlighted the interplay between salinity, ionic strength, and divalent ions in modifying the surface charge of carbonate rocks.