Effects of Salinity and Individual Ions on Crude-Oil/Water Interface Physicochemical Interactions at Elevated Temperature
Authors: Ayirala Subhash C. , Li Zuoli , Saleh Salah H. , Xu Zhenghe , Yousef Ali A. 
Published: December 2018

Advanced waterflooding processes through injection of optimized-chemistry waters have recently become popular for enhanced oil recovery (EOR) in carbonate reservoirs. In this study, we describe the results from a series of experiments (including interfacial tension (IFT), interfacial shear rheology, and droplet-coalescence times) at elevated temperature to determine the effects of both salinity and individual water ions on different physicochemical interactions occurring at the crude-oil/water interface. In addition, the results from microscopy imaging of the interface and ζ-potential/average oil-droplet size in crude-oil/water/brine emulsions are reported at ambient temperature. Reservoir crude oil was used for these measurements, and ionic strengths of different brines were varied from high-salinity seawater (56,000 ppm total dissolved solids) to pure deionized (DI) water (zero salinity).

The dynamic and equilibrium IFTs of crude-oil/brine interfaces were found to be lower than those of the crude-oil/DI-water interface. Both the 10-times-reduced-salinity seawater and 10-times-reduced-salinity seawater enriched with sulfates showed relatively higher IFT compared with high-salinity multivalent-ion brines, whereas the initial rate of IFT change with time was observed to be the lowest with sulfate-rich brine. The interfacial-rheology results showed that the transition times for the interface to become elastic-dominant from a viscous-dominant regime were found to be the lowest for low-salinity sulfate-rich brine and the greatest with high-salinity multivalent-ion brines. The coalescence times determined for crude-oil droplets in different brines showed delayed coalescence with sulfate-rich brine and DI water, whereas the fastest coalescence was observed with the high-salinity multivalent-ion brine containing magnesium (Mg2+) and calcium (Ca2+) ions. All these results on IFT, interfacial shear rheology, and crude-oil-droplet-coalescence times obtained at elevated temperature agreed well with previously reported ambient-temperature data to indicate similar trends for different water ion interactions occurring at the crude-oil/water interface. However, faster kinetics of interfacially active molecules together with reduced viscosities of fluid phases resulted in lower IFTs, shorter interfacial-film-transition times, and rapid coalescence times with DI water and different brines at elevated temperature. Microscopy-imaging results at ambient temperature showed formation of aggregates with DI water and two low-ionic-strength brines to demonstrate the better activity of interfacially active molecules to result in stable interfaces. The highest negative ζ-potentials were observed in DI water and sulfate-rich brine at ambient temperature. The oil-droplet diameters measured in crude-oil/brine emulsions showed the smallest droplet sizes with DI water and sulfate-rich brine. These measured data from different experimental techniques at both elevated and ambient temperatures therefore showed good consistency with each other, demonstrating the importance of having certain salinity levels and key ions, such as Mg2+ and Ca2+, in the injection water to enhance the coalescence between released oil droplets. Considering the importance of snap-off and oil-droplet coalescence on improving oil-phase connectivity, injection-water salinities and ionic compositions should be designed for optimal interactions at the crude-oil/water interface to capitalize on both of these effects in advanced waterflooding.