Effects of Salinity and Individual Water 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: March 2018

Abstract
SmartWater flooding processes through injection of optimized chemistry waters are lately becoming popular for enhanced oil recovery (EOR) in carbonate reservoirs. In this study, we describe the results from a series of experiments, including interfacial tension, interfacial shear rheology, and droplet coalescence times, at elevated temperature to determine the effects of both salinity and individual water ions on different microscopic scale interactions occurring at the crude oil-water interface. In addition the results from microscopy imaging of the interface and zeta potential/oil droplet size distributions in crude oil-water/brine emulsions were also reported at the ambient temperature.
We measured various crude oil-brine physicochemical changes and effects using a variety of experimental techniques including attention tensiometer, double wall ring shear rheometer, Brewster angle microscope, ZetaPhoremeter, Zetasizer Nano and integrated thin film drainage apparatus (ITFDA). The reservoir crude oil was used and ionic strengths of different brines were varied from high salinity seawater (56,000 ppm TDS) to pure deionized water.
The dynamic and equilibrium interfacial tension (IFT) of crude oil-brine interfaces were found to be lower than that 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, while 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 were with high salinity multivalent brines. The coalescence times determined for crude oil droplets in different brines showed delayed coalescence with DI water and sulfate-rich brine, whereas the fastest coalescence was observed with the high salinity multivalent brine containing calcium and magnesium 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. The 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 zeta 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 lowest droplet sizes with DI water and sulfate-rich brine, respectively. The measured data from different experimental techniques at both elevated and ambient temperature therefore showed good consistency with each other to demonstrate the adverse impact of sulfate ions and importance of having certain salinity and specific key ions, such as calcium and magnesium, in the aqueous phase to result in favorable interactions at crude oil-water interface. Such tailored water recipes can promote oil phase connectivity in SmartWater flood by enhancing the coalescence between released oil droplets.