An insight into the effect of surface-active agents on the interfacial viscosity and stability of oil-in-water emulsions

Emulsion formation is a common occurrence in oil production due to the presence of connate water in the reservoir and during various water-based enhanced oil recovery (EOR) methods. This study explores how the rheology of the water–oil interface, specifically interfacial viscosity, influences the stability of emulsions in the presence of surface-active materials. This parameter has been relatively overlooked in previous studies. The surface-active materials included CTAB as a cationic surfactant and CAPB as an amphoteric one and also monovalent and divalent salts commonly found in the Persian Gulf brine. Various tests were conducted to determine critical micelle concentration (CMC) by electrical conductivity method, interfacial tension (IFT) by pendant drop method, and interfacial viscosity by a rheometer. Furthermore, the emulsion stability was assessed by analyzing microscope images and investigating the changes in the oil droplet area over time. The results showed that the CMC of both surfactants is 300 ppm. When CTAB is dissolved in brine, its CMC is reduced to 200 ppm but CAPB showed no alteration. Also, monovalent salts had a more pronounced effect on reducing interfacial tension and enhancing emulsion stability for saturated synthetic oil compared to divalent salts. Moreover, adding high salinity brine to 25 ppm solution of CTAB decreased the IFT from 10 mN/m to 1 mN/m, while adding high salinity brine to 25 ppm solution of CAPB increased the IFT from 6 mN/m to 12 mN/m. Moreover, the interfacial viscosity at the interface of the two phases exhibited a strong dependence on the shear rate. In the presence of surfactant, the interfacial viscosity displayed shear thickening behavior and values less than 0.2 Pa·s·m were obtained, while in the presence of both surfactant and HPAM polymer, it exhibited shear thinning behavior. The presence of polymer increased the interfacial viscosity to more than 10 Pa·s·m, leading to improved emulsion stability. These results highlight the positive effect of the polymer as a stabilizer in emulsion systems. The findings from this research have several practical applications in the oil field, particularly in chemical enhanced oil recovery processes. These applications include the selection of suitable surfactants and polymers, optimization of brine formulation, and a deeper understanding of the relationship between interfacial viscosity and stability. This knowledge is essential for modifying emulsions to withstand the shear stresses typically encountered near the wellbore.