Systematic study of the effect of electrolyte composition on interfacial tensions between surfactant solutions and crude oils

• IFT measurements between crude oils and AOT solutions at low ionic strength.
• Equilibrium and dynamic IFT strongly dependent on bivalent/monovalent ratio.
• Minor change of bivalent/monovalent ratio alters IFT by one order of magnitude.
• Alkaline earth ions important to consider in low salinity EOR processes.

Dynamic and equilibrium interfacial tensions (IFT) were measured between four crude oils and aqueous low salinity solutions of the anionic surfactant AOT with different molar ratios between calcium and sodium ions. The ion strength of the electrolyte was kept constant at 20 mmol/L for all experiments. Slight changes of the calcium/sodium ratio altered the equilibrium oil/water IFT by more than one order of magnitude in the presence of surfactant. Experiments, in which either AOT was excluded or crude oil was replaced by pure hydrocarbons, were conducted in order to determine the relative impact of surfactant–electrolyte and crude oil–electrolyte interactions. In the presence of calcium, AOT dominated the interfacial properties. In sodium brine synergistic effects of AOT and surface active oil components on IFT were observed.