Abstract: Sweep efficiency of a post-primary oil recovery process in an oil-bearing subterranean formation containing a high concentration of divalent ions such as Ca.sup.++ and Mg.sup.++ in the connate water is improved by injecting a surfactant system which forms a macroemulsion in situ so as to selectively plug the more permeable zones. The surfactant system is characterized by a relatively low viscosity and is composed of a predominantly sodium chloride brine, a petroleum sulfonate surfactant, a cosurfactant, and no more than 1 percent oil. This composition possesses sufficient fluidity to penetrate quite deeply into the formation and yet tends to form a macroemulsion on contact with high concentrations of divalent ions in the connate water so as to selectively plug the more permeable zones.

Abstract: In the recovery of oil from a formation with an aqueous surfactant solution containing an anionic surfactant such as a petroleum sulfonate, a cosurfactant such as tertiary butyl alcohol, and an electrolyte, a specific surfactant system which is suitable for the given formation, is selected by: (1) analyzing the formation brine and determining the total effective NA.sup.+ concentration, on the basis that each part Ca.sup.++ is equivalent to about 14 parts Na.sup.+ and each part Mg.sup.++ is equivalent to about 16 parts Na.sup.+ ; (2) comparing this total effective Na.sup.+ concentration with previously prepared phasevolume diagrams of surfactant systems partitioning into separate phases as a function of electrolyte concentration; and (3) adjusting the electrolyte concentration, hydrophobic content of the surfactant, or solubility of the alcohol to give a particularly effective system for recovering oil.