Abstract: A method of characterizing a petroleum sulfonate of unknown average equivalent weight, comprising:(a) identifying at least one substantially pure hydrocarbon which forms three phases, comprising; a predominantly water phase, a microemulsion phase and a predominantly oil phase, when a mixture of said petroleum sulfonate of unknown average equivalent weight, water, an electrolyte and said pure hydrocarbon is equilibrated; and(b) assigning an effective average equivalent weight to the petroleum sulfonate of unknown average equivalent weight which is substantially equal to the average equivalent weight of a similarly prepared petroleum sulfonate of known average equivalent weight which when equilibrated with water, the electrolyte of step (a) and at least one pure hydrocarbon, having an equivalent alkane carbon number substantially equal to the equivalent alkane carbon number of the pure hydrocarbon identified in step (a), in the same proportions and under the same conditions as step (a), forms three phases, comp

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.