Abstract: In the displacement of crude oil through a subterranean reservoir with an aqueous displacement fluid, improved oil recovery is obtained by providing a salinity contrast between the displacement fluid and aqueous formation fluid. The salinity of the displacement fluid is selected such that at least one mixture of the aqueous displacement and the aqueous formation fluid will have the minimum interfacial tension between the displaced crude oil and mixtures of these fluids. The minimum interfacial tension between crude oil and mixtures of aqueous displacement and formation fluids can be determined by mixing these fluids such that some mixtures contain from essentially none to essentially all displacement fluid and determining the interfacial tension between the mixtures and the crude oil. The displacement fluid is a micellar fluid followed by a polymer-containing aqueous fluid.

Abstract: Any method for tertiary recovery of oil from an oil reservoir penetrated by injection and producing wells in which injected fluid, either the micellar or polymer bank, is formulated so that the front portion of the bank is miscible with oil and the rear portion is miscible with water. In one method of formulating the graded bank or banks, the salinity of the bank is graded from a high salinity at the front to a low salinity at the rear. Another method of formulating the bank, the cosurfactant concentration is graded from a low cosurfactant concentration at the front of the bank to a low concentration of surfactant at the rear.