Oil recovery method employing alternate slugs of surfactant and fresh

Abstract

Disclosed is a petroleum recovery method for recovering petroleum from subterranean formations containing water having high salinity and/or divalent ion concentration, employing an aqueous surfactant-containing fluid which is designed to effect low surface tension displacement of petroleum in the formation in the presence of high salinity water. The improvement comprises injecting the total desired volume of surfactant in the form of a plurality of relatively small slugs of surfactant fluid, and alternatingly injecting similarly small pore volume slugs of fresh, low salinity solution of hydrophilic polymer. The total pore volume of surfactant-containing fluid injected is ordinarily from 0.01 to 1.00 and preferably from 0.20 to 0.50 pore volumes. This total amount of surfactant fluid is injected in from 2 to 15 and preferably from 3 to 6 separate discrete slugs. Each slug of surfactant fluid is followed by injecting a quantity of low salinity, relatively fresh water solution of polymer, e.g. of salinity less than about 10,000 and preferably less than 1000 parts per million total dissolved solids. The volume of each slug of lower salinity polymer solution is ordinarily from 0.5 to 5.0 and preferably 1 to 2 times the volume of the preceding surfactant fluid slug.