Oil recovery method using in situ-partitioning surfactant flood systems

Claim 1

A process for recovering oil from a subterranean oil-bearing reservoir comprising the steps of: (a) injecting into said reservoir through at least one injection well a surfactant system corresponding in composition to one which causes a separation into at least two phases upon mixing as follows: mixing samples of oil from said reservoir with a series of surfactant systems, each of said systems comprising: water; 3 to 12 weight percent based on the weight of said water of a petroleum sulfonate surfactant having an average equivalent weight within the range of 375 to 500; 250 to 100,000 parts per million by weight of an electrolyte based on the weight of said water; and 1-12 weight percent based on the weight of said water of a cosurfactant, which cosurfactant has a solubility in water within the range of 0.5 to 20 grams/100 grams of water and wherein either the equivalent weight of said sulfonate and/or the solubility of said cosurfactant and/or the concentration of said electrolyte is different in each of said systems making up said series; and wherein one of said two phases is an oil rich microemulsion containing at least 85 percent of said petroleum sulfonate; (b) contacting the thus injected surfactant system with said oil in said reservoir to form in situ a bank comprising (1) a phase of said reservoir oil containing a small amount of said cosurfactant, (2) a microemulsion phase comprising (a) oil from said reservoir and (b) water, surfactant and cosurfactant from said injected surfactant system, said surfactant being in substantially higher concentration in said microemulsion phase than in said injected surfactant system; and (3) a phase comprising a majority of said water from said injected surfactant system, a portion of said cosurfactant from said injected surfactant system and a minor portion of said surfactant from said injected surfactant system; (c) thereafter injecting a mobility buffer fluid behind said bank; and (d) thereafter injecting a drive fluid behind said mobility buffer thus forcing said oil on toward a recovery well.