Recovering oil by injecting aqueous alkali, cosurfactant and gas

Claim 1

An oil recovery process comprising: injecting an oil-displacing fluid inclusive of gas and aqueous alkaline liquid into a subterranean reservoir containing an oil which is significantly acidic but forms monovalent cation soaps of only relatively low interfacial activity when reacted with an aqueous liquid containing a stoichiometric excess of dissolved alkaline material; using as said oil-displacing fluid a mixture which, before or soon after entering the reservoir, is substantially homogeneous and contains significant proportions of (a) an aqueous alkaline solution containing kinds and amounts of dissolved inorganic alkaline material, polyalkoxyalcohol sulfate cosurfactant, and substantially neutral salt providing an alkalinity exceeding the stoichiometric requirement for neutralizing substantially all of the acidity of the reservoir oil and a total salinity which is substantially optimum for lowering the interfacial tension between the surfactant system which is formed by reaction with the reservoir oil at the reservoir temperature and (b) a fluid which is gaseous at the temperature and pressure encountered in the reservoir; and recovering oil displaced by the injected fluid.