Process of displacing oil in subterranean reservoir employing aqueous surfactant systems

In a process in which oil within a subterranean reservoir that contains an aqueous solution of multivalent cations is displaced from one location to another by injecting into the reservoir an aqueous anionic surfactant system which is capable of providing an interfacial tension against the oil of less than about 0.01 dyne per centimeter, the improvement which comprises:

• dissolving in the surfactant system (a) a sodium salt of amino tris(methylphosphonic acid) in a concentration of from about 0.001 to 1 percent by weight of the system, and (b) a multivalent cation-precipitating compound of the group consisting of sodium or potassium salts of carbonates, sulfates, phosphates, or fluorides, in a concentration of from about 0.01 to 0.1 moles per liter of the system;
• and
• dissolving said reactants in relative proportions such that, when the system is mixed with the aqueous solution in the reservoir, multivalent cations are reacted with a crystaline-precipitate-forming material to form a substantially colloidal suspension of crystal-growth-inhibited crystals having average diameters that are maintained at less than about 1 micron.