Salinity tolerant surfactant oil recovery process

Claim 1

In a method of recovering petroleum from a subterranean, petroleum-containing, permeable formation penetrated by least two wells in fluid communication with the formation, said formation also containing water of known or determinable salinity and divalent ion concentration, comprising injecting an aqueous surfactant-containing fluid into the formation by means of at least one of the wells to displace petroleum toward at least one other well from which it is recovered to the surface of the earth, said surfactant fluid comprising an aqueous fluid having salinity and divalent ion concentration about equal to the formation water, containing an organic sulfonate primary anionic surfactant and at least one solubilizing co-surfactant selected from the group consisting of ethoxylated aliphatic alcohols, ethoxylated alkylphenols, ethoxylated alkylthiols, ethoxylated alkylarylthiols, alkylpolyethoxyalkyl sulfonates having from 8 to 22 carbon atoms in the first alkyl chain and the second alkyl is ethyl, propyl or hydroxypropyl and from 2 to 12 ethoxy groups per molecule, alkylarylpolyethoxyalkyl sulfonates having from 8 to 15 carbon atoms in the first alkyl chain and the second alkyl is ethyl, propyl or hydroxypropyl and from 2 to 12 ethoxy groups per molecule, alkylpolyethoxy sulfates having from 8 to 22 carbon atoms in the alkyl chain and from 2 to 12 ethoxy groups per molecule, alkylarylpolyethoxy sulfates having from 8 to 15 carbon atoms in the alkyl chain and from 2 to 12 ethoxy groups per molecule, wherein the improvement comprises (a) identifying the ratio of solubilizing co-surfactant and primary anionic surfactant at which the above mixture of surfactants is borderline soluble in the fluid by a method comprising: (1) preparing a series of at least four samples having a total surfactant concentration between 1.0 and 4.0 percent by weight and weight ratios of solubilizing co-surfactant to primary surfactant from 0.1 to 4.0; (2) measuring the electrical conductivities of the samples creating a graphical plot of electrical conductivity versus weight ratio of solubilizing co-surfactant to organic sulfonate primary anionic surfactant, and identifying the ratio of surfactant concentration corresponding to the first inflection in the function of electrical conductivity versus concentration of solubilizing co-surfactant as the concentration of solubilizing co-surfactant is increased from a value insufficient to render the surfactants soluble in the fluid; (b) blending the anionic primary surfactant and the solubilizing co-surfactant in a ratio at which they are borderline soluble as determined above.