A large percentage of the oil in petroliferous formations is held within the rock of the formation by the surface forces between the rock, the oil therein, and the aqueous liquid in the formation. As a result, a substantial portion of such oil usually remains in the rock of the formation even when wells traversing the formation are no longer productive. Various secondary techniques, such as thermal recovery, gas injection, and waterflooding have been suggested for the recovery of this fixed oil which remains in the formation after primary recovery methods are no longer feasible to produce additional oil. As a secondary recovery technique, waterflooding is quite commonly employed, and a multiplicity of methods have been suggested for improving the efficiency and economy of oil recovery by the practice of waterflooding. Such methods frequently include incorporation of a water-soluble surfactant in the waterflood liquid, which is injected into the petroliferous formation.
More recently, tertiary recovery methods have been used to recover any oil still remaining after the conclusion of the secondary recovery. There are many such methods. A typical method uses an aqueous surfactant followed by the addition of a polymer solution (such polymers are well-known in the art).
Many types of materials have been suggested as the surfactant for both secondary and tertiary recovery methods. For example many types of sulfonates have been proposed. U.S. Pat. No. 3,302,711 teaches sulfonates derived from a hydrocarbon residue containing 1 to 50 carbon atoms.
I have found that sulfonates prepared from linear alkanes containing from 20 to 24 carbon atoms provide improved results. The results are better than those obtained on a C.sub.18 alkane sulfonate and much better than that obtained on a C.sub.16 alkane sulfonate.
Heretofore it has been thought that one property required in a sulfonate for use in surfactant waterflooding was a low interfacial tension between an aqueous surfactant-containing phase and the particular oil. Typically, workers in this field have striven for interfacial tension (IFI) values of 10.sup.-3 dynes/cm. or less. I have found that a surfactant having an IFI of 0.01 to 0.05 dyne/cm. provides excellent results. This is surprising.
My invention is suitable for use in both secondary and tertiary waterflood method which uses a surfactant.