Patent ID: 7576043

Claim:
A wellbore fluid configured for use in hydrocarbon recovery, comprising an aqueous solution of: a surfactant, the surfactant in said solution consisting of a thickening amount of surfactant which is soluble in aqueous solutions and has the formula (R 1 —X) n Z, wherein: R 1 is an aliphatic group comprising a C 16 -C 24 principal straight chain bonded at a terminal carbon atom thereof to X, and comprising at least one C 1 or C 2 side chain; and X being a charged head group, Z being a counterion, and n being an integer which ensures that the surfactant is charge neutral; and wherein: the charged head group X is selected to provide that the surfactant is soluble in oil; and at least one part of the charged head group is anionic wherein the wellbore fluid is a viscoelastic gel and wherein said gel undergoes a reduction in viscosity on contact with oil.