Viscoelastic wellbore fluid comprising an anionic surfactant

1n1 1822 12 A wellbore fluid comprising a surfactant, the surfactant having the formula (R-X)Z, wherein Ris an aliphatic group-comprising a C-Cprincipal straight chain bonded at a terminal carbon atom thereof to X, and comprising at least one C-Cside chain-X is a charged head group, Z is a counterion, and n is 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.