A wellbore fluid comprising a surfactant, the surfactant having the formula (R1—X)nZ, wherein R1 is an aliphatic group—comprising a C18-C22 principal straight chain bonded at a terminal carbon atom thereof to X, and comprising at least one C1-C2 side 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.