Wellbore treatment with hydrocarbon-responsive fluid containing oligomeric viscoelastic surfactant

A method of treating a subterranean formation which comprises at least one hydrocarbon-bearing zone, comprising

• providing a hydrocarbon-responsive thickening agent which is an oligomeric surfactant with a structure consisting of linked surfactant monomer subunits, each monomer subunit having a formula (R1—X)pZm, where X is a charged group, R1 is a C10-C50 organic tail group comprising a C10-C25 straight chain, Z is a counterion, and p and m are integers so that each surfactant monomer subunit is charge neutral, the molecules of said oligomeric surfactant containing not more than 8 said subunits
• mixing a thickening amount of said oligomeric surfactant with an aqueous liquid to make a viscoelastic treatment fluid which is an aqueous micellar solution of said surfactant wherein micellar aggregation of said oligomeric surfactant makes said solution viscoelastic,
• pumping said viscoelastic treatment fluid through a wellbore and into the subterranean formation, where contact with hydrocarbons within the formation dissipates the viscosity of said treatment fluid and causes said treatment fluid to selectively invade a hydrocarbon-bearing zone of the formation.