Patent ID: 8887804

Claim:
A method for assisting recovery of petroleum from a subterranean formation comprising: a) injecting into the subterranean formation, a drive fluid comprising a mixture of: an aqueous medium comprising a salt; and 0.05 to 5% by weight of a mixture of two viscosity-improving zwitterionic surfactants, or a mixture of two populations of zwitterionic surfactants each comprising a narrow distribution of R 1 groups; wherein the zwitterionic surfactants comprise a compound of formula: wherein: A − comprises a carboxylate COO − or sulfonate SO 3 − group; R 1 comprises a hydrophobic moiety of an alkyl, alkoxyalkyl, alkylaminoalkyl, or alkylamidoalkyl group, further wherein: R 1 is linear or branched and comprises about 16 to 30 carbon atoms, and the difference between the number of carbon atoms in radicals R 1 of the two zwitterionic surfactants, or of the centers of the distributions of R 1 groups of each of the two populations of zwitterionic surfactants, is equal to or greater than 4; R 2 and R 3 independently comprise an aliphatic chain having from 1 to 30 carbon atoms, wherein said aliphatic chain is linear or branched, saturated or unsaturated; and R 4 comprises an alkylene group having from 1 to 4 carbon atoms, optionally substituted with a hydroxyl group; further wherein the drive fluid exhibits: an oil/water interfacial tension of about 10 mN/m or less at 25° C., and a viscosity of at least about 10 cP at 25° C. and with a shear gradient of 10 s −1 , and b) recovering the drive fluid and the petroleum from a different location than where the drive fluid was injected.