Patent ID: 7985718

Claim:
A method of tertiary mineral oil extraction, which is a method for surfactant flooding, comprising injecting a surfactant formulation through at least one injection bore into a mineral oil deposit, thereby lowering an interfacial tension between water and mineral oil, and removing crude oil from the deposit through at least one production bore, said surfactant mixture comprising at least the following surfactants: from 50 to 99% by weight of at least one surfactant (A) of the general formula R 1 —X where R 1 is a hydrocarbon radical having from 12 to 30 carbon atoms and X is an anionic or nonionic hydrophilic group, and from 1 to 50% by weight of at least one cosurfactant (B), where the amounts stated are based in each case on the total amount of all surfactants in the mixture, wherein the cosurfactant (B) has the general formula R 2 —O—(R 3 —O) n —R 4 (I) and where the R 2 , R 3 and R 4 radicals and the number n are each defined as follows: n is from 2 to 20, R 2 is a branched hydrocarbon radical which has from 6 to 11 carbon atoms and an average degree of branching of from 1 to 2.5, selected from the group of R 2a saturated, aliphatic hydrocarbon radicals, or R 2b phenyl or R 6 -phenyl, where R 6 is an alkyl radical having from 1 to 5 carbon atoms R 2c phenyl-substituted hydrocarbon radicals of the general formula phenyl-R 7 where R 7 is a divalent hydrocarbon radical having from 1 to 5 carbon atoms, R 3 are each independently an ethylene group or a propylene group, with the proviso that the ethylene and propylene groups—where both types of groups are present—may be arranged randomly, alternately or in block structure, R 4 is hydrogen or a group selected from the group of —SO 3 H, —PO 3 H 2 , —R 5 —COOH, —R 5 —SO 3 H or —R 5 —PO 3 H 2 or salts thereof, where R 5 is a divalent hydrocarbon group having from 1 to 4 carbon atoms, with the proviso that the amount of surfactants (A) and cosurfactants (B) together is at least 25% by weight based on the amount of all surfactants in the mixture, wherein the surfactant mixture is used in the presence of formation water which comprises more than 150 g/l of dissolved salts.