VISCOELASTIC COMPOSITION WITH IMPROVED VISCOSITY

The present invention relates to a method for the assisted recovery of crude oil from an underground formation, comprising: a) injecting, via at least one injection means in contact with the underground formation containing the crude oil, a liquid containing a mixture of at least: i) a salty aqueous medium, ii) a mixture of two zwitterionic viscosising surfactants or a mixture of two populations of said surfactants having a bimodal characteristic of the narrow distribution of Ri groups as defined below, in a weight content of between 1 and 0.05 wt %, preferably between 0.5 and 0.1 wt %, and more preferably between 0.4 and 0.15 wt %, such that the liquid has an oil/water interface tension of about 10 mN/m (milinewton per meter) or less as measured at room temperature (25° C.) and a viscosity of about 3 cPs or more as measured at a temperature of 80° C. for a shearing gradient of 10 s−1, and corresponding to formula (1) where the two Ri radicals of the two surfactants or surfactant populations have a difference in the number of carbon atoms thereof that is at least greater than or equal to 4. The method comprises using at least one production means located at a different location than that at which the liquid or the polymer has been fed in order to recover the liquid containing the crude oil.