Cosurfactant blends for oil recovery surfactant system

A process for recovering oil from a subterranean oil-bearing reservoir comprising the steps of injecting an aqueous surfactant system into said reservoir, thereafter injecting a drive fluid into said reservoir and forcing said oil towards at least one recovery well and recovering said oil, said surfactant system corresponding to one prepared by the steps of:

• (a) preparing a series of aqueous surfactant-cosurfactant-electrolyte systems at varying electrolyte concentrations using a relatively water insoluble cosurfactant, mixing oil to be displaced or its equivalent with the thus produced systems, allowing the resulting mixtures to equilibrate and determining the salinity at which the microemulsion phase formed on said equilibration has approximately equal volumes of oil and water thus giving the optimal salinity concentration for this surfactant-cosurfactant combination;
• (b) preparing a series of aqueous surfactant-cosurfactant-electrolyte systems at varying electrolyte concentrations using a relatively water soluble cosurfactant, mixing oil to be displaced or its equivalent with the thus produced systems, allowing the resulting mixtures to equilibrate and determining the salinity at which the microemulsion phase formed on said equilibration has approximately equal volumes of oil and water, thus giving the optimal salinity concentration for this surfactant-cosurfactant combination;
• (c) preparing at least one series of aqueous surfactant-cosurfactant-electrolyte systems using cosurfactants with a solubility intermediate those of (a) and (b), mixing oil to be displaced or its equivalent with the thus produced systems, allowing the resulting mixture to equilibrate and determining the salinity at which the microemulsion phase formed on said equilibration has approximately equal volumes of oil and water, thus giving the optimal salinity concentration for this surfactant-cosurfactant combination;
• (d) utilizing said systems of (a), (b) and (c) at optimal salinity to remove oil from a core and plotting the percentage oil recovered versus the salinity of each system;
• (e) determining from said plot of (d) the unique salinity at which maximum oil recovery is obtained;
• and
• (f) using the relationship that increasing more water soluble component of the cosurfactant increases the optimum salinity value and decreasing the more water soluble component of the cosurfactant decreases the optimum salinity, preparing an aqueous surfactant-cosurfactant-electrolyte system using a mixture of cosurfactants so as to give a surfactant system having an optimal salinity essentially corresponding to said unique salinity.