Roles of Co-Surfactant and Co-Solvent in Surfactant Waterflooding

Residual oil displacement and surfactant retention were measured in Berea cores with well-characterized surfactant systems; phase and interfacial tension behavior was determined as well. The results, interpreted in terms of what is known about the different surfactant-rich microstructures present in aqueous sodium 4-(l'-heptylnonyl) benzenesulfonate (SHBS) alone or in conjunction with the co-surfactant sodium dodecylsulfate (SDS) or the co-solvent n-butanol (NBA), indicate that the large retention by Berea rock of liquid crystalline dispersions can be greatly reduced by sonicating them to produce tiny vesicles or by adding a suitable co-solvent or co-surfactant to dissolve the liquid crystallites. The core tests show that high oil recovery with low retention can be achieved by injecting isotropic solutions of alcohol-solubilized surfactant or ultradispersions of vesicles, although the former performed better than the latter. These are able to form, in situ upon contact with residual oil, a surfactant-rich third phase with low interfacial tensions against both aqueous and oleic phases so that it can mobilize the oil.