In order to select a surfactant formulation for chemical flooding, the surfactant has to be evaluated at reservoir conditions to determine its compatibility with the reservoir to be injected in. This is to avoid formation of gels and precipitation in the reservoir which can make surfactant enhanced oil recovery unsuccessful. In several studies, surfactants have been tested in the laboratory at room temperature using only sodium chloride salt in the brine. While in oilfield scenario, the temperature is higher and the reservoir brine contains divalent ions. In this study, very low concentration alcohol alkoxy sulfate with and without a co-surfactant in hard brine and medium crude oil has been evaluated. The results from the salinity scan, phase behaviour and core flooding experiments at 60°C shows that alcohol alkoxy sulfate is tolerant to divalent ions and its stability can be improved with the addition of methyl ester sulfonate and internal olefin sulfonate as co-surfactants. These co-surfactants were able to reduce the viscosity of microemulsion phase, create a lower interfacial tension by increasing solubilisation ratio and also increase oil recovery by at least 20%.


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