This paper features an experimental investigation of the effect of single and mixed alkyl ether carboxylate (AEC) surfactants having different molecular structure in altering the wettability of oil-wet carbonate and reducing the interfacial tension (IFT) between oil and brine. All tests were performed at a representative carbonate reservoir conditions; at brine salinity of 196 g/L and temperature of 75 °C. Preliminary compatibility tests revealed that only AEC surfactants having high hydrophilicity; molecular weight ratio of hydrophilic to hydrophobic groups ≥1, were compatible at reservoir harsh conditions and were considered for further analysis. IFT measurements showed that better IFT reduction could be achieved with surfactants having longer branched alkyl chain. Interestingly, inclusion of polypropylene oxide (PO) groups was found to reduce the IFT reduction performances. Only AEC surfactant with branched alkyl chain was able to generate microemulsion Winsor type III. Contact angle measurements using calcite surfaces showed that wettability alteration from strong oil-wet state toward strong water-wet state was obtained with surfactant having the shortest alkyl chain and, correspondingly, highest hydrophilicity. Moreover, this surfactant exhibited the lowest adsorption level on calcite. Adsorption results were further confirmed with thermogravimetric analysis (TGA). Mixing of AEC surfactants resulted in synergistic effect and ultra-low IFT in the order of 10−4 mN/m was obtained. While, no improvement in wettability alteration was observed for the mixed surfactants system. Overall, selection of proper surfactant type is crucial to maximize oil recovery. The novelty of this work resides in quantifying the effect that a particular structural difference could have on the surfactant capabilities in IFT reduction and wettability alteration, which could provide flexibility of tailoring the structure of the surfactant to meet the different reservoir conditions.
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