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Unique Synergistic Surfactant Mixture to Match Ultralow Interfacial Tension with Solubility at High Temperature and High Salinity


 Chemical enhanced oil recovery offers the huge benefit to reduce the carbon footprint in oil production. Mature oil fields with existing infrastructure can be turned into fields with higher productivity. Thereby, costly exploration of new fields can be avoided. The rejuvenation of mature oil fields can be achieved by surfactant flooding. Well-designed surfactants can reduce the tension between crude oil and water to ultralow values and thereby mobilize physically trapped oil. With respect to fields of high salinity and elevated temperature, it is particularly challenging to match the reduction of interfacial tension with the surfactant solubility as well as the chemical stability. In this paper, the solubility of surfactants in model reservoir brine (rich in calcium and magnesium ions) at elevated temperature was evaluated. The capability of surfactants to reduce interfacial tension between light crude oil and model reservoir brine was tested by phase behavior tests as well as by spinning-drop experiments. The thermal stability of stored surfactant solutions was analyzed by use of high-performance liquid chromatography (HPLC). The focus of the investigations was a temperature above 100°C and salinities of at least 100,000 ppm TDS for fields with light crude oils. Alkyl ether carboxylates are chemically very stable and offer a good potential to reduce interfacial tension. However, they usually lack sufficient solubility. In contrast, cationic surfactants are well soluble under such harsh conditions. However, they typically lack chemical stability and sufficient potential to reduce the interfacial tension. By taking the typical degradation pathways of cationic surfactants into consideration, it was possible to identify a cationic surfactant that is stable under harsh conditions. A well-designed formulation of the alkyl ether carboxylate with the cationic surfactant allows to match reduction of interfacial tension with solubility. Several other surfactant classes and their combinations were evaluated, but they were not able provide similar performance. This study reveals a unique synergistic surfactant mixture, which provides ultralow interfacial tensions against light crude oil at elevated temperature of 125°C and at high salinity 139,000 ppm TDS. In addition, the surfactant mixture is soluble and chemically stable under such harsh conditions.