Design of salinity tolerant in alkaline surfactant systems for high salinity reservoirs

Chemical oil recovery from offshore reservoirs which are usually characterised by harsh conditions such as high salinities have remained a source of concern. This study assesses chemical compatibilities of alkalis –surfactant systems under high salinity and divalent ions (Ca2+ and Mg2+). It analysed the synergy between alkali-surfactant slugs and crude oil, leading to the design of an optimal low-cost alkaline- surfactant flooding. Three alkali systems: sodium hydroxide, sodium metaborate and ethylene-diamine-tetracetic acid-(EDTA/NaOH) were evaluated with two high salinity synthetic brines; one without divalent ions (soft brine) and the other containing divalent ions (hard brine). Compatibility of these alkali systems with the commercial-grade surfactant was evaluated based on fluid-fluid interaction. Phase behaviour experiments were conducted to determine the alkali–surfactant compatibility with brine. Solutions free of precipitation were used for the interfacial-tension (IFT) and phase separation analysis, obtained level readings were used to calculate optimal salinity. Results showed that all alkali systems were compatible with soft brine, while compatibility with hard brine was highly controlled by the pH. Alkali-crude oil failed to produce Type III micro-emulsion; however, the addition of surfactant achieved an ultra-low IFT. EDTA/NaOH effectively sequestered the divalent ions like sodium metaborate hence increasing the solubilisation of surfactant at higher salinity. Comparison of both alkalis with commercial surfactant, alcohol alkoxy sulphate at controlled pH exhibited a higher optimum salinity for EDTA/NaOH than for sodium metaborate. This study shows that at pH ≥ 11 and an increased salinity above optimal salinity, chemical precipitation occurs resulting in reduced oil recovery.