Effects of Salinity and Polar Components on Middle-Phase Microemulsion Formation and Oil Recovery

Middle-phase (Winsor III) microemulsion flooding is a promising technique for enhanced oil recovery (EOR). However, the presence of natural polar components in crude oil and varying reservoir salinity complicate the formulation of optimal microemulsion systems. This study experimentally investigates the effects of salinity and organic acids (lauric acid and caprylic acid) on the phase behavior of a microemulsion system composed of sodium dodecyl sulfonate (SDS), n-butanol, and different oils. Phase behavior experiments revealed that the optimal salinity window for middle-phase formation increases with the carbon number of the oil phase, expanding from 3–6.6 to 4–8.5 and 6–11.8 g/100 mL for octane, decane, and dodecane, respectively. The addition of polar acids was found to narrow this optimal salinity range; in particular, for decane, an increase in lauric acid concentration from 0.1% to 0.9% narrowed the salinity window from 4–8 to 4–6 g/100 mL, making the microemulsion systems more sensitive to salinity changes. Subsequent core flooding experiments demonstrated that the presence of polar components enhances the ultimate oil recovery, with higher acid concentrations leading to faster production rates. Furthermore, the EOR performance was shown to be highly dependent on salinity, peaking within the optimal salinity range that promotes a stable Winsor III microemulsion, leading to an ultimate recovery as high as 68.8%. These findings provide crucial insights for designing robust surfactant formulations for EOR in reservoirs containing polar crude oils.