A Quantitative Study Relating Surfactant Concentration to Middle-Phase Microemulsion Phase Behavior and Oil Recovery Factor

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Abstract
Middle-phase microemulsions have garnered increasing attention for enhanced oil recovery (EOR) owing to their capacity to achieve ultra-low oil-water interfacial tension (IFT). However, the quantitative correlations between surfactant concentration (CS), middle-phase volume (jM), optimal salinity, and oil recovery factor remain inadequately explored, impeding the rational design of cost-effective formulations. In this study, model microemulsions were fabricated using a surfactant pair of sodium dodecyl benzene sulfonate and alkyl propoxy sulfate, with paraffin oil serving as a crude oil surrogate. Salinity scans, 100-day phase behavior tracking, and core flooding tests were conducted across a CS range of 0.5–14.0 wt% to establish key parametric relationships. Results show that elevating CS shifts the middle-phase salinity window from 10.5–13.0 wt% to 9.0–12.0 wt% NaCl, while optimal salinity decreases linearly. At optimal salinity, a linear relationship emerges between CS and equilibrium jM: . The microemulsion flooding recovery factor (EM) obeys an exponential relationship with CS: , rising from 7.8% to 48.1% as CS increases from 0.5 wt% to 8.0 wt%. Recovery enhancement is governed predominantly by IFT reduction at low concentrations (0.5–2.0 wt%) and by the synergistic effect of sweep efficiency and ultra-low IFT at higher concentrations (2.0–8.0 wt%). These integrated relationships provide a practical framework for balancing technical performance with economic viability in field-scale EOR deployment.

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