Interfacial migration–diffusion and oil displacement mechanism of middle-phase microemulsion

Abstract

Research on the phase transition mechanism of surfactant-emulsified crude oil and the oil displacement mechanism of microemulsions remains relatively scarce. In this study, phase behavior experiments were conducted using three surfactant complex systems with crude oil. Gas chromatography was employed to quantitatively characterize the changes in the carbon components of the solution during the phase transition. The oil displacement mechanism of the microemulsions was determined based on oil displacement visualization images within a microfluidic chip. The results showed that the optimal complex electrolyte concentrations for forming middle-phase microemulsions were 1.70% for the fatty alcohol polyoxypropylene ether sulfate (AES) and long-chain alkylbenzene sulfonate (ABS) complex system, 1.40% for the emulsifier/petroleum sulfonate complex system, and 1.90% for interfacially obstructive surfactants. When oil–water ratios of 3:7, 4:6, 5:5, and 6:4 were tested, the optimal complex electrolyte concentrations for forming middle-phase microemulsions decreased with increasing oil content, narrowing the range of optimal concentrations. The solubilization range of the AES/ABS complex system was C₉–C₁₅, with a solubilization ratio of 22.67%. For the emulsifier/petroleum sulfonate complex system, the solubilization range was C₁₀–C₁₇, with a solubilization ratio of 6.27%. The solubilization range of the interfacially obstructive surfactants was C₁₀–C₁₈, with a solubilization ratio of 11.02%. Based on the dynamic oil displacement images collected within the microfluidic chip, the oil displacement mechanism of the microemulsions was determined as emulsification into small droplets and stripping, trapping, and activation of oil droplets that aggregate and migrate as oil bands. This study innovatively quantified the internal carbon components of middle-phase microemulsions, clarified the target carbon component ranges for solubilization by different surfactants, and revealed the migration–diffusion theory of phase transition in middle-phase microemulsions at the microscopic scale, filling a gap in theoretical research at the microscale. The findings of this study provide technical guidance and theoretical support for the large-scale field applications of surfactant complex systems.