System in low-permeability reservoirs study on the mechanism of enhanced oil recovery in low permeability reservoirs by modified heavy alkylbenzene sulfonate middle phase microemulsion system

As oil and gas development shifts toward low-permeability and ultra-low-permeability reservoirs, conventional oil recovery technologies struggle to effectively recover residual oil. Although the study of modified middle-phase microemulsion systems is an important direction for enhanced oil recovery (EOR) via chemical flooding, their microscopic mechanism of action in low-permeability reservoirs remains unclear. In this study, the low-permeability reservoir of Gansu Huachi Block B was selected as the research object. A systematic investigation into the EOR mechanism of a middle-phase microemulsion system formulated by compounding heavy alkylbenzene sulfonate (main agent) and hydroxysulfobetaine (co-agent) was conducted, combining interfacial tension measurement, contact angle measurement, static and dynamic oil washing experiments, core flooding tests, microscopic visualization, and laser confocal microscopy analysis. The results show that the system can significantly reduce the oil-water interfacial tension and realize the wettability reversal of rock from oil-wet to water-wet. The static oil washing efficiency increases remarkably with the increase in system concentration (only 10.66% for the formation water control group). In dynamic flooding tests, the oil recovery rate of formation water flooding was only 25.85%, while the microemulsion groups exhibited higher recovery rates, with the enhancement becoming more significant as the concentration increased. At the microscopic level, the residual oil after water flooding is dominated by large oil droplets/continuous phases (trapped residual oil accounts for 39.3%). In contrast, the microemulsion flooding converts it into tiny emulsions. Three-dimensional image scanning of core slices indicates that after middle-phase microemulsion flooding, the trapped residual oil in the core pores decreases to 29.5%, while the free residual oil increases to 57.5%. In summary, this compound system differs from conventional single-surfactant systems, with its core innovation lying in enhancing the synergistic effect of ‘interfacial tension reduction, wettability modification, and emulsification-solubilization’ via a compounding ratio of 7:3. Future studies should further investigate its adaptability to different reservoirs and explore its synergistic application with other oil displacement technologies.