Experimental Study on the Mechanism and Effect of Middle-Phase Microemulsion Flooding in a Conglomerate Reservoir

To address the characteristics of strong heterogeneity, complex pore-throat structures, and poor adaptability of conventional flooding systems in conglomerate reservoirs, a systematic comparative study on the oil displacement efficiency of middle-phase microemulsion and conventional binary systems (polymer + surfactant) was conducted using a combination of long-core displacement experiments, interfacial tension measurements, and polarizing microscopy observations. Representative cores with permeability ranging from 50 to 500 mD were selected for displacement experiments under varying conditions of injection rate, permeability contrast, timing of chemical injection, and surfactant concentration. A three-stage experimental procedure comprising waterflooding, chemical flooding, and post-waterflooding was employed to compare and analyze the injectivity, enhanced oil recovery (EOR) magnitude, and water control performance of the two flooding systems. The results indicate that the middle-phase microemulsion exhibits significantly superior oil displacement efficiency compared to the binary system across the entire permeability range. It demonstrates excellent injectivity in medium-permeability reservoirs above 50 mD, rapidly achieving an ultra-low interfacial tension on the order of 10^-3 mN/m, which effectively mobilizes residual oil trapped in fine pore throats. The final enhanced oil recovery increment is 3-7 percentage points higher than that achieved with the binary system. The applicability boundaries for middle-phase microemulsion flooding are defined as follows: permeability should not be lower than that of conventional low-permeability reservoirs; injection conversion is preferably implemented during the high water-cut stage; the permeability heterogeneity ratio should not exceed 10. Additionally, supporting measures for channeling control are required to accommodate reservoir stress sensitivity. The middle-phase microemulsion effectively inhibits water channeling, and its stability and profile control capability are significantly superior to those of the binary system. The research findings provide technical support and a theoretical basis for enhanced oil recovery in conglomerate reservoirs and analogous highly heterogeneous reservoirs.