Characteristics and efficient imbibition-oil displacement mechanism of gemini surfactant slickwater for integrated fracturing flooding technology

The integrated fracturing-energizing-displacement technology is considered a critical means for efficient development of ultra-low permeability reservoirs. To meet the technological requirements of "low dosage, strong drag reduction, high oil displacement recovery and low damage", a gemini surfactant slickwater fracturing fluid was developed based on hydrophobic association and electrostatic shielding effect. The system consists of 0.21% gemini surfactant YZS18-YZS18, 0.17% betaine additive COHSB and 0.17% counterion additive NaPts. It exhibits excellent fracturing and interface control characteristics, with a drag reduction rate of more than 75% and a retained drag reduction rate of more than 98% after continuous cyclic shearing at 80 ℃ for 30 min. The damage rate in the ultra-low permeability core is only 3.2%, 78.9% lower than that of the polymer slickwater. The interfacial tension of gel breaking liquid can be reduced to 10^-1mN/m, and the strong wettability alteration ability can reverse oil wetting to water wetting. The imbibition and oil displacement mechanism was further explored by the combination of physical simulation and nuclear magnetic resonance technology. The results show that compared with water flooding soak and conventional polymer slickwater system, the gemini surfactant slickwater system can achieve a pressure reduction rate up to 73.5% ; the oil displacement recovery is 15.5%, 2.9 and 1.6 times that by water flooding soak and polymer slickwater, respectively. Compared with the direct flowback after filtration, well soaking can increase the oil displacement recovery by 27.0% to 31.1%. The gemini surfactant slickwater system demonstrates the high-efficiency oil displacement mechanism of "long-distance filtrating, thorough drainage, and high recovery". Well soaking can enhance the oil-water imbibition replacement effect. During imbibition process, the crude oil in small holes shows the highest utilization rate of 72.1%. At the initial stage of imbibition, the crude oil was mainly recovered from small holes; in the mid-to-late stage of imbibition, the oil was mainly originated from the medium and large holes. Optimizing oil displacement process can help achieve a high utilization rate of crude oil in medium and large holes.