Objective Aiming at the problem of high injection pressure in tight reservoir, a depressurization and augmented injection system with low interfacial tension suitable for low mineralized water flooding was developed.

Methods Fluorocarbon surfactant FEC was synthesized from polyethylene glycol and fluoroalkanoic acid with low carbon number, and its structure was characterized by IR. The compounding properties of fluorocarbon surfactant with Gemini surfactant and organic ammonium salt anti swelling agent were studied. The influences of FEC content in the compound surfactant on anti swelling performance, interface performance and wettability were discussed. The surfactant increasing injection system was developed and its depressurization and increasing injection performance were evaluated.

Results The combination of fluorocarbon surfactant FEC and Gemini surfactant can effectively improve the anti swelling effect. Adding 0.8% ammonium salt anti swelling agent into FEC/GC composite system can reduce the interfacial tension to 10^-3 mN/m, For the system with the FEC/GC mass ratio of 1∶1-3∶2, and the core contact angle increases to 54°-59 °, which can effectively reduce the adhesion work of water on the mineral surface. The optimized compound injection system has good erosion resistance, and the anti swelling rate reaches 75.24% after 10 times of distilled water washing.

Conclusion The core experiment shows that the pressure decreases by 34.7% after injecting the compound system 1 PV. The field test of three wells shows that the system has good effect of depressurization and increasing injection, and has achieved the goal of reducing water injection pressure in tight reservoirs, which is instructive for depressurization and increasing injection in low permeability reservoirs.