New clean fracturing fluid VES⁃YF was prepared by mixing the betain type amphoteric surfactant YF⁃1 and cosurfactant YF⁃2 with KCl. By measuring the influence of different concentration of YF⁃1, YF⁃2 and KCl on the viscosity of fracturing fluid, the optimized formulation was obtained, which is 2.0%YF⁃1+1.0%YF⁃2+2.0%KCl. The research and evaluation of the system's construction mechanism and its applicable performance were conducted. The results show that VES⁃YF clean fracturing fluid has shear thinning property and good viscoelasticity. The rheological measurement shows that the network structure of worm⁃like micelle exists in the system and can be recovered after shear failure. The apparent viscosity of VES⁃YF clean fracturing fluid is still higher than 80 mPa·s after shearing for 2 h at the temperature of 45 ℃ and the shearing rate of 170 s-1. The system has the advantages of strong sand carrying capacity, good gel breaking property and low core damage rate. VES⁃YF clean fracturing fluid has a broad application prospects in the fracturing operation of low permeability reservoir.

To solve the problem of polymer plugging of polymer injection well area in the bohai sea, a kind of efficient composite depolymerization additive is screened in the paper. Laboratory experiments show that the degradation rate of 5 000 mg/L polymer solution can reach more than 99%, and the polymer scale samples with crude oil from oil well in Bohai can be degraded in 72 h because of the low interfacial tension and good oil washing effecthigh effect of washing oil. Meanwhile, the high chelating ability of this depolymerization additive is good for the depolymerization of the crosslinking polymer, and the degradation progress can be as less as 3 h.The plug removal application in one oil well well shows that the efficient depolymerization additive has remarkable capacity of enhancing liquid and oil.