An associative seawater⁃based fracturing fluids with temperature resistance of 180 ℃ was formed by thermostable and salt⁃resistant thickening agent containing hydrophobic groups, physical crosslinking agent with ionic surfactant, high temperature stabilizer and high efficiency chelating agent. The fracturing fluid forms a high⁃strength network structure through the physical association between the hydrophobic groups on the molecular chain of thickener and cross⁃linker, which enhances its viscoelasticity and takes elasticity as the dominant factor. The viscosity of the fracturing fluid can be maintained above 30 mPa•s for 90 minutes at 180 ℃ and 170 s-1, which has good static sand⁃carrying and wall⁃building properties. The viscosity of gel breaking fluid is 3.3 mPa•s, the residual content is 45 mg/L, and the damage rate of core permeability is less than 10%. The seawater⁃based fracturing fluid meets the requirements of fracturing for offshore ultra⁃high temperature deep reservoirs.

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.