Development and study of a salt tolerance Gemini cationic viscoelastic surfactant with spacer of neopentyl-glycol applied in seawater-based clean fracturing fluid

A viscoelastic surfactant (VES) clean fracturing fluid is an excellent alternative to polymer-based fracturing fluids for fracturing low-permeability reservoirs. However, the existing VES either has poor salt resistance or involve complicated preparation steps. Herein, a cationic Gemini VES with high salt tolerance, named neopentanediol di-(erucamide propyl ammonium bromide) (NEPB), was synthesized through a one-step reaction to simplify the process, using erucamide propyl dimethylamine and pentaerythritol dibromide dissolved in DMF. The double hydroxyl groups on the spacer imparts NEPB superior salt tolerance to the Gemini VES with a single hydroxyl (JS-(OH)-JS). Salinity-sensitive evaluation experiments consisting of salinity cloud point, DLS, and fluorescent probe tests; rheological experiments including apparent viscosity, viscoelasticity, and heat and shear resistance measurement; and comprehensive tests of fracturing fluids such as proppant suspending, gel-breaking, and core damage rate were carried out to study the performance of NEPB used in a fracturing fluid. The apparent viscosity of 3 wt% NEPB solution rises to the peak value at the NaCl concentration of 5 wt%, much higher than that of the JS-(OH)-JS solution. Oscillatory measurements exhibit that the NEPB solution with the peak apparent viscosity exhibits superior viscoelasticity to the JS-(OH)-JS solution, and the relaxation time and the plateau of the storage moduli are 17.69 s and 53.71 Pa. And the NEPB VES fracturing fluids prepared by the simulated seawater of the South China Sea, with NaSal added, exhibits best heat and shear resistance, and the remain viscosity is above 40 mPa·s. Settling velocity proppant in the NEPB VES fracturing fluid with NaSal added is 1.22 mm/s. Ethylene glycol butyl ether exhibits a superior gel-breaking performance to crude oil, and the insoluble residues of the ethylene glycol butyl ether gel-breaking fluids are all below 3 mg/L. The development of NEPB not only simplifies the preparation course of VES but also improve the salt tolerance of cationic Gemini VES, which is meaningful to the development of low-permeability oil and gas reservoirs offshore.

Graphical Abstract