Objective To satisfy the needs of reservoir stimulation in tight sandstone gas reservoirs and solve the problem of fracturing flowback fluid treatment in the field, a variable viscosity slick water (VSW) system has been developed through self-associating emulsion polymerization. This fluid mainly consists of one multi-functional thickener.

Methods The on-line conversion was realized by concentration control of slick water and sand-carrying fluid, the resistance reduction, temperature and shear resistance, suspended proppant capability, gel breaking and core damage of fracturing fluid were evaluated. Finally the fracturing fluid was applied in Sulige gas field.

Results The results show that the viscosity of the high-viscosity slick water is 93 mPa·s and 64 mPa·s with a formula of 1.0% VSW in clean water and standard saltwater. The viscosity after shearing at 90 ℃ and 170 s^-1 for 1 h is 78 mPa·s, the sand-carrying performance of this system is good, the sedimentation velocity is 0.84 mm/s for 0.425-0.850 mm ceramsite proppant. With a formula of 0.1%-0.3% VSW, the resistance reduction rate of low-viscosity slippery water can exceed 75%. The viscosity of the high-viscosity slick water gel breaker is 1.74 mPa·s, and the destruction rate to the core matrix is less than 10%. The maximum sand concentration of on-site fracturing construction is 700 kg/m³, and the average unblocked flow rate is 104.69×10⁴ m³/d. The recovery rate of flowback fluid reaches 97.5%.

Conclusions The fracturing fluid system has good viscosity increasing and salt resistance, excellent resistance reduction performance, good temperature and shear resistance, high flowback utilization rate and significant yield increase effect.