Preparation and evaluation of salt tolerant delayed- crosslinking fracturing fluid system in ultra-deep high temperature wells

The Tarim area, characterized by deep reservoirs, high temperatures, and limited fresh water resources, necessitates a fracturing fluid system that exhibits excellent temperature shear resistance, low friction, and salinity tolerance. This study presents the development of a zwitterionic hydrophobic polymer, HPC-5, as an effective thickener using five types of polymeric monomers, including AM, AA, DMC, AMPS, and a non-ionic hydrophobic monomer. The method employed for synthesis was free-radical polymerization in solution. The zwitterionic design imparts great salt tolerance to HPC-5, and the apparent viscosities of HPC-5 solutions can maintain comparably high values with 6×10 ⁴ ppm NaCl or CaCl ₂ concentration. Meanwhile, the molecules of HPC-5 associate with each other to form tight net structures, resulting in an excellent viscoelasticity of the solution. To achieve high pump rate during hydraulic fracturing operation in ultra-deep reservoirs, the delayed crosslinking agent ZDC-L was prepared for forming a delayed crosslinking gel fracturing fluid system using reservoir brine, and the drag reduction rate can reach over 70% before crossing link within 4minutes. Under pH = 4 conditions, the crosslinking time can be significantly delayed to over 4 minutes while maintaining exceptional temperature resistance up to 160 ℃ for the gel. These properties make it highly suitable for hydraulic fracturing operations in ultra-deep wells with temperatures reaching up to 7000m depth at pump rates of 4 ~ 5m ³ /min.