Rheological properties and ultra-high salt resistance of novel hydrophobically associating copolymers for fracturing fluids

A hydrophobically associating copolymer (HPAOS) with synergistic interactions between the monomers, 2-acrylamido-2-methylpropanesulfonic acid, octadecyl methacrylate, octadecyldimethylallyl ammonium chloride, and acrylamide was synthesized by free radical polymerization, followed by hydrolysis. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy (¹H NMR) were employed to characterize the structure of HPAOS. The thermogravimetric analysis and viscosity–temperature curves proved that the copolymer had good temperature resistance and the decomposition temperature corresponding to 50% weight loss was 412 °C. The viscosity retention value of 0.7 wt% HPAOS solution was 87.0 mPa·s after continuous shearing for 80 min at 180 °C. Fluorescence spectroscopy confirmed the self-assembly of hydrophobic regions in HPAOS solution, and the critical micelle concentration was determined as 0.15 wt%. The salt resistance and shear resistance properties of HPAOS were studied by apparent viscosity and rheological tests. As compared to HAPM and EM30S, HPAOS showed higher viscosity retention in sodium chloride, calcium chloride, and standard brine solutions. After continuous shearing for 80 min at 90 °C, the values of apparent viscosity retention reached 81.33 mPa·s and 50.98 mPa·s for 0.5 wt% HPAOS solutions in 2 × 10⁴ mg/L and 4 × 10⁴ mg/L standard brine, respectively. Rheological tests showed that HPAOS solution behaved as a non-Newtonian pseudoplastic fluid, which was beneficial for the formation of wide fractures. 0.5wt % HPAOS solution exhibited high viscoelasticity with storage modulus higher than the loss modulus, and HPAOS showed excellent shear recovery and instantaneous cross-linking recovery characteristics. Therefore, this fracturing fluid can tolerate extreme environmental conditions and provides a practical choice for the exploitation and production of fossil energy.