Abstract: To meet the requirements for hydraulic fracturing of deep and ultra-deep offshore oil and gas wells while ensuring the thermal stability and proppant-carrying capacity of fracturing fluids, a high-temperature and high-salinity hydrophobic associative polymer, TH-200, was synthesized using acrylamide, acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and the hydrophobic monomer DM. The structure of polymer TH-200 was characterized by infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and thermogravimetric analysis, and its main performance as a fracturing fluid thickener was further evaluated. The results show that polymer TH-200 is a quaternary copolymer with excellent thermal stability. It exhibits a viscosity increase rate of 92 % in seawater, indicating good viscosity enhancement and salt resistance, and is therefore suitable for high-mineralization environments such as seawater. The critical intermolecular association mass fraction is approximately 0.25%. Molecular dynamics simulations revel that as the AMPS monomer concentration increases, the gyration radius of polymer segments significantly improves, with minimal influence from temperature and mineralization. 0.1% polymer solution prepared in seawater achieves a drag reduction rate of up to 78%. The seawater-based fracturing fluid formulated with 1.0 % polymer TH-200, 0.5% chelating regulator, and 1.0% organic zirconium cross linking agent maintains a viscosity of 110 mPa$\cdot$s after shearing at 200 ℃ and 100 s$^{-1}$ for 90 minutes. These results demonstrate favorable delayed cross linking performance, excellent thermal and shear resistance and outstanding proppant-carrying capabilities, meeting the demands for fracturing operations in offshore reservoirs at 200 ℃.

Key words: high temperature resistance, seawater based fracturing fluid, hydrophobic association, rheological properties, delayed cross-linking, temperature and shear resistance