Zwitterionic nanocomposites grafted onto titanium dioxide enhanced water-based drilling fluid for high-temperature and high-salinity deep resource development.

With increasing challenges in high-pressure high-temperature (HPHT) and saline drilling environments, improving the thermal and filtration stability of water-based drilling fluids (WBDFs) is critical. In this study, a zwitterionic polymer-grafted titanium dioxide (TiO₂) nanocomposite was developed as a multifunctional additive. The nanocomposite was synthesized via aqueous free-radical polymerization of acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), N-vinylpyrrolidone (NVP), and dimethyldiallylammonium chloride (DMDAAC), which were covalently bonded to silane-modified TiO₂ nanoparticles. Structural and thermal analyses confirmed successful grafting, uniform dispersion, and stability up to 300 °C. When 0.11-1.00 g per 350 mL of bentonite-based WBDF was incorporated, the rheological performance of the nanocomposite improved, increasing the plastic viscosity from 7 to 15 mPa·s (7-15 cP), the apparent viscosity from 25.5 to 55 mPa·s (25.5-55 cP), and the yield point from 8.86 to 19.15 Pa (18.5-40 lb/100 ft²), while maintaining shear-thinning behavior. Under low-pressure low-temperature conditions, fluid loss decreased by ~ 35%, and filter cake thickness decreased by 75%, whereas HPHT tests (120-180 °C, 3.45 MPa/500 psi) revealed up to 23% lower fluid loss and ~ 62% thinner cakes. Comparative tests with the pure zwitterionic polymer confirmed that TiO₂ functionalization further enhanced the gel strength, filtration control, and salt resistance. The additive also maintained stability up to 30 wt% NaCl, and cyclic HPHT aging tests demonstrated superior long-term performance relative to that of the polymer alone. Zeta potential (-75 mV), DLS, and SEM analyses revealed high colloidal stability and compact, low-permeability filter cakes, driven by synergistic electrostatic-steric stabilization and nanoparticle-polymer reinforcement. Rheological modeling revealed that the Herschel-Bulkley model best captured flow behavior (R² > 0.99). These findings highlight the TiO₂-zwitterionic nanocomposite as a robust thermal stabilizer and filtration controller, offering next-generation performance for WBDFs in HPHT and saline drilling environments.