Nano-SiO₂/Polymer Composite Gel Fluid-Loss Reducer for High-Temperature, Salt-Resistant Water-Based Drilling Fluids:...

Polymeric gel fluid-loss reducers for water-based drilling fluids often undergo performance deterioration under combined high-temperature and high-salinity conditions. In this study, an organic-inorganic nanocomposite gel fluid-loss reducer, SiO₂@P-DAS, was synthesized through aqueous free-radical polymerization using N,N-dimethylacrylamide, 2-acrylamido-2-methylpropane sulfonic acid, sodium styrene sulfonate, and KH-570-modified nano-SiO₂. The corresponding polymer gel without nano-SiO₂, P-DAS, was used as the control. FTIR results were consistent with the presence of modified nano-SiO₂ in the composite gel. Thermogravimetric analysis showed that its initial decomposition temperature increased from 245 °C for P-DAS to 252 °C for SiO₂@P-DAS. At a concentration of 3.0 wt%, the post-aging API filtrate volume of SiO₂@P-DAS was 3.8 mL, compared with 4.8 mL for P-DAS after aging at 220 °C for 16 h. Under HTHP conditions of 220 °C and 3.5 MPa, the corresponding filtrate volumes were 33.33 and 49.67 mL, respectively. SiO₂@P-DAS also maintained lower fluid loss under NaCl and CaCl₂ contamination. At 5 wt% CaCl₂, its post-aging API filtrate volume was 6.8 mL, 58.0% lower than that of P-DAS. In weighted drilling fluids with densities of 1.4-2.0 g/cm³, the post-aging API and HTHP filtrate volumes remained within 2.4-3.2 and 24.6-26.2 mL, respectively. Zeta-potential, particle-size, and filter-cake observations suggest that polymer adsorption and hydration, together with the contribution of modified nano-SiO₂ to particle packing and filter-cake continuity, improve filtration control. These results demonstrate the potential of SiO₂@P-DAS as a nanocomposite gel fluid-loss reducer for high-temperature and salt-contaminated water-based drilling fluids.