SiO2-g-Poly(sulfobetaine methacrylate) Inorganic–Organic Hybrids as Fracturing Fluids Under Harsh Reservoir Conditions

Zwitterionic polymer-grafted inorganic nanoparticles have emerged as promising functional additives for hydraulic fracturing fluids under harsh reservoir conditions, yet achieving precise control over graft chain length and understanding its structure–performance relationship remain challenging. Herein, well-defined SiO2-g-poly(sulfobetaine methacrylate) (SiO2-g-PSBMA) inorganic–organic hybrid nanoparticles with tunable chain lengths were synthesized via surface-initiated atom-transfer radical polymerization (SI-ATRP) at 80 °C. The number-average molecular weight (Mn) of the grafted PSBMA chains was precisely tuned from 3.1 to 14.2 kg mol−1 with narrow dispersity (Đ = 1.64–1.78), and the corresponding grafting density was calculated to range from 0.12 to 0.45 chains nm−2 by combining TGA, GPC, and BET analyses. Interfacial tension measurements showed that the SiO2-g-PSBMA14k fluid reduced the oil–water interfacial tension from 28 to 5.1 mN m−1 at 90 °C and retained effective tension reduction (7.8 mN m−1) even at 20 wt% NaCl. Rock-adsorption studies on tight sandstone revealed that longer graft chains achieved a higher saturated adsorption capacity of 2.1 mg g−1. Core-flooding experiments using natural Xujiahe tight sandstone cores at 90 °C demonstrated that the SiO2-g-PSBMA14k fracturing fluid at an optimal concentration of 1.0 g L−1 achieved ultimate oil recovery (Roil) of approximately 41% for low-permeability cores (D = 5.1 × 103 μm2) and approximately 52% for higher-permeability cores (D = 112.5 × 103 μm2). These results establish the chain-length-dependent structure–performance relationship of SiO2-g-PSBMA hybrids under simulated reservoir conditions (up to 90 °C and 20 wt% NaCl) and provide guidance for the rational design of zwitterionic nanofluid additives for fracturing applications.