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.