Nano-SiO2 Grafted with Temperature-Sensitive Polymer as Plugging Agent for Water-Based Drilling Fluids

Combining smart polymers and nano-silica is an effective method to prevent free water intrusion into micro-nanopores of shale, maintaining the wellbore stability in the drilling process. A core–shell structure nano-plugging agent (NPA) for water-based drilling fluid was synthesized through modifying nano-SiO₂ by using N-isopropylacrylamide, 2-acrylamide-2-methylpropane-sulfonic acid and KH-570. The structure and properties of NPA were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, particle size distribution, and transmission electron microscopy methods. Results show that NPA presents a good thermal stability with the decomposition temperature of 370 °C. The median particle size of NPA is 193 nm. The results of the pressure transfer experiments indicate that NPA presents a good sealing performance. The permeability of the cores after NPA sealing was significantly reduced to 0.031 × 10^–7 μm². After reaching the response temperature, the wetting angle of the shale surface treated by NPA is 77.22°. After aging at 150 °C for 16 h, the FL_HTHP of the water-based drilling fluid system was controlled within 5 mL. NPA can enhance wellbore stability and effectively reduce the leakage of the drilling fluid during drilling process.