Synthesis and performance evaluation of thermo-thickening/Nano-SiO2 as a viscosifier for bentonite-free water-based drilling fluids

Recently, conventional viscosifiers exhibit limited effectiveness under deep formations due to their poor salt tolerance and low thermal resistance. To address the limitations, a thermo-responsive macromonomer (DAM) consisting of N,N-diethylacrylamide and N,N'-methylenebisacrylamide was copolymerized with 2-acrylamido-2-methylpropane sulfonate and chemically modified nano-silica (N-np) to obtain an effective thermo-thickening/Nano-SiO₂ polymer composite (N-DPAM) via in-situ polymerization under optimal conditions. The molecular structure of N-DPAM was analyzed by FTIR and ¹H NMR, while rheological and rheometric responses under high temperature, salt dosages, and shear resistance were investigated. The rheometric results demonstrated that DPAM exhibits a viscosity increase of 235% from 65 to 160 °C, but rapidly decreased at 180 °C, whereas N-DPAM displayed stabilized thickening responses of 218% above 160 °C due to intercalation and self-assembly of N-np within the polymer matrix as temperature increases. The viscosity retention rate (VRR) at a high shear rate of 1021 s⁻¹ and 200 °C indicated that the solution viscosity of N-DPAM was observed at 55 mPa s, which is 13 times higher compared to DPAM solution at 4 mPa s. From the rheological results, the VRR of N-DPAM fluid observed at 68% was slightly lower than that of HE300 at 73%, a commercially available viscosity additive in a salt-free environment at 200 °C, but three times higher (63%) than HE300 (25%) in the salt-saturated environment (20% NaCl). Additionally, a study of N-DPAM fluid contaminated by shaly soil from Dagang Oilfield demonstrated excellent compatibility with a filtration control agent to control the viscosity and filtration volumes (< 10 mL) at 200 °C.

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