Temperature-Responsive Nano-SiO2 Composite for Improving Shale Stability in Oil-Based Drilling Fluids

The development of nanomicrometer pores and microfractures in shale gas formation leads to frequent wellbore instability during shale gas drilling, which seriously restricts the safe and rapid drilling of shale gas. In this work, N-isopropylacrylamide (NIPAM), N-hydroxyethyl acrylamide (HEAA), styrene (St), and nano-SiO2 were used as raw materials to prepare the temperature-responsive nanocomposite named NHS/SiO2. NHS/SiO2 was highly sensitive and exhibited reversible temperature sensitivity, with a lower critical solution temperature (LCST) of 82 °C. The amphiphilicity of NHS/SiO2 positively affected the stability of oil-based drilling fluids (OBDFs). The effects of NHS/SiO2 on the plugging property of shale were tested by experiments on fluid loss, pressure transfer rate, shale specific surface area, shale pore volume, and shale strength. NHS/SiO2 significantly plugged shale pores at 130 °C, with a specific surface area reduction rate of shale from 22.19% to 65.95% and pore volume reduction rate of shale from 30.73% to 68.51%. NHS/SiO2 exhibited obvious oil adsorption in white oil and the water-in-oil emulsions after reaching the LCST. The maximum oil adsorption rate of NHS/SiO2 in the water-in-oil emulsion was about 23.41 g/g. Finally, the mechanism of NHS/SiO2 to improve the shale stability in OBDFs was further proposed. At high temperatures, NHS/SiO2 increased its elastic deformation ability by adsorbing oil for a long time, thus improving the plugging efficiency.