Core–shell structure acrylamide copolymer grafted on nano-silica surface as an anti-calcium and anti-temperature fluid loss agent

The copolymer (PAAN-SiO 2 ) of acrylamide (AM), 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS), N -vinylpyrrolidone (NVP) and modified nano-silica (M-SiO 2 ) was synthesized by free radical polymerization in a water solution. The composition, micro-morphology and thermal stability properties of PAAN-SiO 2 were characterized by Fourier transform infrared spectroscopy, thermal gravity analysis and transmission electron microscopy (TEM). The results showed that AM, AMPS and NVP were successfully grafted onto the surface of M-SiO 2 and formed a spherical core–shell structure copolymer. A significant reduction in the filtration volume was achieved after PAAN-SiO 2 added, and this phenomenon was even more pronounced after aging at the high temperature. The filtration volume of base slurry containing 2 wt% of calcium chloride after aging at 180 °C was reduced from 186 to 6 ml after adding 2 wt% of PAAN-SiO 2 , which exhibited a special property of anti-calcium contamination at high temperature. The interaction mechanism between PAAN-SiO 2 and bentonite and calcium ions was analyzed by particle size analysis, scanning electron microscopy, TEM and X-ray diffraction. Because of the stretching of polymer chain under 180 °C that more amide, sulfonic acid groups and cyclic rigid groups were exposed and the strengthening of the connection between PAAN-SiO 2 and clay prevents a large amount of ion exchange between Ca 2+ and clay layers, reducing the agglomeration of clay. Meantime, the distribution of clay particles was more extensive and some particles with a size of 1–10 μm plugged the micro-nano-pores on the filter cake, and eventually the thin and compact filter cake was formed.