Mechanism analysis of enhancing the temperature and shear resistance of hydroxypropyl guar gum fracturing fluid by boron-functionalized nanosilica colloidal crosslinker

With the increasing depth of reservoir development, the temperature and shear resistance of fracturing fluids needs to be further improved. In comparison with traditional crosslinking agents, nano-crosslinkers have superior crosslinking properties and can enhance the temperature and shear resistance of fracturing fluids. However, the synthesis of nano-crosslinkers is complicated and not sufficiently explained to elucidate the crosslinking mechanism. In this study, a nanosilica colloidal crosslinker with a larger size and more crosslinking sites was synthesized by a simplified method using nanosilica. The structure and crosslinking mechanism of nanosilica crosslinkers were explored using FTIR, XPS, TGA, and SEM. The results indicated that the nanosilica and organoboron were bonded through B-N bonds. The nanosilica crosslinker has good dispersion and excellent thermal stability. The thermal and shear stability of nano-crosslinked fracturing fluids was better than that of fracturing fluids formed with organoboron crosslinkers. Specifically, temperature resistance is enhanced by 7–18 °C and shear resistance by 5–20 mPa s. In addition, the gel-breaking fluid changes the wettability of the core surface and enhances the flow capacity of the oil. The results of FTIR showed the synergistic effect of hydrogen and covalent bonding between the crosslinker and guar gum molecules. The results of SEM indicated that the guar gum molecules are transformed from a linear structure to a membrane structure during the transformation of the fracturing fluid from a base fluid to a gel, which gave the gel water retention capacity and good proppant-carrying capacity. Furthermore, the nanosilica crosslinker is distributed on the surface of the membrane or located in the membrane by binding to the membrane. The interaction between the nanoparticles and the membrane structure enhances the strength of the membrane structure, which allows for better shear stability of the nano-crosslinked fracturing fluid at high temperatures.