Guar gum and its derivatives based-gels crosslinked with boron have been utilized in hydraulic fracturing for decades. However, these gels have a noticeable limit-large polymer loading which causes problems including large quantity of residues and low regained permeability. Besides, to achieve optimal thermal stability, conventional boron crosslinker must be utilized at high pH. To overcome the above limitation of conventional boron crosslinker, we proposed highly efficient nano boron crosslinker for low polymer loading fracturing fluids.

Boron-modified nanoparticles were synthesized by two-step surface modification as nano boron crosslinker (NBC). First, (3-Aminopropyl) triethoxysilane was coated onto the silica nanoparticles surface obtaining amino-modified nanoparticles JT-1. Second, Organic boron (OBC) was coated onto JT-1 resulting in boron-modified nanoparticles (NBC). Characterization was given using FTIR spectroscopy to prove the presence of surface modifier and transmission electron microscopy (TEM) to study the particle size. The effect of pH and mass ratio of JT-1 and OBC on rheological properties of hydroxypropyl guar (HPG) gels crosslinked with NBC was investigated. The microstructures of NBC crosslinked-gels was visualized via Scanning Electron Microscope (SEM).

The results of FTIR spectroscopy illustrate the presence of amino on the surface of JT-1 and OBC was bounded onto the surface of JT-1 by B-N. The particle size of NBC ranges from 20 to 40 nm. NBC crosslinked0.3 wt.% gels exhibit a unique rheological performance at 100°C. The viscosity of OBC crosslinked-gels shows a tendency of first increase and then fall as pH ranges from 7 to 14 and peak viscosity was achieved at pH 10. Whereas, viscosity of NBC crosslinked-0.3 wt.% gels increased continuously from 44.72 mPa•s to 327.8 mPa•s as pH rises from 7 to 14. The remaining viscosity (viscosity after shearing at 170s−1 for 60 min) at 100 °Cof NBC-crosslinked 0.3 wt.% HPG gels remains higher than 50 mPa•s at pH 8 -14, indicating that NBC crosslinked-gels possessed great thermal stability at low polymer loading over a wide pH range. The difference between microstructures of NBC crosslinked-gels at different pH may be responsible for the effect of pH on the thermal stability of NBC crosslinked-gels. Gels crosslinked at higher pH gave a more coiled 3D microstructures with considerably higher structure density compared with those crosslinked at pH 7.

The hydraulic fracturing fluid system crosslinked with surface modified nano boron crosslinker showed great thermal stability at low polymer loading (0.3 wt.%) over wide pH range (8-14). Due to its unique crosslinking properties with HPG, nano boron crosslinker may provide an excellent solution to the limits of conventional boron crosslinked gels.

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