Highly Efficient Nanocrystalline Cellulose Cross-Linker for Fracturing Fluid System

摘要

Guar gum and its derivative based-gels cross-linked with boron have been used in hydraulic fracturing for decades. In order to achieve gel strength requirements, conventional fracturing requires the use of a large amount of thickener and cross-linking agent, which results in more residue and difficulty in the recovery of permeability. At the same time, the gel can be used to achieve the best thermal stability in a high pH environment. Therefore, we proposed a highly efficient organoboron nanocellulose cross-linker for low polymer loading fracturing fluids. Nanocrystalline cellulose (NCC) resulted from sulfuric acid hydrolysis of cellulose microciystalline. Boron-modified nanoparticles were synthesized by one-pot reaction as nano boron cross-linker (NBC). Nanocrystalline cellulose (NCC), (3-Aminopropyl) triethoxysilane, Organic boron (OBC) was mixed at a ratio of 1:4:4 and stirred at a constant temperature of 85°C for 5 hours. The presence of surface modification was shown with FTIR spectroscopy and the particle size was characterized by Transmission Electron Microscopy. The microscopic morphology of the NBC crosslinked gels was observed by SEM. The changes in gels rheology in different pH environments were investigated. In addition, the effect of mass ratio of NCC and OBC on the rheological properties of NBC crosslinked HPG gel was also investigated. The results of FTIR spectroscopy illustrated OBC was bonded onto the surface of NCC by B-N bond. The particles have a large aspect ratio with a diameter of 5-20 nm and a length of 50-300 nm. NBC cross-linked 0.25 wt.% gels exhibited excellent rheological properties at 90°C. When the pH was increased from 7 to 11, the viscosity of the NBC crosslinked gels increased from 56.64 mPa·s to 348.7 mPa·s, and as the pH continued to increase, the viscosity of the gels began to decrease. The gels maintained a shear rate of 170s~(-1) at 90°C and the residual viscosity after shearing for one hour was higher than 50 mPa·s, which is in accordance with the requirements of fracturing construction. Due to the thermal stability of the NBC cross-linked-gels, pH was affected. The NBC cross-linked-gels had different microscopic morphology at different pH and the microstructures become more complex at higher pH. NCC is a kind of natural macromolecular materials made from microcrystalline cellulose, which has the properties of abundance, biodegradability and low price, and has greater economic advantages compared with other nanoparticles. The NCC prepared by sulfuric acid hydrolysis has good dispersion due to electrostatic repulsion. The NBC crosslinker has excellent cross-linking properties, which may provide a possible solution to reduce the amount of cross-linking agent used, thereby reducing formation damage.