Crosslinking of Guar and HPG Based Fracturing Fluids Using ZrO2 Nanoparticles

摘要

Hydraulic fracturing requires the use of highly viscous gels which are achieved by crosslinking guar gum or its derivatives with boron, zirconium and titanium compounds. Commonly used zirconium complexes contain chelate ligands such as e.g. triethanolamine or lactate. While many different Zr crosslinkers have been developed, their working mechanism has not received much attention. In our study, we focused on the crosslinking of guar and hydroxypropyl guar gum (HPG) with Zr(IV) triethanolamine and lactate complexes. Their interaction with the polysaccharides was studied via particle size measurements, spectroscopic methods and transmission electron microscopy (TEM). It is commonly perceived by the industry that the crosslinking effect of Zr complexes is based on a ligand exchange reaction involving the cis-hydroxyl groups present in guar and HPG. To investigate on this concept, HPG and zirconium lactate were chosen as model system. The polysaccharide and the crosslinker were mixed at low pH where no crosslinking occurred and at higher pH values (> 7) where crosslinking took place. When crosslinking had occurred, IR spectroscopy confirmed that the Zr com- plexes had hydrolyzed and the lactate ligand had been released suggesting that potentially a ligand exchange reaction had taken place. However, TEM images clearly evidenced the formation of almost monodisperse ZrO2 nanoparticles. Numerous such nanoparticles were found within the gelled HPG. In contrast, when no crosslinking had occurred, almost no ZrO2 nanoparticles could be detected. Based on the observation that the nanoparticles are responsible for the crosslinking of guar, zirconia nanoparticles were synthesized separately and added to guar solutions. Their crosslinking performance was comparable to that of the Zr lactate complex, thus proving that the nanoparticles induce the crosslinking effect. Additional tests revealed that the size of the ZrO2 nanoparticles is a key factor for their crosslinking effectiveness. Particularly small particles (d ~ 3 nm) are most effective while larger particles (d ≥ 10 nm) no longer can crosslink guar.