Behavior of Titania Nanoparticles in Cross-linking Hydroxypropyl Guar Used in Hydraulic Fracturing Fluids For Oil Recovery

Abstract

Fluids pumped in hydraulic fracturing operations constitute highly viscous gels achieved via cross-linking of the polysaccharide derivative hydroxypropyl guar (HPG), e.g., with titanium or zirconium complexes. In this study, the mechanism underlying the cross-linking effect was clarified experimentally for the titanium system. It was found that the cross-linking effect is not based on a ligand exchange reaction with the cis-hydroxy functionalities present in HPG but instead relies on TiO2 nanoparticles resulting from hydrolysis of the Ti complexes. For example, 6 nm titania nanospheres (anatase polymorph) synthesized via acid hydrolysis of tetraisopropyl orthotitanate can increase the viscosity of a HPG solution by a factor of 25. However, this effect was observed only at pH = 2–4 where the nanoparticles are stable. At higher pH values, electrostatic repulsion between the nanoparticles decreases, resulting in agglomeration as was observed via zeta potential and pH-dependent particle size measurement. Such...