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 agglomerates exhibit a lower surface area and thus produce a weaker or no cross-linking at all. Similar effects were obtained from TiO2 particles when their diameter was increased stepwise from 6 to 14 nm. Minor additions of citric acid can stabilize the nanoparticles even at pH 5–11, thus allowing efficient cross-linking of HPG also in an alkaline environment while excessive amounts of citric acid (>5 mmol/g TiO2) retard the cross-linking process. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed that the cross-linking mechanism is based on the formation of hydrogen bonds between OH groups present on the surface of titania and along the galactomannan chain of HPG. The study confirms that TiO2 nanoparticles represent the active species responsible for the cross-linking of HPG in fracturing fluids when titanium complexes are used.

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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 complexes 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.

The interaction of guar gum, a galactomannan polysaccharide, and its derivative hydroxypropyl guar (HPG) with MO2 nanoparticles whereby M exhibits a decreasing metallic character (Ti → Sn → Si) and MO2 different sizes (6 – 15 nm) was studied by applying isothermal titration calorimetry (ITC) and rotational viscosimetry. It was found that the crosslinking ability of the oxide nanoparticles towards galactomannans is controlled by the electropositive character of M and the size of the nanoparticles which should be as small as possible (< 10 nm) to produce extremely viscous gels. The thermodynamic parameters ΔH, ΔS and ΔG for the association reaction obtained from ITC revealed that the interaction generally is entropy driven, slightly stronger for guar gum than for HPG and is decreasing from TiO2 to SnO2 to SiO2. The results explain the observations made in the viscosimetric measurements.

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