Borate cross-linked guar gum gels are both shear-thinning and thermally thinning; but on cooling, gel restoration is rapid and complete. These fluids are considered reversible gels since the cross-link junctions are not permanent covalent bonds; they are short-lived, temporary junctions. The cross-linking agents do not undergo any permanent chemical change during junction dissociation at higher temperatures or shear. The properties of the gel to be improved include the control of the cross-linking rates, sand suspending properties, and extended thermal stability . During a hydraulic fracturing treatment the fluid is transported down the tubing into the formation. The subsequent temperature rise will alter the chemical equilibrium and the number of cross-linked bonds; therefore, changing the fluid’s viscosity. If the fluid’s viscosity becomes too low, the setting rate of the proppant may increase, which would give an undesirable proppant distribution over the fracture . Along with borate, zirconate cross-linkers have been used extensively in fracturing fluids. While the borates lack thermal stability (above 200oF), zirconium cross-linkers provide reliable performance up to fluid temperatures of ~350oF in fresh and salt water.
However, the borate-guar bond is readily reversible providing for a more predictable fluid viscosity, and the zirconium cross-linking bonds are not reversible. Therefore, the fluid performance of the borate based fluids is much less shear sensitive, than the performance of the fluids formed from the transition metal-polymer bond. Other disadvantages for the borates include the need for high pH (9 – 10) and high friction & pressures because of the lack of adequate cross-linking delay mechanisms (for cross-linking to occur down hole and not in the tanker or during mixing of materials)
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