 Indeed, it is often used in conjonction with guar gum as cross-linking and gelling agent for controlling the viscosity and the rheology of the fracking fluid injected at high pressure in the well.Indeed, it is important to control the fluid viscosity for keeping in suspension on long transport distances the grains of the propping agents aimed at maintaining the cracks in the shales sufficiently open to facilitate the gas extraction after the hydraulic pressure is relieved.The rheological properties of borate cross-linked guar gum hydrogel mainly depend on the pH value.