A second type of fracking water, containing gel-like polymer additives, is often referred to as "crosslink". It is more viscous, can tolerate a higher ionic strength (i.e., a higher concentration in total dissolved solids (TDS), as commonly found in concentrated brines) and preferred to carry more easily and deeply in the fractures the granular material used as proppant to maintain the fractures open after the fluid injection and the hydraulic pressure release. Guar gum mixed with borate is often used as a viscosity modifier. The fluid viscosity, depending on the degree of cross-linking of the guar gum with boric acid, is controlled by varying the pH of the solution, and therefore the chemical speciation of the boric acid/borate anion system.
