**3.1 Water-based fracturing fluids**

These are aqueous-based fluids composed of water mixed with proppants and chemical additives such as friction reducers. Water-based fracturing fluids can be categorised as slickwater, linear, crosslinked and viscoelastic surfactant fluids. Slickwater is mainly water; the proportion of water is normally dominant and might be up to 99% of the composition of the fluid. Other constituents (proppants and additives) account for less than 2% of the total volume [110]. The friction reducers (e.g., acrylamide-based polymers, surfactants and biocides) lower the viscosity to values below that for normal water. Because of its low viscosity and proppant concentration, it is possible to inject slickwater at high velocities to create narrow fractures [111].

Linear fluids are uncrosslinked solutions based on polymers (i.e., biopolymers or synthetic polymers or polysaccharides) [111]. Guar, cellulose and their derivatives are examples of biopolymers. Generally, linear fluids are higher in viscosity and thus better than slickwater in proppant conveyance and suspension. Crosslinked solutions are formed when two polymer chains are bonded to enable a fluid type with improved physical characteristics. Examples are crosslinked polymer (guar, guar derivatives, cellulose and cellulose derivatives, etc.) fluids. Typical crosslinkers include borate and other metal-based (Aluminuim, Zirconate, Titanate, etc.) ions [46, 111]. In comparison to linear fluids, crosslinked fluids have higher gel viscosity (hence, proppant carrying capacity) and stability at high temperatures [46, 111]. The high viscosity of crosslinked fluids

*Production from Unconventional Petroleum Reservoirs: Précis of Stimulation Techniques… DOI: http://dx.doi.org/10.5772/intechopen.106318*

and their tendency to form filter cakes at the fracture walls means that they must be degraded and removed at the end of the operation using breakers (enzymes and oxidisers), to avoid damage to formation conductivity [46].

Viscous fluids are suitable where high fracture conductivity is desired. Viscoelastic surfactant (VES) fluids are not formed with crosslinkers but mainly reflect the distinctive characteristics of surfactants. They contain less residues and are viscous under shear—i.e., they become highly viscous at low shear rates [46, 112, 113]. For VES fluids, crosslinkers are not necessary; rather, when these fluids are mixed with water the surfactants create micelles that increase its viscosity. Viscosity is reduced when VES fluids are in contact with hydrophobic and organic fluids (e.g., oil and gas); hence, breakers are not required to lower the fluid viscosity during clean-up [46]. VES fluids also facilitate reduction in surface tension of the reservoir fluid, which enables the outflow of water trapped within the rock pores. This is crucial in formations sensitive to water [114]. The deficiencies of VES fluids are their high costs, excessive leak-off rates in very permeable formations (> 200 mD) due to their inability to build a filter cake at the fracture wall, and their instability and decrease in viscosity at high temperatures (> 135°C), [46, 112, 114].
