**3. Fracturing fluids and fluid systems**

 Fracturing fluids are used for hydraulic fracturing and are injected into formations either as highly pressurised fluids or as acid-based fluids used to etch the walls of existing or newly formed fractures, creating additional flow channels. There are four main categories of fracturing fluids: water-based, oil-based, acid-based and foam-based fracturing fluids. The characteristics of fracturing fluids affect the pattern of fractures formed. Viscosity and density are the major properties that primarily determine the fluid behaviour. When designing a fracturing fluid system, it is imperative that, at least, the following are taken into consideration: fluid viscosity, fluid rheology, rock conductivity, cost, the impact on the environment, proppant carrying capacity, friction loss, compatibility of the fluid with the formation rock and the net pressure drop in the fractures.

The first type of fluids preferred for hydraulic fracturing was oil-based, including hydrocarbons such as gasoline and kerosene [23, 24]. Oil-based fracturing fluids are low in viscosity and generally need to be mixed with chemicals for its quality to be improved. They are excellent fracturing fluid alternatives for water-sensitive formations. Oil-based fluids are shown to be recyclable and compatible with drilling fluids and can be fully recovered during clean-up [25].

 Water-based fluids are the most predominant fracturing fluids and in many ways better alternatives to oil-based fluids. The advent of water-based fracturing fluids introduced the petroleum industry to safer and cheaper substitutes to oilbased fluids. They can be classified as slickwater, cross-linked fluids, uncross-linked (linear) fluids and viscoelastic surfactant (VES) fluids [24]. Water-based fracturing fluids are aqueous, consisting mostly of water mixed with varying proportions of chemical additives and proppants [26]. The added chemicals may serve as viscosifiers or friction reducers. Acid-based fluids are suitable for formation rocks that are acid-soluble.

*Introductory Chapter: Developments in the Exploitation of Unconventional Hydrocarbon… DOI: http://dx.doi.org/10.5772/intechopen.86625* 

Acid-based fracturing fluids frequently used in practice are hydrochloric acid (HCL), hydrofluoric acid (HF) and organic acids. Carbonate rocks (e.g. limestone, dolostone and carbonate-rich shale) and silicate-rich rocks (e.g. sandstone) are soluble in HCL and HF, respectively [13, 27]. Most formation rocks are not exclusively one or the other; therefore, in many instances, an acid blend (mud acid) comprising a combination of more than one type of acid is used.

 Foam-based fluids are composed of a mixture of gas and liquid phases with a very high percentage of the gas fraction within the range of 52% ≦ Fg ≦ 96%, where Fg is the percentage composition of gas [28]. Nitrogen (N2) and carbon dioxide (CO2) are the common gas phases used in practice, while either acids or water or polymer or alcohol (methanol) constitute the liquid phase [29]. Foam-based fluids are also appropriate for water-sensitive formations and have been successfully applied in shale gas reservoirs [29, 30]. The proppant carrying capacity of foambased fluids greatly exceeds (by≈ 85%) that of water-based fluids. Their application requires a considerably less amount of water, and there is less liquid to recover at the end of the fracturing operation. Moreover, foam is recyclable and reusable, implying a reduction in waste and cost [29, 30]. The demerits are mainly the high initial costs and logistic requirement and the decrease in viscosity in high temperatures [30].
