**3.2 Oil-based fracturing fluids**

Oil-based fracturing fluids are principally applied in formations that are water sensitive. The earliest practices of hydraulic fracturing were conducted using oilbased fracturing fluids [115]. These were mainly hydrocarbons including kerosene, diesel and crude oils. These fluids are generally low in viscosity, which is normally increased by the addition of chemicals such as fatty acids, aluminium phosphates and aluminium esters [114, 116]. An increase in viscosity is imperative for improved stability and proppant-carrying capacity at high temperatures. Oil-based fracturing fluids can perform better than their water-based counterparts. Comparative studies completed by Perfetto *et al*. [116] show that for wells stimulated by oil-based fracturing fluids, there is a slower long-term decline in production, shorter clean-up times and improvements in economic returns. Other notable benefits are minimal contamination, lower specific gravity, lower pipe frictional losses, stability at high shear rates, and less difficulty in optimising proppant pumping and the fracturing process. The drawbacks of oil-based fracturing fluids is the hazard they pose due to high flammability and negative environmental impacts. Additional efforts to combat health and safety concerns are vital.

## **3.3 Foam-based fracturing fluids**

Foam fluids are fundamentally gas/liquid composed of significantly higher proportion of gas in comparison to the liquid fraction. They are viscous fluids. The gas fraction forms the internal phase since it is suspended in the external phase (the liquid). It is differentiated from other gas/liquid mixtures (e.g., dispersions and mist) by the percentage of gas in the total volume. Typically, the gas fraction (Fg ) of foams is in the range 52% < = Fg < = 96% [117]. Dispersions (normally classified as energised fluids) and mists consist of gas fractions below 52% and above 96% respectively [114, 117, 118]. **Figures 8** and **9** are schematic representations of these. Foams are characterised by three main parameters: rheology, quality and texture [117]. *Quality*, herein, refers to the percentage of gas in the mixture. The *texture* is the bubble size distribution of the dispersed gas.

Obviously, foams are also preferred for water-sensitive formations because they aid flowback and the amount of water needed for treatment is lower. Carbon dioxide (CO2) and nitrogen (N2)—as gas phases—and water, polymers (e.g., guar)

#### **Figure 8.**

*Foam qualities depicted by different gas–liquid compositions [117].*

#### **Figure 9.**

*Classification of gas–liquid mixture depending on the proportion of gas fraction [119].*

and acids—as liquid phases—are common components of foams fracturing fluids [120, 121]. **Table 2** presents the main categories of foam-based fracturing fluids. These are water-based, hydrocarbon/oil-based, alcohol-based, acid-based and

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

