**4. Hydraulic fracturing**

Hydraulic fracturing entails high rate injection of pressurised fracturing fluids into low permeability formations often targeted at specific horizontal sections of a wellbore in order to induce failure, consequently fracturing rock formation and creating a fracture network that can provide permeability in otherwise almostimpermeable rocks. Studies have shown that the fractures induced by hydraulic fracturing fluids are formed normal to the direction of minimum horizontal stress in the horizontal section of the wellbore. Horizontal wells are normally drilled in trajectories parallel to the minimum horizontal stress in a given reservoir. However, branch-like networks of micro-fractures are formed in all directions, resulting in a hydraulic connectivity that provides permeability form otherwise impermeable shale matrix. The majority of fractures are kept open by proppants which are transported by the injected fluid into the formation. Proppants ensure that fractures remain open thus enhancing the contact area between reservoir and wellbore which consequently serve as a conduit for hydrocarbon recovery, from otherwise low permeability shales [48, 49].

Hydraulic fracturing entails lots of activities, thus, research is fine-tuned on investigating and understanding certain key issues about hydraulic fracturing. For example, Rikards et al. [50] indicated that one of the biggest problems in hydraulic fracturing has to do with ability to find balance between proppantquality and proppant-transport efficiency. They intimated that high density proppants pose proppant transport challenges whilst low density proppants present issues of strength of the proppants. Also, the importance of fluid viscosity in terms of providing sufficient fracture width to enable transport and proper placement of proppants is another issue in hydraulic fracturing highlighted by Montgomery et al. [51].

#### **4.1 Hydraulic fracturing fluids (HFF)**

Since inception of the concept of hydraulic fracturing, a lot of fluids have been developed and experimented as possible suites for various formation types and even geographical locations. These are discussed below.

#### *4.1.1 Water-based fracturing fluid*

Water-based fracturing fluids are the most common hydraulic fracturing fluids in use today. This is due to their low cost, availability and their ability to transport

proppants in place to maintain fracture conductivity. Though water-based hydraulic fluids have several advantages over other types of fracturing fluids, they are more susceptible to causing formation damage due to hydration of clays which may lead to lower recovery rates for hydrocarbons. Ribeiro and Sharma [52] contend that water-based fracturing in unconventional wells, most of which contain substantial clay mineral component, presents significant challenges. One of the most effective ways of dealing with this drawback, thus, has been to use energised water-based fracturing fluids in which the fracturing fluid is energised with CO2 or N2. This significantly reduces the amount of water needed for fracturing and thus improves the fracturing job in water-sensitive formations. Some water-based fracturing fluid types are discussed below.

Slickwater fracturing fluids are primarily composed of water, sand proppants and other chemicals to deal with friction, corrosion, clay swelling and other adverse reactions due to injection of fluids into the subsurface. These fluids are characterised by lower viscosities and the ability to generate complex fractures which generally reach deeper into target formations. The drawback with this type of water-based fracturing fluid is its poor proppant transport capacity. This is often compensated for with higher pumping rates in order to maintain optimal velocities that prevent settling of proppants.

Linear fracturing fluids were developed as a solution to the poor proppant carrying capacity of slickwater fluids. This was achieved by increasing the viscosity of fracturing fluid through addition of polymers in the fluids. These polymers are capable of turning the aqueous solutions into viscous gels capable of transporting proppants effectively but may also adversely affect the permeability of low permeability formations by forming filter cakes on the walls of fractures. Linear fracturing fluids are good in controlling fluid loss in low permeability formations but prone to higher fluid losses in high permeability formations.

Cross-linked fluids were developed to obtain increased viscosity and performance of gelled polymers without necessarily increasing the concentration of polymers. To develop these fluids, Aluminium, Borate, Titanium and Zirconium compounds may be used to crosslink hydrated polymers in order to increase the viscosity of resulting fluid. The main advantage of these fluids is the reversibility of crosslinks based on pH adjustments. This enables better clean up and consequently improved permeability following fracturing treatment. Borate crosslinked fracturing fluids have been reported to show rheological stability, good clean-up and low fluid loss up to temperatures of over 300°F.

In viscoelastic surfactant gel fluids, increased viscosity and elasticity is obtained by adding surfactants and inorganic salts into water-based fracturing fluids to create ordered structures. These fluids exhibit very high zero-shear viscosity and are capable of transporting proppants with lower loading and without the comparable viscosity requirements of conventional fluids [53].
