**2.1 Heavy oil reservoirs**

As aptly defined by its name, heavy oil formations contain heavy oils typically characterised by high viscosity and density, and capillarity pressure effects [1, 5]. These peculiar properties make it virtually impossible to exploit heavy oil formations without stimulation. Exploitation can be accomplished by cold production and thermal stimulation. Cold production is a primary recovery method performed at the native reservoir temperature and can achieve a recovery factor between 1 and 10% [5, 29]. This may be carried out by injecting a diluent into the reservoir to reduce the viscosity of the hydrocarbon or by encouraging the initiation and continuous sand production throughout the completion process; the latter is known as *cold heavy oil production with sand* (CHOPS) [5, 30]. Sanding produces high permeability channels referred to as 'wormholes' which

enhances recovery [30]. For both approaches, artificial lifts are vital because they lower the producing bottomhole pressure (BHP) thereby increasing the flow rate. Artificial lift systems may consists of pumps (e.g., progressing cavity pumps (PCP) and electrical submersible pumps (ESP)) or a gas lift, whereby injected gas is used to reduce the fluid density of the tubing which is then lifted as the gas expands [31].

Thermal stimulation is an alternative method applied where cold production is not effective or economical. The dependency of oil viscosity on temperature is inverse. Which means it is possible to enhance fluid flow by raising the reservoir temperature. There are several ways this can be realised—for instance, cyclic steam injection and steam flooding [5]. Cyclic steam injection involves two main phases: injection of steam followed by the production of heavy oil with the condensed steam. This is carried out alternatingly with a new cycle started when the rate of oil production declines below a critical level [32]. This method is favoured in the following conditions: in reservoirs that can withstand high-pressure steam and in the presence of thick pay zones (> 10 m) containing sands with high porosity (> 30%) [32]. Steam flooding is the injection of steam into the reservoir to raise the temperature of the oil whilst reducing its viscosity [33, 34]. This method aids the distillation of the light constituents of the oil [35], which further decreases the parent oil viscosity. Steam flooding also reduces the interfacial tension between oil and rock surfaces at the vicinity of the wellbore due to the liberation of the immiscible fluid phase (oil) attached to the host solids by the wetting phase (water) [36].

#### **2.2 Oil shales**

Oil shales have very low permeability and porosity. Recovery from such reservoirs can be achieved by the use of horizontal wells [37], hydraulic fracturing, surfactant treatment, matrix acidisation, water imbibition, thermal treatment or a hybrid of these techniques.

#### *2.2.1 Horizontal wells*

Horizontal wells have several advantages over vertical wells and are generally more effective in enhancing reservoir performance (**Figure 1**). These include the following: greater and more efficient reservoir drainage and detainment of water production; reduction in gas and water coning; greater rate of production because of the increased reach of the wellbore in the pay zone, since penetration to discrete compartments is possible in complex reservoirs; and reduction in sand production [40, 41]. However, there is a higher cost associated with horizontal wells, which can be up to 2.5 greater than vertical wells [40]. Hence, it is likely that a cost-benefit assessment will be necessary, especially where the increase in reservoir performance is not expected to be intense [41].

#### *2.2.2 Hydraulic fracturing*

Hydraulic fracturing is one of the foremost and traditional ways of enhancing fluid flow in oil shales (**Figure 1**). The process induces the initiation and propagation of cracks through the injection of high-pressure fluids with magnitudes that exceed the rock failure stress [42]. The shape, orientation, size and conductivity of the fractures are functions of the direction and magnitude of the formation principal stresses and rock anisotropy, amongst other factors [42]. The in situ principal stress conditions determine the minimum pressure necessary for crack initiation and propagation. The fracturing fluids influence the pattern and behaviour of created fractures. An increasing number of fracturing fluids with

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

**Figure 1.**

*Hydraulic fracturing of unconventional reservoirs [38] (a) fracture layout for vertical and horizontal wells [39] (b) fissures created by hydraulic fracturing.*

different properties are being demonstrated to be appropriate [42–45] with each category of fluid causing dissimilar effects. This is mainly caused by the differing properties of the fluids. Examples of fracturing fluids include water, CO2 and oil; they can be generally classified as water-based, oil-based, acid-based and foambased fluids [46–48]. Fluid density and viscosity are, amongst other primary physical properties, considered when selecting a fracturing fluid. Low-viscosity fracturing fluids produce fractures that are expansive with the tendency to split into several branches. CO2, for instance, is a low-viscosity fluid that is suitable as a fracturing fluid in oil shales because it creates fractures with surface areas that

are more extensive in comparison to those created by fluids with higher viscosities—e.g., water [44, 49]. High-viscosity fluids such as viscous oils or liquid CO2 tend to generate shorter and thicker planar fractures with a small number of branches [44].

Hydraulic fracturing is not usually applied as a stand-alone strategy to improve flow in oil shales. It is often used in tandem with other techniques such as horizontal/inclined wells, thermal dissolution, and the use of special blends of fracturing fluids like surfactants and other chemicals that aid the recovery of shale oils [45, 46].
