**2.2 Surfactant treatment/flooding**

Surfactant flooding is a technique used to lower the amount of oil entrapment in the rock matrix. Surfactants are amphiphilic organic chemicals comprising hydrophobic and hydrophilic compounds. When injected into ultra-low permeability hydrocarbons rocks, they reduce the oil viscosity, pore capillary forces and the interfacial tension between water and oil and decrease oil-wet wettability [7–9].

### **2.3 Thermal stimulation/treatment**

 Thermal stimulation and treatment are particularly useful for reservoirs with high-viscosity fluids and for the release of methane-rich gases from gas hydrates. Heavy oil formations contain high-density and high-viscosity fluids, which make them even more challenging to produce. Viscosity is heat dependent, having an inverse relationship with temperature. This implies that when there is a rise in temperature, the heavy reservoir oils become less viscous and, thus, more mobile. Thermal stimulation methods for heavy oils include steam flooding, steam-assisted gravity drainage (SAGD) and cyclic steam stimulation [10, 11]. Methane-rich gases naturally trapped in gas hydrates can be produced by raising the temperature of the formation. In practice, steam or hot brine is injected to heat the deposit; however, other innovative methods, e.g. microwave heating and electromagnetic heating, may be adopted [12].

#### **2.4 Acidisation**

 This is the injection of acids into a reservoir to dissolve the rock matrix. Dissolution of portions of the rock creates wormholes while increasing its permeability and porosity [13]. Acid fracturing and matrix acidisation are the two main acidisation methods adopted as stimulation techniques [14]. The most commonly used types of acid are hydrochloric acid (HCL) and hydrofluoric acid (HF). These are often not applied singly; rather they are combined together or with other types of acids (e.g. organic acids) (e.g. [15, 16]). Carbonate rocks such as limestone,

dolomites and carbonate-rich shales are readily soluble in HCL. On the contrary, silicate or quartz-based rocks (e.g. sandstones) are not soluble in HCL; they react favourably with HF.

## **2.5 Water imbibition**

 This is the process of absorption of water as the wetting phase into rock, in which saturation by the wetting phase rises while that by the non-wetting phase reduces. This phenomenon can be induced by water flooding [17] or surfactant flooding [18, 19]. A shift in wettability from oil-wet to water-wet increases water imbibition. Water imbibition into porous rock improves oil recovery by displacing trapped hydrocarbons at both the rock surface and the pores. An example of this is *spontaneous imbibition* which is used successfully for oil recovery in shale reservoirs [20, 21].
