**1. Introduction**

In the oil and gas industry, wettability studies are extensively carried out on reservoir rocks as it is one of the primary factors that control location, saturation distribution, and flow behavior of reservoir fluids [1]. Consequently, it influences several petrophysical properties such as capillary pressure, relative permeability, waterflood behavior, electrical properties, and enhanced oil recovery (EOR) [1–3].

Traditional methods, devised by researchers over time, have classified wettability of rocks into three main states: (i) oil-wet, (ii) water-wet, and (iii) neutral wet. Oil-wet refers to the state of the reservoir rock when oil, in presence of other fluids, preferentially covers the rock surface. As a consequence, oil forms a thin layer on the surface of the rock, while other fluids remain in the center, as seen in **Figure 1(a)**. For the water-wet scenario, water in the presence of other fluids (i.e., oil) preferentially covers the rock surface. As a result, water forms a thin layer on the surface of the rock, while oil remains in the center, as seen in **Figure 1(b)**. In the case of neutral wetting, the reservoir rock surface does not show any preferential covering for any fluid. It is not necessary that a reservoir rock strongly adheres to either one of these phenomena; rather cases of heterogeneity may exist where many other states of wettability may be defined, such as mixed wettability, fractional wettability, "Dalmatian" wetting, and speckled wetting [3].

**Figure 1.** 

*Schematic diagrams of (a) oil-wet and (b) water-wet states of wettability on a microscopic level.* 

 In the case of mixed wettability, some parts of the rock surface are water-wet, while others are oil-wet [4]. For the fractional wettability scenario, a fraction of the internal rock surface area is wetted by a fluid, either oil or water [5]. "Dalmatian" is a type of nonuniform wettability where oil-wet and water-wet regions, smaller than

