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Read the text "Wettability" and make the annotation of it.

Wettability

Wettability is the term used to describe the relative adhesion of two fluids to a solid surface. In a porous medium containing two or more immiscible fluids, wettabilty is a measure of the preferential tendency of one of the fluids to wet (spread or adhere) the surface.

In water-wet brine-oil-rock system, water will occupy the smaller pores and wet the major portion of the surfaces in the larger pores. In area of high oil saturation, the oil rests on a film of water spread over the surface. If the rock surface is preferentially water-wet and the rock is saturated with oil, water will imbibe into the smaller pores, displacing oil from the core when the system is in contact with water.

If the rock surface is preferentially oil-wet, even though it may be saturated with water, the core will imbibe oil into the smaller pores, displacing water from the core when it is contacted with water. Thus, a core saturated with oil is water-wet if it will imbibe water and, conversely, a core saturated with water is oil-wet if it will imbibe oil.

Actually, the wettability of a system can range from strongly water-wet to strongly oil-water depending on the brine-oil interactions with the rock surface. If no preference is shown by the rock to either fluid, the system is said to exhibit neutral wettability or intermediate wettability, a condition that one might visualize as being equally wet by both fluids (50% \ 50% wettability)

Other descriptive terms have evolved from the realization that components from the oil may wet selected areas throughout the rock surface. Thus, fractional wettability implies spotted, heterogeneous wetting of the surface, labeled "Dalmatian wetting" (by Brown and Fatt). Fractional wettability means that scattered areas throughout the rock are strongly wet by oil, whereas the rest of the area is strongly water-wet. Fractional wettability occurs when the surfaces of the rocks are composed of many minerals that have very different surface chemical properties, leading to variations in wettability throughout the internal surfaces of the pores. This concept is different from neutral wettability, which is used to imply that all portions of the rock have an equal preference for water or oil. Cores exhibiting fractional wettability will imbibe a small quantity of water when oil saturation is high and also will imbibe a small amount of oil when the water saturation is high.

The term "mixed wettability" commonly refers to the conditions where the smaller pores are occupied by water and are water-wet, but the larger pores of the rock are oil-wet and a continuous filament of oil exists throughout the core in the larger pores. Because the oil is located in the large pores of the rock in a continuous path, oil displacement from the core occurs even at very low oil saturation; hence, the residual oil saturation of mixed-wettability rocks is usually low.

Mixed wettability can occur when oil containing interfacially active polar organic compounds invade a water-wet rock saturated with brine. After displacing brine from the larger pores, the interfacially active compounds react with the rock surface, displacing the remaining aqueous film and, thus, producing an oil-wet lining in the large pores. The water film between the rock and the oil in the pore is stabilized by a double layer of electrostatic forces. As the thickness of the film is diminished by the invading oil, the electrostatic force balance is destroyed and the film ruptures, allowing the polar organic compounds to displace the remaining water and react directly with the rock surface.

Wettability has a profound influence on all types of fluid-rock interactions: capillary pressure, relative permeability, electrical properties, irreducible water saturation and residual oil and water saturations. On the other hand, the wettability is affected by minerals exposed to fluids in the pores of the rock, chemical constituents in the fluids and the saturation history of the samples. Wettability presents a serious problem for core analyses because drilling fluids and core-handling procedures may change the native-state wetting properties, leading to erroneous conclusions from laboratory tests.