Wettability is a tendency of fluid to stick to the surface of formation when other types of fluid are present. Wettability of rock is measured by a core analysis in a laboratory and typically a laboratory measure contact angle between the fluid and the rock.
Wettability of rock is classified by the angle of contact, which is divided into 3 categories.
Water wet – contact angle (θ) is less than 90 degrees
Figure 1 – Water Wet
Naturally wet – contact angle (θ) is equal to 90 degrees
Oil wet – contact angle (θ) is more than 90 degrees
Figure 2 – Oil Wet
Even though wettability will not be directly used in a material balance equation, it helps determine fluid distribution in a reservoir and potential flow restriction. Several rock properties have a direct relationship to wettability, for instant relative permeability, capillary pressure and electrical properties. When testing a core in a laboratory, it is very vital to consider the type of drilling fluid used in a drilling operation because it can change wettability of rock at the contact area in wellbore. Then, a result of testing will not be valid to use.
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