Page 392 - Petrophysics 2E

CHAPTER 6

W ETTABI LITY

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, wettability is a measure of the preferential tendency of
one of the fluids to wet (spread or adhere to) the surface. In a 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 areas 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-wet depending on the brined 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 [ 11. Fractional wettability means that scattered areas throughout
the rock are strongly wet by oil, whereas the rest of the area is