Page 393 - Petrophysics 2E

WETTABILITY 361

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 the oil saturation is high
(for example, at the irreducible water saturation, Li), and also will
imbibe a small amount of oil when the water saturation is high (for
example, at the residual oil saturation, S,).
The term “mixed wettability“ commonly refers to the condition
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 [2-41. 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 unusually low. Mixed wettability can occur when oil containing
interfacially active polar organic compounds invades a water-wet rock
saturated with brine. After displacing brine from the larger pores, the
interfacially-active compounds react with the rocks 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 [5].
The wettability of a rock-fluid system is an overall average characteristic
of a heterogeneous system with microscopic relative wetting throughout
the porous medium [6]. The rock pore surfaces have preferential
wetting tendencies toward water or oil leading to establishment of
the various states of overall wettability. This overall wettability has
a dominant influence on the fluid flow and electrical properties of
the water-hydrocarbon-rock system. It controls the capillary pressure
and relative permeability behavior and thus the rate of hydrocarbon
displacement and ultimate recovery [7- 101.

INTERFACIAL TENSION

When two immiscible fluids (gas-liquid or liquid-liquid) are in contact,
the fluids are separated by a well-defined interface, which is only a few
molecular diameters in thickness. Within the fluid and away from the