Page 419 - Petrophysics 2E

WATER-OIL-ROCK INTERFACIAL ACTIVITY 387

process (crude oils treated with light hydrocarbons precipitate asphalts
and asphaltenes). Rocks also are wet by water and oil in a spotty
(fractional and mixed wettability) fashion. Thus, certain regions of the
surface may be wet by oil and the remainder by water; the overall
wettability depends on the ratio of the surface area wet by water to
that wet by oil [ 1, 57, 72, 75-77].
Anderson presented a thorough review of the literature on the effects
of wettability on relative permeability curves [3, 46, 78-81]. In a
water-wet system, water occupies the small pores and coats most of
the large pores with a thin film. Inasmuch as most of the flow occurs
through the larger pores where the oil is located and water is not present
to impede the flow of oil, the oil effective permeability, relative to water,
is very high. On the other hand, the water effective relative permeability
is very low, even when the oil saturation has been reduced to Sor,
because residual oil in the large pores remains to effectively block the
flow of water (Figure 6.10). When a water-wet core is waterflooded from
an initial saturation equal to the irreducible saturation (Siw), only oil
is produced until a critical average water saturation is attained where
water breakthrough begins. Water breakthrough is indicated when
water production first begins at the outlet. Prior to water breakthrough,
piston-like displacement of oil occurs because for every volume of
water injected an equal volume of oil is produced. Just after water
breakthrough, the water-to-oil production ratio increases dramatically,
reaching a point where oil production almost ceases and a practical
residual oil saturation is reached. To attain the true (or ultimate) residual
oil saturation requires that waterflooding continues until production of
oil completely stops. This limit may require hundreds of pore volumes of
injected water; therefore, the limiting So, is only investigated for special
research applications. For a strongly water-wet system with a moderate
owwater viscosity ratio, the three average saturations-breakthrough
saturation, practical Sor, and ultimate So,-are almost equal [82]. For
intermediate or oil-wet systems, the three saturations can vary greatly.
In an oil-wet system, theoretically, the locations of the two fluids are
reversed. Even at low water saturations, the effective permeability to oil is
much lower than in water-wet systems (at any given saturation) because
water in the larger pores is blocking the flow of oil. This becomes more
pronounced as the water saturation increases during a waterflood, and
it eventually results in a final residual oil saturation higher than it would
be in a water-wet system (Figure 6.10). The effective permeability to
water should be high in an oil-wet system because, theoretically, the
oil is located in the small pores and is coating the larger pores with a
thin film and is not interfering very much with the flow of water. The
relative permeabilities are controlled by the distribution of the fluids in
the pores of rock. The relative permeability of a fluid at any saturation is