Page 92 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

80 Reservoir Engineering

I WATER-WET
OIL- WET
s,, = 10% PV

I
I
I
I I I I I
0 20 40 so 80 100
OIL RECOVERY, *lo OIL IN PLACE
Figure 5-56. Comparison of oil recovery for oil-wet and water-wet cores [133].

and several investigators have suggested better oil recovery from cores of
intermediate or mixed wettability [ 106,108,121,123,171]. More recent evidence
by Morrow [172] and Melrose and Brandner [173] suggests that mobilization
of trapped oil is more difficult in the intermediate wettability region, but the
prevention of oil entrapment should be easier for advancing contact angles
slightly less than 90".
With water-wet cores in laboratory waterfloods, the oil production at water.
breakthrough ceases abruptly and water production increases sharply. With
systems that are not water-wet, water breakthrough may occur earlier, but small
fractions of oil are produced for long periods of time at high water cuts. In
strongly water-wet systems, the residual oil that is permanently trapped by water
resides in the larger pores, whereas in oil-wet systems trapping occurs in the
smaller capillary spaces [106,133].
Relatlve Permeability Characteristics. For a system having a strong wetting
preference for either oil or water, relative ermeability of the wetting phase is
a function of fluid saturation only [76,13{,160]. Details of the effect of wet-
ability on relative permeabilities have been discussed by several authors [llS,
135,174,1751. In a detailed study using fired Torpedo (outcrop sandstone) cores,
Owens and Archer [174] changed wettability by adding surface active agents to
either the oil or water. Firing of the cores stabilized any clay minerals present