Page 77 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Basic Principles, Definitions, and Data 65

called the U.S. Bureau of Mines (USBM) test, by measuring the areas under
capillary pressure curves. The USBM method has the advantage of working well
in the intermediate wettability region. A multitude of techniques for the
qualitative indication of wettability that have been proposed will not be described
but have been discussed in the literature [133].
In a fairly extensive examination of 55 different reservoirs, Treiber, Archer
and Owens [ 1141 arbitrarily assigned water-wet conditions for contact angles of
0" to 75" and oil-wet conditions for contact angles of 105" to 180" with contact
angles of 75" to 105" representing an intermediate (referred to as neutral by
others) wettability. With these designations, 27% of the samples were water-wet,
66% were oil-wet, and the remaining 7% were of intermediate wettability.
Subsequently, Morrow 11151 has defined an intermediate wettability when neither
fluid spontaneously imbibes in a "squatters' rights" situation. Morrow found that
for contact angles less than 62", the wetting phase would spontaneously imbibe,
and for contact angles above 133", the nonwetting phase would spontaneously
imbibe; therefore, the intermediate wettability condition would be operative for
contact angles from 62" to 133'. Using Morrow's guidelines, the data of Treiber,
Archer and Owens indicate that 47% of the samples were of intermediate
wettability, 27% were oil-wet, and 26% were water-wet. The distribution in
wettability according to lithology is given in Table 5-15. In either case, it is
apparent that a majority of the samples were not water-wet.
Using 161 core samples representing various carbonate reservoirs, Chilingar
and Yen [116] found that 8% were water-wet (e, > SO"), 12% were intermediate
(9, = 80" - IOO"), 65% were oil-wet (e, = 100" - ISO"), and 15% were strongly
oil-wet (e, = 160" - 180"). The arbitrary definitions of wettability differ from
Treiber et al. [114] and Morrow [115], but the distributions appear to be similar
to the carbonate data in Table 5-15.
In the previous discussion, it was implied that pore surfaces within a reservoir
rock are uniformly wetted. The concept whereby a portion of the reservoir
surfaces are preferentially oil-wet while other portions are preferentially water-
wet was termed fractional wettability by Brown and Fatt [117] and Fatt and
Klikoff [118]. Fractional wetting was believed to be a result of the varying
amount of adsorption of crude oil components on the different minerals present
in a reservoir. Other evidence [119,120] supported the existence of a hetero-
geneous wetting (also called spotted or Dalmation wettability in the literature).
Salathiel [121] introduced the concept of a mixed-wettability condition, a special
case of fractional wetting, in which the fine pores and grain contacts are

Table 5-15
Wettability of 55 Reservoir Rock Samples
~~ ~~ ~
Water-wet Intermediate Oil-wet
(0, c 75op (e, e 750 - iosoy (0, > lOS0)*
Sandstones 43% 7% 50%
Carbonate 8% 8% 84%
Total samples 27% 7% 66X
Water-wet Intermediate Oil-wet
(0, e 62")+* (0, = 62" - 133')** (e, > 1330~
Total samples+* %YO 4796 27%
'Based on contact anglehettability relation suggested by Treiber, Archer and Owens [114].
**Based on contact angldwettability relation suggested by Morrow [115].