Page 407 - Petrophysics 2E

EVALUATION OF WE'M'AF3ILITY 375

ratios of the areas under the capillary pressure curves (between Si, and
S,,,) is a direct indicator of the degree of wettability. The logarithm
of the area ratio of oil displacing water, A1 (from Sor to Si,), to water
displacing oil, A2 (from Si, to So,-), was used as a convenient scale for
the wettability index (Iu):
Iu =log (2) (6.14)

where:

(1) Increasing positive values to +cc indicate increasing preferential
water wetting to infinite water wettability.
(2) A value of zero represents equal wetting of rock by both fluids
(neutral wettability).
(3) Increasing negative values to -m indicate increasing preferential oil
wetting to infinite oil wettability.

The USBM method does not depend on spontaneous imbibition
and, therefore, is sensitive to wettability throughout the range from
complete water-wetting (+m) to complete oil-wetting (-m). For
example, if a water-oil-rock system being tested repeatedly becomes
progressively more water-wet, A1 will become larger while A2 will
decrease. Eventually, A2 will vanish as the hysteresis loop rises above
the line representing Pc = 0. In this case, A2 is zero and the wettability
index (defined in Equation 6.14) is infinite, meaning 100% wetting of the
surface by water. Infinite oil wettability also is possible, in which case
A1 = 0 and the hysteresis loop is below the line where Pc = 0.
Kwan developed a centrifuge core holder for unconsolidated sands
and used it to examine capillary pressure and wettability of viscous
bitumen [51]. The tests were conducted with a heated (40°C) centrifuge
to maintain mobility of the bitumen.

COMBINED AMOTT-USBM WETTABILITY TEST

A procedure has been developed for combining the Amott and USBM
methods that yields both the USBM wettability index and the Amott
ratio. According to several authors, the resolution of the USBM index
is improved by being able to account for saturation changes that occur
at zero capillary pressure [46, 471. Figure 6.8 illustrates this combined
method. At each point where the capillary pressure is equal to zero, the
sample is immersed in the displacing fluid for 20 hours and the amount of
fluid imbibed is recorded and used to determine the Amott ratios. For the
combined test, the capillary pressure data are plotted versus the average