Page 408 - Petrophysics 2E

376 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

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0 I
Sequence of Steps
I- Sample 100KC Saturated Wlth Wster
2- Oil Dlsplaclng Water as Positlve
Caplilary Pressure Incresses
3- Water Free lmblbitlon
4- Water Displacing Oil as Negatlve
Caplllary Pressure increases
5- Oil Free imbibition
6- 011 Dlsplacing Water as Posltlve
Caplilary Pressure Increases
Figure 6.8. Illustration of the combined USBM-Amott method for determination of
the wettability index.

saturation (not the saturations at the inlet face of the core). Thus, the
procedure has six steps:

(1) saturation of the core with water,
(2) initial displacement of water to Si, (oil drive),
(3) spontaneous imbibition of brine,
(4) displacement of oil by brine (brine drive),
(5) spontaneous imbibition of oil, and
(6) final displacement of water by oil (oil drive).

The Amott index is calculated from the spontaneous and total water
and oil displacements, whereas the USBM index is calculated from the
areas under the curves.
Data presented by Sharma and Wunderlich are shown in Table 6.2 [48].
Spontaneous imbibition of oil does not occur in strongly water-wet
samples that are 100% saturated by water (point 1, Figure 6.8). Therefore,
the displacement of water by oil ratio for the Amott test is zero for
the three water-wet cores. A USBM wettability index greater than 1.0