Page 404 - Petrophysics 2E

372 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

the average wettability of the core, using a procedure that involves five
stages:

(1) The test begins at the residual oil saturation; therefore, the fluids are
reduced to Sor by forced displacement of the oil.
(2) The core is immersed in oil for 20 hours, and the amount of water
displaced by spontaneous imbibition of oil, if any, is recorded as
vwsp .
(3) The water is displaced to the residual water saturation (Siw) with oil,
and the total amount of water displaced (by imbibition of oil and by
forced displacement) is recorded as V,.
(4) The core is immersed in brine for 20 hours, and the volume of oil
displaced, if any, by spontaneous imbibition of water is recorded
as vosp -
(5) The oil remaining in the core is displaced by water to Sor and the
total amount of oil displaced (by imbibition of water and by forced
displacement) is recorded as Vot .

The forced displacements of oil to Sor , and water to Si, may be conducted
using a centrifuge or by mounting the core in fluid-flow equipment and
pumping the displacing fluids into the core.
The Amott wettability index is expressed as a relative wettability index
defined as the displacement-by-oil ratio (Vosp/Vot = &) minus the
displacement-by-water ratio (Vwsp/Vwt = 6,):

I, =Vosp/Vot - vwsp/vwt = 6w - 60 (6.13)

Preferentially water-wet cores are characterized by a positive
displacement-by-water ratio, 60, and a value of zero for the displacement-
by-oil ratio, &. A value approaching 1.0 for the displacement-by-water
ratio, 60, indicates a strongly water wet-sample, whereas a weakly
water-wet sample is characterized by a value approaching zero. Neutral
(or 50%/50%) wettability is characterized by a value of zero for
both ratios. Cores that are oil-wet show a positive value for the
displacement-by-oil ratio, &, and zero for the displacement-by-water
ratio, 8,. A strongly oil-wet sample is characterized by a value approach-
ing one for the displacement-by-oil ratio. Thus, the Amott wettability
index varies from + 1 for infinitely water-wet to - 1 for infinitely oil-wet
rocks, with zero representing neutral wettability [44, 451.
The 20-hour arbitrary time limit for the two periods of imbibition were
probably chosen to allow completion of the test in a reasonable length
of time. Completion of imbibition, however, can sometimes take several
weeks, and when the system is near neutral wettability, spontaneous
imbibition may be very slow [46, 471. If the imbibition is not allowed to