Page 430 - Petrophysics

398 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

crude oil for 30 to 40 days, produces better results, but the long time
required may be a constraint.
McGhee et al. and Donaldson et al. found that reproducible restoration
of wettability can be attained repeatedly, using the same cores, with the
following treatment sequence [77, 801 :
(1) cleaning with toluene;
(2) cleaning with steam (they found that steam does not disturb the clay
minerals);
(3) saturating with brine and crude oil to SWi; and
(4) aging in the crude oil for at least 100 hours at 65°C

EFFECT OF WETTABILITY ELECTRICAL PROPERTIES
ON

Keller showed that different values of resistivity can be obtained at the
same water saturation in rocks if the wettability is changed [ 1161. His
values of the saturation exponent n ranged from 1.5 to 1 1.7 for the same
rock. Oil-wet rocks have a high resistivity because oil is an insulator. Even
at very low water saturations, a water-wet sand will have a continuous
water film along the surfaces of the sand grains from the entrance to
the exit, which furnishes a conductive path for the electric current. In
an oil-wet sand, however, oil is the continuous phase and is in contact
with the pore walls. Since water is the discontinuous phase in this case,
the electrical path is interrupted by the insulating oil. Consequently,
the resistivity of an oil-wet sand is very high, and the Archie saturation
exponent n is considerably greater than 2.0.
Sweeney and Jennings obtained variations of n from 1.6 to 5.7 for
carbonate rocks treated with acids to make them preferentially water-wet
[ 1 171. Even after cleaning the carbonate surfaces with acid, polar organic
compounds from the crude oil apparently adsorbed on the surface of
many of the samples, resulting in high values of n. Morgan and Pirson
reported a very wide range of values for n, from 2.5 for strongly water-wet
samples to 25.2 for strongly oil-wet packs of glass beads treated with
progressively higher concentrations of silicone solutions [47]. Donaldson
and Siddiqui confirmed previously reported results showing that Archie’s
saturation exponent increases from values near 2.0 for strongly water-wet
to values higher than 8.0 for strongly oil-wet systems [log]. A linear
relationship was observed between the USBM wettability index and
the saturation exponent. They showed that the water-oil-rock systems
become more water-wet when the temperature is increased. Figures 6.13
and 6.14 show a wettability index increase of 0.4, corresponding to a
temperature increase from 25°C to 78°C. The difference in the slopes