Page 429 - Petrophysics

ALTERATION OF WETTABILITY 397

1.0 1 ELGlN SANDSTONE

x
W
- 0.5-
>
c
2 O-
m
I c
c
w
' -0.5-
25' C
-1.0 , I
I
0 I I 2 I 3 4 I I I 6 r I a
5
SATURATION EXPONENT
Figure 6.14. Change of the Archie saturation exponent as a,function of wettability
and temperature for Elgin outcrop sandstone, Oklahoma.

Zhou et al. used variable aging times to obtain cores with different
degrees of wettability to develop a correlation between spontaneous
imbibition and wettability [55]. The rate and amount of water imbibition
decreases as the cores change from strongly water-wet toward neutral
or oil-wet conditions. Thus a correlation to the wettability index could
be made from the area under the imbibition capillary pressure curve
(displacement energy or pseudo work) and the advancing contact angle
at the point of 50% oil recovery by imbibition.

RESTORATION OF ORIGINAL WETTABILITY

After the wettability of an oilfield core has been determined and
perhaps waterflood tests conducted with reservoir fluids, the core
is generally cleaned to determine permeability, porosity, and other
parameters. Many methods have been suggested for cleaning cores, but
by far the most used method is to place the core in a Soxhlet extractor
and extract with toluene; this is frequently followed by extraction with
ethanol to remove the toluene. The cores are then dried and used for
various tests. If the tests require restoration of the original wettability,
the cores are generally saturated with the oilfield brine and oil and aged
by various procedures prior to use. The principal problem with cores
treated in this manner is that not all of the adsorbed high molecular weight
resins and asphaltenes are removed. Therefore, the restored cores have
variable wettabilities that are more oil-wet than the original. Extraction
with chloroform and methanol for three weeks, followed by aging in