Page 259 - Petrophysics

232 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

TABLE 4.5
ARCHIE SATURATION EXPONENTS A FUNCTION OF SATURATION
AS
FOR A CONDUCTING NON-WETTING PHASE [ 131
Air-Brine Oil-Brine
Brine Saturation (% PV) n Brine Saturation (% PV) n
66.2 1.97 64.1 2.35
65.1 1.98 63.1 2.31
63.2 1.92 60.2 2.46
59.3 2.01 55.3 2.37
51.4 1.93 50.7 2.51
43.6 1.39 44.2 2.46
39.5 2.11 40.5 2.61
33.9 4.06 36.8 2.81
30.1 7.50 34.3 4.00
28.4 8.90 33.9 7.15
31.0 9

no longer contributes to the current flow, because it is surrounded by
non-conducting oil, thereby increasing electrical resistivity of the porous
system.
The exponent n must be measured at reservoir wetting conditions, i.e.,
on native or restored-state cores, otherwise the water saturation in the
reservoir obtained from well logs would be underestimated. Figure 4.19
shows the effect of cleaning on the saturation exponent. Extraction
lowered the value of n from 2.71 to 1.91. The effects of wettability
on carbonate cores were investigated by Sweeney and Jennings [ 141.
They found that the saturation exponent ranged from 1.6 to 1.9 for
water-wet cores, whereas the oil-wet cores exhibited two different types
of behaviors as shown in Figure 4.20. In some cores, n was about 8
even when S, was very high. In other cores, the behavior of n was
similar to the water-wet and neutrally wet, i.e., 1.5 e n -= 2.5, until a
brine saturation of nearly 35% was reached, at which point n increased
rapidly to 12. When a core is extracted with toluene it is considered to
have neutral wettability, but, in actuality, its wettability is somewhere
between mildly water-wet and mildly oil-wet.
Longeron et al. and Lewis et al. showed that the saturation history
of the formation has a considerable effect on the saturation exponent,
particularly in water-wet porous systems [15, 161. Both studies found
that there is a significant resistivity and saturation exponent hysteresis
between drainage and imbibition saturation cycles. Drainage tests, i.e.,
tests in which water saturation decreases, describe the process that
probably took place when hydrocarbons migrated from the source rock