Page 427 - Petrophysics

ALTERATION OF WE'ITABILITY

Cores with fractional, or mixed, wettability have other effects that
have been discussed previously. Experiments conducted to evaluate the
effect of wettability on residual oil saturation show that the residual oil
saturation is less for systems that are at neutral wettability (probably
fractional or mixed wettability). Residual oil saturation (at its minimum
at neutral wettability) increases as the system becomes more water-wet
or oil-wet (Figure 6.11) [44, 104-1081.
A strongly water-wet core will produce most of the oil before water
breakthrough, which will occur soon after one pore volume of water has
been injected. The water/oil ratio will increase rapidly after water
breakthrough to an infinite value; thus production will diminish to an
insignificant amount.
An oil-wet core will produce water early at a low water/oil ratio which
will continue to increase gradually. After about two pore volumes of
water have been injected, production will continue for a long time with
gradually increasing water/oil ratio.

EFFECTS OF TEMPERATURE PRESSURE
AND

The wettability of a water-oil-rock system becomes progressively more
water-wet as the temperature of the system is increased. Lorenz et al.,
working with outcrop cores saturated with brine and crude oil, observed
an average USBM wettability index increase of 0.3 for water-oil-outcrop
sandstone systems when the temperature was changed from 25 to 65°C
(Table 6.4) [95].
Work conducted by Donaldson and Siddiqui to examine the effect
of wettability on the Archie saturation exponent at two temperatures
also showed the change to a more water-wet system that occurs when
the temperature is increased [log, 1101. Figures 6.13 and 6.14 show
an increase of the USBM I, for water-oil-rock systems when the test
temperature is increased from 25" to 78°C. The observed wettability
index change with respect to temperature is strongly influenced by the
chemical and physical properties of the rock surface; the Berea sandstone
(k = 325 mD), registered an increase of I, of about 0.4 over that of the
Elgin sandstone (k = 1900mD), which exhibited a change of 0.9. The
change of wettability to a more water-wet system as the temperature
is increased has also been observed, directly and indirectly, by other
investigators [102, 111-1141.
Donaldson et al. have shown conclusively that an increase of
temperature produces a more water-wet system [7, 1091. The test for
wettability should therefore be conducted at reservoir temperature using
reservoir fluids. The presence of trace metals in the actual formation
water, if used, renders the cores more water-wet.