Page 78 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

66 Reservoir Engineering

preferentially water-wet and the large pore surfaces are strongly oil-wet. Salathiel
concluded that the oil-wet paths can be continuous to provide a means for oil
to flow even at very low oil saturations; these results were offered to explain
the very good oil recovery noted in some field projects. More recently, Morrow,
Lim, and Ward [122] introduced the concept of a speckled wettability in which
a rationale is presented whereby oil tends to be trapped in pore throats rather
than pore bodies. Speckled wettability mimics behavior of strongly water-wet
conditions observed during waterflooding: water breakthrough is abrupt, relative
permeability to water at residual oil saturation is low, water is imbibed spon-
taneously, and oil is not imbibed spontaneously.
When cores are obtained for laboratory tests where wettability is important,
precautions must be taken to ensure the wetting preference of the formation is
not altered during coring. Mud additives, such as dispersants, weighting agents,
lost circulation materials, thinners or colloids, that possess surface-active
properties may drastically change core wettability. Surface active agents should
be avoided so that the core samples have the same wettability as the reservoir
rock. Listed in Table 5-16 are the effects of various mud additives on wettability
of water-wet and oil-wet cores [107].
In the case of water-wet sandstone or limestone cores, rock-salt, bentonite,
carboxymethylcellulose (CMC), and barite had no effect on wettability. However,
oil-wet sandstone cores were reversed to a water-wet condition when exposed
to CMC, bentonite, or lime solutions. Additional tests with bentonite solutions
indicated that wettability of oil-wet cores is not reversed if the solution pH is
lowered to a neutral or slightly acidic value. These results suggest that from a
wettability standpoint, the best coring fluid is water (preferably formation brine);
if bentonite is used, mud pH should be neutral or slightly acidic. If appreciable
hydrogen sulfide is suspected in the interval being cored, it may be undesirable
to lower pH. In fact, a very alkaline mud (pH 10-12) may be used to keep the
sulfide in the ionized state for safety and corrosion considerations. Subsequent
work [123] suggests the preferred system to obtain fresh cores is a natural water-
base mud with no additives, or a bland mud consisting of bentonite, salt, and
CMC. However, recent results [124] conclude that bland muds may not, in fact,
be bland. While none of the bland additives altered wettability of water-wet rock

Table 5-16
Effect of Water-Base Mud Additives on the Wettability of Cores
Wettabillty of Test Cores after Exposure to Filtrate
Water-Wet Water-Wet 011-Wet
Com ponent Limestone Sandstone Sandstone
~~~~
Rock-salt No change No change No change
Starch Slightly less water-wet Slightly less water-wet -
CMC No change No change Water-wet
Bentonite No change No change Water-wet
Tetrasodium
pyrophosphate No change Less water-wet -
Calcium
lignosulfonate No change Less water-wet -
Lime No change Slightly more water-wet Water-wet
Barite No change No chanoe -
From Reference 107.