Page 332 - Geology and Geochemistry of Oil and Gas

PERMEABILITY 293
Fig. B-3, the critical saturation for water decreases (point a moves to a ) and that for
0
oil increases (point b moves to b ) with increasing concentration of polar substances
0
in the oil. With decreasing concentration of polar substances in oil, the relative
permeability to water sharply increases (point b 000 moves to b ), whereas that to oil
00
decreases (point a 000 moves to a ). This is mainly due to the fact that attraction of
00
non-polar oil to solid surfaces is negligible and that the mobile oil presents less
resistance to flow of water than does the immobile oil.
According to Babalyan (1956), in the case of polar oil the water, and oil pro-
duction is greater when the water is alkaline than when it is hard. This is due to the
change in the critical saturation of both phases (Fig. B-4). As shown in Fig. B-4, the
relative permeability curves of oil+alkaline water lie above those of oil+hard water,
because the following is true in the case of alkaline waters: (1) low interfacial tension
between oil and water, (2) low values of contact angle, (3) slow coalescence of oil
droplets in water, and (4) greater degree of dispersion of oil in water. In the case of
hard water, on the other hand, the oil becomes a dispersed phase at higher water
saturation in a porous medium than in the case of alkaline waters. The intensity of
the transformation of oil into a dispersed phase is greater in the alkaline than in hard
waters.
In the case of non-polar oil, the attachment of oil to solid surfaces is negligible in
the presence of both alkaline and hard waters. When non-polar oil flows with either
alkaline or hard water, there is no change in critical saturations and hence the
recovery of oil and water is the same in each case. The relative permeability curves
with alkaline water, however, lie above those with hard water (Fig. B-5).
Some of the controversial aspects of the above discussion will be resolved by
future research.
For details on the relative permeability of carbonate reservoirs see Honarpour
et al. (1992).