Page 319 - Geology and Geochemistry of Oil and Gas

280 APPENDIX A

Fig. A.6. Shape of water drop resulting from interfacial tension forces.

Fig. A.7. Illustration of 01, 901, and 1801 contact angles.

y ¼ 901 and cos y ¼ 0, and thus
(A.15)
s so ¼ s ws
On the other hand, if the solid is completely wetted by oil (Fig. A.7C), y ¼ 1801,
cos y ¼ –1, and
(A.16)
s so ¼ s ws s wo
If yo901, the surfaces are called hydrophilic and when y4901, they are called
hydrophobic. An interfacial tension depressant lowers s wo , whereas a wetting agent
lowers y or increases cos y. A decrease in s wo does not necessarily mean an increase
in cos y, or vice versa, because of the changes in s so and s ws .
If a rock is completely water-wet (y ¼ 01), water will tend to envelop all of the
grains and force all of the oil out into the middle of the pore channel. Even though
some oil may still be trapped in this case, the recovery would be high. On the other
hand, if all of the solid surfaces were completely oil-wet (y ¼ 1801), oil would tend to
envelop all of the grains and force all of the water out into the center of the pore
channel. In this extreme case, recovery would be very low by water drive. Many oil-
wet reservoirs are known to exist.
In the usual case (01oyo1801), to improve waterflooding operations the contact
angle y should be changed from 4901 to o901, through the use of surfactants. This
would move the oil from the surface of the grains out into the center of the pore
channels, where they would be produced more readily.