Page 44 - Primer on Enhanced Oil Recovery

Fluid-rock interaction 35

The capillary pressure (this is in the presented picture a pressure which drags the
liquid up against the gravitational force) can be expressed as follows:

2γcosα
p 5
r
where γ is interfacial tension, α is contact angle and r is the capillary radius. As
radius (size) of the capillary gets smaller the pressure is getting bigger. In other
words, the wetting phase gets into a small opening in the rock and it is very diffi-
cult to get it from there.
In general terms, wetting phase coats the rock surface.
If we talk of rock types wettability then fresh carbonate, quartz and dolomite are
water-wet. But life is newer as simple as that after contact with oil the mentioned
minerals can become oil-wet. At the same time, as temperature rises quartz
becomes more oil-wet while calcite switches it affiliation to water.
It has been seen that small pores can be water-wet and are filled with water,
while the larger pores are oil-wet and filled with oil. In many cases wettability is
defined on an ionic level it all depends on ions on a rock surface. In a slightly
bigger scale the wettability depends on surface roughness and grain properties (size
and shape). The wetting phase is difficult to displace and this is why wettability is
important for oil recovery. In water-wet rock an oil can be trapped in large pores.
The wettability can also change as the chemistry of water (dissolved minerals) is
modified. One can see that the wettability is a very variable phenomenon.
What makes life even more difficult is that it is generally agreed that there is no
method to measure the wettability reliably.