Page 316 - Geology and Geochemistry of Oil and Gas

WETTABILITY AND CAPILLARITY 277
volume. The difference in pressure across the interface between two immiscible fluid
phases jointly occupying the interstices of a rock is known as the capillary pressure.
2
The value of the capillary pressure (P c in dyn/cm ) depends on the interfacial tension
and the curvature of the interfacial surface. Mathematical expression of the capillary
pressure is as follow:

P c ¼ 4sðcos yÞ=d (A.1)

where P c is the capillary pressure, s is the interfacial tension, y is the angle of contact
of the liquid with the capillary tube wall, and d is the diameter of capillary tube.
A phenomenon associated with surface tension, which occurs in capillary tubes, is
known as the capillarity, which is represented by the elevation (or depression) of
liquid in the capillary tube.
The chief effect of capillary pressure is its control over the static distribution of the
fluids within a non-producing reservoir. The static capillary pressure within the voids
of a reservoir, which may be considered as a bundle of capillary tubes, is a function
of the relative fluid saturation. The capillary pressure of a water–oil system may be
plotted as a function of the water saturation, as shown in Fig. A.3.
If it is assumed that reservoir rocks are originally water-wet, the water is displaced
by oil. Obviously, not all of the water is displaced, for all oil reservoirs contain
varying amounts of interstitial water, even if only oil (or gas) is produced. The
displaced water is called the free water to distinguish it from the thin layer of
adsorbed water held to the mineral grains by capillary pressure. This water is called
the residual water or the irreducible water, S iw (see Fig. A.3).

Fig. A.3. Typical method for plotting capillary pressure vs. saturation for a water-wet system (modified
after Tiab and Donaldson, 2004, Fig. 5.9, p. 325). P ct is the threshold pressure, S iw is the irreducible water
saturation, and S wor is the residual oil saturation. 1 – Water is displaced from the core by oil. 2 – Oil is
displaced from the core by water.