Page 345 - Petrophysics 2E

CHAPTER 5

APILLARY

RESSURE

CAPILLARY PRESSURE

Capillary pressure is the difference in pressure between two
immiscible fluids across a curved interface at equilibrium. Curvature
of the interface is the consequence of preferential wetting of the
capillary walls by one of the phases. Figure 5.1 illustrates various wetting
conditions. In Figure 5.la, two immiscible fluids are shown in contact
with a capillary. The water wets the walls of the capillary, but the oil
is non-wetting and is resting on a thin film of the wetting fluid. The
pressure within the non-wetting fluid is greater than the pressure in
the wetting fluid and, consequently, the interface between the fluids
is curved convex with respect to the non-wetting fluid. The capillary
pressure is defined as the pressure difference between the non-wetting
and wetting phases:

In Figure 5.lb, the two fluids wet the walls of the capillary to the same
extent, and the pressure of each fluid is the same. Therefore, the interface
between the immiscible fluids is straight across (-90") and the capillary
pressure is equal to zero. If the pressure in the water is greater than in the
oil, the curvature of the interface is directed into the oil and the capillary
pressure is positive (Figure 5.1~).
The radii of curvature between water and oil in the pores of the rock
are functions of wettability, saturations of water and oil, pore geometry,
mineralogy of the pore walls, and the saturation history of the system.
Therefore, the radii of curvature and contact angle vary from one pore
to another, and the average macroscopic properties of the rock sample
apply *
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