TABLE OF CONTENTS 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 5.1a, 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 5.1b, 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 5.1c).
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...
