Secondary Recovery Conference, 2–3 May, Wichita Falls, Texas
Soil scientists were probably the first research investigators to study the role of capillarity and capillary forces in determining the static distribution of water and air in porous rocks. These workers developed the "and pressure" concept, as applied to porous sands, based on the fundamental Laplace equation of capillarity relating the pressure difference across a curved fluid interface to the interfacial tension and principal radii of curvature. In addition to interfacial tension it was recognized that the contact angle at the solid water interface played an important role in determining the distribution of fluids within porous sands or soils, but the assumption generally was made that this contact angle as measured through water was zero. In a petroleum reservoir, however, consisting of a porous rock system containing oil, water, and gas the angle of contact cannot be neglected, when considering the capillary pressure, because there is considerably less certainty that it is always near zero as measured through the water phase.
In 1940 Leverett developed the idealized capillary model of static equilibrium between fluid phases in a sandstone rock. This work, together with that of Buckley and Leverett, has been used as a basis for the approximate solution of reservoir engineering problems involving the calculation of reserves or the advance of a displacing gas or water front.
The displacement of one fluid by another on the surface of the solid involves changes in the energy of the system. In systems such as an interface between two immiscible fluid phases in a capillary tube, contact angle measurements permit the quantitative prediction of the pressure required for one fluid to displace another. When an attempt is made, however, to apply this concept to study the behavior of a petroleum reservoir, numerous difficulties arise which detract from its usefulness.
