Page 346 - Petrophysics 2E

3 14 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Wotor-Wit Syrtom
O<BC70

Neutrally-Wet Syrtom
70<e<110

011-Wet System
I io<e<i~~o

Figure 5.1. Various wetting conditions that may exist for water and oil in contact in
a capillary, using the contact angle method.

DERIVATION THE CAPILLARY PRESSURE EQUATION
OF

A fundamental property of liquids is the tendency to contract and
yield the smallest possible surface area, producing a spherical form in
small drops. The explanation for this behavior can be illustrated as
an imbalance of molecular attractive forces at the surface of a liquid.
Consider a liquid coexisting with a gas: Molecules in the interior are
surrounded by others on all sides, subjecting them to uniform molecular
attraction in all directions. On the surface, however, the molecules are
attracted inward and on all sides, but there is no outward attraction
to balance the inward tug on the surface molecules. This imbalance of
forces causes the surface to contract to the smallest possible area and
produces a surface tension (0) expressed as newtons per meter. Work
must be done to extend the surface in opposition to the surface tension