Page 395 - Petrophysics 2E

WE'M'ABILITY 363

the three contact angles. Writing the equation for the three conditions:
(1) water, oil, solid; (2) water, gas, solid; and (3) gas, oil, solid, one
obtains

0, = os, + owo COS e,,
osg = bsw 1- owg cos 0,
osg = oso+oOg~~Seog

Algebraic elimination of the solid-fluid interfacial tensions yields:

The conditions of wetting can be determined from the easily measured
interfacial tensions and contact angles, using Equation 6.3 [12, 131.

Adhesion tension, z, is defined as the difference between the solid-oil
and solid-water interfacial tensions:

Three conditions of wettability are apparent from Equation 6.4 and are
illustrated in Figure 6.2:

(1) when the adhesion tension is positive, the system is water-wet
(0 < go", COS^ = +);
(2) when z is zero, the system is neutrally wet (8 = 90°, cos 8 = 0); and
(3) when z is negative, the system is oil-wet (e > 90", cos 8 = -).

When a liquid spreads on a solid, or on the surface of another
immiscible liquid, the imbalance of forces is defined as the spreading
coefficient, C, as follows:

(1) Water spread on a solid in the presence of oil (water-wet):

cswo = os0 - ow0 - osw (6.5)

(2) Oil spread on a solid in the presence of water (oil-wet):

(3) A liquid spread on a solid surface in the presence of gas: