Page 326 - Geology and Geochemistry of Oil and Gas

WETTABILITY AND CAPILLARITY 287
rocks), F c ¼ 2prðs wg cos y wg Þ, where s wg is the water–gas interfacial tension and y wg is
2
the water–gas–rock contact angle. Thus, W g þ F c ¼ B, or ðpr hÞg þ 2pr
g
2
ðs wg cos y wg Þ ¼ ðpr hÞg . Solving for h, one obtains
w
h ¼ ð2s wg cos y wg Þ=rðg g Þ ¼ ð2s wg cos y wg Þ=rDg (A.37)
w
g
and in as much as ð2s wg cos y wg Þ=r ¼ P ðwgÞ , which is the capillary pressure in the
c
water–gas system:
h ¼ P ðwgÞ =Dg (A.38)
c
In gas-wet system (possibly some carbonates), the gas starts moving without
reaching this minimum height because the capillary pressure aids the upward
movement. (For analysis of oil-water system, see Chilingar et. al., 1972, pp. 355–356.)
The writers believe that petroleum geologists and engineers must be familiar with
both the SI and FPS systems of weights and measures and be able to easily make
necessary conversions.