Page 322 - Geology and Geochemistry of Oil and Gas

WETTABILITY AND CAPILLARITY 283

n 1 =n 2 d 1 =d 2 (A.24)
As shown in Fig. A.12, the sum of forces acting on the trapped oil globule may be
expressed as
X
F ¼ F 1 þ F 2 F 3 (A.25)
where
X 2
F ¼ pd Dp =4 (A.26)
t
2
F 1 ¼ pd Dp =4 (A.27)
i
F 2 ¼ pdð4s a cos y a Þ (A.28)
and
F 3 ¼ pdð4s b cos y b Þ (A.29)
Thus,
Dp ¼ Dp þ ð4s a cos y a Þ=d ð4s b cos y b Þ=d (A.30)
t i
Because the receding angle is usually lower than the advancing angle, the capillary
pressure does not help, but hinders the flow. The term ð4s b cos y b Þ=d is usually
greater than ð4s a cos y a Þ=d because y b oy a . If a surfactant was added at the left to
reduce s a , Dp would become less and the oil globule may eventually move to the left
t
as shown in Fig. A.12 when Dp becomes negative. The quantity of trapped oil is
t
dependent upon the value of s cos y at each end of the globule as well as upon Dp i
(imposed pressure drop).
Inasmuch as the contact angle depends upon the interfacial tensions, which, in
turn, may be influenced by surfactants, these chemicals may alter recovery by
altering both the contact angle and interfacial tension. As the oil is displaced by
water, which wets the rock surface, capillary pressure is a driving force. If, on the
other hand, water does not wet the rock surface, then the capillary pressure is a
retarding force, which must be overcome.
The magnitude of capillary pressure in pores having a radius of around 15 mm is
not large and, therefore, capillary pressure is not an important force during
the movement of OWC, providing there is no mixing. The movement of oil

Fig. A.12. Forces acting on a trapped oil globule in a capillary.