Page 397 - Petrophysics 2E

WETTABILITY 365

where the solid plate is suspended horizontally below the surface of
a clear (or refined) oil and a drop of water is placed on the solid.
A photograph is taken of the system for accurate measurement of the
contact angle. By convention, the contact angle is measured through the
denser phase.
The second method is to suspend the plate horizontally in the water
and place a drop of oil on the bottom of the plate (Figure 6.2 D, E, F>.
The contact angle is measured through the water phase and the same
analysis is applied.
A modification of the sessile drop method was introduced by
Leach et al. to measure the water-advancing contact angle [ZI]. Two
polished mineral plates are mounted horizontally with a small gap
between them; one plate is fixed and the other can be moved smoothly
with a screw (Figure 6.3). A drop of oil is placed between the plates and
allowed to age until the contact angle no longer changes; then the mobile
plate is moved, creating the advancing contact angle. This angle changes
gradually and eventually reaches a stable value after a few days.
The sessile drop method was used by Yan et al. to determine the
mechanisms of contact angle hysteresis and advancing contact angles [22].
Mica surfaces equilibrated with crude oil, which was diluted with

I I

~ATER
ADVANCING
OIL CONTACT
ANGLE

I

Figure 6.3. Method used to measure the advancing and receding contact angle.