Page 399 - Petrophysics 2E

WETTABILITY 367

as the advancing and receding contact angles [23]. As the plate is moved
into and out of a liquid, the change of force, F, due to the adhesion
tension is:

and the contact angle is:
0 = arc cos (E) (6.9)

where: F = force, mN.
1 = 2 x (width + thickness), m.
O = interfacial tension, mN/m.

The plate is attached to a microbalance and its rate of movement is
controlled by a computer that calculates the instantaneous value of the
contact angle. The advancing contact angle is obtained as the plate is
moved into the liquid and the receding angle when the plate is pulled
from the liquid [23-251.
The principal constituents of sandstones and limestones are quartz
and calcite, respectively. Consequently, polished plates of quartz
and calcite are used as representative surfaces for sessile drop and
advancing contact angle measurements. These are not representative
of reservoir rocks, however, because the plates do not account for
surface roughness, the large variety of minerals, or thin layers of
organic materials. The wettability of these heterogeneous surfaces can
be measured only by using one of the methods that measure the
average core wettability, such as the Amott and USBM methods. The
sessile drop and advancing contact angle measurements are therefore
qualitative, rendering information on the behavior of the reservoir
fluids and the gradual deposition of interfacially active compounds onto
the solid surface. The methods are most useful for measurement of
wettability effects of solutes in pure fluids. One can quickly observe
wettability changes of the smooth plate toward different oils and aqueous
solutions.

SURFACE CHEMICAL PROPERTIES

The chemical compositions of the fluids and the rock surfaces
determine the values of the solid-fluid and fluid-fluid specific surface
energies. Thus, the mineralogy of the rock surface has an influence on
the relative adhesive tensions and contributes to the overall wettability
of the fluid-rock system. Polar organic compounds in crude oil can react