Page 82 - Geology of Carbonate Reservoirs

SATURATION, WETTABILITY, AND CAPILLARITY 63

Oil

θ Water

cos θ =

Figure 3.5 The interfacial tension values (identified by σ ) and the contact angle ( θ ) between
the two liquids in a system of oil and water in contact with a solid surface. These relationships
provide the basis for calculating values of adhesion tension ( A T ) at different liquid – solid
interfaces. (Adapted from an illustration in Amyx, Bass, and Whiting (1960) .)

A T = σ so −σ sw = σ wo cos θ wo

A positive adhesion tension indicates that the denser liquid is the wetting phase
and an adhesion tension of 0 indicates that both liquids have equal affinity for the

solid. If θ is small, the denser liquid will spread readily and coat the solid surface.

If θ is large, an external force will be required to make the denser liquid spread

across the solid surface. The interfacial contact angles and attendant spreading
characteristics of several different liquid systems in contact with quartz and calcite
are shown in Figure 3.6 . In the case of water and isooctane, water is the wetting fl uid
for both quartz and calcite. When naphthenic acid is used on both quartz and calcite,
it preferentially wets calcite with a contact angle of 106 ° , but water is the wetting
fluid on quartz where naphthenic acid forms a contact angle of 35 ° . This illustrates

that for a water – oil – solid system, it is possible to have an oil - wet or water - wet
medium, depending on the composition of the liquids and solids and the surface
chemical reactions that may occur. In gas – water, or gas – oil reservoirs, gas is always

the nonwetting fluid. Reservoir rocks that were deposited in aqueous environments,
as most were, start out as water - wet. However, after oil is trapped in the reservoir,
polar organic compounds in the oil can adhere to pore surfaces and, with surface
chemical reactions over time, change the reservoir to oil - wet or partially oil - wet

(Pittman, 1992 ) . Tiab and Donaldson (2004) point out that the chemically basic
nature of carbonates causes them to react readily with organic acids in crude oils,
which in turn causes the carbonates to become neutral to oil - wet.

3.1.3 Capillarity

Capillary attraction is defined in the American Heritage Dictionary (1992) as “ the

force that results from greater adhesion of a liquid to a solid surface than internal
cohesion of the liquid itself and that causes the liquid to be raised against a vertical
surface, as water is in a clean glass tube. It is the force that allows a porous material
to soak up a liquid . ” This interactive force depends on surface tension and on the
ability of certain liquids to wet the surface of solids, or wettability. Wettability is
manifested by a curvature of the liquid surface that forms a contact angle different