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Appendix A

WETTABILITY AND CAPILLARITY

A.1. INTRODUCTION

The pore space of reservoir rock is filled with oil, gas, and/or water. The interfaces
between fluids, and between fluids and solids, are the loci of many physical and
chemical phenomena, including surface free energy, surface tension, adsorption,
cohesion, adhesion, wettability, and capillarity.
The fundamental basis for explaining many interfacial phenomena is the fact that
all molecules exert attractive forces on each other. These attractive forces, generally
known as the van der Waals forces, are weak forces between molecules, which vary
inversely as the sixth power of the intermolecular distance, and are due to
momentary dipoles caused by fluctuations in the electronic configuration of
the molecules. These forces oppose the molecular agitation (kinetic energy of the
molecules that increases with increasing temperature) and cause contraction of
the surface of a liquid due to the surface free energy. The surface area is reduced to a
minimum, and for a given volume the minimum is a sphere. The tension at a liquid
surface in contact with either air or its vapor is called the surface tension. The surface
tension is equal to the surface energy.
In terms of molecular interaction, one should distinguish between the terms of
‘‘cohesion’’ and ‘‘adhesion:’’ the term ‘‘cohesion’’ is applied to the attractive forces
between the like molecules, whereas ‘‘adhesion’’ is applied to forces between unlike
molecules. The liquid that has the higher molecular interaction (higher adhesion)
with the solid is the wetting liquid, whereas the liquid that has the lower molecular
interaction (lower adhesion) with the solid is the non-wetting liquid.
When two immiscible liquids, such as oil and water, fill the pore space of reservoir
rock, they compete for a place on the solid surface. In solving many production and
reservoir engineering problems, it is very important to know which liquid (oil or
water) wets the rock surfaces. The relative ability of water and oil to wet the rock is
theoretically the same as the relative degree of adhesion between the two liquids and
the reservoir rock. Because the pore throats and canals in the reservoir rock are
mainly of capillary size (o0.5 mm), capillary phenomena help explain the behavior
of fluids in the oil and gas reservoirs.

A.2. WETTABILITY
The manifestation of molecular interaction at the interface between three phases:
solid, liquid, and gas (or another liquid not miscible with the first) is called the
wettability. In the reservoir rock, the wettability is the appearance of molecular
interaction of a liquid with the pore space surface, the magnitude and nature of
which is related to the nature of the surface and geometry of the pore space.