The concept of fractional wettability is examined. Fractional water wettability of a reservoir rock is defined as the fraction of the internal surface area that is in contact with water. Capillary pressure and relative permeability of unconsolidated sand are shown to be functions of fractional wettability.


The petroleum industry has long recognized that wettability of reservoir rock has an important effect on multiphase flow of oil, water and gas through reservoirs. As early as 1928 the American Petroleum Institute sponsored a study of wettability as part of API Project 27 at the U. of Michigan. Despite 30 years of research, there is still little exact knowledge of the wettability of reservoir rocks.

There are two parts to the wettability problem. After agreeing to a uniform nomenclature in regard to wettability, the first question to be answered is, "What is the in situ wettability of a given reservoir rock?" If this can be answered the next question is, "What part does wettability play in determining the characteristics of multiphase fluid flow through the rock?" This paper represents an oblique attack on the problem of wettability. No attempt is made here to answer the basic question of wettability in situ. Instead the consequences of the concept of fractional wettability are examined. Multiphase flow in sandpacks is shown to be highly influenced by fractional wettability.

Jennings has given a definition of wettability and the other terms used in discussing wettability. These terms must be applied to the physical situation existing in reservoir rock. A survey of the pertinent literature from 1928 to 1956 indicates that the concept of a contact angle was applied to reservoir rock in the same way it would be applied to a flat, homogeneous surface. Attempts were made to state quantitatively the wettability of a reservoir rock in terms of a contact angle which was presumably constant at all points on the very rough and heterogeneous interior surface of a porous rock. Calhoun, et al, prepared synthetic consolidated and unconsolidated porous media in which they claimed there was a known uniform contact angle. They then showed the effects on the capillary pressure and relative permeability characteristics of varying this angle.

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