Capillary Pressures--Their Interpretation and Use

Jeffrey B. Jennings

Capillary pressure techniques have been available to the petroleum industry for over 40 years, but have been used primarily in engineering applications. With the search for petroleum becoming more difficult, exploration and development geologists should take a closer look at capillary pressures as another tool with which to improve the odds.

A capillary pressure curve is obtained by injecting mercury into a rock sample plug to produce a plot of injection pressure vs. mercury saturation. The resulting capillary pressure curve is a valuable piece of data for exploration and development. In exploration, the data can be used to "high-grade" prospects, define areas to concentrate further exploratory efforts, study oil migration, and make regional studies that can be used in much the same fashion as stratigraphic studies. All of these can be integrated into geological, geophysical, and engineering programs. In development, capillary pressure data can locate oil-water contacts, calculate oil columns, give relative permeabilities, define multiple reservoir systems, and explain some reservoir tilting. Stratigraphic traps result fr m capillary pressure differences, and understanding the concept of a "capillary pressure release valve" can help exploit this type of trap. For enhanced recovery programs, capillary pressures can provide a relatively inexpensive source of essential data.

By understanding how capillary pressures work within a reservoir and how they affect migration and entrapment, valuable insight can be gained and applied to the search for petroleum deposits. Capillary pressure data are neither difficult nor expensive to acquire and may yield answers unobtainable by any other method.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988