ABSTRACT: Gas Sorption on Coal

YEE, DAN, and JOHN P. SEIDLE, Amoco Production Company, Tulsa, OK, and WILLIAM B. HANSON, Amoco Production Company, Denver, CO

One feature that makes coal different from conventional gas reservoirs is the manner in which the gas is stored. In conventional reservoirs, the gas exists in a free state in the pores of the reservoir rock, and its behavior can be described by the real gas law. In contrast, nearly all of the gas in coal exists in a condensed, near liquid-like state owing to physical sorption.

These differences bring up two important questions. How do you describe the behavior of the sorbed gas, and how do you determine the sorbed gas content? Much literature has been written concerning these two subjects, and the following discussion reviews and summarizes the results and presents the current answers to these questions. First, the behavior of the sorbed gas is described, in terms of the physical sorption process and a sorption isotherm. The changes in gas content with pressure, temperature, mineral matter, moisture, rank, petrology, different pure gases, and multicomponent sorption are discussed. Techniques for measuring isotherms are also presented. Second, methods for determining the gas content are described in terms of direct methods that actually measure the amount of gas present and indirect methods in which the gas content is inferred from the sorption isotherm.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)