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ABSTRACT: Variation in pore size distribution with coal rank and composition: potential for substantial resources of free gas in matrix porosity in low rank

Bustin, R. Marc , UBC, Vancouver, BC

To establish the contribution of free-gas stored in coal matrix and fracture porosity to the gas storage capacity of coal, a series of experiments have been performed on coals of various ranks and compositions. Conventional gas storage, defined as compressed gas in matrix/fracture porosity, is often overlooked as a storage mechanism for coal. Experimental data indicate that free-gas in coal matrix meso-/macroporosity may comprise a substantial component (up to 50%) of the total amount of gas stored in low-rank coal at typical reservoir pressures and temperatures depending on water content (i.e. 150 to 400 psi, 60 to 75º F). Coal matrix meso- and macroporosities, determined through a combination of helium and mercury porosimetry, and low-pressure nitrogen adsorption analysis, may be as large as 7% excluding fracture porosity. Matrix meso- and macroporosity in general decreases with increasing rank and in high rank, vitrinite rich coal matrix porosity is about 2%. In contrast coal microporosity, as determined by low pressure carbon dioxide adsorption, in general increases with coal rank and vitrinite content which account for the higher adsorption capacity of higher rank, vitrinite rich coals. Coal matrix porosity and pore size distributions are also highly dependent upon coal composition.

In low rank coals such as those of the Powder River Basin, and of the Western Canadian Sedimentary Basin, free gas in meso- and macroporosity including fractures may comprise a substantial component (up to 50%) of the total produceable gas depending on moisture content. Free gas is not measured by adsorption isotherms, desorption analyses nor predicted by lost gas techniques.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia