CLARKSON, C. R., and BUSTIN, R.M., Department of Earth and Ocean Sciences, The University of British Columbia, Vancouver, B.C. Canada, V6T 1Z4
Coalbed methane and mixed-gas equilibrium isotherms provide a measure
of coal seam reservoir capacity and provide insight into potential production
rates at reservoir temperatures and pressures. Traditionally isotherms
have been modeled using the two-parameter Langmuir equation but our data
suggests that equations based on pore volume filling/potential theory have
general validity and provide a better fit to high-pressure (up to 10 MPa),
high-temperature (> 1.5 Tc) methane isotherm data for a variety of coals
of varying composition. The assumption of an energetically homogeneous
surface is in doubt for coal and additionally the Langmuir model overestimates
adsorbed volumes in the knee of the isotherm, and underestimates them in
the high-pressure region. Application of potential theory to the methane-coal
system results in temperature-invariant methane characteristic curves,
obtained with the assumption of liquid molar volume of the adsorbate and
extrapolated vapor pressures. A single characteristic curve may be used
to model methane adsorption isotherms at several different temperatures
for the same coal.
Effect of coal composition on methane and mixed isotherms and thus
reservoir characteristics of coal is marked. Maceral composition and mineral-matter
content have an important influence on adsorption characteristics as indicated
by carbon dioxide surface areas and methane adsorption isotherms. Amount
of methane adsorbed increases with vitrinite content reflecting the predominantly
microporous nature of vitrinite as compared to inertinite which is meso-
to macroporous.
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