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Abstract: High Pressure Adsorption/Desorption Isotherms: Application to Predicting Reservoir Capacity and Drainage


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.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky