GASCHNITZ, ROLAND; KROOSS, BERNHARD M. and LITTKE, RALF, Institute for Petroleum and Organic Geochemistry, Forschungszentrum Juelich GmbH D-52425, Juelich, Germany

Since 1992 the coalbed methane potential of Upper Carboniferous coal seams in the Ruhr Basin has been examined. Gas contents vary both laterally and with depth betweeen 0 and 16 m³/t of coal. With increasing distance from coal mines data becomes increasingly sparse and prognoses of the gas potential become rather unreliable. Our research aims at a genetic understanding of coalbed methane in this area.
The adsorptive gas storage capacity is determined by the accessibility of the internal surface and by the amount of gas molecules that is attachable to the available surface area at given reservoir temperature and pressure. Upper Carboniferous coals show an internal surface area between 150 and 250 m³/g. Increasing maturity, vitrinite-rich petrography and low humidity of the organic matter positively influence the accessible surface area.
The influence of pressure and temperature is currently investigated using a newly designed apparatus that allows for the first time to measure adsorptive gas storage capacity at pressures up to 25 MPa and temperatures of up to 175 °C, i.e., close to reservoir pressure and temperature. Current results show that increasing pressure initially increases adsorption capacity of coals. However, at elevated pressure adsorption isotherms flatten out and even higher pressures would not significantly increase the adsorption capacity. At low pressure (5 MPa) and low temperature the adsorption capacity of coals is high and decreases rapidly towards higher temperatures whereas this decrease is less pronounced at higher pressures.

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