Application of the mono/multilayer and adsorption potential theories to coal methane adsorption isotherms at elevated temperature and pressure

Accurate estimates of gas-in-place and prediction of gas production from coal reservoirs require reasonable estimates of gas contents. Equations based on pore volume filling/potential theory provide a better fit than the Langmuir equation to both high-pressure (up to 10 MPa), high-temperature (>1.5T(c)) methane isotherm data, and low-pressure (<0.127 MPa) carbon dioxide isotherm data for 13 Australian coals. The assumption of an energetically homogeneous surface as proposed by Langmuir theory is not true for coal. 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 vapour pressures. Temperature-invariant characteristic curves are obtained for carbon dioxide, although further testing is required. The application of isotherms equations based upon pore volume filling/potential theory, in particular the Dubinin-Astakhov equation, have general validity in their application to high-pressure supercritical methane-coal systems as well as providing a better fit to isotherm data. (C) 1997 Elsevier Science Ltd.