Desorption as a Criterion for the Estimation of Methane Content in a Coal Seam

For the purpose of methane extraction the pressure and volume parameters of methane contained in a coal seam are usually evaluated using its high-pressure isotherm determined under laboratory conditions. Volumes interpolated on the adsorption branch of methane isotherm measured on the evacuated exploited coal is in many cases not identical with the real gas content in the seam at the same pressures. Practical arguments for this statement are results of the canister test, where the volume of the released gas is frequently more or less lower than the volume read from the adsorption isotherm. Assuming that the canister test at lossless sampling includes the total real quantity of gas contained in the coal seam, the cause of such a difference is to be looked for in the interpretation of the sorption isotherm. Although several sources quote that — during the sorption of methane on coal — the desorption branch coincides with the adsorption one [1–3], while the reversibility occurs only exceptionally [4], it cannot be precluded, that the actual methane content in coal lies rather on the desorption branch of the sorption isotherm.