Prediction of gas content of coalbeds, and therefore potential producibility, has relied primarily on its observed relationship to coal rank, pressure, and the methane adsorption capacity of a given coal. This relationship has been illustrated by the construction of adsorption isotherms for various coals from experimental data. These adsorption isotherms were developed for sized, clean coal particles. The value and limitations of these adsorption isotherms are addressed by relating them to observed coalbed methane gas content data.

Coalbed gas content determined by the "Direct Method" includes three separate figures: lost gas, desorbed gas, and residual gas. The residual gas and its relationship to the total gas content is discussed. The ratio of lost and desorbed gas to residual gas has been determined for some ranks of coal.

The ratio is very dependent on rank. Residual gas increases as rank increases, reaches a maximum at the rank of high-volatile A bituminous and decreases rapidly as rank increases to medium-volatile.

The general conclusion is made that the cause of the pattern of correlation of residual gas to rank is a change in the internal structure of the coal and the moisture content of the coal.

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