Introduction
Coal gas is predominantly methane, with minor amounts of nitrogen, CO2, and heavier hydrocarbons.
Composition of gas produced from a coal varies with depletion. Weakly sorbed species, such as methane and nitrogen, are released from the matrix before strongly sorbed species, such as CO2 and ethane .
Consequently, the produced gas stream from a well, project, or basin is characterized by steadily rising fractions of CO2 and heavier hydrocarbon gases.
References
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King, G. R., Material-Balance Techniques for Coal-Seam and Devonian Shale Gas Reservoirs With Limited Water Influx 1993, SPE Reservoir Engineering
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Clarkson, C. R., Unconventional Reservoir Rate-Transient Analysis: Volume I and II, 2021, Gulf Professional Publishing
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John Seidle, Fundamentals of Coalbed Methane Reservoir Engineering, PennWell Corporation, 201