Gas Absorbed in Coal Bed Methane Reservoir

In a coal bed methane, abbreviated as CBM, reservoir, the majority of the methane gas is not stored as free gas within pore spaces, like in conventional gas reservoirs.  Instead, it exists in an adsorbed state, meaning the methane molecules are attached to the extensive internal surfaces of the coal matrix.  This adsorption occurs within the micropores and on the surfaces of the cleats (natural fractures) present in the coal seam.  The amount of gas that can be adsorbed is primarily controlled by the pressure and temperature of the reservoir, as well as the inherent properties of the coal itself, such as its rank, maceral composition, and moisture content.  This unique storage mechanism is a defining characteristic of CBM reservoirs and fundamentally influences how the gas is produced.  To extract the methane, the pressure within the coal seam is reduced, typically by pumping out the water that often saturates the cleats, which allows the adsorbed gas to desorb from the coal matrix, become free gas, and then flow to the wellbore.