An Introduction to Coalbed Methane

The process by which plant material is converted to coal generates large quantites of methane gas, which is often trapped within the coal. The presence of this gas is very well known due to underground coal mining, where it presents a serious safety risk. This is coalbed methane, often refered to as CBM.

Coalbed methane differs from a typical sandstone natural gas reservoir in the following ways:

• Methane is stored within the matrix (the coal) by a process called adsorption. The methane is in a nearly liquid state, lining the inside of the coal pores.
• The porosity of the matrix typically refers to the size of the cleats (fractures running throughout the coal), not the porosity of the coal rock itself. Porosity is typically very low compared to a traditional reservoir (less than 3%).
• The gas is often, but not always, sealed in the coal by 100% water saturation of the cleats. The reservoir must be dewatered before the gas can begin to desorp from the coal.