Coalbed Methane

Coalbed methane (CBM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy throughout the world. The term refers to methane adsorbed into the solid matrix of the coal. It is called 'sweet gas' because of its lack of hydrogen sulfide. The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk. Coalbed methane, often referred to as CBM, is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, lining the inside of pores within the coal (matrix). The open fractures in the coal (cleats) can also contain free gas or can be saturated with water.

Permeability is key factor for gas reservoirs, including CBM. Coal itself has a low permeability. Almost all the permeability of a coal bed is usually considered to be due to fractures, which in coal are in the form of cleats. The permeability of the coal matrix is negligible by comparison. Coal cleats are of two types: butt cleats and face cleats, which occur at nearly right angles. The face cleats are continuous and provide paths of higher permeability while butt cleats are non-continuous and end at face cleats. Hence, on a small scale, fluid flow through coal bed methane reservoirs usually follows rectangular paths. The ratio of permeability in the face cleat direction over the butt cleat direction may range from 1:1 to 17:1. Because of this anisotropic permeability, drainage areas around coal bed methane wells are often elliptical in shape.

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