How Coal Bed Methane is Formed

The formation of coal over time from the gradual breakdown of organic materials creates methane, which in turn finds itself trapped under increasing depths of burial. As a result, and unlike conventional natural gas reservoirs found in rock such as sandstone, the methane is not held in a void of space, but instead clings on to the micropore surface of the coal.

The jagged coal surface is made up of what are known as "cleats" and it is through this uneven surface that the natural gas circulates. The pressure of the surrounding water and rock - known as hydrostatic and lithostatic pressure - holds the gas in place.

During mining activity, the disruption to the coal seam can cause the release of significant amounts of methane - and this can continue, often long after the mine has been abandoned.

Indeed, historically, onshore mines have often had to drill boreholes to release the natural gas during extraction activity in an effort to avoid explosions. The concept of pumping away this gas for commercial use has long been mooted both at working and abandoned mines. As a result, the practice, known either as Coal Mine Methane (CMM) or Abandoned Mine Methane (AMM) has had some success for a few private operators.

The commercial viability of this type of gas extraction is dependent on the amount of gas being given off by the coal. This in turn depends on the total thickness of the coal seams, gas content, speed of extraction and permeability.

Please also see our introduction to Coal Bed Methane.