Coal Bed Methane, sometimes called Coal Seam Gas is methane gas trapped in coal seams underground.
To extract the gas, the basic method is to drill into the coal seam. Then, if the seam is permeable enough, pumping water out of the seam will be enough to start gas flowing from the well. If not, it is often also necessary to frack the seam to extract the gas.
The process uses huge amounts of water and water contamination is one of the largest concerns, both from the ‘produced water’ in the process but also from leaking wells. As coal seams are much closer to the surface methane contamination increases in likelihood.
With the necessity of thousands of wells to ensure continuing production, the cumulative potential risks and impact of unconventional gas operations is undeniable.
Methane gas within deep coal seams is held on the coal surface (adsorbed) by pressure of surrounding water and rock. If pressure is released, gas can flow through rock fractures, and be extracted from boreholes.
In Somerset, coal bed methane could potentially be present within the north Mendip boundary zone of the Bristol-Bath Coal Basin, the Chew Valley and Keynsham areas, amongst others.
The main target zones are unworked coal seams thicker than 400mm at between 200-1200m depth.
Techniques to release gas include:
1) Dewatering (pumping large amounts of water out of the coal seam over several months)
2) Nitrogen gas fracturing/fracking by injection of nitrogen under pressure into the coal seams
3) Higher pressure hydraulic fracturing / fracking
4) A series of techniques may be necessary to reactivate flow, as the initial gas pressure decreases.
Click here to read about the potential impacts of coal bed methane exploration.