Coal Bed Methane (CBM) is methane (natural 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.
An even more extreme technique called cavitation can be used, which involves pumping water and air or foam into the well at a very high pressure, before suddenly releasing the pressure (sometimes likened to opening a shaken fizzy drink). This causes gas, water, coal and rock fragments to explode out of the well. Cavitation can be repeated dozens of times, resulting in an enlargement of the well bore by as much as 16 feet in diameter in the coal seam, as well as producing fractures that extend from the well bore. Even if it isn’t necessary at first, wells are often cavitated later on once the gas that is easier to get at has run out.
CBM has been widely developed in the United States and Australia, where it is known as Coal Seam Gas. There is planning permission for around 60 CBM wells in the UK. Dart Energy recently submitted planning applications to Falkirk and Stirling County Councils for the first CBM production wells in the UK.
Extracting Coal Bed Methane can cause water contamination. If hydraulic fracturing is used, this can contaminate the groundwater. The process uses huge amounts of water mixed with toxic chemicals, a large fraction of which are never recovered.
However, even if the CBM well is not fracked, problems with water contamination can still occur. CBM almost always involves pumping huge quantities of water out of the coal seam – this is water that has been marinading in coal for thousands of years, and can contain a wide variety of toxic contaminants. The cost to treat and dispose of the produced water can be a critical factor in the viability of a coal bed methane project, producing a strong economic incentive to cut corners and dump water illegally. Dart Energy, who have applied for planning permission to drill 22 CBM production wells in Airth, Scotland, plan to dump all their produced water from these sites into the Firth of Forth via an outfall pipe, after minimal treatment.
Coal Bed Methane wells could leak. Both Duke University and the US Environmental Protection Agency have correlated gas in water with proximity to fracking sites. CBM exploration requires a very large number of wells to be drilled. These wells can never be removed, and the steel and concrete structures plunged deep into the earth will decay slowly over time. All gas wells will leak eventually. As coal seams tend to be closer to the surface than shale gas reserves, the chances of methane leaking are much higher.
Very large numbers of wells need to be drilled to extract Coal Bed Methane. In Queensland, Australia, over 3,000 wells have been drilled with projections of 40,000 to come. If CBM goes into full production, there will be thousands of wells across the UK.
For more information, see Coal Bed Methane: The Evil Twin Of Shale Gas.