Coalbed Methane is methane and other gases formed during the coalification process. In the Chew Valley coal was laid down as organic matter about 300 million years ago. Since then gases have been formed and held in the micro pores on the surface and within the coal structure. Undisturbed it will generally just stay just where it is, as it has for millions of years.
The requirements for extraction of coalbed methane are:
- an unworked coal seam more than 40cm thick
- a coal seam between 200m and 1200m depth from the surface
- suitable porosity, sorption, permeability and natural fracture pattern of the coal
Coalbed Methane is extracted by drilling vertically to the coal strata and then horizontally into the seam, or seams. Methane gas can be mobilised by reduing pressure by extracting water from the coal seam by pumping and if necessary by hydraulic fracturing.
There are numerous environmental concerns relating to Coalbed Methane and shale gas extraction.
(Source: British Geologic Survey)
In Somerset the coal seams are not nice and horizontal but are contorted and deformed into complex structures.
The coal acts as reservoir for the gas, which is bound to the coal surface throughout its structure. The gas is liberated by reducing pressure in the coal seam by pumping out water. As water is extracted gas desorbtion increases and then tails off with time, as shown below.
The large quantities of water extracted from the coalbed is generally saline and needs to be disposed of responsibly.
- UK Methane intend to drill a test bore hole at Hicks Gate, Kensham – NB UK Methane are looking to test drill at two additional sites, possibly at Compton Martin and Ston Easton
- UK Methane intends a rapid development of CBM from 2015
- UK Methane consider there is potential in the coal seams in the Keynsham area between at 600 – 1000m depth
- Moving from exploration to production: horizontal bore(s) would be drilled up to 400m horizontally to collect methane
- Water would be extracted to lower pressure and enable gas flow
- Additional bore holes would be drilled 2-3 miles part where methane production was considered economic
(Source: Transition Keynsham)
The following video is a talk by Dr Mariann Lloyd-Smith on the toxic risks of Coalbed Methane (known as coal seam gas in Australia). Dr Lloyd-Smith will be speaking at a public meeting at The Salvation Army Hall, Bath Citadel, Green Park Road BA1 1XE, 7-9pm Monday 13th May 2013
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