Coal Bed Methane

Coal Bed Methane, an unconventional reservoir

In conventional natural gas reservoirs such as sandstone, gas is trapped in the tiny void spaces between the rock particles. The gas is formed following the decay of microscopic plant and animal life which sank to the bottom of the oceans and over millions of years were buried under ever increasing layers of mud and sand. The heat and pressure converted the decaying organic matter to oil and gas, which being mobile and light migrated upwards. The vast majority of these hydrocarbons would have been lost, but in some cases the migration path was blocked by a layer of hard impermeable rock such as shale. The oil and gas molecules became trapped in the sandstone reservoir. Gas is generally formed when the temperatures and pressures are greater. A conventional gas play therefore requires a source, reservoir, trap and a seal.

Coal on the other hand is often formed from the remains of land vegetation in lowland swamps which is similarly buried under layers of sediment and subjected to the same heat and pressures. The hydrocarbon compounds in the decaying matter break down into what will later become coal and mainly methane gas. The methane does not however migrate away, but is trapped in the matrix of the coal and on the uneven surface by the pressure of the surrounding water and rock – hydrostatic and lithostatic pressure. This uneven coal surface is made up of ‘cleats’, or natural fractures through which the gas is released when these pressures are released. The coal seam therefore acts as the source, trap and seal, and  the size of the recoverable reserves are dependent on the thickness and cleat density of the coal seam. However because coal has such a large internal surface area on which the gas is stored, coal can produce up to seven times the volume of gas of a similar sized conventional gas reservoir.

Production from CBM reservoirs requires the release of these hydrostatic pressures which allows the gas to become mobile, and flow to the well bore. CBM production wells therefore typically produce a lot of water initially, before a stabilised gas flow is achieved. Because the coal seam may be only a few feet thick, a number of lateral wells are drilled to follow the seam. These lateral legs may be a few thousand feet long, and there may be a number of producing lateral legs from the one well head.

Because of the low permeability of coal, CBM projects require more wells than conventional gas reservoirs to maintain production, but these tend to be shallower. The development expenditure therefore is more evenly spread over the production period, unlike a conventional gas project which requires high initial expenditure, and has high initial production. In most cases the methane quality is such that it can be fed directly into the gas pipeline system, or where this is uneconomic due to distances or production volumes, used directly to generate electricity. 

CBM in the UK

The CBM industry is well advanced in the USA, Canada and Australia. In the USA approximately 10% of all natural gas production is from coal, and in some parts of Australia this is considerably higher. According to the independent industry commentator, Wood Mackenzie, CBM could provide up to 50% of the Australian east coast gas supply by 2020.

The decline in conventional gas production in the North Sea and increased understanding of exploitation techniques has resulted in a far greater interest in the industry in the UK. The UK is also keen to encourage alternative domestic energy sources rather than being reliant on imported gas which can be subject to  supply disruptions. The UK government’s Energy White Paper in 2007 recognised this fact and is likely to provide a much more supportive regulatory framework in the future. In addition, the experienced North American CBM companies are now viewing the UK as an exciting prospect and forging partnership with the existing domestic players in what is still a fairly undeveloped, but hugely attractive market.

How is it Produced?

Coal Bed Methane is produced by drilling a well which contacts the coal seams. Once water is pumped out, the pressure is lowered and gas migrates through the coal, up the well bore to surface.

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