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- Coal seam gas is also commonly called coal bed methane or coal seam methane
- It is formed by the geological process of heating and compressing plant matter to create coal.
- Over millions of years, methane forms within the coal.
- The methane is trapped by water in the gaps and cracks between the coal molecules. These gaps are known as cleats.
How is the methane trapped?
A weak chemical reaction occurs between the molecules in the coal when they are under pressure from water. The methane is “adsorbed” by the coal – this means that the methane molecules stick to the coal molecules.
By removing the water and reducing pressure, the chemical bond between the methane and the coal is broken and the methane naturally flows out of the rock. This process is called desorbing. | |
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BG Group is investigating the potential of coal seam gas in various locations from Scotland to Australia.
In alliance with Queensland Gas Company, BG Group is exploring one of Australia's largest stores of coal seam gas, the Surat Basin in southern Queensland.
Producing coal seam gas involves drilling wells and pumping water out of the coal bed. This allows the methane in the coal to flow through the natural fissures or cleats in the coal, to be produced as natural gas. This means that coal seam gas requires a lot of small wells, to pump out water and collect gas from within a relatively small region of the coal bed.
To make the process commercially attractive, the coal bed needs to have high levels of associated gas - usually within the region of 5-20 cubic metres of gas per ton of coal. The coal seam also needs to have relatively high-permeability, to enable the gas to flow freely.
But it's a critical balance - too shallow and the pressure is not sufficient to make the gas flow, too deep and the pressure is too much and the pressure on the coal is so great that it is not possible for the gas to flow successfully. This is usually the case in coal beds deeper than 1500 metres.
See how the process of drilling for coal seam gas works (link opens in a new window).
Developing LNG from Coal Seam Gas - Queensland Curtis LNG
Queensland Curtis LNG plans to take coal seam gas from the Surat Basin and pipe it to Gladstone on the Australian coast. From there, the gas will be super-cooled to create LNG. This gas contains 98.75% methane and 1% nitrogen and only 0.25% carbon dioxide.