CSG

Coal seam gas occurs naturally when coal is formed deep underground by a process of heating and compressing plant matter.

Over millions of years, natural gas, or methane, becomes trapped in the coal seams by water, typically 300-600 metres underground.

When the water is removed, the pressure that has kept the gas in place changes, allowing the gas to flow.

Methane, which is odourless, colourless and flammable, is a serious safety risk for coal mining, particularly when miners are underground, and is usually vented to the atmosphere as part of the mining process.

Coal seam gas in the Surat Basin of southern Queensland is more than 98% methane with only small amounts of nitrogen and carbon dioxide.

As such, it requires relatively little treatment before being liquefied.

Gas extraction

Coal seam gas is extracted through wells drilled through the coal seams.

Water is pumped out and the coal seam gas is released from the coal.

If the pressure within the seam is high the gas may flow to the surface unaided.

Conversely, if the pressure is lower the gas may have to be pumped to the surface.

Various techniques have been developed to enhance gas flow, including the pumping of carbon dioxide underground to increase field pressure.

This "sequestration" of carbon dioxide underground may also have environmental benefits if the carbon dioxide would otherwise be released into the atmosphere.

At the wellhead

When coal seam gas comes to the surface, any remaining water is separated from the gas which is compressed and sent by pipeline to customers.

In southern Queensland, water from coal seams is generally stored in large evaporation ponds.

Companies are investigating how best to treat the water and ensure its beneficial use for communities affected by drought.