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Natural gas is a colourless, odourless mixture of gases made up mostly of methane. It’s used every day to cook meals, warm homes and fuel buses, cars and power plants.
Apart from methane (which is given the chemical symbol CH4), other gases that can form part of natural gas include small amounts of ethane (C2H6), propane (C3H8) and butane (C4H10).
As early as 2000 years ago, the Chinese piped natural gas into their homes through bamboo pipes to fuel lights.
Today, most natural gas used in Australia comes from onshore fields such as the Cooper Basin, or offshore petroleum fields such as the North West Shelf and Bass Strait. In recent years, coal seam gas has become an important source of natural gas for Queensland.
Coal seam gas
Coal seam gas, or CSG, is a natural gas.
It occurs when coal is formed deep underground over millions of years of heating and compressing decomposing plant matter.
Over time, the gas becomes trapped in coal seams by water, typically 300-600 metres under ground.
When the water is removed, the pressure that has kept the gas in place changes, allowing the gas to flow.
Apart from methane, coal seam gas contains little or no other amounts of hydrocarbon gases such as ethane, propane and butane. Coal seam gas typically has only small amounts of carbon dioxide and nitrogen.
As such, it is considered a cleaner gas that requires relatively little treatment before being used in industry and households.
QGC and a number of other companies currently produce coal seam gas from the Surat Basin in southern Queensland, one of Australia’s largest stores of coal seam gas.
Apart from increasing the availability of natural gas, extracting the coal seam gas serves another useful purpose.
Methane, which is odourless, colourless and flammable, is a serious safety risk for coal mining, particularly when miners are underground.
It is usually vented to the atmosphere as part of the mining process and, because methane is one of the most potent of greenhouse gases, it has a significant impact on global warming.
Coal seam gas extraction
Coal seam gas is extracted through wells drilled into coal seams.
When water is pumped out, 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, the gas may have to be pumped to the surface if the pressure is lower.
When coal seam gas comes to the surface, any water in the gas is separated and the gas is compressed and sent by pipeline to customers.
In southern Queensland, water from coal seams is generally stored in evaporation ponds. Companies including QGC are investigating ways to treat the water and make it available to communities affected by drought.
Coal seam gas to LNG to natural gas
QGC is proposing to take coal seam gas produced from the Surat Basin and pipe it to Gladstone. From there the gas will be fed into the proposed liquefaction plant where it will be super-cooled to create liquefied natural gas, otherwise known
as LNG.
The coal seam gas feedstock contains 98% methane and 1% nitrogen and 1% carbon dioxide. Because these small amounts of other gases are more difficult to liquefy and are not able to be burnt when the LNG is returned to natural gas, they are removed at the gas plant prior to the liquefaction process.
The LNG process reduces the natural gas to 1/600th of its original volume, allowing large quantities to be readily transported by specially-built LNG tankers to markets all
over the world.
When it is delivered to these markets it undergoes “regasification”, a process which returns the LNG to natural gas. Now it can be used as a fuel source for power generation and domestic consumption, including heating and cooking.
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