Natural gas was formed hundreds of millions of years ago from decomposing microrganisms, plants and animals that was gradually covered by sand, sediment and rock. As it slowly compressed, it was exposed to increasing temperatures which caused its molecules to break down thus producing methane, which is carbon and hydrogen, and is the major component of natural gas. Large volumes of gas can be trapped in the rock in sedimentary basins which are usually composed of porous sandstone capped by impermeable rock. Australia has a number of large areas where gas is found, both onshore, such as in the Cooper Basin in central Australia, and offshore such as the Gippsland Basin off Victoria and the North-west Shelf off Western Australia. Natural gas is also found in shale, coal seams and tight sand. Gas found in these rocks is usually referred to as unconventional gas because the methods used to extract it are different to those used when the gas is in sandstone.
Once exploration has revealed the likely presence of gas, a well is drilled through the earth and rock to reach the basin. If the gas is in sandstone, the well is often drilled vertically. Wells to extract gas held in shale or tight sand require horizontal drilling as well, and gas in coal seams may be extracted by hydraulic fracturing of the rock, but this is becoming less common in Australia. Wells are cased with steel pipes to ensure a single pathway for the gas to come to the surface. The gas is released via a tube inserted into the well casing and flow is regulated by valves and fittings on the surface. This assembly is connected to holding tanks where any water is removed from the gas. The gas is then sent to a processing facility—in Australia, the processing facility is often nearby, collecting gas and oil from all the wells in the field. For instance, around 5600km of pipelines and flowlines carry gas from around 700 gas wells and 360 oil wells to the processing facility in Moomba in South Australia’s Cooper Basin.
Gas processing facilities
The natural gas used in Australian homes is almost entirely composed of methane, but the gas that comes out of the well also contains some other compounds and these are separated out in the processing facility.
Gas is often found with oil or mixed in with condensate, and if so, these two components are removed first, often by using the force of gravity in an enclosed tank to separate the light gas from the heavier oil and condensates. The oil and condensates are also valuable and these are sent for further processing. The gas that has been separated typically contains other hydrocarbons, such as propane and butane, which are also separated out. For instance, the Moomba processing facility removes the ethane from natural gas and sends it via a dedicated 1160km pipeline to the ethylene plant at Botany in Sydney where it is used as a feedstock in the manufacture of polyethylene.
After the other compounds have been removed from it, the natural gas is ready for transport to large consumers of gas such as energy companies, or manufacturing industry that uses gas as a feedstock or for manufacturing processes such as high-temperature firing of building products.
Gas transmission pipelines
Australia’s 35,000km of gas transmission pipelines carry all of the natural gas used in Australia. These pipelines form a major network on the east coast (SA, Queensland, NSW, Victoria and Tasmania) and cover vast distances in Western Australia and the Northern Territory.
Transmission pipelines often have a wider diameter than gathering (pipelines in the field) or distribution (pipelines in cities) pipelines — typically 300mm or more – and in Australia they are made of steel. The gas is pressurised for transport, usually from 10,000 kilopascals to 15,000 kilopascals or almost 150 times greater than atmospheric pressure at sea level. The pressure is maintained by using compressor stations along the route of the transmission pipeline. Transmission pipelines in Australia are buried underground to increase safety.
Some transmission pipelines deliver directly to end users such as manufacturing facilities or electricity generators and others deliver the gas to an energy distribution company which supplies gas to retail customers including businesses and households. The gas from the transmission pipeline is depressurised before it enters the distribution system which uses a lower pressure throughout the system. In a large city like Sydney or Melbourne, the natural gas distribution system could be made up of thousands of kilometres of pipelines criss-crossing the city underground.
Natural gas can be converted to liquefied natural gas (LNG) and this is the process used when gas is exported. Transmission pipelines carry the gas to an LNG facility where it is cooled to minus 161 degrees Celsius, a process which also reduces its volume by about 600 times. It can then be loaded into LNG tanker ships and sent overseas. It is delivered to a regasification facility where it is heated so that it is returned to a gaseous state and can be distributed.