Natural gas is mostly methane, although other gases such as ethane can also be present in small amounts. Natural gas formed hundreds of millions of years ago when organic material decayed and was compressed as it was covered by sand, sediment and rock. Until recently, much of the natural gas extracted in Australia came from underground basins where the gas was contained in porous sandstone and trapped there by an impermeable rock cap. It has been conventionally extracted by drilling a hole through the rock and inserting a well via which the gas comes to the surface. However, natural gas can also be trapped in coal seams, in shale and in tight sands and these all require slightly different methods, unconventional techniques, to be used to extract the gas.
Coal seam gas
Natural gas can also be trapped in coal that lies in seams hundreds of metres below the Earth’s surface. This natural gas is part of the supply used in homes on the east coast of Australia and has been since commercial production began in Queensland in 1996. Australia’s major resources of coal seam gas are in Queensland’s Bowen and Surat basins, while NSW has reserves in the Gunnedah, Gloucester and Sydney basins, and on the NSW-Queensland border in the Clarence-Moreton Basin.
The gas is extracted by a range of methods including vertical drilling, horizontal or directional drilling and hydraulic fracturing. Hydraulic fracturing, or fracking, is used when it is the most efficient method to stimulate or improve the flow of gas from formations that are difficult to access due to depth and rock composition. A review by the Independent Expert Scientific Committee on Coal Seam Gas and Large Coal Mining Development found that of the 1844 coal seam gas wells drilled in Australia over 15 months during 2012 and 2013, six per cent were subject to hydraulic fracturing.
Tight gas and shale gas
Tight gas and shale gas are also types of natural gas, the difference is that more complex technology is required to extract them. Tight gas is found in rock of low porosity with few fractures, and this means that both horizontal or directional drilling and hydraulic fracturing may be required to extract it. As yet, Australia has no commercial operations extracting tight gas which has been found onshore in Western Australia, South Australia and Victoria.
Shale gas is found in sedimentary rock of low permeability, usually between 1000 and 2000 metres underground. Australia is believed to have significant shale gas resources in the Cooper Basin in South Australia and Queensland; the Maryborough Basin in Queensland; the Perth and Canning basins in Western Australia; and the Georgina Basin and Beetaloo Basin in the Northern Territory, but they are yet to be fully assessed. Some gas is produced from shale in South Australia’s Cooper Basin.
With hydraulic fracturing, fractures in the rock are created or increased to enable the gas to escape. In Australia, this is mostly done by injecting water and sand into the coal seams. The fluid also often contains a gelling agent to help the sand stay in the seams. Benzene, toluene, ethyl-benzene and xylenes (BTEX chemicals) are banned from use in hydraulic fracturing in New South Wales and Queensland, the two States where CSG activities occur, and all chemicals used by operators are approved by State regulators (including volume, concentration and potential toxicity) before fracturing begins. Coal seams also often contain a considerable amount of water, so hydraulic fracturing means that more water is recovered than is used as the gas and water is extracted from the well. In Queensland where most CSG extraction occurs, this water is treated if required to remove any additional minerals and salts that may be present and then the water can be used for a beneficial purpose, including for irrigation.
Fugitive emissions is the name given to gas that is unintentionally released during gas exploration and extraction operations. Legislation requires that any fugitive emissions from the gas industry are reported to the Department of the Environment and the Clean Energy Regulator. Fugitive emissions total around seven per cent of all Australia’s emissions, and the gas industry accounts for less than a third of all fugitive emissions – most come from coal extraction and production. While more research into fugitive emissions from coal seam gas production is required, a pilot study undertaken by CSIRO in 2014 which measured emissions at 43 CSG wells in Queensland and New South Wales found that overall, emission rates were very low. The study examined well pads only; future studies will examine emissions from other parts of the production cycle.
Fugitive emissions may rise through the ground and disperse in the air or through water such as in rivers or ponds. This type of emission can occur naturally, and has been observed over centuries. Queensland’s Gasfields Commission has compiled a list of historical natural gas seeps that occur around the Bowen, Surat, Galilee, Georgina, Eromanga and Cooper basins, their location and the volume of the seepage.
Extraction of natural gas by unconventional means has been extensively investigated, both in Australia and overseas, in a significant number of independent studies. Some of the important inquiries in Australia have been undertaken by the NSW Chief Scientist and Engineer, the Independent Expert Scientific Committee on Coal Seam Gas and Large Coal Mining Development and the Australian Council of Learned Academies. Overwhelmingly, the inquiries both in Australia and overseas, have found that the risks of extracting gas using unconventional means can be managed though appropriate industry land use regulation, high standards of professionalism and engineering, comprehensive data collection and monitoring, appropriate workforce training and application of new technology as it becomes available. This type of monitoring regime and regulation is applied to a range of industries that operate with risks in Australia, for example, the construction industry, the health sector and manufacturing industry.
Queensland Department of Natural Resources and Minerals
Commonwealth Department of the Environment
National Industrial Chemicals Notification and Assessment Scheme
NSW Chief Scientist and Engineer
Gas Industry Social and Environmental Research Alliance
Queensland Gas Fields Commissioner
APPEA’s Shale Gas Australia website