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The United States Geological Survey (USGS) recently indicated that CSG activities in the US appear to have contributed to an increase in the number of earth tremors. Back in 1970, the US midcontinent experienced about 21 events a year, but this has abruptly increased to 31 events a year around 2000 and 151 events around 2008. Before we conjure up images of toppling cities, note that a geologist defines an earthquake as any detectable rumbling in the ground, including those only recorded by sensitive instruments locked away in basements. The important point is that the USGS research showed that earth movements that are detectable by people (not just instruments) have increased along with CSG activities, and this highlights the extent of underground environmental disturbance that CSG activities can potentially cause.
Australia is well aware of concerns around CSG. On 22 March 2012, the government introduced legislation to Parliament to establish an Independent Expert Scientific Committee on Coal Seam Gas and Large Coal Mining Development. (By this time, an interim committee was already in place, having been established by Environment and Water Minister Tony Burke on 27 January 2012.) During the debate, the Australian Greens and Bob Katter, MP both proposed moratoria on new CSG proposals (of five and one year respectively) but these amendments were not passed. At the time of writing, the legislation has passed the House and is continuing its passage through the Senate. The purpose of the committee proposed in the legislation is to provide government with robust independent scientific advice regarding CSG activities. One issue that has been flagged as important are earthquakes: could CSG mining activities trigger them? And, if so, what does this mean for Australia?
The CSG industry profits from the extraction of methane trapped in pockets and fractures inside coal beds hundreds of metres underground. The methane is purified and marketed as natural gas (for more information see this Parliamentary Library Background Note). Often, a process known as fracking will be used to help release the methane by fracturing the coal beds allowing the gas to escape. The fracking processes involves pumping a pressurised mixture of sand, water and chemicals into bores that have been drilled into coal beds. The pressurised mixture builds up stresses in the earth until the rock suddenly breaks, like bending a wooden ruler slowly till it snaps. Upon failure, the energy stored in the rock is converted to sound, heat and vibrations that fracture the coal beds and may be felt on the surface as an earthquake. Earthquakes occur naturally from the release of stresses that have been built up in the earth from natural processes. From January to June 2012, a total of 228 earthquakes were recorded in Australia (about 1.5 a day). However, there is now uncertainty over whether CSG operations may increase that daily average.
According to the USGS, the fracking process generates earthquakes small enough that only the attentive will feel them. However, more serious earthquakes may be triggered not by the fracking itself, but by disposal of wastewater used to fuel the fracking process. The wastewater that is recovered from fracking operations is dirty, saline, and not fit for consumption by humans or livestock. While the wastewater can be treated, this is an expensive operation and it is often more economical to dispose of the wastewater by reinjecting it back into the earth. These reinjection points are known as disposal wells. Injection of wastewater down disposal wells widens pre-existing cracks deep in the earth. However, the danger is that this wastewater can lubricate these pre-existing cracks. This allows rocks to relieve stress by shifting their position, and an earthquake may result. A good analogy for why this happens is the act of trying to remove a stubborn lid from a jar. From the strain against friction between the lid and jar, stresses build up proportional to how much effort is put in. It is possible to strain unsuccessfully to open a lid, but if the same amount of effort is put into a lubricated lid, the jar suddenly and unexpectedly opens.
Studies in the US have suggested that localised earthquakes, in some cases up to magnitude 5.2 on the Richter scale (a measure of the energy an earthquake releases), can be induced by injection of wastewater. Magnitudes around 5 will be felt by everyone, china may break and doors may swing. Magnitudes of less than three are unnoticed or discounted as a passing truck. Note this does not translate directly to ‘damage’, because that is determined by a combination of the earthquake characteristics (energy, depth of origin, geology, shaking time, and distance from civilisation), and the nature of affected buildings (e.g. wooden structures survive better than brick and mortar).
How many earthquakes could CSG activities be responsible for? It’s hard to know exactly, but the USGS suggests that only about a dozen out of 40,000 disposal wells actually trigger significant events; more research is investigating the how, why and what can we do (on the management side) to minimise the risk.
The results of the USGS study highlight the potential risks of several CSG developments in Australia. Geological details vary from place to place, and Australia is not the same as the US, but it could still be at risk. Until recently, the reinjection of wastewater was not seriously contemplated here, but this is beginning to change. Last month, CSG company Santos released a booklet describing its research into the injection of treated waters into aquifers and trialling wastewater injection into the deep earth. While there are some benefits to the reinjection of water, such as to offset the environmental impacts of a water-hungry industry, there are now obvious implications for water quality and human-induced seismicity. The CSIRO is also researching the possibility of injecting carbon dioxide (CO2) into the coal beds to enhance methane gas extraction and, at the same time, permanently store CO2. While these activities have the potential to generate earthquakes, differences in the geology, maturity of technology, methods, and management may lead to improved outcomes. Importantly, the experiences of the US have placed Australia in a better position to prevent or minimise potentially damaging effects of CSG activities while continuing to enjoy its benefits.
*Updated 20 June 2012