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High Levels of Methane Around Coal Seam Gas Fields

Southern Cross University academics have reported unusually high levels of methane around coal seam gas fields in northern NSW and the Tara gas fields in southern Queensland.

“A team from Southern Cross University recently reported high concentrations of methane in the Tara basin CSG field. I have seen these data and the methods that underlie them. The data themselves are very solid; this is state-of-the-art equipment and contamination is very unlikely, especially given the large spikes they observed.

Concentrations, however, are not what we're really interested in. One can get high concentrations near even small leaks, especially in an enclosed space. We are really interested in emissions.

Here the task becomes harder since we need to know how fast an emission dilutes into the atmosphere then back-calculate the emission from the measured concentration...To do it we run a computer model of the atmosphere like those that calculate pollution in cities. The model accounts for the movement of pollutants by the winds and the problem of trapping pollutants under inversion layers.

The Tara basin measurements are a tough challenge since they were taken at night and under calm conditions. These conditions usually produce high concentrations for a given emission.

Our team at the University of Melbourne hasn't modelled the Tara measurements yet but we have modelled another gas field under somewhat similar conditions and made a simple extrapolation. If we believe the extrapolation, the Tara emissions are several thousand tonnes of methane each year. Of course we don't trust the extrapolation but it does tell us the effect is big enough to test.”

Dr Peter Rayner is Australian Professorial Fellow at The University of Melbourne.

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“This a preliminary study that clearly indicates the need for further investigation on the source of the high emissions observed in this one-off spatial survey. Scientifically, I would be careful to point at the CSG wells as the culprit straight away. The problem is that atmospheric studies are really difficult due to the high variability and different boundary conditions that could influence any observation. These are a spatial snapshot observation, admittedly with a lot of observations, but it is only one single snapshot. So the data and study is very valuable as a preliminary study and points at the need for further investigation.”

Associate Professor Willem Vervoort is Senior Lecturer in the Faculty of Agriculture and Environment at The University of Sydney.

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“My first impression is that people need to be very cautious about this result. It is a common fallacy that oil and gas leaks are entirely man-made. In reality, far more oil and gas is leaked from the earth naturally at hydrocarbon seeps than is leaked by hydrocarbon exploration and production activities. Indeed, such natural seeps are very common, particularly near shallow hydrocarbon sources, and are routinely used to explore for hydrocarbons. Because of this, it is extremely common to have oil and gas fields close to natural seeps. This makes the source of high levels of methane in the air difficult to determine.

CSG is a very shallow hydrocarbon source, and thus there is a higher likelihood of natural seepage of gases. Hence, it is hard to demonstrate the cause of high levels of such gases without baseline analysis and detailed repeated measurements over time. This is particularly the case for CSG, in which large amounts of pressure depletion are required to produce the gas. This subsurface pressure is partially what drives oil and gas seeps, and it has been proven in many places that pressure depletion from oil and gas production usually causes emissions from natural seeps to decrease.

In short, the common proximity of natural seeps and producing hydrocarbon fields makes it very hard to conclusively demonstrate increased fugitive ground emissions caused by hydrocarbon production. What this really highlights is the need for industry baseline readings of ground water and air quality before production. Without baseline readings, it is very difficult to distinguish natural emissions from human-triggered emissions.”

Professor Mark Tingay is Senior Lecturer at the Australian School of Petroleum Science at the University of Adelaide.

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“Having visited SCU last week and met the researchers involved, I am very impressed with this research. I believe this is crucial research which should have been part of the environmental assessment process and ongoing monitoring. The fact that there is no pre-CSG data on methane levels in air means these researchers have had to do some cross-checking of possible explanations, such as methane from cows or wetlands, and it seems to me they have been very careful in ensuring that the only realistic explanation for the considerable methane levels they measured is CSG field leakage.

At present, there is considerable more work to do to understand the processes causing this degree of leakage – it could be caused by leaking bores or pipelines, or just be diffuse leakage from the geology due to lower groundwater pressures – and especially to quantify diffuse emissions on a life cycle basis for CSG production, but I certainly view this research as a great breakthrough in documenting the real processes occurring in CSG fields.”

Dr Gavin Mudd is Senior Lecturer in the Department of Civil Engineering, Monash University.

Source: AusSMC