Pollution: Shale gas vs Coal

Shale gas: more or less polluting than coal

Research into the extent of leakage during the fracking process could seriously dent claims that shale gas is a relatively 'clean' fossil fuel. But the evidence still isn't clear. More than two years after one study called shale gas "more polluting than coal", academics are still wrangling over the fuel's impact on the climate.

Escaping gas 

Some academics argue that gas leaks during the process of extracting shale gas from rock - known as fracking - could make the fuel far more climate-polluting than its supporters claim. 

Natural gas releases about half the carbon emissions that coal does when burnt. But some gas escapes into the atmosphere while wells are being built and during the fracking process itself. These so-called fugitive emissions are primarily made up of methane - which is approximately 25 times more climate-polluting than carbon dioxide over a 100-year timescale. 

Leakage occurs when gas is extracted by conventional methods as well - but the fear is that it's a bigger problem when fracking is used to extract shale gas. American lobby group the Environmental Defence Fund (EDF) published a study concluding that if more than 3.2 per cent of natural gas leaks out during the production and transportation of shale gas, then it's more polluting than coal. 

More polluting than coal?

Two years ago, Cornell University released a study arguing that 3.6 to 7.9 per cent of the methane from shale gas production escapes into the atmosphere - meaning that shale gas could be more environmentally polluting than coal. 

But the study's research methodology was criticised as " seriously flawed" by another set of researchers - who argued that it over-estimated the fugitive emissions from shale gas and undervalued the potential for new technologies to reduce those emissions. The critique also said Cornell's study inappropriately based its comparison of coal and gas fuels on heat rather than electricity generation, and measured the climate impact of the two fuels over too long a time period. At one seminar Carbon Brief attended, the speakers suggested the study's main advantage was it was so bad it prompted others to look at the same question. 

Unswayed by the criticism, the Cornell researchers released another paper in 2012, defending their methodology. They said their analysis covered both electricity and heat and it was appropriate to assess greenhouse gas emissions over a long, 100-year timescale. 

Enter NOAA   

Back in 2011, the main problem in the debate over emissions from shale gas seemed to be a dearth of actual data on fugitive emissions. Hypothetical modelling studies - including one from the European Commission - appear to mostly agree that fugitive emissions aren't that much of a problem. Counting up the methane emissions from individual shale gas wells is however rather a labour-intensive business, so there just weren't that many numbers out there. 

In 2012, data started to emerge. Researchers at the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado suggested that one shale gas well was losing about four per cent of its gas to the atmosphere. This was far higher than official estimates and roughly in line with the Cornell study.

But again, the research was criticised. In a peer reviewed piece, energy Professor Michael Levi dismissed the study as "great data; wrong interpretation". Professor Levi said the calculations used to convert the observations of gas in the atmosphere into gas leakage were flawed. 

In January of this year, the same set of researchers released new results. One shale gas field in Colorado was, they claimed, leaking an "eye popping" nine per cent of its methane into the atmosphere. And on one day in February, another field in Utah's Unitah Basin leaked six to 12 per cent. 

Scaling up?

The NOAA studies have prompted a storm of commentary from concerned environmental campaigners. But they're not all convinced - Washington NGO the Environmental Defence Fund (EDF) says that the results from individual sites can't be scaled up to draw general conclusions: 

"While the Colorado and Utah studies offer valuable snapshots of a specific place on a specific day, neither is a systematic measurement across geographies and extended time periods  and that is what's necessary to accurately scope the dimensions of the fugitive methane problem. For this reason, conclusions should not be drawn about total leakage based on these preliminary, localized reports." 

Dr Colm Sweeny tells Carbon Brief that he agrees with EDF's critique "to a certain extent":

"Our point was that our estimate was much higher than the inventory estimates for this region … Our study provides a very nice example of how direct measurements of atmospheric measurements can be used to verify inventory estimates which use a variety of assumptions to estimate leakage over a wide range of basins where rock formations, drilling practices, regulatory environments and oil and gas composition vary tremendously."

Dr Sweeny also points out that his study reports on the total emissions from a basin where fracking is being used. But it doesn't identify what part of the process - fracking, liquid unloading, transferring or compressing the gas - are contributing to the high emissions. 

EDF has launched a multi-part study - part-funded by industry - to look at the issue as a whole. Campaigning website desmogblog points out, however, that it's long-delayed.

Not the whole story 

If NOAA is right, then shale gas could be more polluting than coal. But there is now other research out there that disagrees - for example a peer-reviewed study part-funded by ExxonMobil used a life cycle assessment of shale gas to conclude that the carbon footprint is shale gas is 53 per cent lower than coal. 

So - despite claims on both sides - the question remains unresolved, and is likely to remain so until more data emerges. In this country, the chief scientific advisor to the Department for Energy and Climate Change is currently leading a study on the issue, according to the Telegraph.

It's also important to remember that this debate isn't the whole story. Whoever's right about fugitive emissions, the climate-impact of shale gas is also affected by a nest of other factors - whether it will out-compete coal or renewables, for example, or whether carbon capture and storage technology is ever developed enough on a global scale to capture the emissions from burning natural gas. As different countries develop a shale gas industry, these questions will become clearer. In the meantime, the academic argument over fugitive emissions looks set to continue.

By Robin Webster.  Our thanks to Carbon Brief