Environ. Res. Lett. 6, 044008 (2011)

Extracting shale gas trapped in rocks by hydraulic fracturing, or fracking, produces just 11% more greenhouse-gas pollution than drilling for natural gas in wells, according to an analysis by Nathan Hultman, of the University of Maryland, and his colleagues. The finding comes after another study placed the carbon footprint of shale gas alongside that of coal.

The greenhouse-gas content of the differently located gases is the same — both are mainly methane and both extraction processes leak some of this gas. However, drawing up shale gas involves injecting high-pressure fluids to force the gases embedded in rock to diffuse into fractures, which requires slightly more energy and gas leakage. Hultman's group calculated the leakage rates using individual data provided by the US Environmental Protection Agency for more than 400,000 wells, but they acknowledge that methane leakage estimates for shale-gas production are 'uncertain'.

In a life-cycle analysis, the per kilowatt greenhouse-gas impacts of shale gas are 56% of those of coal when both are used in electricity generation for the US grid, the authors calculate.