Shale gas, an energy source enjoying a boom in North America and Europe, carries a greater carbon footprint than oil, coal and conventional gas over at least a 20-year period, according to a study released on April 12.
Scientists led by Robert Howarth from New York's Cornell University looked at greenhouse-gas emissions from the extraction of shale gas in the US, drawing on data from the oil and gas industry and from a federal auditing agency, the General Accountability Office (GAO).
Shale gas holes up in a dense sedimentary rock which is fractured by large volumes of water and chemicals that are piped in horizontally under high pressure.
After the fracturing, large amounts of water returns to the surface within a few days, along with significant amounts of methane, which comprises the bulk of the shale gas.
The problem, though, is that methane is a potent greenhouse gas as well as a fuel, and large amounts of it leak from shale-gas extraction, said the study. During the life cycle of an average shale-gas well, between 3.6 percent and 7.9 percent of the well's total production is emitted to the atmosphere as methane, it said.
This comes through routine venting, equipment leaks and emissions that are included in the water flowback.
These leaks are at least 30 percent more than-and perhaps more than twice as great as-those from conventional gas, although their emissions from routine production and downstream operations are the same, it said. Methane is more than 20 times more efficient than carbon dioxide (CO2), the principal greenhouse gas emitted by fossil fuels, for trapping solar heat.
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