Shale gas has a greater greenhouse gas footprint than fossil fuels over a 20-year period, according to new research.
Natural gas extracted from shale formations produces more methane emissions than conventional gas, oil and coal, according to Robert Howarth and colleagues at Cornell University in New York.
Their research, published in Springer's journal Climatic Change Letters, questions the logic of using shale gas as a climate-friendly alternative to fossil fuels.
"The large greenhouse gas footprint of shale gas undercuts the logic of its use as a bridging fuel over coming decades, if the goal is to reduce global warming," Howarth said.
Shale gas has become an increasingly important source of natural gas over the past decade.
The greenhouse gas footprint of natural gas was evaluated by Howarth and his team by obtaining high-volume hydraulic fracturing of shale formations and focusing on methane emissions.
Data analysed included the technical background document on greenhouse gas emissions from the oil and gas industry (EPA 2010) and a report natural gas losses on federal lands from the General Accountability Office (GAO 2010).
They calculated that during the life cycle of an average shale-gas well, between four to eight per cent of the total production of the well is emitted to the atmosphere as methane through routine venting and equipment leaks, as well as with flow-back return fluids during drill out following the fracturing of the shale formations.
Routine production and downstream methane emissions are also large, but comparable to those of conventional gas.
While methane is a far more potent greenhouse gas than carbon dioxide, it has a 10-fold shorter residence time in the atmosphere meaning its effect on global warming falls more rapidly.
Methane dominates the greenhouse gas footprint for shale gas on a 20 year horizon, contributing up to three times more than direct carbon dioxide emission.
The footprint for shale gas at this time-scale is at least 20 per cent greater than that for coal, and perhaps twice as great.
"The full greenhouse gas footprint should be used in planning for alternative energy futures that adequately consider global climate change," Howarth said.
Read the Climatic Change Journal online