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Natural gas locked up in water crystals could be a source of enormous amounts of energy – and if a new technology delivers what scientists are claiming, then it could even be emissions-free too.

To the naked eye, clathrate hydrate looks like regular ice. However, while it is made up partly of water, the water molecules are organised into "cages", which trap individual molecules of methane inside them.

Compared to other fossil fuels, methane – also known as natural gas – releases less carbon dioxide per unit of energy generated. Nevertheless, burning it still releases carbon dioxide and thus drives climate change.

However, according to research presented this week at the national meeting of the American Chemical Society, a new method of extracting the methane could effectively make it a carbon-neutral fossil fuel.
'Bridging fuel'

Due to their physical structure, clathrate hydrate cages "prefer" to have carbon dioxide at their cores, so if carbon dioxide is pumped into the hydrate, it spontaneously takes the methane's place. As a result, it should be possible to simultaneously extract methane and store carbon dioxide.

"Methane from hydrate could be a bridging fuel, to lead towards more renewable energies," says Tim Collett of the United States Geological Survey.

According to the results in Collett's presentation, the exchange process has been shown to work in the lab. Pumping carbon dioxide into rock cores containing hydrate successfully released the methane, and stored the carbon dioxide.

The US Department of Energy is now working with the oil company ConocoPhillips on a field trial in Alaska (pdf), to test whether the technique can be scaled up.
Rival technology

Previous attempts to obtain methane by heating up the hydrate were not effective, but pumping fluids out of the hydrate to release the pressure does release the methane. To be put into commercial practice, it is likely that the carbon storage method will need to outcompete this depressurisation technique.

Deborah Hutchinson of the USGS says that the technique "could make it possible to sequester CO2."

Natural gas normally contains a percentage of CO2, which under industry regulations must be pumped back into the gas wells when it is extracted.

"The first CO2 to be utilised in this [new] methodology would be the CO2 'cleaned' from raw natural gas produced in nearby wells," says Hutchinson. In other words, the CO2 sequestered in the extraction of methane in ice will most likely be the stuff separated from conventional gas reservoirs.

Globally, there are thought to be from 1015 to 1017 cubic metres of methane stored in hydrates – a vast store, large quantities of which should be recoverable.
'Limited sequestration'

Much of it is in sediments just below the sea floor, or trapped under permafrost. Some of the best-studied reservoirs are in Alaska, and beneath the Gulf of Mexico and the Sea of Japan.

The deposits on the North Slope of Alaska are among the richest. A 2008 USGS study showed that there are 2.4 trillion cubic metres (85 trillion cubic feet) of methane in hydrate form, which could be recovered using existing technology.

The US, Canada, Japan and Korea are all looking into clathrate hydrates as a possible energy source.

"A lot of countries are getting very serious about this," says Ray Boswell of the US National Energy Technology Laboratory. "Something that used to be more hype than reality is becoming something people are seriously talking about."

Bahman Tohidi of Heriot-Watt University's Institute of Petroleum Engineering says the Alaska trial is "a step in the right direction", but that the potential for sequestering carbon would be limited by the remote locations of the hydrate reservoirs. "You're talking about long distance CO2 transport," he says.

Neil Crumpton of UK environmental campaign group Friends of the Earth is sceptical. "It's a technology we think is best avoided. The US should be focusing its efforts on concentrated solar power in its southwestern deserts."
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