Published online 12 December 2007 | Nature | doi:10.1038/news.2007.375

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Oil-eating bacteria make light work of heavy fuel

Unpicking the route by which microbes produce methane could help to boost the process.

Getting usable fuel out of the heavy oil of Canada's tar fields takes a lot of energy.Ian M. Head

Researchers have worked out how natural bacteria deep within the Earth break down crude oil and produce methane. This knowledge could help with projects to encourage these bacteria to covert more oil, faster. And it could point towards a way to produce hydrogen — an even cleaner fuel — by using these natural fuel-processing plants.

Microbes living on the crude oil in petroleum reservoirs usually start by biodegrading the simpler oil fractions, leaving behind a sticky residue called 'heavy oil'. They will then start breaking down this heavier substance too, all the while producing methane as a product.

There are around six trillion barrels of heavy oil across the globe, lurking beneath the Earth’s surface, and it causes headaches for oil companies. "To get heavy oil out you’ve basically got to melt it," says Steve Larter, a petroleum geochemist at the University of Calgary in Alberta, Canada. This means using energy to produce steam to extract the gunk, he says. "It’s like turning gold into lead." Only 17% of the oil can usually be recovered.

Methane gas, on the other hand, simply rises to the surface. But it has been unclear how methane is produced by microbes in heavy-oil fields.

The first type of bacteria to attack crude oil can break down the long-chain hydrocarbons into acetic acid, carbon dioxide and hydrogen. In the second step of biodegradation, one set of microbes can turn acetic acid to methane, and another set acts on carbon dioxide and hydrogen to produce methane.

The bacterial way

To work out which process dominates in an oil field, Larter and his colleagues, recreated methanogenesis in the lab. They sealed different samples of oil from the Gullfaks field in the North Sea in glass jars, and watched them over a couple of years. They analysed the isotopic ratios of the methane produced and compared that to the methane produced by the actual oil field, and discovered that in the field the hydrogen route dominates. They think this to be true of all heavy oil fields.

“Why not just speed up the natural process by lobbing in some fertilizer?”

Steve Larter

Larter and his team have been working with oil companies to try and take advantage of their new-found knowledge. The micro-organisms in the subsurface reservoirs have plenty to break down, but not enough key nutrients such as phosphorous and trace elements. This is why the bacteria take so long to break down the oil. To recover more energy from the oilfields, more efficiently, Larter suggests encouraging the bacteria to biodegrade the oil more quickly and then collect the methane they produce. "We’ve got a process that naturally turns oil into natural gas," he says. "Why not just speed up the natural process by lobbing in some fertilizer?"

Larter estimates that methane recovery could convert at least 20% of the heavy oil to methane. His team is hoping to run some field tests in 2009.

Gas collectors

The practicalities of such a methane-recovery scheme, however, are not straightforward. "How would you ever collect the methane?" asks Robert Burruss, a geologist at the US Geological Survey.

Should the problem of gas collection be solved, Larter's team and Burruss both advocate an even more ambitious goal than collecting methane. "If you could stop the [hydrogen-using] methanogens, and speed up the other guys [the acetic acid-processing bacteria], you could get gas enriched in molecular hydrogen." Hydrogen has long been touted as a clean fuel.

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To get this to work will take a lot of extra research to understand how to manipulate the bacterial process, says Larter.

Burruss thinks an additional scheme might have merit: "perhaps CO2 could be added from an external source to be converted to methane," he says. "If that could be done at the scale of a large depleted oil reservoir, then one could inject CO2, have the bacteria make methane, and have a renewable source of natural gas. That could be an interesting new aspect of geological sequestration of CO2." 

  • References

    1. Jones, D. M. et al. Nature doi: 10.1038/nature06484 (12 December 2007)
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