Bob Holmes, New Scientist 15 Apr 09;
IN THE Canadian province of Alberta the ground is skinned and gutted. Rising oil prices and dwindling reserves have pushed oil companies to exploit what was once considered unexploitable: tar sands, the dirtiest oil on Earth and the most expensive to extract.
This strip-mined landscape is bad enough, but another method of extracting the oil is on the rise, and it is even more damaging to the environment. Yet new technologies offer hope that tar sands could one day be transformed into one of the cleanest fossil fuels.
The Canadian tar sands contain an estimated 170 billion barrels of recoverable oil, second only to Saudi Arabia's reserves. As the name suggests, the fuel must be separated from sand. Today, most operations dig up the tarry bitumen in gigantic open pit mines, then separate and refine it. The process destroys habitat and creates vast lakes of toxic residues. Worst of all, processing it requires large amounts of energy. The Canadian government estimates that oil from tar sands takes three to five times as much energy to produce as conventional oil.
That carbon cost is likely to get even higher. Only about 20 per cent of the tar sands lie shallow enough to be mined. If you drive an hour south of Fort McMurray in Canada's gigantic Athabasca Tar Sands, you leave behind the vast strip mines and enormous processing plants where sandy bitumen is shovelled out of the bowels of the earth and turned into oil. The boreal forest once again dominates the landscape. In a bulldozed clearing amid the spruce trees, accompanied by the gentle hum of pumps and turbines, you will find a few low, metal-clad industrial buildings surrounded by a tangle of piping.
This is ConocoPhillips's Surmont production site, the new face of the Athabasca Tar Sands. Within a few years, most plants will probably look more like this than the strip mines. At first sight, that seems a good thing, and it is, if you live nearby. But Surmont's benign appearance belies the fact that from a global perspective, it is even worse than the strip mines.
The facility pumps huge amounts of steam at 305 °C several hundred metres underground to separate fuel from sand in situ, a process called steam-assisted gravity drainage (SAGD). At room temperature the bitumen is more viscous than peanut butter, but after several months of heating it becomes as runny as heavy cream and can be pumped to the surface.
Generating all that steam takes energy - more than mining, says Simon Dyer of the Pembina Institute, an environmental group based in Calgary, Alberta. In fact, if you include the energy needed to upgrade and refine the fuel, every three barrels of oil extracted with SAGD consumes nearly a whole extra barrel, says Pedro Pereira, a chemical engineer at the University of Calgary.
Matt Fox, who manages oil sands operations for ConocoPhillips, is optimistic that improvements can be made as engineers gain more experience with SAGD, which has only been in commercial use for eight years. But he admits that the process will always consume more energy than pumping conventional oil. "The oil sands have got nowhere to hide on the greenhouse gas issue," says Fox. "We have a major challenge we have to address."
There's hope on the horizon. Nascent technologies may further reduce the greenhouse gas cost of tar sands extraction. Pereira is leading a team that is trying to transform the bitumen into lighter oil underground, before it is pumped to the surface. The process requires higher temperatures than SAGD, but compensates by saving more energy further down the line when the fuel is processed. And many of the toxic sulphur and nitrogen compounds remain underground, which removes the surface pollution.
So far, the team has developed a catalyst for use in the underground deposits and is working on proving the concept in the lab. Eventually, they hope to complete most of the refining process underground, yielding usable oil at a carbon cost comparable to conventional crude oil.
A much cleaner and truly inventive solution, though, can be found in the laboratory of Stephen Larter, also at the University of Calgary. Together with colleagues at Newcastle University, UK, he thinks it might be possible to induce bacteria to digest the bitumen into methane, which could be extracted just like conventional natural gas. The process occurs naturally underground: bacteria called Syntrophus digest the oil and release hydrogen as a waste product. A second group of archaebacteria called methanogens then converts the hydrogen to methane. "They're already there. All you do is add fertiliser," says Larter.
Last year, Larter's team showed that by adding nitrogen, phosphorus, vitamins and trace minerals to oil-field samples in the lab, they can convert the hydrocarbons to methane in about 600 days (Nature, DOI: 10.1038/nature06484). Their newly founded company, Profero Energy, is about to begin field tests to see whether the method can produce methane economically from a natural underground reservoir. "If it's an economic success, it is a game-changer. You have suddenly got a huge amount of a relatively clean fuel, methane," says Larter. "But it has absolutely not been proven in the field yet."
Even if Larter's microbial conversion proves practical, of course, no amount of technological trickery can get around the fact that burning any fossil fuel adds carbon dioxide to the atmosphere. But, "the oil industry is not going to disappear overnight", says Larter. "Cleaning up the tail end of the industry is a crucial thing, otherwise we'll have really serious problems." Burning methane provides more useable energy per tonne of CO2 emitted than any other fossil fuel (see chart), making it the best choice for weaning society off fossil fuels altogether.
In the long run, Larter notes, a similar process could someday offer the ultimate in clean energy: a hydrogen economy. Methanogens are very efficient at using up the hydrogen Syntrophus produces, so it does not last long enough to tap. But if the researchers could find a way of inhibiting the methanogens, their system would offer the possibility of producing immense quantities of hydrogen, a zero-carbon fuel.
