Going underground to go green

Carbon emissions are one of the hottest topics in science and politics today – but with an addiction to fossil fuels that looks unlikely to wain – could it be that we need to put carbon back where it came from in order to keep it away from the atmosphere?

CO2 is captured from Statoil's Sleipner Project in the North Sea and injected back underground

Going green has never been so prominent in the life of the western world. From government policy to personal recycling habits – we have never been so focussed on the damage that we are doing to the planet. And while some think that the green agenda has been high-jacked and turned into political rhetoric for the power mongers of government and big business – the fact remains that our climate is changing, and so it seems, must our reliance on fossil fuels.

Or must it? If you subscribe to the theory – and let’s remember that it is still a theory – that rising levels of CO₂ are the root cause of global warming, then it follows that if you could find a way of reducing the CO₂ produced by burning fossil fuels - then we can burn away to our hearts content. Intuitively this might feel like a quick fix, and many would say that is exactly what it is. But a quick fix is better than no fix at all, and if rising CO₂ levels are the cause of a changing climate, then maybe we don’t have the time to wait for humanity to wean its self off oil.

The question then becomes – what to do with the CO^₂ that would othe

"Humanity is unlikely to break its addiction to fossil fuels anytime soon, so reducing the amount of CO2 released into the atmosphere due to that addiction is becoming an ever more attractive option."

rwise be churned out into the atmosphere? The answer, according to a growing number of scientists, is lurking right below our feet.

Geologic sequestration is the process of injecting CO₂ directly into underground geological formations for the purpose of storing it and keeping it away from the atmosphere. This is not a new idea – CO₂ has been injected into declining oil fields for more than 30 years to increase oil recovery. However, using CO₂ to gain yet more fossil fuel is not the soundest ecological use of the technology, despite the fact that it might be the most pragmatic.

The overriding problems of geo-sequestration have been leakage and location. Pumping CO₂ into disused oil fields may be well trodden technology, but doing so presumes that the geological barrier preventing the upward migration of the oil – common to the majority of oil fields – will also prevent the leakage of CO₂ back into the atmosphere. In many cases this is a presumption too far, and leaking CO₂ is something that environmental groups are particularly nervous about. The other problem that has dogged carbon sequestration is the fact that oil fields are just not very common – so the chances of having one near an industrial development that wished to sequester its CO₂ is unlikely.

Ecologists argue that what is needed is a ubiquitous way of storing CO₂, long term, without any real effect on the environment. A tough ask, but one that Dr Ruben Juanes of the Massachusetts Institute of Technology thinks he may have an answer for.

In a recent paper published in Water Resources Research he claims that saltwater aquifers - underground layers of water-bearing permeable rock – hold the key to leak-free CO₂ storage.

According to Juanes’ work, it is the porous nature of the aquifers that makes them such good sequesters of CO₂. When injected into the aquifer, CO₂ forms a plume and starts to rise through the rock. Because the rock minerals have an affinity for water, the water will coat the crevices and pore walls of the rock in a thin film, then when injection stops, the water closes up at the bottom of the plume, turning it into what is essentially a long bubble. The water films in the rock pores swell, eventually touching and closing off the carbon dioxide’s escape.

The fact that CO₂ sequestration relies on a bubble may seem disconcertingly unstable given the apparent importance of reducing the CO₂ in our atmosphere. However, Juanes is confident that the mechanism he has described will be effective enough to prevent the gas from rising and escaping the aquifers.   

He explained to Laboratory News: “The main concern in any sequestration project is the leak of the CO₂ back to the atmosphere. This can happen if fractures develop in the impermeable barrier or if abandoned wells are present. This risk is exacerbated if the CO₂ is present in its own mobile phase. In this case, the amounts that can be released are much larger, and the time scales for such a release much shorter. If, on the other hand, the CO₂ phase is trapped within the porous medium, it remains immobile and slowly dissolves into the brine. The storage of CO₂ is then subject to the residence times of deep aquifers, which can be as large as thousands or tens of thousands of years.”

He added: “The novelty lies in incorporating the mechanism of trapping of the CO₂ while it migrates upwards due to buoyancy. This mechanism is known from the pore-scale physics of multiphase flow. We argue that leaking is unlikely if most of the CO₂ is trapped in the porous medium, rather than present as a mobile phase.”

Combined with the fact that saline aquifers are fairly ubiquitous, Juanes’ theory could well prove to be a useful tool in the battle against atmospheric CO₂.
 
Of course, the technology is aimed at large industry and power plant emissions rather than small scale domestic use – and this is where Juanes hopes it could make the most difference, even if it is viewed as a stop-gap solution until fossil fuel consumption is replaced by renewable energy solutions.

Indeed, the technique has been used for the past 10 years at Statoil’s Sleipner Project in the North Sea. Here, CO₂ separated from the project’s natural gas production stream is captured and injected back into the permeable rock beneath the seabed.

The more cynical environmentalists would say that this is merely a token offering from one of the world’s largest sellers of crude oil, but the fact remains that the technique prevents the release of over 1 million tonnes of CO₂ a year into the atmosphere.

So, will we see the power plants of the future plumbed into the nearest aquifer?

“I believe that geological sequestration may play a role in the mitigation of atmospheric emissions of carbon dioxide and possibly other toxic or greenhouse gases,” said Juanes. “The main barrier is not really technical - CO₂ injection has been used in the oil and gas industry for a long time - but economical. A regulatory structure must be in place for CO₂ sequestration to be pursued vigorously. This will most probably require that the governments impose some form of carbon emission tax.”

This is something that in the current socio-political landscape might not be so far fetched. In this country, the Government has unveiled plans to set a legally binding target to cut carbon emissions by 60% by 2050. Part of the draft Climate Change Bill seeks to introduce carbon “budgets”, capping emissions of CO₂ to set levels.

In a statement, Environment Secretary, David Miliband said: “The debate on climate change has shifted from whether we need to act to how much we need to do by when, and the economic implications of doing so.”

Encouraging, albeit very vague, words – but it is hard to imagine how the government hopes to achieve this without sequestering carbon emissions from fossil fuel burning power plants. And this is something that has been picked up by The Tyndall Centre – a multidisciplinary research centre working to develop sustainable responses to climate change. In January last year they released a report - “An assessment of carbon sequestration in the UK” – in which they estimated the storage potential of the Bunter Sandstone Formation, a saltwater aquifer located in the southern North Sea, to be 2811 million tones of CO₂.

This is enough to have a real impact on the country’s carbon emissions. Indeed the authors go on to conclude that “there is huge potential for CO₂ storage in the southern North Sea, in both depleted gas fields and the Bunter Sandstone Formation.”

However, there was a problem. According to the Protocol to the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter – an international convention that governs the dumping of wastes at sea – storage of CO₂ under the seabed was prohibited. This was a real barrier to the development of carbon geo-sequestration and, as suggested by Juanes, it was an economical rather than technical one.

All that changed in February this year with the introduction of an amendment to that all important protocol. It means that a basis has been created in international environmental law to regulate carbon capture and storage in sub-seabed geological formations, for permanent isolation – exactly what the experts had been calling for.

This then, is perhaps the opening of a new front in the war against carbon emissions. In theory it looks good – the technology is there and the regulatory basis is in place - in practice, geo-sequestration has to go hand in hand with other methods of reducing atmospheric CO₂.

“Rather than comparing carbon sequestration with other alternatives, what I would say is that all of them at this point in time should be investigated,” said Juanes.

It may be a quick-fix, but sequestration of CO₂ in saltwater aquifers represents one of the best techniques currently available to reduce carbon emissions. Humanity is unlikely to break its addiction to fossil fuels anytime soon, so reducing the amount of CO₂ released into the atmosphere due to that addiction is becoming an ever more attractive option.

It seems that, for know at least, to go green we need to go underground. 

Phil Prime
Phil is assistant editor of Laboratory News. He holds a degree in Neuroscience from Nottingham University and an NCTJ in journalism from City College, Brighton.