Original article by Andy Chadwick, British Geological Survey In this article the principles and practicalities of geosequestration are described. Is this a viable means of producing ‘clean power’ from traditional fossil fuel sources? The example of Utsira gas field in Norway is providing lessons for further use of this technological solution.

Introduction

The most viable short- and medium-term carbon dioxide reduction strategy could be the development of ‘clean' power generation from fossil fuel sources. A more complete strategy involves moving towards a so-called ‘hydrogen economy'. This is where naturally occurring carbon-rich fossil fuels are transformed into hydrogen, a pollution-free fuel, suitable for power generation and many of our transport needs. The work I describe here involves exploring a way to remove the carbon by-products of such processes and to store rather than release them into the atmosphere.

The Sleipner rig in the North Sea, where the Norwegian oil company Statoil are storing Carbon dioxide underground as part of a major geological sequestration research project.

Sections in this article

• Storing carbon in rock

• Ongoing research

• Some of the results so far

• How much can be stored?

• Future potential elsewhere

• Research outside the UK

• Studying leakage

• Conclusions from the project

• Where Next?

Storing carbon in rock

The key requirement of any clean power generation or ‘feeder' to a hydrogen-based industrial economy, is the ability to remove (sequester) carbon from the fossil fuel and store it so that it remains isolated from the atmosphere for a long period of time. This can be accomplished by pumping carbon dioxide underground, in such a way that it becomes trapped in the pore spaces between grains of sedimentary rocks in exactly the same way that hydrocarbons are trapped in oil and gas fields.

The technique offers the opportunity to remove quantifiable, monitorable and ultimately secure amounts of carbon dioxide (CO₂) to a non-atmospheric sink, using technologies which are both currently available and constantly improving. It has the potential to completely isolate the CO₂ from the atmosphere in the long-term and is particularly suited to the cleansing of emissions from large industrial sources.

Ongoing research

Britain is very much at the forefront in developing this line of research. The British Geological Survey (BGS) is leading or participating in a number of major international projects aimed at testing and verifying the long-term viability of underground carbon sequestration.

A major subsurface carbon dioxide sequestration project is currently underway in the North Sea under the direction of the Norwegian oil company Statoil, with a similar operation planned for the Barents Sea. Natural gas from the Sleipner West gas field contains about 9% natural carbon dioxide. After separation from the methane content, this would normally be vented to the atmosphere.

At Sleipner it is injected into a deep saline aquifer (a large volume of highly porous and permeable sand, the pore spaces of which are filled with unusable salty water), called the Utsira Sand, which lies about 1000 metres beneath the seabed. The injection operation started in October 1996, with over 3 million tonnes of carbon dioxide injected since then.

A demonstration project, jointly funded by the EU, industry and national governments, is currently evaluating the geological aspects of the subsurface disposal operation. The project is assessing the capacity, storage properties and performance of the Utsira reservoir, modelling carbon dioxide migration within the reservoir, and monitoring the subsurface dispersal of the carbon dioxide using sophisticated time-lapse seismic techniques.

Some of the results so far

Spectacular seismic images (shown in the diagram below) of the injected carbon dioxide have been obtained showing that the carbon dioxide is currently trapped within the reservoir above and around the injection point. Advanced seismic and reservoir modelling techniques are now being used to further determine the carbon dioxide subsurface distribution and predict its future behaviour.

Diagram showing the injection of CO2 into the Utsira formation.  Seismic monitoring in the October 1999 clearly shows where the CO₂ has been injected.

How much can be stored?

Regional mapping of the Utsira Sand  indicates that it has a potential storage volume of about 5.5 x 10¹¹ m³. A typical 500 MW coal-fired power plant fitted for carbon dioxide capture would produce around 4.3 million tonnes of carbon dioxide per year for sequestration. An equivalent figure for a similar size natural gas-fired combined cycle power plant would be around 1.7 million tonnes of carbon dioxide. At reservoir conditions in the Utsira Sand one tonne of carbon dioxide, present as a supercritical fluid would occupy about 1.4 m³ of pore space in the reservoir rock. Even if only about 1% of the storage volume of the Utsira Sand were utilised for carbon dioxide storage, this would be sufficient to sequester the annual output of over 900 coal-fired, or about 2300 gas-fired 500 MW power stations.

Map of the Utsira Sand.

Future potential elsewhere

The Utsira Sand is by no means an unusual geological formation in terms of its storage potential, and the Sleipner operation represents just one of many subsurface storage scenarios. The possibility of storing carbon dioxide in exhausted oil or gas bearing structures, which form proven long-term traps for buoyant fluids and gas, is another option.

Underground carbon dioxide injection is routinely used by the oil industry to assist with enhanced oil recovery (EOR), in the effective exploitation of oilfields. An EOR project in southern Canada is currently using carbon dioxide stripped from the emissions of a coal gasification plant in North Dakota to improve recovery in an ageing oilfield. BGS is leading a European team as part of a multinational research and monitoring project, whose aim is to further develop effective methods of carbon dioxide storage whilst at the same time increasing understanding of EOR technology.

Research outside the UK

Many other countries have active research programmes aimed at identifying potential carbon dioxide underground disposal sites. Notable amongst these are a major programme in Australia and various initiatives funded by the US Department of Energy and the European Union. In Europe, BGS is involved in a regional evaluation project, GESTCO, partially funded by the EU, which is assessing the practicality and economics of storing carbon dioxide underground in the U.K., Belgium, Denmark, France, Germany, Greece, the Netherlands and Norway . The major industrial sources of carbon dioxide in these countries have been identified, and the capacity of some of the most promising sites for long term underground storage is being determined.

Studying leakage

There are many naturally occurring underground accumulations of carbon dioxide (CO₂) which provide valuable natural analogues of the man-made CO₂ storage scenario. It has been suggested in the journal Nature, that in the Permian Basin of West Texas CO₂ has been effectively trapped for 300 million years. A multinational project NASCENT, led by the British Geological Society, is looking at, and seeking to understand these long-lived natural CO₂ accumulations (in Hungary, Greece, France, Germany, and Italy). At some of these locations CO₂ naturally emerges from the ground, both dissolved in spring waters and as pure CO₂ gas - areas around Rome, in northern Hungary and northern Greece and the Mammoth Mountain area of California are examples. These leakage sites are being studied to determine their effects on man and the natural environment. Lessons learned will be applied to the selection of suitable, safe underground repositories for CO₂.

Conclusions from the project

The bottom line governing the viability of carbon dioxide sequestration is economical rather than technical. The cost of fitting suitable carbon dioxide ‘scrubbing' equipment and storing the carbon dioxide underground is estimated to increase power generation costs by up to 50%, but as a percentage of the final electricity cost to the consumer, the figure would be considerably lower than this. Where storage is linked to an enhanced oil recovery operation, costs are reduced much further, indeed sequestration may be a profitable business. The overall economics compare favourably therefore with the much-touted alternatives.

To conclude, irrespective of how quickly alternative, renewable energy technologies are realised, adherence to the Kyoto protocol, and the medium-term necessity of limiting atmospheric carbon dioxide levels to an environmentally acceptable limit, will make carbon dioxide sequestration a necessary fact of life. Underground storage offers a safe, verifiable, technologically feasible and, ultimately affordable option.

Where Next?

On ClimateX.org

To find out more about other types of sequestration, see the article ‘Carbon Sequestration' in this section.

External links

Recent developments in CO2 capture and storage in 2007 can be examined via the IEA (International Energy Agency) project website http://www.co2captureandstorage.info/.

The British Geological Survey

http://nts1.cgu.cz/geocapacity this project is an EU wide assessment of the potential for geological storage of carbon dioxide throughout Europe, replacing the earlier GESTCO_project.

International Energy Agency Greenhouse Gas R&D Program