Accelerating rise of global temperatures, an increased incidence of unusual and damaging weather patterns, freak storms, and sustained thinning of the Arctic sea ice (Geophysical Research Letters, Dec. 1999) are intensifying the debate about possible worsening environmental consequences of global warming.
Carbon dioxide (CO2 ) concentrations in the atmosphere (currently 370 ppm by volume) are higher now than they have been for millions of years, and if they reach 550 ppm, the consequences for global climate could be catastrophic. Unless current and future emissions of greenhouse gases are drastically reduced the 550 ppm threshold will be exceeded during the next 200 years (Royal Commission on Environmental Pollution, Report 21).
Urgent remediative action is required. It is unlikely that energy generation from renewables and non-fossil fuel technologies are going to be developed quickly enough or provide sufficient energy for the growing world economies. Therefore, CO2 production is unlikely to fall.
Dams accelerate CO2
Some renewable power sources, for example hydroelectric, actually accelerate global warming through the production of methane (Bioscience, vol. 50, p.766). This gas is 20 times more potent as a greenhouse gas than CO2, and hydroelectric dams are estimated to account for 7% of global warming (that is higher than aircraft emissions).
Nuclear power is the only option among known technologies that can generate the quantities of energy required, without contributing to greenhouse emissions. Governments and the public need to take account of the balance of risks between the potential damage of radioactive emissions from nuclear generation, and the consequences of climate change from fossil fuel burning.
In western Europe (apart from France), and many other western countries, current energy policy is to abandon nuclear power generation. This means that CO2 emissions will be significant for several decades and will need to be prevented from reaching the atmosphere. In the next decade, there will be financial as well as environmental incentives to achieve this (Kyoto protocol), as carbon permits/levies become enforced, first in Europe (Norway) and then on a global scale.
Such action requires several parallel appr-oaches, including more efficient energy use, reduction of reliance on fossil fuels, removal of CO2 from the atmosphere (through cultivating biomass storage), and sequestration of CO2 emissions to a non-atmospheric sink.
Ocean storage is a controversial option as the ocean system is closely linked and interchanges with the atmosphere, carbon storage is insecure, and we do not know the consequences of CO2 injection on the marine ecosystem. On the other hand, subsurface storage is an appropriate and proven option, which provides a particularly effective supplement/adjunct to biomass storage, as well as having several significant advantages over it.
Tree planting uncertain
A recent article by Fred Pearce (New Scientist 26 August 2000) highlighted some of the difficulties and drawbacks associated with proposed passive biomass CO2 sequestration strategies based on tree planting. Pearce's arguments were partially theoretical, based on uncertainties in the understanding of the global carbon cycle. More practical considerations, particularly the worrying increase in the frequency of damaging windstorms, cast further doubt on tree planting as a globally effective CO2 sequestration option.
In October 1987, a relatively small storm destroyed 17 million trees in southeast England. This event was dwarfed by the storm which swept across NW Europe on Christmas 1999, destroying over 270 million trees in France alone.
Forest fires provide further problems with the reliability of biomass storage. This summer fire devastated over 18 million hectares in the western US. Vast areas of drought stricken rainforest were destroyed in Borneo and Sumatra during 1997-1998.
The likely increased frequency of such events as climate belts and weather patterns will render a strategy of CO2 sequestration by tree planting progressively more insecure - young trees in particular are susceptible to fire damage (Science, vol. 41(3), 430-451).
Quantification and verification of carbon removal is a key aspect of the Kyoto protocol and any foreseeable future greenhouse gas reduction agreements. A viable CO2 removal strategy will need to:
- Quantify the amount of CO2 initially sequ-estered
- Be able to monitor and verify the subsequent fate of this CO2, particularly in terms of the amounts leaking back into the atmosphere.
As Pearce pointed out, biomass storage by tree planting is both difficult to quantify, and in many parts of the world, likely to become increasingly unreliable as a permanent CO2 store.
Biomass fuel storage
Using biomass as a fuel source by working in tandem with subsurface CO2 sequestration is an option. Fuel gas derived from biomass can be pressurized and have its carbon chemically stripped to produce CO2, and then removed to a suitable underground rock reservoir for permanent storage.
The remaining hydrogen can be burned, or used to power a fuel cell, resulting in zero carbon emissions. Subsurface sequestration of CO2 involves pumping it underground in fluid form, such that it becomes trapped in the pore spaces between grains of sedimentary rock 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 CO2 to a non-atmospheric sink, using technologies which are both currently available and constantly improving.
It has the advantage over using the oceans or biomass as sinks, in that underground storage completely isolates the CO2 from the atmosphere, capturing it at the point of emission, and is ideally suited for large industrial point sources.
A major subsurface CO2 sequestration project now is running in the North Sea under the direction of Statoil. In the Sleipner field, natural gas is contaminated by about 9% CO2, which after separation from the methane, would normally be vented to the atmosphere. At Sleipner, it is injected into a deep saline aquifer, the Utsira Sand, which lies about 1,000 meters beneath the seabed. The sequestration operation started in October 1996, with over 2 million tons of CO2 injected since then.
A demonstration project, SACS, jointly funded by the EU, industry, and national governments, is evaluating the geological aspects of the subsurface disposal operation. This involves assessing the capacity, storage properties and performance of the Utsira reservoir, modeling CO2 migration within the reservoir, and monitoring the subsurface dispersal of the CO2 using time-lapse seismic techniques.
It is clear from the illustrations that the underground situation is well imaged. The CO2 is trapped within the reservoir above and around the injection point. Sophisticated seismic and reservoir modeling is now being carried out to further quantify and constrain the CO2 subsurface distribution and predict its future behavior.
Regional mapping of the Utsira Sand by SACS indicates that it has a potential storage volume of about 5.5 x 1,011 cu meters. A typical 500 MW(e) coal-fired power plant fitted for CO2 capture would produce around 4.3 million tons of CO2 per year for sequestration. An equivalent figure for a similar size natural gas-fired combined cycle power plant would be around 1.7 million tons of CO2. The density of CO2 at reservoir conditions in the Utsira Sand is about 725 kgm-3. Thus, one ton of CO2 would occupy about 1.38 cu meters of pore space in the reservoir rock. So, even if only about 1% of the storage volume of the Utsira Sand were utilized for CO2 sequestration, this would be sufficient to sequester the annual output of about 925 coal-fired, or about 2,340 gas-fired 500 MW(e) power stations.
The Utsira Sand is by no means unusual in terms of its storage potential, and the Sleipner operation represents just one of many potential subsurface storage scenarios. The possibility of disposing of CO2 in exhausted oil or gas bearing structures, which form proven long-term traps for buoyant fluids, is another option.
Underground CO2 injection is routinely used by the oil industry to assist with enhanced oil recovery (EOR) in the effective exploitation of oilfields. In October 2000, PanCanadian Resources will begin an unusual EOR project in Canada.
This will involve using CO2 stripped from emission gases from a coal gasification plant in North Dakota to improve recovery in the aging Weyburn oilfield in Saskatchewan. A multinational research and monitoring project is being planned at Weyburn to further develop effective methods of CO2 disposal while at the same time increasing understanding of EOR technology.
Many other countries have active research programmes aimed at identifying potential CO2 underground disposal sites. Notable among these are the Geodisc program in Australia and various initiatives funded by the US Department of Energy and the European Union. In Europe, a recently started project, GESTCO, partially funded by the EU, will evaluate the practicality of subsurface storage of CO2 in a number of countries. As part of this project, a public hearing will be held, to gauge the reaction of the general public to the idea of CO2 storage underground.
There are many naturally occurring underground accumulations of CO2, which provide valuable natural analogues of the man-made CO2 storage scenario. The CO2 in one such accumulation, the Pisgah Anticline of Central Mississippi, is thought to have originated through the heating of Jurassic limestones by the Jackson Dome igneous intrusion.
This intrusion was emplaced during Late Cretaceous times, some 65 million years ago, indicating that the CO2 has remained underground since then. The study of these natural analogues is the subject of the part EU-funded NASCENT Project, which is looking at natural CO2 accumulations (in eastern Europe) that have remained in the ground for millions of years.
With subsurface storage, realistic assessments indicate CO2 retention times are likely to be of the order of thousands to millions of years. For all practical purposes, storage at a suitable site will mean that the CO2 is removed from the atmosphere until all recoverable fossil fuels are likely to have been exhausted and any consequent greenhouse crisis has passed.
To conclude, regardless of how quickly alternative renewable energy technologies are realized, adherence to the Kyoto protocol, and the medium-term necessity of limiting atmospheric CO2 levels to an environmentally acceptable limit, will make CO2 sequestration a necessary fact of life.
Subsurface storage offers a safe, verifiable and technologically feasible option. Costs will clearly be a significant factor, but where storage is linked to an enhanced oil recovery operation, or a biomass sourced fuel plant, these can be much reduced. Furthermore, decommissioning strategy of offshore fields and installations needs to take into account the possible change of use of abandoned fields and associated infrastructure as potential recipient sites for CO2 storage.
It is one of our concerns that some North Sea sites which may be identified as suitable for CO2 storage by the GESTCO project risk being lost unless decommissioning policy takes this potential change of use into account.