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Carbon storage
Three main storage options exist for CO2: Depleted oil and gas fields, saline aquifers and unmineable coal seams.
On the long term, it can be assumed that formations that have already proven themselves capable of trapping gas or liquids for a very long time have a high potential for storing CO2 safely. This is the case with depleted oil and gas reservoirs and with deep saline aquifers. Oil and gas fields have the added benefit of having been extensively surveyed over the production period of the fields.
Another much discussed method of storage is injection of CO2 into the ocean at great depths. CO2 will dissolve in water, providing a temporary retention. However, over time it enters the global carbon cycle and eventually equilibrates with the CO2 in the atmosphere, returning most of the deposited carbon to the air. Also, injection of CO2 into the oceans may have very severe environmental effects. Ocean storage should therefore be considered an undesirable solution.
3. Geographical distribution and capacity
Geological storage
Main article: Geological storage, CO2 in EOR
Natural accumulations of virtually pure CO2 can be found all over the planet in a wide range of geological settings: particularly in sedimentary basins, intra-plate volcanic regions and in faulted areas, or in dormant volcanic structures. Studies of natural gas and oil reservoirs have demonstrated that a great many formations are capable of storing gas for millions of years. Further geological surveys are necessary to map the global storage potential, but in areas of petroleum activity the geology is already well understood and charted. Gas and liquid injections into such formations for enhanced petroleum recovery has also provided valuable information on how CO2 can be safely stored for global warming mitigation purposes. The storage capacity of discovered oil and gas fields has been estimated to be at least 675 GtCO2, subject to increase when more oil and gas fields are discovered and produced.
Saline aquifers are geological formations that contain saline water. Liquified CO2 have properties similar to water, and formations that can trap water are therefore good candidates for CO2 storage. These formations must be deep, at least 800 meters, for CO2 to be kept liquified by natural pressure. The greater the depth, the greater the density of CO2 relative to water, thus reducing buoyancy forces of stored CO2 and increasing storage safety. However, this may be offset by increasing temperatures at greater depths, which reduces density.
Another form of geological storage is injections into unmineable coal seams. This is primarily a method for enhanced production of coal bed methane, as CO2 preferentially displaces methane from the coal and is adsorbed into the coal. The released methane can then be produced. This could be considered a climate change mitigation measure under some circumstances, such as if the produced methane is used in a gas power plant with carbon capture. Other geological structures have been suggested for storage of CO2, including basalts, oil and gas shale, salt caverns and abandoned mines. None of these storage mediums have large global potentials, but may provide a local niche option in areas where there are few other possibilities for storage.
Ocean Storage
Main article: Ocean Storage
When injected into the ocean at depths of 1000 meters or more, most of the CO2 is isolated from the atmosphere for hundreds of years. This has been suggested as a short-term solution, assuming that future technology may be capable of removing CO2 from the atmosphere at a greater rate than the CO2 stored in the ocean returns to the atmosphere, or that the climate will be able to adapt to the higher levels of CO2 over time.
Storage in the ocean is not permanent. Even when stored as deep as 3000 meters, as much as half of the injected CO2 may have reentered the atmosphere within 500 years. Over longer time, all CO2 stored in the ocean will enter the global carbon cycle.
Injection of large volumes of CO2 would produce a measurable change in the ocean chemistry. The full environmental consequences of this are not well understood, but may cause significant damage to ocean ecosystems.
Geographical distribution and capacity
Not all sedimentary basins are suitable for CO2 storage; some are too shallow and others are dominated by rocks with low permeability or poor confining characteristics. Basins suitable for CO2 storage have characteristics such as thick accumulations of sediments, permeable rock formations saturated with saline water (saline formations), extensive covers of low porosity rocks (acting as seals) and structural simplicity.
Storage capacity estimates are usually highly approximate and based on the spatial extent of potentially suitable formations. Capacity can be assessed on different scales, from national scale rough estimates, through to basin and reservoir scale for more precise calculations that take into account the heterogenity and complexity of the real geological structure.
The IPCC special report on CCS estimates the worldwide technical potential for storage in geological formations to be at least 2,000 GtCO2. This is only the lower bound, and the IPCC believes the capacity may be many times higher, but the upper limit estimates are uncertain due to insufficient charting and disagreements on methodology. The capacity for storing CO2 in depleted petroleum reservoirs is known with much greater certainty.
| Storage option | Global capacity, lowest estimate (Gt CO2) | Global capaity, highest estimate (Gt CO2) |
|---|---|---|
| Depleted oil and gas reservoirs | 675* | 900* |
| Deep saline aquifers | 1000 | Uncertain, but possibly 10,000 |
| Deep unmineable coal seams | 3-15 | 200 |
* These estimates may increase by 25 per cent, when undiscovered oil and gas fields are included (IPCC 2005).
Experiences in CO2 storage
The engineered injection of CO2 into subsurface geological formations was first undertaken in Texas, USA, in the early 1970s, as part of enhanced oil recovery (EOR) projects.
There are four industrial-scale storage projects in operation today: the Sleipner project and the Snøhvit project in Norway, the In Salah project in Algeria, and the Weyburn EOR project in Canada. Annually, about 4 MtCO2 that would otherwise have been released into the atmosphere, are captured and stored in geological formations.
Storage safety
Main articles: Geological storage
At the global level, the risk involved in CO2 storage is the possibility that significant amounts of the CO2 that has been stored returns to the atmosphere, contributing to global warming. If one assumes that all the captured CO2 would otherwise have been released directly into the atmosphere, even retaining a small fraction of the stored CO2 is better than not capturing it in the first place. However, if not most of the stored CO2 is retained, the overall effect of CCS decreases, reducing its function as a climate change mitigation solution.
According to the IPCC (2005), the fraction retained in appropriately selected and managed reservoirs is very likely to exceed 99% over 100 years and likely to exceed 99% over 1000 years. As the risk of leakage decreases over time, similar fractions are expected for even longer time periods.
At the local level, there are two types of risks that must be addressed. Abrupt releases of CO2 may occur if the case of injection well failure or through abandoned wells. This type of injection is likely to be detected quickly and stopped using techniques that are available today for containing well blow-outs. Slow leakages may occur through undetected faults, fractures or leaking wells. In this case, the most likely hazard is to drinking-water aquifers and ecosystems in the soil between the surface and the top of the water table. Careful storage system design and siting combined with good systems for early detection of leakage are effective ways of reducing this risk.
In a worst case situation, where a storage site is deemed to be unsuitable for safe long-term storage after injection, CO2 can be produced back, in the same way as natural gas is produced from gas fields. Produced CO2 can then be transported to a different site for re-injection.
