Side-alternativer
Geological storage
Geological carbon storage refers to the permanent storage of CO2 in geological formations. Underground accumulations of natural CO2 is a widespread geological phenomenon, demonstrating that certain geological formations are capable of trapping CO2 for a very long time. Similarly, geological reservoirs of water, oil and natural gas have shown that geological trapping mechanisms can hold gases and liquids for millions of years.
Enhanced oil recovery (EOR) projects, where CO2 is injected into oil reservoirs to increase production in the later stages of production life time, has provided much knowledge and experience concerning CO2 injection and CO2 migration in deep geological formations. This information is highly applicable for permanent CO2 storage projects. In the later years, several ongoing projects that inject CO2 into geological formations aiming to store it permanently have increased the experience in the field.
Permanent geological storage of CO2 is still a technology in its infancy, and further research is necessary to increase knowledge about CO2 migration in underground rock formations, to improve the methods for monitoring stored CO2, to study the effects of possible small scale CO2 leakages and to improve the methods for surveying deep geological formations so that possible storage formations can be identified an confidently assessed for capacity for permanent storage.
Content
3. Storage mechanisms in geological formations
Storage formations
A great many formations suitable for permanent CO2 storage are likely to exist within sedimentary basins all over the world. A sedimentary basin is a depression in the crust of the Earth formed by tectonic activity that is subsequently filled with sediments over time. As the sediments are subjected to pressure from the higher sediment levels of the basin, they gradually form solid rock, through a geological process known as lithification. This causes deep structures of layered sedimentary rock, which is the most important prerequisite for forming formations that can store CO2 permanently.
A formation has potential for CO2 storage when it is made up of a layer of porous rock with high liquid permeability, covered by a layer of solid rock with very low permeability. The above layer, called the caprock, acts as a seal, stopping any stored gas or liquid from migrating upwards.
Depleted oil and gas reservoirs are prime candidates for CO2 storage. Oil and gas is formed from biological material accumulated in deep layers of sedimentary basins. Formed hydro-carbons will migrate towards the surface of the basin, unless trapped by an unpermeable caprock layer. Hydrocarbons accumulated over time under a caprock layer form reservoirs than can be produced.
As these reservoirs have proven themselves capable of trapping hydro-carbons, they will most likely also be able to trap CO2. An added advantage is the fact that petroleum reservoirs have been extensively surveyed before and during the production period. A potential risk is abandoned wells that have not been properly sealed. In the case of old oil fields, the location of all wells may not even be known, so efforts must be made to locate all wells and resealing them if necessary.
Much experience exists on injecting CO2 into petroleum reservoirs. Most of this knowledge comes from EOR projects, where permanent trapping of the CO2 is not a goal. However, a few projects are in operation where CO2 is injected for the purpose of permanent storage.
Deep saline aquifers are deep sedimentary rocks containing water saturated with salt. These formations are widespread and contain enormous amounts of water. Aquifers have properties very similar to petroleum reservoirs, and are strong candidates for CO2 storage. The water in these formations is not suitable for agriculture or human consumption, making it highly unlikely that a need to produce the water would ever occur (if the water in an aquifer that stores CO2 is produced, some or all the CO2 would be released back into the atmosphere).
Care must be taken to avoid injecting the CO2 at too high pressure, as a rapid pressure increase might potentially cause cracking of the caprock, compromising long term storage safety.
Unminable coal beds has some potential for CO2 storage. The permeability of coal is very low, but in some coal beds fractures (cleats) in the coal are widespread enough to allow sufficient permeability. CO2 diffuses into micropores in the surface of the cleats, where it is adsorbed. This is a tight chemical binding, storing the CO" permanently unless the coal is later mined or otherwise distrubed.
Coal usually contains large quantities of adsorbed methane. CO2 will displace the methane, relasing it into the coal bed. Therefore, coal bed storage is primarily useful in a coal bed methane recovery system. As CO2 is injected, methane is produced from the same seam. This naturally eliminated the the climate change mitigation effect of the storage, but if the produced methane is used on site in natural gas power plants with CCS, where the CO2 is reinjected into the same coal bed, near zero emission power generation may be achieved.
Other geological media are available for CO2 storage, but provide little storage capacity in a global scale. However, such alternative storage options may provide a local option for storage in areas with few other suitable storage sites. Basalts, oil shale, salt caverns and abandoned mines may all offer niche opportunities for CO2 storage. None of these options are likely to ever see widespread use, for different reasons.
Storage site selection
Not all sedimentary basins will contain formations that are suitable for CO2 storage. Certain criteria must be met for storage to be possible. The storage site must have the capacity to store large volumes of CO2 for injection to be economically viable. The storage formation must also be porous enough for CO2 to diffuse into the formation from the injection point.
The caprock above the storage formation must be without cracks or other irregularities that could cause a leak of stored gas. A sufficiently stable geological environment is required to avoid compromising the integrity of the storage site.
In addition, outside factors such as infrastructure, industrial maturity, level of development, economy and environmental concerns are important determining factors in picking storage sites.
Storage mechanisms in geological formations
Once injected underground into a suitable reservoir rock, the CO2 accumulates in the pores between grains and in fractures, thus displacing and replacing any existing fluid such as gas, water or oil. Suitable host rocks for CO2 geological storage should therefore have a high porosity and permeability. Such rock formations are commonly located in so-called “sedimentary basins”. In places, these permeable formations alternate with impermeable rocks, which can act as an impenetrable seal. Sedimentary basins often host hydrocarbon reservoirs and natural CO2 fields, which proves their ability to retain fluids for long periods of time, having naturally trapped oil, gas and even pure CO2 for millions of years.
The subsurface is often depicted as an over-simplified, homogenous, layer-cake structure in illustrations showing the possible storage options for CO2. In reality, it is composed of unevenly distributed and locally faulted rock formations, reservoirs and cap rocks forming complex, heterogenous structures. In-depth knowledge of the site and geoscientific experience are required to assess the suitability of underground structures that are proposed for long-term CO2 storage.
Potential CO2 storage reservoirs must fulfill many criteria, the essential ones being:
* Sufficient porosity, permeability and storage capacity
* The presence of overlying impenetrable rock (cap rock), which prevents the CO2 from migrating upwards
* The presence of trapping structures: features that can control the extent of CO2 migration within the storage formation
* Location deeper than 800 m, where pressures and temperatures are high enough to enable the storage of CO2 in a compressed fluid phase and thus maximize the quantity stored
A number of different mechanisms contribute to the trapping of CO2 in geological formations. Increasingly strong trapping mechanisms set in over time, in effect making storage security higher with longer storage time. Trapping mechanisms can be separated into physical and geochemical mechanisms. The physical mechanisms will start the permanent trapping of some or all of the CO2 immediately, while the geophysical mechanisms take place over hundreds and thousands of years. CO2 that has been geochemically trapped will deposit at the bottom of the formation, posing no risk of escape storage ever.
The CO2, held in the supercritical phase by the formation pressure, is both more buoyant and more mobile than the in situ formation fluids. This causes most of the CO2 to migrate upwards through the formation until stopped by a physical barrier og low permeability rock. However, for a caprock to provide sufficient physical trapping of the CO2 it must also stop CO2 from escaping by moving horizontally along the caprock until it finds an area of high permeability rock to escape through.
Vertical movement can be stopped by structural or stratigraphic traps, or a combination of the two. A structural trap can be a faulting or folding in the caprock that traps CO2 in the same way as air is trapped in a cup that is lowered upside-down into water. A typical structural trap is a an anticline or a folding of the cap rock to form a shape similiar to a bowl turned upside-down. Faults may also form structural traps. Miniature structural traps may exist as as small cracks or indentations in the caprock that traps some of the CO2 as it travels by.
A startigraphic trap is a variation in the permeability in the formation rock. Unconformity trapping is a form of stratigraphic trapping where the formation rock changes gradually from a porous sandstone to an area of clay stone which traps migrating CO2.
Residual trapping is another form of physical trapping where the CO2 is trapped in the pore spaces by capillary forces. This can immobilize significant amounts of CO2. Models have shown that when the degree of trapping is high and CO2 is injected at the bottom of a thick formation, all the CO2 may be trapped by this mechanism even before it reaches the caprock at the top of the formation. The degree of residual trapping is formation specific, but may be as high as 15-25% for typical storage formations.
Solution trapping is a geochemical trapping mechanism that occurs over decades to centuries, depending on water movement in the formation. CO2 dissolves into the formation water, forming carbonic acid. After being dissolved, the CO2 no longer exists as a separate phase, eliminating the buoyant forces that drives it upwards. When saturated with CO2, water becomes slightly denser, causing it to move downwards through the formation.
Mineralization is the last step of CO2 trapping. The CO2, now as carbonic acid, reacts with the minerals in the formation, and some of it will eventually be converted to stable carbonate minerals. This is a very slow process, taking from several centuries to more than a thousand years.
Risk assessment
Numerical simulations on existing storage projects conclude that very long retention times are to be expected with geological storage. A study on the Sleipner Field concludes that no CO2 would migrate into the North Sea for 100 000 years, and that even after a million years, the annual rate of release would be only 10-6 of the stored CO2 (Lindeberg and Bergbom, 2003). A study of the Forties Oilfield on the effects of uncertainties of in paramteters such as the flow velocity in the aquifer and capillary entry pressure into the caprock, showed that less than 0,2% of the CO2 would escape into the overlying layers within 1000 years, and even in the worst case, the maximum vertical distance moved by any of the CO2 was less than halway to the seabed within this period (Cawley et al., 2005). Similarly, one study of the Weyburn storage site showed that within 5000 years there was 95% probability that less than 1% of the stored CO2 would be released into the biosphere (Walton et al., 2005) and another study of the same site found nor release to the atmosphere within 5000 years at all (Zhou et al, 2005).
For large-scale CO2 storage projects, 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.
References
IPCC, 2005: IPCC Special Report on Carbon Dioxide Capture and Storage
CO2GeoNet, 2009: What does geological storage really mean?
