Underground accumulation of carbon dioxide (CO2) is a widespread geological phenomenon, with natural trapping of CO2 in underground reservoirs. Information and experience gained from the injection and/or storage of CO2 from a large number of existing enhanced oil recovery (EOR) and acid gas projects, as well as from the Sleipner, Weyburn and In Salah projects, indicate that it is feasible to store CO2 in geological formations as a CO2 mitigation option. Industrial analogues, including underground natural gas storage projects around the world and acid gas injection projects, provide additional indications that CO2 can be safely injected and stored at well-characterized and properly managed sites. While there are differences between natural accumulations and engineered storage, injecting CO2 into deep geological formations at carefully selected sites can store it underground for long periods of time: it is considered likely that 99% or more of the injected CO2 will be retained for 1000 years. Depleted oil and gas reservoirs, possibly coal formations and particularly saline formations (deep underground porous reservoir rocks saturated with brackish water or brine), can be used for storage of CO2. At depths below about 800-1000 m, supercritical CO2 has a liquid-like density that provides the potential for efficient utilization of underground storage space in the pores of sedimentary rocks. Carbon dioxide can remain trapped underground by virtue of a number of mechanisms, such as: trapping below an impermeable, confining layer (caprock); retention as an immobile phase trapped in the pore spaces of the storage formation; dissolution in the in situ formation fluids; and/or adsorption onto organic matter in coal and shale. Additionally, it may be trapped by reacting with the minerals in the storage formation and caprock to produce carbonate minerals. Models are available to predict what happens when CO2 is injected underground. Also, by avoiding deteriorated wells or open fractures or faults, injected CO2 will be retained for very long periods of time. Moreover, CO2 becomes less mobile over time as a result of multiple trapping mechanisms, further lowering the prospect of leakage.
Injection of CO2 in deep geological formations uses technologies that have been developed for and applied by, the oil and gas industry. Well-drilling technology, injection technology, computer simulation of storage reservoir dynamics and monitoring methods can potentially be adapted from existing applications to meet the needs of geological storage. Beyond conventional oil and gas technology, other successful underground injection practices - including natural gas storage, acid gas disposal and deep injection of liquid wastes - as well as the industry's extensive experience with subsurface disposal of oil-field brines, can provide useful information about designing programmes for long-term storage of CO2. Geological storage of CO2 is in practice today beneath the North Sea, where nearly 1 MtCO2 has been successfully injected annually at Sleipner since 1996 and in Algeria at the In-Salah gas field. Carbon dioxide is also injected underground to recover oil. About 30 Mt of non-anthropogenic CO2 are injected annually, mostly in west Texas, to recover oil from over 50 individual projects, some of which started in the early 1970s. The Weyburn Project in Canada, where currently 1-2 MtCO2 are injected annually, combines EOR with a comprehensive monitoring and modelling programme to evaluate CO2 storage. Several more storage projects are under development at this time.
In areas with suitable hydrocarbon accumulations, CO2-EOR may be implemented because of the added economic benefit of incremental oil production, which may offset some of the costs of CO2 capture, transport and injection. Storage of CO2 in coal beds, in conjunction with enhanced coal bed methane (ECBM) production, is potentially attractive because of the prospect of enhanced production of methane, the cleanest of the fossil fuels. This technology, however, is not well developed and a better understanding of injection and storage processes in coals is needed. Carbon dioxide storage in depleted oil and gas reservoirs is very promising in some areas, because these structures are well known and significant infrastructures are already in place. Nevertheless, relatively few hydrocarbon reservoirs are currently depleted or near depletion and CO2 storage will have to be staged to fit the time of reservoir availability. Deep saline formations are believed to have by far the largest capacity for CO2 storage and are much more widespread than other options.
While there are uncertainties, the global capacity to store CO2 deep underground is large. Depleted oil and gas reservoirs are estimated to have a storage capacity of 675-900 GtCO2. Deep saline formations are very likely to have a storage capacity of at least 1000 GtCO2 and some studies suggest it may be an order of magnitude greater than this, but quantification of the upper range is difficult until additional studies are undertaken. Capacity of unminable coal formations is uncertain, with estimates ranging from as little as 3 GtCO2 up to 200 GtCO2. Potential storage sites are likely to be broadly distributed in many of the world's sedimentary basins, located in the same region as many of the world's emission sources and are likely to be adequate to store a significant proportion of those emissions well into the future.
The cost of geological storage of CO2 is highly site-specific, depending on factors such as the depth of the storage formation, the number of wells needed for injection and whether the project is onshore or offshore - but costs for storage, including monitoring, appear to lie in the range of 0.6-8.3 US$/tCO2 stored. This cost is small compared to present-day costs of CO2 capture from flue gases, as indicated in Chapter 3. EOR could lead to negative storage costs of 10-16 US$/tCO2 for oil prices of 15-20 US$ per barrel and more for higher oil prices.
Potential risks to humans and ecosystems from geological storage may arise from leaking injection wells, abandoned wells, leakage across faults and ineffective confining layers. Leakage of CO2 could potentially degrade the quality of groundwater, damage some hydrocarbon or mineral resources, and have lethal effects on plants and sub-soil animals. Release of CO2 back into the atmosphere could also create local health and safety concerns. Avoiding or mitigating these impacts will require careful site selection, effective regulatory oversight, an appropriate monitoring programme that provides
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