Climate Change 2001: Working Group III: Mitigation

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3.8.4.4.2 Transmission and Storage of CO₂

CO₂ can be transported to storage sites using high-pressure pipelines or by ship. Pipelines are used routinely today to transport CO₂ long distances for use in enhanced oil recovery (Stevens et al., 2000). Although CO₂ is not transported by ship at present, tankers similar to those currently used for liquefied petroleum gas (LPG) could be used for this purpose (Ozaki, 1997).

CO₂ exists in natural underground reservoirs in various parts of the world. Potential sites for storage of captured CO₂ are underground reservoirs, such as depleted oil and gas fields or deep saline reservoirs. CO₂ injected into coal beds may be preferentially absorbed, displacing methane from the coal; sequestration would be achieved providing the coal is never mined. Another possible storage location for captured CO₂ is the deep ocean, but this option is at an earlier stage of development than underground reservoirs; so far only small-scale experiments for preliminary investigation have been carried out (Herzog et al., 2000); the deep ocean is chemically able to dissolve up to 1800GtC (Sato, 1999). An indication of the global capacities of the major storage options is given in Table 3.34. The capacities of these reservoirs are subject to substantial uncertainty, as purposeful exploration has only been conducted in some parts of the world so far. Other published estimates of the global capacity for storage in underground aquifers range up to 14,000GtC (Hendriks, 1994). Other methods of CO₂ storage have been suggested but none are competitive with underground storage (Freund, 2000).

Table 3.34: Some natural reservoirs which may be suitable for storage of carbon dioxide (Freund, 1998; Turkenburg, 1997)
Reservoir type	Storage option	Global capacity (GtC)
Below ground	Disused oil fields	100
	Disused gas fields	400
	Deep saline reservoirs	>1000
	Unminable coal measures	40
Above ground	Forestry	1.2 GtC/yr
Ocean	Deep ocean	>1000

Substantial amounts of CO₂ are already being stored in underground reservoirs:

• Nearly 1Mt/yr of CO₂ is being stored in a deep saline reservoir about 800m beneath the bed of the North Sea as part of the Sleipner Vest gas production project (Baklid and Korbol, 1996). This is the first time CO₂ has been stored purely for reasons of climate protection.
  
• About 33Mt/yr of CO₂ is used at more than 74 enhanced oil recovery (EOR) projects in the USA. Most of this CO₂ is extracted from natural CO₂ reservoirs but some is captured from gas processing plants. Much of this CO₂ remains in the reservoir at the completion of oil production; any CO₂ produced with the oil is separated and reinjected. An example of an EOR scheme which will use anthropogenic CO₂ is the Weyburn project in Canada (Wilson et al., 2000). In this project, 5,000 t/d of CO₂ captured in a coal gasification plant in North Dakota, USA will be piped to the Weyburn field in Saskatchewan, Canada.
  
• At the Allison unit in New Mexico, USA, (Stevens et al., 1999), over 100,000 tonnes of CO₂ has been injected over a three-year period to enhance production of coal bed methane. The injected CO₂ is sequestered in the coal (providing it is never mined). This is the first example of CO₂-enhanced coal bed methane production.

The cost of CO₂ transport and storage depends greatly on the transport distance and the capacity of the pipeline. The cost of transporting large quantities of CO₂ is approximately US$1-3/t per 100km (Ormerod, 1994; Doctor et al., 2000). The cost of underground storage, excluding compression and transport, would be approximately US$1-2/tCO₂ stored (Ormerod, 1994). The overall cost of transport and storage for a transport distance of 300 km would therefore be about US$8/tCO₂ stored, equivalent to about US$10/t of emissions avoided (US$37/tC).

If the CO₂ is used for enhanced oil recovery (EOR) or enhanced coal bed methane production (ECBM), there is a valuable product (oil or methane, respectively) which would help to offset the cost of CO₂ capture and transport. In some EOR or ECBM projects, the net cost of CO₂ capture and storage might be negative. Other ideas for utilizing CO₂ to make valuable products have not proved to be as useful as sequestration measures, because of the amount of energy consumed in the process and the relatively insignificant quantities of CO₂ which would be used.

If no valuable products were produced, the overall cost of CO₂ capture and storage would be about US$40-60/t CO₂ emissions avoided (150-220/tC). As with most new technologies, there is scope to reduce these costs in the future through technical developments and wider application.