Carbon dioxide (CO2) sequestration

Carbon dioxide sequestration in geological media: State of the science
Series	Studies in Geology
Chapter	Geological input to selection and evaluation of CO2 geosequestration sites
Author	John G. Jkaldi, Catherine M. Gibson-Poole, Tobias H. D. Payenberg
Link	Web page
PDF	PDF file (requires access)
Store	AAPG Store

Coal, oil, and natural gas currently supply about 85% of the world's energy needs. Moreover, given the relatively low cost and abundance of fossil fuels together with the huge sunken investment in fossil-fuel-based infrastructure, fossil fuels will likely continue to be used for at least the next 25 to 50 years. The burning of fossil fuels is, however, the major source of anthropogenic (man-made) carbon dioxide (CO₂). Carbon dioxide is the main greenhouse gas released to the atmosphere.

Geosequestration, also known as carbon capture and storage (CCS), is a means to reduce anthropogenic CO₂ emissions to the atmosphere. Geosequestration involves the long-term storage of captured CO₂ emissions in deep subsurface geological reservoirs. Carbon sequestration can be pursued as part of a portfolio of greenhouse gas abatement options, when this portfolio also includes improving the conservation and efficiency of energy use and utilizing nonfossil energy forms such as renewable (solar, wind, and tidal) and nuclear energy. Geosequestration may contribute significant reductions to anthropogenic CO₂ emissions. Estimates by the Intergovernmental Panel on Climate Change indicate that a technical potential of at least about 2000 billion metric tonnes of CO₂ storage capacity in geological formations likely exists (Table 1).

Table 1 Storage capacity for several geological storage options*

Reservoir type	Lower estimate of storage capacity (Gt CO2	Upper estimate of storage capacity (Gt CO2
Oil and gas fields	675**	900**
Unmineable coal seams in ECBM*** recovery	3-15	200
Deep saline formations	1000	Uncertain but possibly 10,000

^{* The storage capacity includes storage options that are not economical.
**These numbers would increase by 25% if undiscovered oil and gas fields were included in this assessment.
***ECBM = enhanced coalbed methane.}

Figure 1 A simplified view of the steps involved in the geosequestration process (image courtesy of Cooperative Research Centre for Greenhouse Gas Technologies [CO2CRC]).

Geosequestration comprises several steps: first, the CO₂ is captured at the source, which can be a power plant or other industrial facility; the captured CO₂ is then transported, typically via pipeline, from the source to the geological storage site; next, the CO₂ is injected deep underground via wells into the geological reservoir; and finally, the CO₂ is stored in the geological reservoir, where its movement is carefully monitored and the quantity stored is regularly verified (Figure 1). The capture, transport, and injection processes do require additional energy to be expended (and hence more CO₂ is emitted); however, the net CO₂ emission reduction is still a significantly large volume to make deep reductions in anthropogenic greenhouse gas emissions. For example, a power plant with CCS could reduce net CO₂ emissions to the atmosphere by approximately 80–90% compared to a plant without CCS (Figure 2).

Comparison of CO₂ emissions from power plants with and without CO₂ capture and storage. A power plant with CCS (lower bar) has increased CO₂ production resulting from loss in overall efficiency because of the additional energy required for capture, transport, and storage. However, available technology can capture 85-95% of the CO₂ processed in a capture plant, resulting in a net CO₂ emission reduction (CO₂ avoided) of 80-90% compared to the reference power plant (upper bar) without capture. CCS = carbon capture and storage.

Carbon dioxide capture

Carbon dioxide capture can be conducted at a point (stationary) source of CO₂ such as a power plant. Carbon dioxide capture involves capturing the produced CO₂ instead of allowing it to be released to the atmosphere. This captured CO₂ is then compressed to make it more dense and much easier, and less costly, to transport to the geological storage site.

Anthropogenic CO₂ that can be captured is produced by three main types of activity: industrial processes, electricity generation, and hydrogen (H₂) production. Industrial processes that lend themselves to CO₂ capture include natural gas processing, ammonia production, and cement manufacturing. Note, however, that the total quantity of CO₂ produced by these processes is limited. A far larger source, accounting for one-third of total CO₂ emissions, is fossil-fueled power production. The types of power plants that are best suited to CO₂ capture are pulverized coal (PC), natural gas combined cycle (NGCC), and integrated gasification combined cycle (IGCC) plants. Finally, a potentially large future source of CO₂ for capture will be [[H₂ production]], whereby the produced H₂ is used to fuel a hydrogen economy, i.e., it is used in turbines to produce electricity and in fuel cells to power cars. Technologies for capturing CO₂ from electricity generation fall into two general categories: postcombustion and precombustion.

Carbon dioxide transport

Carbon dioxide transport involves moving, or transporting, the captured CO₂ from the CO₂ point source to the geological storage site. The CO₂ is typically transported in a compressed form via pipeline, although the CO₂ could also be transported by truck, rail, or in the case of a geological storage site located offshore, ocean tanker.

Carbon dioxide injection

Carbon dioxide injection involves taking the CO₂ from the surface and injecting it deep underground into a reservoir rock. The CO₂ is injected into the reservoir via a single well or array of wells. Both enhanced oil recovery (EOR) using carbon dioxide floods and acid gas injection (AGI) are mature technologies that involve significant quantities of CO₂ being injected underground and are therefore very good analogs for CO₂ injection as part of geosequestration activities. The first project using CO₂ for EOR began in 1972, and by 1999, 84 operational projects worldwide existed (72 in the United States) injecting an estimated total of more than 15 million tonnes of CO₂ per year.

Options for the geological storage of CO2 (image courtesy of Cooperative Research Centre for Greenhouse Gas Technologies [CO2CRC]).

Carbon dioxide storage

Main article: Carbon dioxide (CO2) storage

Carbon dioxide storage involves keeping the CO₂ secured deep underground in a geological reservoir. Carbon dioxide can be stored geologically in a variety of different options (Figure 3). These include depleted oil and gas fields, EOR, deep saline formations, deep unmineable coal seams, enhanced coalbed methane recovery (ECBMR), and other opportunities such as salt caverns.

See also

• Carbon dioxide (CO2) storage

External links

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