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Editing: MOF

Carbon sequestration

Physical processes

Geological sequestration

Geological sequestration refers to the storage of CO2 underground in depleted oil and gas reservoirs, saline formations, or deep, un-minable coal beds.

Once CO₂ is captured from a gas or coal-fired power plant, it would be compressed to ~100 bar so that it would be a supercritical fluid. In this fluid form, the CO₂ would be easy to transport via pipeline to the place of storage. The CO₂ would then be injected deep underground, typically around 1 km, where it would be stable for hundreds to millions of years.^[1] At these storage conditions, the density of supercritical CO₂ is 600 to 800 kg / m³.^[2] For consumers, the cost of electricity from a coal-fired power plant with carbon capture and storage (CCS) is estimated to be 0.01 - 0.05 $ / kWh higher than without CCS. For reference, the average cost of electricity in the US in 2004 was 0.0762 $ / kWh. In other terms, the cost of CCS would be 20 - 70 $/ton of CO₂ captured. The transportation and injection of CO₂ is relatively cheap, with the capture costs accounting for 70 - 80 % of CCS costs.^[2]

The important parameters in determining a good site for carbon storage are: rock porosity, rock permeability, presence of fault lines, and geometry of rock layers. The medium in which the CO₂ is to be stored ideally has a high porosity and permeability, such as sandstone or limestone. Sandstone can have a permeability ranging from 1 to 10^-5 Darcy, and can have a porosity as high as ~30%. The porous rock must be capped by a layer of low permeability which acts as a seal, or caprock, for the CO₂. Shale is an example of a very good caprock, with a permeability of 10^-5 to 10^-9 Darcy. Once injected, the CO₂ plume will rise through buoyant forces, since it is less dense than its surroundings. Once it encounters a caprock, it will spread laterally until it encounters a gap. If there are fault lines near the injection zone, there is a possibility the CO₂ could leak into the atmosphere, which would be potentially dangerous to life in the surrounding area. Another danger related to carbon sequestration is induced seismicity. If the injection of CO₂ creates pressures that are too high underground, the formation will fracture, causing an earthquake.^[3]

While trapped in a rock formation, CO₂ can be in the supercritical fluid phase or dissolve in groundwater/brine. It can also react with minerals in the geologic formation to precipitate carbonates.

Worldwide storage capacity in oil and gas reservoirs is estimated to be 675 - 900 Gt CO₂, and in un-minable coal seams is estimated to be 15 - 200 Gt CO₂. Deep saline formations have the largest capacity, which is estimated to be 1,000 - 10,000 Gt CO₂.^[2] In the US, there is an estimated 160 Gt CO₂ storage capacity.^[3]

There are a number of large-scale carbon capture and sequestration projects that have demonstrated the viability and safety of this method of carbon storage, which are summarized here by the Global CCS Institute. The dominant monitoring technique is seismic imaging, where vibrations are generated that propagate through the subsurface. The geologic structure can be imaged from the refracted/reflected waves.^[3]

Ocean storage

If CO₂ were to be injected to the ocean bottom, the pressures would be great enough for CO₂ to be in its liquid phase. The idea behind ocean injection would be to have stable, stationary pools of CO₂ at the ocean floor. The ocean could potentially hold over a thousand billion tons of CO₂. However, this avenue of sequestration isn’t being as actively pursued because of concerns about the impact on ocean life, and concerns about its stability.^[1]

References

^ ^a ^b Benson, S. M.; Surles, T. (2006-10-01). "Carbon Dioxide Capture and Storage: An Overview With Emphasis on Capture and Storage in Deep Geological Formations". Proceedings of the IEEE. 94 (10): 1795–1805. doi:10.1109/JPROC.2006.883718. ISSN 0018-9219.
^ ^a ^b ^c Aydin, Gokhan; Karakurt, Izzet; Aydiner, Kerim (2010-09-01). "Evaluation of geologic storage options of CO2: Applicability, cost, storage capacity and safety". Energy Policy. Special Section on Carbon Emissions and Carbon Management in Cities with Regular Papers. 38 (9): 5072–5080. doi:10.1016/j.enpol.2010.04.035.
^ ^a ^b ^c Smit, Berend; Reimer, Jeffrey A.; Oldenburg, Curtis M.; Bourg, Ian C. (2014). Introduction to Carbon Capture and Sequestration. London: Imperial College Press. ISBN 978-1-78326-328-8.

[:0-1] Benson, S. M.; Surles, T. (2006-10-01). "Carbon Dioxide Capture and Storage: An Overview With Emphasis on Capture and Storage in Deep Geological Formations". Proceedings of the IEEE. 94 (10): 1795–1805. doi:10.1109/JPROC.2006.883718. ISSN 0018-9219.

[:1-2] Aydin, Gokhan; Karakurt, Izzet; Aydiner, Kerim (2010-09-01). "Evaluation of geologic storage options of CO2: Applicability, cost, storage capacity and safety". Energy Policy. Special Section on Carbon Emissions and Carbon Management in Cities with Regular Papers. 38 (9): 5072–5080. doi:10.1016/j.enpol.2010.04.035.

[:2-3] Smit, Berend; Reimer, Jeffrey A.; Oldenburg, Curtis M.; Bourg, Ian C. (2014). Introduction to Carbon Capture and Sequestration. London: Imperial College Press. ISBN 978-1-78326-328-8.

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