Geological storage

The minimum period of time during which CO2 needs to be stored is long but not unlimited. It has to last over one to two centuries, in order to cover the period during which the issue of CO2 emission is likely to remain critical. Beyond such a period of time, CO2 stored underground might be used as a source of carbon, and could be recycled by using a low carbon energy which should become widely available by that time. In order to keep the option fully safe, storage time durations which are presently considered amount generally to one or several thousand years.

Storage in deep oceans has also been considered as a potential option, but suffers two major drawbacks: firstly, the long term behaviour of such storage is difficult to model; secondly, the impact of an increase of the CO2 concentration on marine ecosystems is difficult to assess and potentially dangerous. An accumulation of CO2 might lead to an excessive acidification of the ocean. An increase of the temperature might

1 Above a certain pressure, called the 'critical pressure' in the case of a pure component, it is no longer possible to obtain two phases, one liquid and one vapour, by lowering the temperature.

reduce the CO2 solubility, leading to massive release of CO2. For all these reasons, this option is not considered as acceptable by the European Union.

Underground geological storage is therefore the solution generally favoured. Numerous natural underground CO2 reservoirs already exist in different regions of the world. Such storage is based upon the fact that sedimentary rocks are, in general, porous. Sandstone, which is formed by agglomerated sand particles, is a good example of such sedimentary rocks. The rock porosity is generally filled by water (salt aquifer). Carbon dioxide is injected through a well in the sedimentary rock and forces out the water. Using such a porous layer as an underground reservoir also requires the presence of a cap rock above the sedimentary layer, which acts as a seal preventing upward migration of CO2.

The main possible options for CO2 underground storage are illustrated in Figure 8.1. Captured CO2 can be stored in depleted oil and gas fields, deep saline aquifers and unexploited coal seams.

Figure 8.1 Carbon dioxide underground storage modes (Source: IFP-ADEME-BRGM [90]). Reproduced with permission from Energie & Climat: Réussir la transition énergétique by Alexandre Rojey, Editions Technip, Paris, 2008
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