Storage options

The main options for geological storage of CO2 in Germany are depleted oil and natural gas storage sites and deep saline sandstone layers (aquifers). Other options, such as storing CO2 in coal strata or caverns, are not currently being discussed to any significant degree (Geotechnologies Science Report, 2009).

The use of any potential storage formation involves the following prerequisites:

  • sufficient storage capacity (absorption capacity)
  • presence of a geological barrier (cap rock) that will securely seal the reservoir over a long period of time and remain stable even under increased pressure
  • predictable interactions between the CO2 and the reservoir that do not hinder the storage process

Storage capacity is defined by the size of the storage reservoir, geographic distribution, porosity and permeability. High porosity means that the rock contains a high proportion of pores relative to its overall volume, and permeability measures the ability of such porous rocks to conduct a liquid.
Another important factor is the density of the CO2, which is defined by the pressure and temperature conditions within the storage formation and by the purity of the CO2. Since CO2 has a lower density than the water in the pores, it starts to move upward after being injected. Thus the most important requirement for storage is a layer of barrier rock, for instance salt or clay, that acts as a seal. The shape of the storage formation and the cap rock determines the maximum permissible storage pressures. Anticlinal formations (arches) are advantageous for storage because they can be used as natural traps where the CO2 collects, as if under a bell.

Schematic of large CO2 sources and potential CO2 storage reservoirs in Germany

The injected CO2 displaces some of the formation water that is present in the rock. It also interacts with the various materials in the reservoir (liquids, gases and rock).
New mineral formations, binding the CO2 in small pores and dissolving the CO2 in the existing saline formation water all create possible retention mechanisms that can bind the CO2 in the long term.

Above all, it is important to know where and to what extent there are appropriate storage capacities, and where natural or anthropogenic disruptions (e.g. drilling) must be taken into consideration. To this end, the Bundesanstalt für Geowissenschaften und Rohstoffe (German Federal Institute for Geosciences and Natural Resources, BGR) founded an initiative in 2008 in close cooperation with the official state geological services (Catalogue of Storage Capacities in Germany).
By far the greatest potential for deep geological CO2 storage in Germany is in the North German Plain (see ill. at right). Reaching across all of northern Germany, it is extremely significant for the geotechnical and energetic use of the deeper underground level (Geotechnologies Science Report, 2009).

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