Once injected underground into a suitable reservoir* rock, the CO2 accumulates in the pores (between grains) and in natural cracks, thus displacing and replacing any existing fluid such as gas, water or oil. Suitable host rocks for CO2 geological storage should therefore have a high porosity* and permeability*. Such formations, which are the result of the deposition of sediments in the geological past, are commonly located in “sedimentary basins”. These permeable formations alternate with impermeable rocks, which can act as an overlying seal (or caprock). Sedimentary basins often host hydrocarbon reservoirs and natural CO2 deposits, which proves their ability to retain fluids for long periods of time, having naturally trapped oil, gas and even pure CO2 for millions of years.
The subsurface is often illustrated as an over-simplified, homogeneous, layer-cake structure. In reality, it is composed of unevenly distributed and locally faulted rock formations, reservoirs and caprocks forming complex, heterogeneous structures. In-depth knowledge of the site and geoscientific experience are required to assess the suitability of underground structures that are proposed for long-term CO2 storage.
Potential CO2 storage reservoirs must fulfil several key criteria:
Sedimentary basins are widespread throughout Europe, for example offshore in the North Sea or onshore surrounding the Alpine mountain chains (Fig. 6). Many formations in the European basins fulfil the criteria for geological storage, and are currently being mapped and characterized by experts. Other European areas are composed of ancient consolidated crust, such as parts of Scandinavia, and do not host rocks suitable for CO2 storage.
One example of an area with potential for storage is the Southern Permian Basin, which extends from England to Poland (represented on Fig. 6 by the largest ellipse). The sediments have been affected by rock-forming processes that left some of the pore space filled with saline water, oil or natural gas. The clay layers that exist between the porous sandstones have been compacted to low-permeability strata, which prevent fluid ascent. Much of the sandstone formations are located at depths between 1 and 4 km, where pressure is high enough to store CO2 as a dense phase. The salt content in the formation waters increases in this depth interval from about 100 g/l to 400 g/l, i.e., much saltier than seawater (35 g/l). Movements in the basin have caused plastic deformation of the rock salt, creating hundreds of dome-shaped structures that subsequently trapped natural gas. These traps that are being studied for CO2 storage sites and pilot projects.
Storage capacity estimates are usually highly approximate and based on the spatial extent of potentially suitable formations. Capacity can be assessed on different scales, from national scale for rough estimates, through to basin and reservoir scale for more precise calculations that take into account the heterogeneity and complexity of the real geological structure.
Volumetric Capacity: Published national storage capacities are generally based on calculations of the formations' pore volume. In theory, the storage capacity of a given formation can be calculated by multiplying its area by its thickness, its average porosity and the average density of CO2 at reservoir depth conditions. However, because the pore space is already occupied by water, only a small part can be used for storage, generally assumed to be about 1-3%. This storage capacity coefficient is then applied when assessing the volumetric capacity.
Realistic Capacity: More realistic capacity estimates can be made on single storage sites through detailed investigations. Formation thickness is not constant, and reservoir properties can vary over short distances. Knowledge of the size, shape and geological properties of structures allows us to reduce the uncertainties in the volume calculations. Based on this information, computer simulations can then be used to predict CO2 injection and movement within the reservoir in order to estimate a realistic storage capacity.
Viable Capacity: Capacity is not merely a question of rock physics. Socio-economic factors also influence whether or not a suitable site will be used. For example, moving CO2 from the source to the storage site will be governed by transportation costs. Finally, political choices and public acceptance will have the last say as to whether or not the available reservoir capacity will actually be exploited.
We know that the capacity for CO2 storage in Europe is high, even if uncertainties exist related to reservoir complexity, heterogeneity and socioeconomic factors. Several EU projects carried out an estimation of European storage capacity potential. The EU project GESTCO (2000-2003)* estimated the CO2 storage capacity in hydrocarbon fields in and around the North Sea at 37 Gt, which would enable large installations in this region to inject CO2 for several decades. Updating and further mapping of storage capacities in Europe is a matter of research, in individual member states and through the EU projects for Europe at large. Some examplses are Geocapacity* or CO2StoP* (2012-2013) – “CO2 Storage Potential in Europe”-, a public georeferenced database of CO2 storage locations in Europe and a tool to compute storage capacities and injection rates.