Since 2004, we have been investigating the storage of carbon dioxide (CO₂) in deep geological formations. The focus of our work lies in understanding the processes that take place during injection and the CO₂ migration in the storage formation. The project work is integrated with national and European research programs.

There are four central objectives for the CO₂MAN research project:

• Monitoring the migration behavior of the injected CO₂,
  • Determining the sensitivity of individual monitoring methods
  • Developing geophysical monitoring concepts for the CO₂ storage
• Characterizing and quantifying the CO₂-induced interactions between fluids, stone and the microbial community in the storage system
• Validating the tools for statistical modeling and dynamic simulation at the real-life storage site in Ketzin
• Transferral of knowledge to the public and to interest groups, policy-makers and approval agencies

All of the approvals for the Ketzin pilot site were provided by the Landesamt für Bergbau, Geologie und Rohstoffe Brandenburg (Brandenburg State Office of Mining, Geology and Raw Materials, (LBGR) in Cottbus.

• The CO₂ storage in Ketzin has been operated safely and reliably since the start of injection (June 2008) until the end of injection (August 2013).
• The geophysical methods used allow us to localize very small volumes of CO₂ within the reservoir. These sensitive measurement methods will therefore also work reliably at future sites with significantly larger CO₂ volumes.
• Computational simulations correspond very well to the monitoring results.

The CO₂ storage process is accompanied by a detailed geophysical and geochemical program that monitors the reservoir at a depth of 650 m from the earth's surface. The monitoring methods used at the Ketzin pilot site are among the world's most extensive and innovative methods used in the field of CO₂ storage.

The current planning period for this research extends to 2018.

Since 2008 till August 2013 67,271 tons CO₂ were injected.

CCS stands for "Carbon Capture and Storage" and describes the capture, transport and permanent geological storage of CO₂ in rock layers of the deep underground. CCS aims to avoid CO₂ emissions which are formed by the combustion of fossil fuels and in industrial processes to the atmosphere. There, CO₂ works as a greenhouse gas and is substantially responsible for the climate change. Worldwide, work is currently in progress to use CCS as an innovative technology to mitigate the climate change and its negative consequences.

It is not certain whether the development of renewable energy and the more efficient use of energy as a key strategy against the climatic change in the future will be sufficient. Despite all efforts for an environmentally friendly energy supply, fossil fuels will very probably play a central role in the energy supply worldwide and particularly in today's fast-developing nations for many decades to come. For many CO₂-intensive industrial processes (e.g. cement, steel industry), there is also no prospect of low-carbon alternatives. Therefore, many industrial countries investigate the CCS technology to accomplish the global climate targets.

Until the measures for an expansion of the renewable energies and a more efficient use of the fossil fuels work effectively worldwide, capture and storage of CO₂ could, within a transition period of some decades, play a central role for the climate protection.

According to model calculations of the IPCC (Intergovernmental Panel on Climate Change), CO₂ storage can prospectively make an essential contribution to greenhouse gas reduction (within the limits of) by reducing global CO₂ emissions up to 30%.

As the numerous natural CO₂ storage sites prove, CO₂ storage is an invention of nature.
Decades of experiences with the storage of natural gas in porous rocks can be used technologically. Germany is the EU's largest and worldwide the fourth largest user of gas storage.

Numerous process steps are already known from gas and petroleum industry and state of the art, others must be adapted to the special requirements of the CO₂ storage.

Furthermore, knowledge was gained particularly in the following ongoing large demonstration projects and smaller pilot sites for geological CO₂ storage:

• In the Norwegian North Sea about 1 million tons of CO₂ are injected annually into a sandstone formation at about 800-1000 meters depth in the Sleipner gas field since 1996.
• In the Algerian In Salah up to 1 million tons of CO₂ are stored annually in a sandstone formation at depths of 2000 meters since 2004.
• At the Ketzin pilot site about 67,271 tons of CO₂ were injected since 2008 (status 08.2013).

With regard to climate protection large amounts of CO₂ must be captured. An economic separation of large quantities of CO₂ is currently only feasible at large point sources. These are mainly coal and gas-fired power plants, refineries, cement and steel plants. First research approaches also investigate CO₂ capture as a direct separation of CO₂ from the air.

For large quantities of CO₂ from industrial plants, particularly at power plants, transportation by pipelines is cheapest. An extensive pipeline net for the CO₂ transport already exists in the USA. For the transport of large quantities of CO₂ by sea, liquefied natural gas (LNG) tankers can be used, as they are already used for natural gas. Smaller amounts of CO₂ to about 100,000 m³ as they are used for example by the food industry are transported by truck, train or ship using pressure vessels.

Carbon dioxide is a chemical compound composed of one carbon atom and two oxygen atoms (sum formula CO₂₎. Under normal atmospheric temperature and pressure conditions CO₂ is gaseous. With a density of 1.98 kg/m³ (under atmospheric conditions) it is about 1.5 times heavier then air. The chemically inert gas is colorless and odorless, does not burn or explode and dissolves rapidly in water producing carbonic acid.

By increasing pressure or decreasing temperature gaseous CO₂ becomes liquid. Cooled down to about -20 °C and pressurized to about 18 bar, liquid CO₂ is transported for example with fuel tankers for the beverage industry. At temperatures below -78 °C and atmospheric pressure, the gas sublimates to solid CO₂, known also as dry ice.

CO₂ is used for example in the production of beverages or in the form of dry ice as coolant in the industry. Since it is not combustible and not explosive, it is used as a fire-extinguishing agent due to its oxygen displacing effect.

CO₂ is essential for life on earth because plants need CO₂ for their metabolic processes in order to build organic matter. In the global carbon cycle CO₂ has a key role.

CO₂ as one of the greenhouse gases is decisively responsible for the global climate since the emergence of the earth's atmosphere. CO₂ accounts for about 20 % of the natural greenhouse effect. The natural greenhouse effect bases on the partial retention of energy (released by the Earth´s crust) by so called greenhouse gases in the atmosphere. This effect is responsible for the life-friendly environment on earth. Otherwise, the average temperature on the Earth's surface would be only about -18 °C. However, more than 60 % of the additional greenhouse effect caused by man is due to CO₂.

The concentration of carbon dioxide is about 0.04 volume % or about 400 ppm (status 2012) in today's earth's atmosphere. Plants absorb this carbon dioxide to produce oxygen and carbohydrates (photosynthesis). CO₂ is therefore vital for any faunal and vegetable life and growth.
Increased carbon dioxide concentrations in the air seem to be even growth-promoting to plants. Therefore, CO₂ is frequently added to the air in greenhouses. However, too high CO₂ concentrations (about 0.1 volume %) can harm plants.

In reversal of the photosyntetic process humans and animals produce CO₂ during their metabolic processes. To generate energy they turn over organic substances (food) using oxygen.Therefore, the CO₂ concentration in the breathing air during exhalation has risen a hundredfold to 4 volume %.

In low concentrations CO₂ is ubiquitous and not harmful. However, higher CO₂ concentrations are harmful to health and concentrations of about 5 volume % lead to dizziness, headaches and shortness of breath. The extended-stay (about 30 minutes) at CO₂ concentrations higher than 8 volume % can lead to death.
Among others, accidents are known from wine cellars, food silos, wells or cesspools where considerable amounts of CO₂ form by fermenting processes. At insufficient ventilation harmful CO₂ concentrations can form at ground level primarily due to his higher density in comparison with air.

Deep porous rock formations onshore or offshore are considered as possible storage sites.

The most important options for the geological CO₂ storage in Germany, but also worldwide, are permeable, porous rock layers whichpores are filled with salt water. These are so-called saline aquifers. Depleted gas or oil fields offer considerably lower storage capacity but can be used, too.

When storing the CO₂ the naturally existing pore space within the rock formation is used. Therefore reservoir rocks must have porosity as high as possible and a good connection between the pores, so that gases or liquids can migrate quickly in the rock. In addition, the rock layer has to have a sufficiently large extension in order to provide adequate storage capacity.

For CO₂ storage, suitable rocks are considered with a minimum depth of 800 meters. Under the corresponding pressure and temperature conditions CO₂ has a comparatively high density (approx. 600 kg/m³) and thus the pore space of the storage rocks can be used efficiently.

Since the density of the stored CO₂ is lower than the density of the saline formation water (~1100 kg/m³), buoyancies have an effect on the stored CO₂. In order to hold back the CO₂ in the storage reservoir, they must therefore be overlain by at least one (ideally multiple) impermeable cap rock. Such an impermeable barrier can be e.g. mudstones or salt rocks.

The location of the storage site in a suitable geological structure (e.g. an on-curvature of the rock layers in underground) can limit the lateral CO₂ migration.

Porous and well permeable rock formations are primarily found in extended sedimentary basins of the world. Sediment basins are large-scale areas of subsidence. In the course of long geological time periods thick sedimentary layers have piled up there.

The underground in Northern Germany, for example, is part of a large sedimentary basin that extends from England to Poland. This area offers in principle good prerequisites for the geological storage of CO₂. Further, smaller sedimentary basin structures in Germany are the Rhine Rift, the Thuringian Basin, or the so-called Molasse basin on the northern edge of the Alps. Every potential CO₂ storage site must be carefully checked for its actual suitability.

For Germany, the experts of the Federal Institute for Geosciences and Natural Resources (BGR) currently estimate the CO₂ storage capacity in porous rocks at approximately 6 to 12 giga-tons (6-12 billion tons). In contrast about 0.4 giga-tons of CO₂ are produced at large point sources per annum (e.g. energy, steel, cement and petroleum industries). This means that national capacities would be available for some decades.

The worldwide storage capacities are assessed with at least 2,000 billion tons of CO₂. Optimistic estimates suspect even 11,000 billion tons of storage potential. In comparison with the global emissions which were more than 34 billion tons of CO₂ in the year 2011, a theoretical storage capacity of considerably more than 50 years exists here.

CO₂ is injected under pressure into the reservoir formation via one or more wells. Thereby, the salt water in the pores will be displaced. Due to the impermeable cap rock which located above the saline aquifer, the CO₂ and the salt water can not escape. Before starting the storage of CO₂ it must be ensured that the cap rock can withstand the resulting pressure increase.

The injected CO₂ will rise to the highest point of the storage formation and accumulate below the impermeable cap rock due to its low density compared to that of the formation water. A part of the CO₂ is already held back by capillary forces on its way up. Futher CO₂ will dissolve in the formation water in the course of the time and new minerals can form which also bind CO₂ durably.

Two possibilities exist for CO₂ storage in oil and gas deposits:

1. Nearly depleted deposits, where, with the help of the injected CO₂ additional quantities of natural gas and oil can be produced, that would otherwise remain in the deposits. This is usually referred to the term "Enhanced Gas Recovery (EGR)" and "Enhanced Oil Recovery (EOR)".
2. Completely depleted gas and oil deposits provide space in the rock pores for the storage of CO₂.

The cap rocks of gas and oil deposits are proven to be impermeable as they have already kept the gas and oil in the underground for millions of years.

Partly yes, because the underground is already today used by the mining (coal and salt), the production of oil and natural gas or geothermal energy extraction. The underground storage of mineral oil and natural gas in porous rocks or in artificial caverns in salt rocks (to ensure the raw material supply) also adds to this. In future, the storage of methane or hydrogen (produced by renewable energies) will play a role, too.

The advantages and disadvantages of the different options of usage must be weighed up at all locations. It is necessary to check whether a joint use of the underground is possible, too.

There is no competition to the shallow geothermal energy extraction which uses the underground to max. 400 meters depth or usages that require caverns.

For the storage of CO₂ in saline aquifers, at some locations it might be necessary to drill relief wells in some distance of the CO₂ injection well. Thereby, the resulting pressure increase can be reduced by extracting the displaced salt water. Currently, it is checked whether such wells could also be used for geothermal energy extraction. Thus two usage options could even complement each other.

At the Ketzin pilot site in the State of Brandenburg and at other locations around the world it turned out that CO₂ storage on the research scale can be done safely and reliably. The two large industrial storage projects Sleipner (Norway) and In Salah (Algeria) also underpin that this technology could contribute to climate protection in future.

Natural CO₂ resources exist worldwide and many of them are examined very well. CO₂ reservoirs in which the gas was held back in the underground over millions of years are important references for mechanisms and favorable geological prerequisites of the CO₂ storage. On the other hand, gas emissions from natural CO₂ sources allow interferences about inappropriate geological conditions and leakage pathways and mechanisms.

Natural CO₂ sources that have existed for millions of years are frequently found in large sedimentary basins in geologically stable regions. Therefore, these areas are particularly suitable for the storage of CO₂.

Numerous monitoring methods are already available. Many of these are standards of the oil and gas industries and were already partly adjusted to the characteristics of CO₂ storage. Researchers also develop new methods and improve the quality of already existing ones. At the Ketzin pilot site the researchers of the GFZ German Research Centre for Geosciences and the project partners test a variety of monitoring systems on different scales (see http://www.co2ketzin.de).

For a comprehensive storage monitoring not only the combination of different methods such as geoelectric, seismic, temperature and pressure monitoring and the analysis of liquid and gas samples is important, but also the combination of different measurement setups with various temporal and spatial resolutions within each method.
Modelling and simulations are also important tools to describe and to forecast the processes in the storage system.

For each CO₂ storage site an adapted monitoring strategy is needed. It defines the use of appropriate methods and is essential for the risk assessment and the verification of safety issues and the efficiency of a CO₂ reservoir.

If CO₂ should, despite all the precautions, escape to the surface the danger posed by it is low compared to other gases (e.g. natural gas) because CO₂ is non-toxic and neither combustible nor explosive. Depending on the conditions, such as flow rate, type of terrain, wind speed and direction, escaping CO₂ is rapidly mixed with the ambient air and thereby diluted to a safe level. However, if too high CO₂ concentrations are breathed in permanently, CO₂ can be harmful for humans. Therfore, the processes of potential CO₂ leaks and their effects have to be fully understood in order to select and operate locations with the highest security possible.

Knowledge of natural CO₂ leakages can help to estimate the risk potential and to implement risk prevention measures. Many people live in areas with natural CO₂ leakages. Near Rome, people live in 30 m distance to gas leakage locations where about 7 tons of CO₂ escape daily and soil gas concentrations of 90 % are reached.

In principle yes, if natural or anthropogenic leakage pathways for the CO₂ exist above a reservoir. Pre-existing fractures or fault zones in the overlaying cap rocks could be natural pathways for the CO₂ into shallower groundwater levels. This risk must be minimized by careful geological exploration of the potential storage site.

Manmade pathways may occur at active or already closed wells. However, there are already decades of experience with the safe closure of wells in oil, natural gas and natural CO₂ reservoirs. At present, these methods are refined and newly developed worldwide in order to optimize them for the requirements of CO₂ storage.

Dissolution of CO₂ in the salt water of the reservoir rocks forms carbonic acid and makes the water more acidic. The carbonic water can dissolve substances from the rocks of the aquifer and thus the water composition changes. Degree and nature of any possible change cannot be predicted in general. But they primarily depend on the mineralogical composition of the reservoir rock and also on the quality of the water and can be estimated in advance during the geological exploration.

In fresh water, in principle the same processes in contact with CO₂ occur as in salt water. Dissolved CO₂ can have different impacts on the possible use as drinking water. On the one hand, water with dissolved CO₂ and dissolved mineral components (of the rocks of the aquifer) may make the water unsuitable for human use. On the other hand, worldwide many CO₂ enriched groundwater are coveted mineral waters, which are used for medical purposes or sold in bottles.

In general, earthquakes can be generated during the CO₂ storage due to the increasing pressure. Barely perceptible micro-earthquakes are very well known in this context. The risk of cracking and resulting earthquakes is minimized by the slow injection of CO₂ into the geological storage formation. Thereby the pressure is continuously monitored in order to not endanger the integrity of the cap rocks.

The gas storage technique provides knowledge for example for the technology of the gas injection or for the general migration of gases in porous rocks.

The technology used to store large quantities of natural gas in deep underground rock formations in order to compensate seasonal fluctuations in demand has proven successful in many parts of the world for decades. The storage volume of more than 40 gas reservoirs in Germany amounts to about 20 billion cubic meters of natural gas.

At present, the geological storage of CO₂ is explored near Ketzin/Havel west of Berlin. There, CO₂ is injected into approximately 650 meters deep porous sandstone layers. Since 2008 till August 2013 67,271 tons of CO₂ were injected.
The scientific research program focuses on the monitoring methods. With a modern and very comprehensive geochemical and geophysical monitoring program profound knowledge of to the migration of the CO₂ in the underground could be gained. The pilot site Ketzin demonstrates on a research scale that the geological storage of CO₂ is feasible safely and reliably.
(See http://www.co2ketzin.de).