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Frequently Asked Questions

1. What is CCS?

The Carbon Capture and Storage technologies (CCS) open up opportunities to prevent the release of large quantities of carbon dioxide (CO2, a greenhouse gas) into the atmosphere which is caused by the combustion of fossil fuels in power plants or other industrial facilities.
In a first step, the CCS-technology captures CO2 either before or after combustion of fossil fuels by using different techniques (pre-combustion, post combustion, oxy-fuel combustion). Capture is similar to what is already achieved in liquefied natural gas plants (LNG) for decades to only delivered CO2-free natural gas to the gas consumption network.

After the CO2 removal process the gas is transported usually via pipelines to the storage complex where it is pumped into appropriate deep geological formations. Pilot storage sites are primarily deep saline aquifers (e.g. Sleipner, Norway; Ketzin, Germany; Nagaoka, Japan) or depleted oil and gas reservoirs (K12-B, The Netherlands; Rousse, France). However, in the CO2CARE project alternative examples of trapping and storage are also studied such as the massive flood basalts near Wallula (Washington, USA).

2. Why CCS?

Scientific results of the last 20 years indicate that rising concentrations of carbon dioxide in the atmosphere, caused by human activities, significantly contribute to the greenhouse effect and consecutive global warming.
During the conference on climate change in Kyoto, Japan in 1997, the developed countries agreed to specific targets for cutting their emissions of greenhouse gases. In a first step, a general framework was defined which was ultimately filled with more details in the following years. This international agreement is known as the Kyoto Protocol.
Based on this foundation, members of the G8 and the European Union reacted and agreed on concrete actions for the global reduction of greenhouse gas emissions:
-    The G8 summit of 2007 issued a joint statement about the objective of reducing greenhouse gas emissions by 50% referring to levels of 1990. This target should be met by 2050.
-    The reduction in the developed world is planned to be 30% by 2020, and between 60% and 80% by 2050.
-    The leaders of the European Union and the G8 renewed their commitments in July 2009.
Within the portfolio of different measures to achieve these goals, one important option is the CCS technology (carbon dioxide capture (sequestration) and geological storage).
CCS comprises three main components:
-    separation, dehydration and compression of CO2 out of flue gas or other sources in a multistage process.
-    transportation mainly by pipelines or vessels.
-    injection into geological reservoirs, e.g. in deep saline aquifers.

Despite evolving alternative energies, oil, gas, and coal will most likely still be important energy resources in the next decades. Moreover, significant CO2 emissions are not only caused by power plants but also related to production technologies in the steel, chemical or cement industry. Therefore, without a significant and rapid reduction of fossil fuel-derived CO2 emissions reaching the committed goals of the Kyoto protocol and the targets of the G8 summits will be difficult, if not impossible. Information on climate change can be found in the reports of the Intergovernmental Panel on Climate Change (IPCC):

3. What are the main goals of CO2CARE?

According to the EC Directive 2009/31/EC - Guidance Document 3, the life cycle of a CO2 storage site can be generally subdivided into 6 phases as it is shown in the figure below.
life cycle of CO2 storage
CO2 Storage Life Cycle Framework (Source: European Commission: Implementation of Directive 2009/31/EC on the Geological Storage of Carbon Dioxide - Guidance Document 3)
CO2CARE deals with phases 5 and 6, post-closure/pre-transfer and post transfer of liability. The main goals of the project are closely linked to the three high-level requirements of the EU Directive 2009/31/EC, Article 18 for CO2 storage which are: 1. absence of any detectable leakage, 2. conformity of actual behaviour of the injected CO2 with the modelled behaviour, and 3. evolving of the storage site towards a situation of long-term stability.
These criteria have to be fulfilled prior to subsequent transfer of responsibility to the competent authorities, typically 20 or 30 years after site closure.
Considering this, CO2CARE aims formulating robust procedures for site abandonment which will meet the regulatory requirements and ensure long-term integrity of the storage complex.
 Project brochure (pdf)



4. What is the legal framework for Research & Development?

Research in the field of CSS technology has been supported by the European Union in the last two decades and is a priority of the current Framework Programme (FP7). A variety of CCS projects are financed in order to examine the risks and issues of feasibility, safe, secure deployment, and sustainability of the CCS technology. In 2009 the European Parliament and the Council of the European Union released the “CCS Directive 2009/31/EC”. This Directive needs to be transposed into national law in order to establish secure and long-term storage of CO2. However, not all Member States have completed this process so far. The compilation of international regulatory requirements on CO2 geological storage and site abandonment, its comparison and a detection of possible legal gaps is one of the tasks of CO2CARE.

5. What is the Emissions Trading Scheme?

In 2005 the EC introduced its CO2 Emissions Trading Scheme (ETS), a key instrument for the EU climate policy. This initiative includes all large industrial point CO2 sources (any from 20 MW capacity (thermal) upwards). The scheme applies only to CO2 emissions directly to the atmosphere. The trading scheme comprises between 10.000 and 12.000 industrial companies in Europe belonging to energy, steel, glass, and cement production. All these companies are responsible for about 40% greenhouse gas emissions in the EU. From 2013 onwards, ETS will include the aviation sector as well.
Each industrial plant receives permission to release a certain amount of CO2 per year, also called EU allowances (EUA, emission right for 1 tonne of CO2). Any excess allowances can be sold via some of the Carbon Trading Exchanges. If a company exceed the limit, they in turn have to buy extra allowances. Tradable units are the so-called Certified Emission Reductions, abbreviated CER. One CER is equivalent to one metric ton of CO2 emissions (tCO2e). Companies taking part in the CO2 Emissions Trading Scheme can buy or sell directly to other companies or at several Carbon Trading Exchanges, for instance the European Energy Exchange, the Asian Carbon Exchange (ACX-change) or the NASDAQ OMX Commodities Europe.

6. Is there already experience with CO2 storage?

Sound experience already exists at several places in the world, such as the large-scale CCS industrial pilot projects of the Sleipner field in the North Sea, In Salah in Algeria or the research pilot site Ketzin in Germany.

Example 1: At Sleipner, a natural gas field in the North Sea, for instance, CO2 is removed from the natural gas containing approximately 9.5% CO2, which is too much for a saleable quality. Since 1996, the captured CO2 is stored deep underground – at a rate of about one million tonnes of CO2 per year - in very porous and permeable reservoir rock above the gas field. Sleipner, operated by Statoil, was the world`s first demonstration of CO2 capture and storage. CO2CARE benefit from Statoil`s expertise, as full consortium member. (

Example 2: CO2 storage in an industrial scale project is carried out at In Salah, Algeria, which is in operation since 2004 and stores around one million tons of CO2 per year separated during the gas production process (

Example 3: In Germany, at the Ketzin pilot site located 40 km west of Berlin, geological storage of CO2 is being investigated since 2004 with participation of various national and international research groups. The German Research Centre for Geosciences GFZ is coordinating the research programme with a strong focus on monitoring technology (


7. Are there risks for the human health or the environment?

Before CO2 storage becomes operational, the location has to be selected carefully in terms of geological suitability, storage capacity, long-term stability, and long-term safety (Phase 1-3 = assessment, characterisation and development). This is probably the most important step in the lifecycle of a CO2 storage site (also see “What are the main goals of CO2CARE?”).  ..
In order to ensure negligible impacts on humans or the environment in the long term, specific criteria needs to be defined for each individual CO2 storage site. In this respect, international requirements and guidelines for CO2 storage must provide rules for safety. Both are key issues of the CO2CARE project.
Development, operational phase, as well as a transfer of responsibility after site closure will always be escorted, supported, and controlled by the responsible authorities. 
Potential leakage pathways for CO2 have to be considered from the beginning of a project in order to ensure long-term safety and anticipating any remediation action. Such potential pathways are: through and along wellbores, improperly abandoned old wells within the storage complex, and unsealed fault or fracture zones through the natural caprock. Another potential threat for the environment is the displacement of brines (salty water) in storage formations caused by CO2 injection. This could eventually lead to salinisation of potable shallow aquifers in some quite specific circumstances.
All these potential CO2 and saltwater pathways need to be monitored (see FAQ 10) during the injection phase and for some period of time after site closure. Therefore, risk management and the assessment of suitable monitoring tools or methodologies are further key issues in CO2CARE.
Each potential risk scenario of a storage site candidate is analysed in detail in order to ensure the safety of the storage site. The probability of the occurrence of irregularities and related consequences for humans and environment are evaluated so that a safe operation can be carried out.


8. Who will be responsible after site closure?

Generally, CO2 storage site closure needs approval by the responsible authority. Three main prerequisites must be fulfilled before a transfer of responsibility can happen following the EU directive:
  •     No detectable leakage.
  •     Observed behaviour of the injected CO2 conforms to the modelled behaviour.
  •     Site is evolving towards a situation of long-term stability.
“The responsibility for the storage site, including specific legal obligations, should be transferred to the competent authority, if and when all available evidence indicates that the stored CO2 will be completely and permanently contained” (DIRECTIVE 2009/31/EC).

Even if approval for site closure was given, in most cases the operator still holds responsibility. He must maintain all necessary equipment for monitoring pursuant to the monitoring plan which is usually part of the documents for the approval of site closure.
According to the EU Directive, a minimum period, to be determined by the authority, shall be no shorter than 20 years, unless the authority is convinced that all criteria have been met before the end of that period. Therefore, regulations typically contain a provision for liability of the site to be transferred once safety has been demonstrated. In most cases it is a period between 20 and 30 years.

9. What happens with the CO2 in the reservoir complex after site closure?

After CO2 has been injected for instance into a deep lying saline aquifer, the gas is trapped in different ways by physical and chemical mechanisms strongly dependent on time and on geological settings.
Generally, four basic trapping mechanisms for long-term stabilisation and immobilisation of CO2 are: structural and stratigraphic, capillary (residual), dissolution, and mineral trapping.

Structural and stratigraphic trapping
This type of physical trapping of CO2 happens at the top of an anticline or in a tilted fault block below low-permeability seals (caprocks), such as shale or salt beds. It is the principal means to store CO2 in geological formations. Sedimentary basins have such closed, physically bound traps or structures, which are occupied mainly by saline water, oil and/or gas including natural CO2 such as in Montmiral. Structural traps include those formed by folded or fractured rocks. Faults can act as permeability barriers in some circumstances and as preferential pathways for fluid flow in others. Structural/stratigraphic trapping takes place immediately after injection. The CO2 is less dense than the saline water and therefore it rises upwards into the trap.

Capillary (residual) trapping
Simply explained, porous sedimentary rocks behave like a sponge in which the injected CO2 is trapped within the pore spaces initially brine saturated. However, the trapping mechanism is strongly influenced by the geometry of the sedimentary rocks. The arrangement of pore spaces, the interconnectivity of the pore space, and the local capillary forces are responsible for this type of trapping. The most important mechanism for residual capillary trapping is the snap off, which is a disconnection of a small amount of CO2 of a continuous stream of the non-wetting CO2 when it passes through pore constrictions. CO2 is trapped in pore spaces due to surface tension. Over time, the CO2 trapped in pore spaces can dissolve into the formation water.

Solubility trapping
When CO2 comes in contact with brine, as it is the case in a reservoir, part of the gas dissolves in the formation water (CO2(aq) + H2O  ↔  H+ + HCO3- ) at specific conditions of pressure, temperature and salinity. This process is commonly called solubility trapping and is very beneficial because groundwater saturated with CO2 is denser than before and so will tend to sink in the reservoir. CO2 dissolution is a significant trapping mechanism therefore and saturating formation brine with CO2 would create huge CO2 storage capacities. However, complete solubility trapping is a long-term process operating on timescales of hundreds to thousands of years depending on convection effects, reservoir petrophysical properties and aquifer dynamism.

Mineral trapping
Over time, chemical reactions between the surrounding minerals of the reservoir rock and the CO2-rich water will take place and leads to precipitation of new mineral phases such as  calcite (CaCO3), dolomite (CaMg(CO3)2), siderite (FeCO3), or dawsonite (NaAlCO3(OH)2). This is the most stable form of CO2 trapping. However, under favourable conditions this conversion process of CO2 into carbonate minerals is extremely slow and needs thousands of years.


10. How can the propagation of the CO2 be detected and traced?

After a certain quantity of CO2 has been injected, for instance into a deep lying porous sandstone formation, the propagation of the CO2 plume can be traced (monitored) by established geophysical methods. Appropriate are mainly time-lapse seismic and geoelectric monitoring. 
The example below shows 3D-seismic data reflecting the development of the CO2 plume in the highly porous sandstones of the Utsira-Formation at Sleipner (Norway) between 800 and 1200 meters depth. The baseline data were acquired in 1994, followed by repeat surveys in 1999, 2001, 2002, 2004, and 2006 corresponding to 2.30, 4.20, 4.97, 6.84, and 8.4 million tonnes (Mt) of CO2 in the reservoir (Arts, R. et al. 2008).
Development of the CO2 plume over the years imaged with seismic data (Arts, R. et al. 2008).

For long term predictions of the CO2 migration, numerical modelling tools are used. These tools simplify a complex site-specific system and therefore needs to be proved by on site measurements and calibrations during the operational and post closure phase of a CO2 storage life cycle. If the results are satisfying and the measured data conform with the modelled behaviour (high level requirement), then predictions for hundreds or even thousands of years are possible with sufficient certainty.  

Arts, R., Chadwick, A., Eiken, O, Thibeau, S., and Nooner, S. (2008): Ten years’ experience of monitoring CO2 injection in the Utsira Sand at Sleipner, offshore Norway, first break volume 26, January 2008, special topic – CO2 Sequestration, pp. 65-72  2008 EAGE.