Frequently asked questions
- What is CCS?
- What does CCS cost?
- Isn’t it better to spend the money on renewable energies instead of CCS?
- Is there enough storage capacity?
- How safe is the underground storage of CO2?
- Has the CCS technology been sufficiently tested?
- What are BGR’s goals?
What is CCS?
CCS is short for Carbon (Dioxide) Capture and Storage, comprising the whole workflow capture, transport and storage in deep geological formations.
The capture of CO2 is technologically complex. Therefore, it is only viable for large, stationary power plants or industry complexes. There are three main methods of capturing CO2:
- capture of CO2 from the flue gas after combustion (post combustion)
- separation of carbon from the fuel before the combustion (pre combustion)
- combustion with pure oxygen (oxyfuel)
All three methods still imply an increased energy consumption as compared to standard production processes. This means an increased fuel consumption, a decreased efficiency factor and thus higher electricity production costs. Therefore, current R&D efforts are focussed on improving capture technologies and efficiency enhancement.
CO2 density (kg/m3) versus depth (m)
Source: BGR
The captured CO2 then needs to be transported to the storage site, either by pipeline, truck or ship. A pipeline is normally the most convenient medium. To keep transport costs at a minimum, storage sites should be as close as possible to the emission sources, and existing pipeline lengths used wherever possible.
The storage of CO2 requires a porous rock medium in at least 800 m depth. At this depth, the prevailing pressure greatly reduces the volume of the injected CO2, compared to the earth's surface. With further depth, pressure and temperature of the earth continue to increase, but the density of the compressed CO2 remains almost the same.
What does CCS cost?
The costs for CCS are controlled by the three process steps capture, transport, and storage. Of these three, the capture costs in fossil fuel power plants are dominating. In addition, an efficiency loss and an increased primary energy consumption have to be taken into account. The costs for transport and storage depend on the distance and depth of the storage site, but are substantially lower than the costs of capture.
Isn’t it better to spend the money on renewable energies instead of CCS?
BGR is also active in developing and testing the use of deep geothermal energy.
However, 82 % of the primary energy consumption in Germany is based on fossil fuel commodities (see figure). Even though the contribution of renewable energies like wind and water will increase over the next few years, they will not be able to cover our energy need for decades to come. But to mitigate the impact on climate, it is necessary to reduce the emission of relevant gases immediately. Carbon capture and storage (CCS) is one important element in the portfolio of climate protection measures – together with energy saving, increase of energy efficiency, and further development of renewable energy sources. (See also: Integrated Climate and Energy Policy of the German Federal Government.)
Primary Energy Consumption in Germany 2007. Green – Oil, Orange – Gas, Black – Hard Coal, Brown – Lignite, Yellow – Nuclear Energy, Blue – Water and Wind Energy, Olive – Other.
Source: Arbeitsgemeinschaft Energiebilanzen e.V.
Is there enough storage capacity?
Storage capacities in Germany: Blue = aquifers, yellow = depleted gas fields, red = CO2 sources
Source: BGR
Depleted gas reservoirs are considered a very promising CO2 storage option, because they have inherently proven that they can safely store gases for millions of years. Furthermore, the geology of producing fields is well known, and existing infrastructure might be used. Another advantage and economical incentive is the possibility of enhanced gas recovery (EGR) of nearly depleted fields by the injection of CO2. The storage capacity of depleted gas fields in Germany is about 2,75 Gt (billion tons).
Depleted oil reservoirs are a viable storage option for the same reasons. However, in Germany they are too small to make a substantial contribution to CO2 storage. Their storage potential is only about 130 Mt (million tons).
Due to their large extent, deep saline aquifers have the largest potential for CO2 storage in Germany. Because of their depth and high salinity, they cannot be used for drinking water. Their storage potential is estimated to be roughly 20 Gt (billion tons).
For comparison: CO2 emissions in Germany are currently about 850 Mt per year (source: Deutsches Institut für Wirtschaftsforschung), of which 40 % are produced in coal power plants. If every single coal power plant would be equipped with CCS technology, this would yield a maximum of 350 Mt of CO2 for underground storage per year.
How safe is the underground storage of CO2?
It can be said that the storage of CO2 in the deep underground has only minor, manageable risks. Storage safety basically implies two phases: the operations phase during injection, and the long term safety after the closure of the storage site.
During the operations phase, the single most important safety factor is the safe technological handling as known from “best practice” procedures, developed from decades of oil & gas production and the operation of gas storage sites. Compared to natural gas (methane), CO2 is even less dangerous, because it is neither explosive nor flammable.
On the other hand, long term safety is ruled by the geological conditions of the storage site and its vicinity. To ensure an impact on climate and to keep the stored CO2 safely away from the biosphere, it must remain underground for at least 10,000 years. From natural gas reservoirs, we know that certain geological strata are capable to hold gases in place for millions of years. This would also apply for CO2. It is clear that CO2 storage projects can only be approved where reservoirs are safely covered by tight cap rocks. At carefully selected storage sites with good geological barrier(s), the only other important safety factor is the quality of well completion.
Every storage site must be monitored during and after the operation phase by surface, groundwater, and geophysical depth surveys.
What are the geological criteria for a safe storage site?
3D model of geological strata at about 3500 m depth, folded into a closed anticlinal structure. Purple layers – reservoir rocks (sandstone), pink layers – impermeable rocks (claystone).
Source: BGR
The geological criteria for safe storage sites are similar to those found in natural oil and gas rervoirs. For the reservoir (or storage rock) itself, a porous rock with good porosity and permeability is needed, to hold the quantities of CO2 required. The reservoir then needs one or more tight cap rock layers that are impermeable for gases and liquids. A closed anticlinal structure has the advantage of a straightforward capacity calculation and safe CO2 plume migration prediction. The overlying strata should ideally not be cut by faults, and if they are, these faults should have been proven tight (e.g. because of claystone smear).
What impact does CO2 have on groundwater and rock matrix?
The sweet groundwater used for drinking water is not affected at all, because the CO2 will be injected into far deeper, saline water bearing rock layers. These saline aquifers are separated from the sweet water aquifers higher up by several massive impermeable rock layers that keep both the saline waters and the CO2 separate from the surface near groundwater.
Pure CO2 is lighter than water and moves upward, displacing the brine towards the bottom. Slowly CO2 dissolves in the brine, lowering its pH value. Ideally, it reacts with Calcium iones and precipitates as Calcite. On the other hand, the carbonic acid is corrosive and might affect well cements, especially if they are old and badly completed. Such wells must be worked over prior to storage operation.
There are numerous research projects ongoing about the interactions between CO2, the formation waters, and the rock matrix. The results are used in the practical tests of the CCS technology.
How can the storage sites be monitored?
Monitoring CO2 storage sites implies two phases: the operations phase during the injection and the long term safety after the closure of a site.
During the operations phase, injection pressure and plume development should be monitored regularly. This is necessary to make sure that the reservoir pressure does not increase beyond tolerable limits and that the CO2 does not escape the planned storage area. The latter can be done with geophysical methods, both during the injection phase and afterwards.
At the surface, monitoring the soil gas emanations should focus on all wells and known geological faults. To discriminate natural soil gas emanations from CO2 leakages, the CO2 baseline must be known prior to the beginning of storage. The natural CO2 emanations vary with vegetation, seasons, and weather. Also the fluid dynamics and geochemistry of the surface near groundwater should be monitored regularly.
What happens if CO2 does escape the storage site?
The Andernach cold water geysir – a natural CO2 vent
Source: BGR
Should CO2 gas – despite all safety measures – escape the storage site and make its way to the surface, it is relatively harmless compared to other gases, e.g. methane. It is neither flammable nor explosive, nor poisonous. Like other gases, it can cause problems for human health in higher concentrations. However, we know from natural CO2 emanations in the Eifel region that the vented CO2 mixes rapidly with air and is quickly diluted to breathable concentrations.
Can CO2 storage cause earthquakes?
While injecting the CO2 into the geological reservoir, one of the key issues is pressure control. The injection process has to be constantly monitored and controlled so that the overlying seal rock does not crack. Because earthquakes are caused by such cracks, best practice injection procedures would effectively prevent them from happening.
Has the CCS technology been sufficiently tested?
For many years, practical experiences injecting CO2 into geological formations have been gained in the production of natural gas. Many natural gas fields already contain a percentage of CO2 that lowers the fuel value and needs to be separated before sale. At the Norwegian gas field Sleipner, since 1996 roughly 1 million tons of CO2 have been injected yearly into the Utsira Sandstone above the actual gas field. In In-Salah, Algeria, the CO2 is injected back into the reservoir with two to three wells, actually increasing the production rate of the usable natural gas.
Germany is the largest operator of underground gas storage sites in the EU and fourth worldwide after the USA, Russia and Ukraine. Natural gas has been stored underground for many years in order to avoid delivery shortages and demand variations. The storage sites can be subdivided into cavern sites (artificial caves in salt rocks) and porous reservoirs (natural rocks with pores). The storage of gases in porous reservoirs, as will be the case with CO2 sequestration, has therefore been successfully applied for decades. Also the transport of CO2 works basically the same way as with natural gas.
What are BGR’s goals?
BGR is committed to sustainable use of natural resources and protection of the human habitat. As a neutral institution feeling responsible for the future we advise German ministries and the European Community and act as partners in industry and science. The leading motive of our daily work is “Improvement of living conditions by sustainable use of the geo-potentials”.
Regarding CCS this means that BGR supports the goals formulated in the German government’s “Energy and climate protection programme” with research and advice.
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