Myths and facts about CO2 storage

CO2
19.04.2010
The greenhouse gas carbon dioxide (CO 2 ) can be captured, transported and injected into storage areas where it will not leak out and harm the environment. Geologist Ine Gjeldvik of the Norwegian Petroleum Directorate explains how.

“Capturing” CO2 entails  separating the CO2 from other gases before it is compressed. This means that the CO2 is exposed to pressure, transforming it to its liquid form, and as a result it takes up less space. Due to practical and economical reasons, CO2 capture will be carried out at major emission points, primarily fossil fuelbased energy generation and industrial processes. 

In principle, three technologies can be used for CO2 capture in connection with power generation:

  • Cleaning flue gas
    The CO2 is extracted from the flue gases using chemical cleansing. For this purpose, an absorbent (amine liquid, carbonate or similar) is used. The absorbent is cooled down before it makes contact with the flue gas,  then it is reheated to release the concentrated CO2. About 80 to 90 per cent of the CO2 in the flue gas is captured using this technology.

  • Pre-combustion separation of CO2
    The idea behind this technology is to extract carbon from the fuel before the fuel is combusted for energy generation. The separation is done by first creating CO gas in a conversion facility, then the CO gas  and water vapour is converted to hydrogen and CO2. The hydrogen can be used for fuel. About 90 per cent of the CO2 in the fuel can be cleaned using this technology.
     
  • Oxyfuel combustion
    Oxygen which has been separated from the air before combustion is used. The flue gas will then consist of CO2 and water vapour. The water vapour is separated from the flue gas by cooling (condensation), leaving only pure CO2. Using this technology, almost all the CO2 can be removed.


The CO2 is transported in liquid form to the injection well through pipelines or by ship. The liquid could contain impurities such as water, hydrogen sulphide (H2S), oxygen and hydrocarbons. These impurities will affect the transport properties of the CO2 in the pipeline, and at worse could cause corrosion and formation of hydrates. Hydrates are crystals which form when water molecules bond with molecules of hydrate-forming gases under certain pressure and temperature conditions. Examples of hydrate-forming gases include CO2, methane, propane and isobutane.

The water will then form a grid structure which captures the gas molecules in the openings. You then end up with a lumpy mass which can stop the flow in the pipeline.


Looking for storage areas
It is important to understand how the CO2 behaves in various stages. CO2 several hundred metres under the seabed behaves differently from CO2 in the open. The reason is that pressure and temperature are very different. Accurate estimates are needed to store as much CO2 as possible at the depth where the reservoir is located. An estimate made by the EU-led GeoCapacity project suggests that the ideal depth is between 800 and 2500 metres below the seabed.

CO2 can’t be injected just anywhere below the seabed. Geologists are identifying areas that can represent safe storage for a long time. These could include

  • depleted oil and gas fields
  • saline aquifers (sediment rocks which are filled with saltwater in the pore cavities)
  • coal

In a suitable reservoir, CO2 will be accumulated in the spaces between sand grains. The original liquid, such as formation water, oil or gas will then be displaced. It is therefore important that the formation the CO2 is injected into has good porosity and good permeability, i.e. flow properties.

The geologists are looking for structures where dry wells were drilled (wells without traces of oil or gas), to see if properties that make them suitable for storage are present. In addition to porosity and permeability, we need something that stops the liquid from flowing out of the structure in question. We need a dense rock that envelops the reservoir we inject into. These are called sealing rocks, often a dense shale with low or no porosity and no permeability.

Much has been said and written about CO2 storage in the media. It is easy to get the impression that the Utsira Formation is capable of storing all of Europe's CO2 waste, but that is a great exaggeration. 

 

(The article continue under the picture)

Illustration: Alligator film /BUG / Statoil

Illustration: Alligator film /BUG / Statoil 


According to Statoil, ten million tonnes of CO2 have been injected into the Utsira formation in the Sleipner area since the autumn of 1996. The gas has spread to a three square kilometre area in the formation, which has a total area of 26 000 square kilometres.

In comparison, every Norwegian emits on average about ten tonnes of CO2. According to Statistics Norway, total CO2 emissions in Norway are less than 50 million tonnes annually. When we know that ten million tonnes of CO2 have been injected into Sleipner since 1996, or about one tonne per year, this is not much. Massive reservoirs are needed if we are to receive CO2 from the continent. Total European emissions are approximately 3.3 billion tonnes, according to the IEA.

Safe storage is possible
The water in the reservoir is displaced by the injected CO2, which will rise and fill the pore cavities under the sealing rock on top. Eventually, the CO2 will dissolve and form new minerals. It is then permanently stored.

Storing in porous rocks filled with saline water is called saline aquifers. The aquifers’ ability to store CO2 over time can not be precisely determined beforehand, and it will be necessary to monitor the injected CO2 until it has stabilised.

The risk of extensive damage to the reservoir with resulting leakage is fairly low. Both new and older wells can be monitored, and the injection is stopped at the first sign of leaks. Thorough tests are carried out, and the effect of CO2 injection into various geological formations and structures is studied.

The goal is to gain good scientific and technical understanding of both leaks and non-leaks in a reservoir, in order to design storage projects which have the same characteristics as natural reservoirs, which have held CO2 and methane for millions of years.


Profitability still far off

Energy is needed to capture, transport and store CO2. There is no doubt that carbon capture and storage is costly, but it is difficult to say what the final price will be. There are many factors that must be considered -  for instance the type of reservoir and liquid protperties, transport distance and transport method. Another factor is whether there are emission sources near the storage site.