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"Putting it back" - a series on storage of CO2 (part 1)

An introduction to the concept of storing CO2 underground

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An essential part of the chain of Carbon Capture and Storage (CCS) is the S - the storage part.

This is the first article in a series of 5, and the next article will explore the myths and realities surrounding CCS. Read the next article, Myths and Facts about CO2 Storage here.

Understanding CCS means one has to understand what CO2 is. CO2 in itself is not dangerous. It is a part of photosynthesis and nature’s cycle. The problem arises when the burning of fossil fuels creates more CO2 than the natural carbon cycle can handle and the amount of CO2 in the atmosphere rises. This leads to the so-called greenhouse effect and the dramatic changes to our climate.

The goal of carbon capture and storage, also sometimes called sequestration, is to capture this excess CO2 and literally put it back where it came from.

CO2, or carbon dioxide, was already captured and used for enhanced oil recovery (EOR) in the US during the 70s, which means that the technology behind CCS is well tested.

Jonathan Pearce, geologist at the British Geological Survey (BGS), explains: “CCS is necessary as part of the solution to maintain energy supply to users whilst reducing atmospheric CO2 emissions. Whilst other forms of low-carbon energy will become increasingly important in our energy mix, it is still very likely that fossil-fuel based power generation, using both coal and gas, will provide supportive electricity supplies for decades to come. This will be true in Europe as well as other regions.”

The International Energy Agency (IEA) confirms that all projections for the global energy demand show that fossil fuels will be used in large amounts in the future, which is not compatible with the needs to lower emissions to slow down global warming.

CCS also provides the only way to reduce CO2 emissions in some specific industrial processes, such as the cement and steel industries and some chemical manufacturing processes







“In addition, CCS also provides the only way to reduce CO2 emissions in some specific industrial processes, such as the cement and steel industries and some chemical manufacturing processes,” Pearce continues.

CO2 is captured by either retrofitting existing plants or building completely new plants with CCS technology. After the CO2 is captured, it is transported to the final storage site. Any storage site will be carefully investigated and monitored before being selected for this purpose.

“The ideal storage site requires three important characteristics,” says Pearce. “Firstly, enough ‘capacity’ to accommodate the amounts of Co2 expected to be captured and transported to the facility. This storage ‘capacity’ is determined by considering the proportion of the rock that might be filled with saline water within the tiny pores, or holes (like a sponge), and the total volume of rock, which might be present over a large area of the underground. There may be several such rock layers present in one particular area. These rock layers, called strata, also need to be below a certain depth, normally around 800m, because below this depth the CO2 when it is injected occurs in a dense liquid form, enabling significantly more to be stored than if it was injected as a gas at shallower depths. Such rock strata are known to occur in many regions of the world; typically, though not exclusively, in areas which also have known hydrocarbon reserves, such as the North Sea.”

“Secondly,” he explains, “the storage site must be able to contain the CO2 permanently. As the CO2 is less dense than the surrounding water, oil or gas, it tends to migrate upwards. To prevent this, the CO2 must be prevented from moving upwards by overlying rocks that have very low permeability (permeability is a description of the ease with which a gas or liquid can very slowly move through the tiny connected pores) and are more solid. These rocks are commonly called ‘cap rocks’ or ‘seals’ by geologists because they act as seals.”

The third characteristic, according to Pearce is that we must be able to inject the CO2, via deep boreholes, into the target strata at the required rates.  “These rates of injection could be around 1 million tonnes per year or higher, in large scale CCS projects per well, with several million tonnes of CO2 being captured, transported and stored from a typical modern power station each year. Industrial sources would have lower emission rates. The ease, with which the CO2 can be injected, depends on a number of factors, including the rock permeability, porosity and the degree to which it is divided by natural barriers into small zones, which may be isolated from adjacent zones. It is possible to improve the efficiency of the injection process in a number of ways such as the orientation of the borehole, the number of boreholes and careful management of the storage site.”

“Although these three basic requirements might seem difficult to meet, in reality there are many areas around the world where the conditions are right for CO2 storage. It has been estimated for example that the potential storage capacity of the UK part of the North Sea, is sufficient to store the UK’s and some of Europe’s CO2 for many decades into the future.”

There are potential storage sites all over the world, but the lack of data makes it difficult to calculate exactly how much storage space there is. A report from theIntergovernmental Panel on Climate Change (IPCC), suggests that there is about 2000 GtCO2 of storage capacity in geological formations, thousands of GtCO2 in the ocean. In addition you have mineral carbonation and the use of CO2 for EOR. The US Geological Survey recently released a study, the first of its kind, which estimates that the US has the capacity to store more than 1,000 years' worth of CO2 from the power-generation sector.

The US Geological Survey recently released a study, the first of its kind, which estimates that the US has the capacity to store more than 1,000 years' worth of CO2 from the power-generation sector

There are three different types of potential storage sites. The first is depleted gas and oil fields, which are favourable because they can be used immediately as they are. The injection of CO2 into reservoirs is also used for EOR. CO2 is injected into existing oil reservoirs to help mobilize the oil and thus expand the life span of the oil field. Once the oil reaches the surface, the CO2 is extracted and sent back down in the reservoir. The CO2 can be used to extract more oil or it can be left in the reservoir for storage. The cost of storage in gas and oil fields can be offset by the sale of the recovered oil, which can make this kind of storage desirable.

The second option is saline aquifers, which have the potential to store vast amounts of CO2. The third option, unmineable coalfields, have not been explored much yet, but the theory is that the CO2 molecules will attach to the coal and stay put in the coalfields.

Research on CO2 storage has been conducted since the 90s and some of the storage sites have been running successfully since then. These are some of the most successful storage sites, and show the geological and geographical variations of existing storage sites.

  • Sleipner, Utsira formation, North Sea. Sleipner was built in the early 90s to avoid the Norwegian offshore CO2 tax, and the Norwegian oil company Statoil has captured CO2 at the Sleipner project since 1996. They capture around 2600 tonnes of CO2 every day, which means around 1 million tonnes annually since they started.
  • Ketzin, Germany. Ketzin is one of the longest-running storage sites on land in Europe. Very pure CO2 has been injected 650 down in sandstone since 2008, and the operation has been proved reliable and safe since the start of the project.
  • Weyburn, Canada. Canada is one of the leading countries in CCS research and development. The Weyburn project in Saskatchewan was launched in 2000, and is the largest full-scale CCS study ever conducted.

Once the storage site has been identified, the CO2 is transported to the site, and injected into the reservoir, fours different processes occur. There processes are called trapping.

  • Structural trapping. CO2, which is lighter than water, will start to rise upwards, but is stopped by a layer of cap rock.
  • Residual trapping. This occurs when the pores in the rock where the CO2 is captured are so small that the CO2 will not rise. CO2 will be “taken up” like water into a sponge.This can contain some, but not all of the CO2.
  • Dissolution trapping. Some of the CO2 in the rock pores will dissolve into water. This water is heavier than water without dissolved CO2, and will sink. This is a very slow process, and it is estimated that about 15% of the CO2 captured in the Sleipner project will be dissolved over time.
  • Mineral trapping. When the CO2 dissolves in water, it forms a carbonic acid, which can react with the surrounding rock and form solid minerals.


When the CO2 is injected to the reservoir, the developers constantly monitor the site. This is mainly for security purposes, but also to learn more about how CO2 storage works. The developers will monitor anything from the solidity of the cap rock to migration of CO2 to ensure that the CO2 stays where it is injected.


Thank you to Jonathan Pearce and the British Geological Survey (BGS) for their contribution to this article.

Sources:, Co2 Geonet, IPCC


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