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What is CO2 storage?

Part of the solution to the world's climate challenges lies deep underground. Here, CO2 from the large emitters can be stored in the same reservoirs of sandstone from which oil and gas have previously been extracted. The Danish subsoil is suitable for CO2 storage with its many reservoirs in the North Sea.

CO2 storage is a well-tested and well-known technology that has been used several places in the world for more than 40 years. Project Greensand works to be part of a CCS value chain for geological storage of CO2 in Denmark.

The different steps in the CO2 storage process are:


  1. CO2 is captured:
    CO2 can be captured in many ways. It is most efficient to capture it where there is a large and stable supply of CO2. For example, in large emissions such as industry, energy production or incineration plants, where the CO2 is separated from the smoke.

  2. CO2 is liquefied:
    The captured CO2 is made into liquid. This can be done by heating, compressing, and cooling.

  3. CO2 is transported:
    The liquid CO2 is transported by ship to the storage areas at Siri in the North Sea. In the pilot phase, it will take place in tank-containers from Belgium - while it will be on specially built ships when Project Greensand is fully matured.

  4. CO2 is stored underground:
    Liquid CO2 is sent down into the sandstone reservoir approximately 1800 meters underground via CO2 wells.

  5. The CO2 storage is monitored:The fields for CO2 storage in the subsoil have been screened over many years and new groundbreaking technologies are being developed for monitoring CO2 storage.

Frequently asked questions

What is CO2 storage? 

Capture and storage of CO2 goes by the name CCS. It stands for Carbon Capture Storage. The United Nation’s Intergovernmental Panel on Climate Change (IPCC) points out that CCS has the potential to make a significant contribution to achieving the goal of reducing CO2 emissions. You capture CO2 and store it in the subsoil. Project Greensand works to develop and demonstrate that CO2 can be stored underground in the Danish part of the North Sea. This will take place in the Siri field, which is more than 200 kilometers west of the Danish coast.

Why store CO2 instead of reducing emissions?

We are well underway with a green transformation of Denmark. We install photovoltaic systems, build offshore wind farms, and we replace the petrol and diesel cars with electric cars. It is a step in the right direction - but it is not enough. We must also remove CO2 from the known emission sources if possible and store it underground if we are to solve the climate crisis. Capture and storage of CO2 has a great potential to reduce the amount of CO2 emitted into the atmosphere efficiently and quickly. It cannot stand alone, but it is an important tool. CO2 storage is part of the solution.

How much CO2 can Project Greensand store? 

In the short run, Project Greensand can store up to 1,5 million tonnes of CO2 per year in 2025/2026. By the year 2030, Project Greensand can store up to 8 million tonnes of CO2 per year. This corresponds to the emission from approximately 725.000 Danes a year - or more than 13% of Denmark's annual CO2 emissions.

The Danish subsoil is with its many sandstone formations in the North Sea subsoil well suited for storing CO2. There is actually room enough to store all the CO2 that Danish society has historically emitted - and more. The Danish subsoil is estimated to be able to store the next 500 years of CO2 emissions from Denmark at the current emission level.

Where does the CO2 for storage come from?

There are already large amounts of CO2 in the atmosphere for completely natural reasons. CO2 is emitted when volcanoes erupt and when cows burp, but it is difficult, expensive, and inefficient to capture CO2 during volcanic eruptions and in the stables. Therefore, the plan is to capture CO2 at the large point emitters in Denmark, where it will be converted into liquid. This can happen at waste incineration plants or industrial companies that emit a lot of CO2.

Where will the CO2 be stored?

In the pilot project, Project Greensand will investigate the storage potential for liquid CO2 in the reservoir called Nini West. It is located approximately 200 kilometers out in the North Sea. Here, the project will be further developed by physically injecting liquid CO2 into the reservoir. Nini West is a former oil field located 1800 meters below the seabed surface. Here, the CO2 must be stored in a reservoir of sandstone that has overlying layers of impenetrable rocks.

Thereafter, Project Greensand will expand the storage to also include the Nini Main field and subsequently the fields on Siri Fairway.

How does the CO2 get to the North Sea?

Once the CO2 has been converted into a liquid, it must be transported to the offshore platforms on the North Sea. During the pilot phase, the CO2 is transported in special containers, which sail out to the platform on a ship. From here, it is sent into the existing pipes that are already on the platform - and then the CO2 is sent down through a production well to be stored in the reservoir.


The results from this will be used to further mature the project so that it will be possible to store large amounts of CO2 in the future.

How safe is CO2 storage?

The technology behind CO2 storage has been around for more than 40 years and has been used in several parts of the world since the 1970s. In Project Greensand, the CO2 must be stored in carefully selected reservoirs that have held oil and gas underground for thousands of years. The companies and organizations behind the storage know the underground thoroughly from years of experience with the work on the North Sea and in the specific fields.

When should the CO2 be stored in the North Sea subsoil?

The pilot project is already in process and the first CO2 is planned to be stored in the Danish part of the North Sea during the first quarter of 2023. After that, Project Greensand will store up to 1,5 million tonnes of CO2 in the field Nini West and Nini Main, which is a part of the Siri field, in 2025/2026. Thereafter, Project Greensand will further be expanded to include the so-called Siri Fairway. It includes the remaining fields in the Siri area so that up to 8 million tonnes of CO2 can be stored per year from 2030.

How does CO2 storage affect the marine environment?

CO2 storage has no significant impact on the surrounding marine environment. The CO2 is stored more than 1800 meters below the seabed in the North Sea subsoil. If, contrary to expectations, CO2 seeps up from the storage, then part of the captured CO2 will be released into the atmosphere again, and a small part of the climate gain will be lost. Part of Project Greensand's pilot project involves developing monitoring technologies and testing materials that can ensure that CO2 storage can take place safely without affecting the surrounding environment.

What will offshore workers do when it comes to storing CO2?

Denmark has been extracting oil and gas from the North Sea since 1972. It has created a wide range of jobs both on land and at sea. But as part of the green transition, a majority in the Danish parliament has decided that Denmark must stop oil extraction from the Danish part of the North Sea in the year 2050. If we instead start storing CO2 in oil and gas fields in the North Sea, we will not only solve part of the climate challenges. We will also maintain several thousand jobs.


The work tasks in oil extraction are very comparable to the tasks in CO2 storage, and there is a need for the competencies that offshore employees have built up over decades. At the same time, you can reuse the offshore platforms that are already on the North Sea, which the employees know like the back of their hand. 

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