FAQ
On this page you can find answers to the most frequently asked questions about CO2 storage, seismic surveys and the technologies we use in the Greenstore project. Here you can gain insight into how we work with capturing and storing CO2 underground and why it is an important part of Denmark’s green transition. If you don’t find what you’re looking for, you’re always welcome to contact us directly.
CO2
CO2 is a gas that occurs naturally in the Earth's atmosphere and helps retain heat in the atmosphere. Animals and humans exhale CO2, and plants and trees absorb it and use it for their growth. Unfortunately, we emit more CO2 than the plants and trees on earth can absorb. Rising concentrations of CO2 in the atmosphere are among the main causes of global warming and climate change. One way to combat rising concentrations is to capture the CO2 and store it in the subsoil.
CO2 is a natural part of the atmosphere. Plants and trees use CO2 to grow and convert it into oxygen, which humans and animals need to survive. CO2 is therefore not harmful in itself, but becomes a problem when the amount in the atmosphere increases too much. This occurs primarily through human activities, such as the burning of fossil fuels in the energy sector and the production of materials like cement and steel in industries.
A very large part of the Earth's CO2 is underground. And underground, CO2 poses no threat. CO2 in small concentrations is not dangerous in itself but causes climate damage to the planet when it accumulates in excessive amounts in the atmosphere - which is what we are seeing now as a result of the high emissions since the beginning of the industrial revolution. Capturing and storing CO2 is one way to slow down global warming. Geological storage of CO2 is part of a natural cycle.
About CO2 storage
Carbon Capture and Storage (CCS) is a term for the capture, transport and storage of CO2. In 2021, a broad majority in the Danish Parliament decided that CCS should be a key tool in Danish climate policy to help us reach our ambitious climate targets and contribute to achieving the international goals of the Paris Agreement. If you want to know more about CCS, you can read more on GEUS' website here.
Subsurface CO2 storage requires suitable geological formations. CO2 storage is placed up to 3 kilometres below the ground surface in areas where there are geological reservoirs of porous rocks such as sandstone with voids between the sand grains. The CO2 is stored in the cavities of such reservoir rock, and dense impermeable layers of compressed clay above them prevent the CO2 from seeping upwards. Denmark's subsoil is particularly well suited for this purpose because it consists of deep geological layers of sandstone protected by a thick and dense clay layer above.
If a CO2 storage facility is eventually established in the area between Hobro and Randers, the CO2 will be stored 2-3 kilometres below the surface. The area has been thoroughly investigated by the Geological Survey of Denmark and Greenland (GEUS). Harbour Energy will now conduct even more detailed subsurface investigations and collect data to assess whether the area is suitable for storage.
CO2 storage is monitored using advanced technologies such as pressure sensors, geochemical analyses and seismic surveys to ensure safe and efficient storage. A dense layer of compacted clay seals the geological storage of CO2. By conducting seismic surveys, we map the thickness of the clay layer and ensure that it is sealed.
The Geological Survey of Denmark and Greenland (GEUS) estimates that there is a storage potential of between 12 and 22 billion tonnes of CO2 in the Danish underground. The Greenstore licence area is estimated to have a storage capacity of up to 250 million tonnes of CO2, making the area an important part of Denmark's green transition.
Harbour Energy has extensive experience with CCS and operates CCS projects in Norway, the UK and Denmark, where, in addition to the Greenstore licence, we are partners in Greensand Future together with INEOS and Nordsøfonden. There are also a large number of large and small CCS plants in operation around the world, most of which are located in Canada and the USA. In Norway, the UK and the US, several large storage facilities are under construction.
Once CO2 is captured, it is liquefied by being pressurised and cooled. This makes it easier to transport because it takes up less space. The liquefied CO2 can then be transported in tanks on ships, trains or lorries - or sent directly through special pipelines. The most efficient way to transport large amounts of CO2 is by pipeline, which is why it is the most commonly used method.
CCS is not a new technology, but has been around for more than 40 years. The technology has proven effective and more than 200 million tonnes of CO2 have been captured in the time it has been in use. In 1996, a capture plant was installed in the Norwegian North Sea, which has since captured and stored 1 million tonnes of CO2 annually.
CO2 is neither explosive nor flammable. CO2 cannot overheat and explode underground. CO2 is used to extinguish fires as the gas smothers the fire.
CO2 storage is regulated by national and international laws, including the Danish Subsoil Act, the EU Directive on Geological Storage of CO2 and the Environmental Assessment Act.
The layers where CO2 is planned to be stored are hundreds of metres below our drinking water, so an impact on our drinking water supply is very unlikely.
About seismic surveys
Seismic surveys are used to map underground geological structures. In CCS projects, seismic is used to identify and assess suitable geological layers and structures for CO2 storage. CO2 storage, including the density, stability and capacity of the layers.
A three-dimensional (3D) seismic survey maps the geological structures underground by sending vibrations into the ground, in this case from a truck with a vibrator attached. Small sensors, called geophones, are placed in the landscape and pick up the reflected signals from the subsurface. This signal is then converted into a detailed, three-dimensional image of the geological structures and the thickness of the layers. This allows us to assess whether the area is suitable for CO2 storage.
Seismic surveys are crucial to ensure that CO2 storage is completely safe. In 2023, GEUS carried out a series of 2D seismic surveys that showed that the subsurface between Randers and Hobro has potential for CO2 storage. But to get a more precise understanding of the geology, even more detailed data from 3D seismic surveys is needed.
Seismic surveys are conducted in the early investigation phase to identify potential storage sites and assess their suitability.
Data is analysed to identify the thickness, extent and geometry of porous sandstone layers where CO2 can be stored, as well as the overlying dense clay layers that will act as sealing layers. In addition, the study also maps the position and depth of any fractures
We have planned the routes carefully to minimise inconvenience to residents in the area. Buildings, infrastructure and sensitive areas are taken into account and work is only carried out during daylight hours to avoid unnecessary disturbance. During seismic surveys, low-frequency vibrations and some noise may occur, especially when the vibration trucks are in operation. If a vibration truck passes close to your home, you will briefly feel mild vibrations - typically only for 10-20 minutes. Before and during data collection, we will demonstrate the vehicles to citizens. Stay tuned to this page, where we will announce where and when this will take place.
Once the surveys are complete, the data collected is analysed to create a detailed model of the subsurface. This analysis is crucial to assess whether the area is suitable for safe and permanent CO2 storage. The results will be shared with Danish authorities and will form the basis for decisions on the next steps in the Greenstore project.
About capturing and transporting CO2
CO2 can be captured by separating it from the other substances in the flue gas produced when burning waste in an incinerator, for example. This is done through a chemical process where a chemical liquid called "amine" binds the CO2 before the flue gas is released through the chimney. By subsequently heating the fluid, the pure CO2 is released and can then be captured. There are several other methods of CO2 capture, such as a technology called Direct Air Capture (DAC), which makes it possible to capture CO2 directly from the air, not just from industrial plants, but also from the air we all breathe.
Once CO2 is captured, it is liquefied by being pressurised and cooled. This makes it easier to transport because it takes up less space. The liquefied CO2 can then be transported in tanks on ships, trains or lorries - or sent directly through special pipelines. The most efficient way to transport large amounts of CO2 is by pipeline, which is why it is the most commonly used method.
In a few places there will be smaller surface facilities, either in the form of small tanks acting as temporary storage on the way to the underground, or well sites from which CO2 will be pumped into the storage. The well sites are expected to be 1.5-2 hectares in size. Transport of CO2 to the facility will typically be by underground pipelines, but can also be done by lorry or train.