As the helicopter approaches the platform, the pilot's voice comes over the intercom, "Prepare for an offshore landing," before touching down on a rig situated 250km (155 miles) off Denmark's west coast.

The helicopter had previously flown over Nini, a nearby oil rig located in the choppy waters of the North Sea, before heading to its current destination.

The Nini rig is positioned above an oil field that is nearing depletion, but it is slated to be repurposed as a large-scale carbon storage project known as Greensand Future.

Plans are underway to inject thousands of tonnes of CO2, a potent greenhouse gas, into the depleted oil field as part of the project.

After landing, we board the Siri platform, a larger "mother platform" that serves as a control center and is staffed by offshore workers.

Mads Gade, the CEO of Ineos Energy, points out the massive pipes at the wellhead, which have been used to extract oil and gas from beneath the seabed for decades.

Gade explains that instead of extracting oil and gas, the plan is to utilize the existing infrastructure to inject CO2 into the ground, effectively reversing the process.

Carbon Capture and Storage (CCS) technology involves capturing CO2 emissions and storing them permanently, preventing them from entering the atmosphere.

The Greensand Future project, which is being led by a consortium that includes the British multinational chemicals company Ineos, is poised to become the EU's first large-scale offshore CO2 storage site when commercial operations commence in the coming months.

According to the company, the project aims to store approximately 400,000 tonnes of CO2 in the first year, with the potential to increase to eight million tonnes annually by 2030.

Gade notes that this amount is equivalent to nearly 40% of Denmark's emission reduction target, making it a significant contribution to reducing greenhouse gas emissions.

Both the Intergovernmental Panel on Climate Change (IPCC) and the International Energy Agency (IEA) have emphasized the importance of CCS technology in limiting global warming, in addition to reducing emissions.

The EU has also stated that CCS is necessary to achieve its goal of reaching net zero emissions by 2050.

However, some critics argue that relying on CCS technology could detract from efforts to reduce CO2 emissions in the first place.

Additionally, CCS is a costly technology, and some environmental groups argue that emission reductions can be achieved at a lower cost using existing technologies such as wind power, solar energy, and electric vehicles.

Helene Hagel, the head of climate and environmental policy at Greenpeace Denmark, states that while she supports the use of CCS in certain sectors where emissions are difficult to reduce, she is cautious about its broader application.

Hagel believes that CCS may not be the best solution in all cases, and that its use should be carefully evaluated.

She also raises concerns that using the seabed for carbon storage could create problems for future generations.

Hagel argues that if the current generation uses the seabed to store carbon emissions that should have been avoided in the first place, it may limit the ability of future generations to store their own emissions.

Globally, hundreds of CCS initiatives are currently underway or in development, as countries seek to reduce their greenhouse gas emissions.

In Europe, several large-scale CCS projects are advancing in the North Sea region, particularly in Norway, the Netherlands, Denmark, and the UK.

Norway's Northern Lights project, which was launched last August, has begun storing CO2 under the seabed off the coast of Bergen, marking a significant milestone in the development of commercial carbon storage services.

In the UK, several carbon capture clusters are being developed, including the Acorn Project in Scotland and the Viking project off the coast of Lincolnshire.

According to Niels Schovsbo, a senior researcher at the Geological Survey of Greenland and Denmark (GEUS), the North Sea is emerging as a hub for CCS due to its legacy of oil and gas production, which has provided a thorough understanding of the geology of potential storage sites.

The region's existing offshore infrastructure and technical expertise are also major advantages, making it an attractive location for companies like Ineos to establish CCS operations.

Gade notes that the Nini rig is nearing the end of its operational life, but instead of decommissioning it, the plan is to repurpose the facilities and utilize the existing skills and competencies to support the CCS project.

By reusing the existing infrastructure, the company can reduce costs and minimize the environmental impact of the project.

In a large warehouse on the outskirts of Copenhagen, cases of rock samples are stored, providing valuable insights into the geology of the North Sea.

Schovsbo opens a box containing a grainy green rock sample drilled from the seafloor, which he uses to explain the suitability of the area for CO2 storage.

The rock in this region of the North Sea has a high porosity, with numerous small cavities that can hold CO2, making it an ideal location for carbon storage.

A thick layer of clay or cap rock will trap the CO2, preventing it from escaping, just as it has trapped oil and gas for millions of years, Schovsbo explains.

He notes that the same sealing mechanism that has kept oil and gas trapped for so long can be used to store CO2, providing a safe and effective means of carbon storage.

According to Schovsbo, the amount of CO2 that can be stored in the rock is comparable to the volume of oil and gas that has been extracted, suggesting that CCS sites could have an operational lifespan of 10 to 30 years.

For the many offshore workers in the region, the transition to carbon storage offers new opportunities and a chance to adapt their skills to a changing industry.

Peter Bjerre, a maintenance manager, notes that the work involved in maintaining turbines and gas compressors will shift to maintaining high-pressure pumps used to inject CO2 into the storage sites.

Bjerre, who is from Esbjerg, reflects on the changes he has seen in the region, from the fishing industry to oil and gas, and now to the green transition.

He expresses his amazement at the prospect of a new future emerging in the region, driven by the transition to a more sustainable and environmentally friendly industry.