Appearing first as a dot on the horizon, the remote Nini oil field on Europe's rugged North Sea slowly comes into view from a helicopter.

Used to extract fossil fuels, the field is now getting a second lease on life as a means of permanently storing planet-warming carbon dioxide beneath the seabed.

In a process that almost reverses oil extraction, chemical giant INEOS plans to inject liquefied CO2 deep down into depleted oil reservoirs, 1,800 meters (5,900 feet) beneath the seabed.

The Associated Press made a rare visit to the Siri platform, close to the unmanned Nini field, the final stage in INEOS' carbon capture and storage efforts, named Greensand Future.

When the project begins commercial operations next year, Greensand is expected to become the European Union's first fully-operational offshore CO2 storage site.

Environmentalists say carbon capture and storage, also known as CCS, has a role to play in dealing with climate change but should not be used as an excuse by industries to avoid cutting emissions.