CO2 injection in an aquifer at the Sleipner gas-condensate field
The gas from the Statoil operated Sleipner gas-condensate field in the Norwegian North Sea contains approx 9 % CO2. This is higher than the sales gas specifications, so the CO2 content has to be reduced before the gas is exported to the consumers. The normal procedure is to vent the extracted CO2 to the atmosphere. On the Sleipner field, the CO2 is instead compressed and injected into an aquifer, Utsira, 1000 m below the sea bottom. Sleipner is currently the only place in the world where large volumes of CO2 are injected for underground storage.
This solution has been in use since the field came on stream in1996. In 2000 the saline aquifer carbon dioxide storage (Sacs) project was started to demonstrate that the injected gas remains in place rather than leaking out.
One aim of the Sacs work has been to document what happens to the carbon dioxide after it is injected below ground.
Statoil has hosted this project, which is funded by the European Commission as well as a number of major energy companies and national governments around the North Sea.
The spread of carbon dioxide through the aquifer is recorded by seismic surveys. One was conducted before injection started, and another took place in the autumn of 1999.
Many of the specialists working on geological issues doubted whether liquid CO2 could be distinguished on seismic maps from the brine already present in Utsira.
But staff at Statoil's research centre in Trondheim have confirmed their theory that sound waves reflect differently from carbon dioxide and salt water.
Comparing seismic data collected before and after injection started has allowed researchers to show how carbon dioxide deep inside the Utsira formation migrates.
It is held under the layer of shale cap rock, 80 metres thick, which covers the whole formation. This extends for several hundred kilometres in length and about 150 kilometres in width. With a thickness of 250 metres, the formation can store 600 billion tonnes of carbon dioxide. That compares with a mere million tonnes being injected annually from Sleipner.
One question asked by both researchers and other people when they hear about the Sleipner solution is how long the greenhouse gas will remain underground. The results from Sacs indicate that the formation is highly unlikely to leak for several hundred years. Researchers cannot promise it will stay in store forever. But a duration until the next ice age, in 5-10,000 years, should be good enough.Separation of CO2 from natural gas by use of amine contactors has been implemented on an offshore platform for the first time in the world.Injection of CO2 into a geologic formation (an aquifer) for permanent storage. This is still the only place in the world where large quantities of CO2 is permanently deposited in a geologic formation.Statoil will apply the same principles for CO2 removal and underground storage at the Snøhvit field offshore northern Norway, which will come on stream in 2006.The Sacs project has used seismic technology to verify how the CO2 migrates in the geologic formation (aquifer) and that the CO2 most probably will remain in the formation for a very long time.The technology and results from the Sleipner CO2 extraction and injection and from the Sacs project have been widely published.
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