With the start-up of production on Sleipner West this summer, the biggest offshore operation to remove and dispose of carbon dioxide also gets under way. With reserves of 122 bcm of gas (and 185 MMbbl of condensate), Sleipner West will be one of the main sources of Norway's gas exports to Continental Europe over the next 25 years. However, the gas contains a high proportion of CO2 - up to 9.5% - and this must be reduced to 2.5% to meet sales specifications. The wellstream from the field is produced through a wellhead platform, SLB, and piped 12.5 km to SLT, the treatment platform which is located alongside Sleipner A, the production platform for the Sleipner East Field. On arrival at SLT, the gas is first separated out from the condensate and water.
The CO2 is then removed from the gas by an amine absorption process. This involves passing the gas through two 20 meter high absorption columns in which the CO2 is absorbed by the amine; while the gas goes its separate way with a substantially reduced CO2 content, the rich amine solution is then put through a regeneration process in which the CO2 and the amine are separated. Amine absorption/regeneration is a well known process but this is the first time it has been used offshore, and the first time it has been used to process such a large volume of gas, according to Ragnhild Korboel of the Sleipner Petroleum Technology Department. A proprietary Elf system is being used under licence.
When Sleipner West reaches plateau production of some 20.5 MM cm/d of sales gas in early 1997, it will also be producing 1.7 MM cm/d of CO2. Over the field's life-time, the volume amounts to a staggering 20 million tons. Although Norway's tax on CO2 emissions does not cover naturally produced CO2, Statoil considered it environmentally unacceptable to release such a large volume into the air.
A number of options for disposing of the unwanted gas underground were considered, including injecting it into a nearby oil field to assist oil recovery, or into one of the Sleipner East gas/condensate reservoirs. In the end it was decided to inject it into a separate aquifer, the choice falling on the Utsira Formation. The Utsira is used elsewhere for injecting drilling cuttings slurry, but has not been used before for CO2 injection.
The formation is relatively shallow, lying 800-1,000 metres below the sea bed. An injection test showed that it displayed high permeability and porosity, and there should be no problem in using it to dispose of all the unwanted CO2, Korboel says. It is accessed by a 6.5-km highly deviated well drilled from Sleipner A.
The CO2 was originally to be injected in liquid form containing some produced water, but to avoid potential problems of hydrate formation, it was instead decided to inject it in gaseous form. Nevertheless duplex has been used for the wall of the injection well.
Once in the aquifer, some of the CO2 will dissolve in the water and some will rise to the top of the formation, forming a free CO2 gas cap. It was necessary to investigate whether the dissolved CO2 would spread through the aquifer far enough to reach the Sleipner A production and injection wells where they pass through the formation. Simulation studies showed that after 20 years of injection, the extent of migration of CO2 was unlikely to exceed three km in any direction, so the location for injection was fixed at a point more than three km from the nearest Sleipner A wells.
Even so, Korboel says, the precaution has been taken of using corrosion resistant materials in these wells at the Utsira level. Once injection begins, seismic will be used to keep track of the spread of CO2. The cost of removing and disposing of the CO2 does not come cheap. The amine system itself cost NKr 700 million, and the injection well and compression facilities a further NKr 400 million. Part of the jacket and topside costs is also attributable to the CO2 system.
Due to the growing pressures to reduce CO2 emissions to the atmosphere, the CO2 disposal project has attracted a lot of interest from all over the world, Korboel says. Among those who have made contact are the operators of gas-fired power stations in Japan and Australia who are considering removing CO2 from fuel gas.
The Sleipner West experience should also prove valuable for future gas production projects. Although the CO2 content of its gas reserves is unusually high, there are other fields with similar levels. These include the Natuna Sea gas reserves in Indonesia, and gas finds in Norway's Barents Sea.
SLT platform sets lifting record
The SLT platform, which consists of a 6,500-ton jacket, 7,000-ton CO2 module and 8,000-ton gas processing module, was installed in April by Saipem's S7000 crane barge - the heaviest jacket and module lifts to date by a vessel in DP mode.
Eighteen wells are being drilled by Smedvig jackup West Epsilon through the SLB wellhead platform, which has the first steel jacket on the NCS fixed to the seabed by means of bucket skirts. It was built under Norway's first partnering arrangement, between Statoil, HMV, and Maritime Tentech.
Start-up was scheduled for early August with five wells, but because drilling has been interrupted by a strike, that number might be reduced to four, according to Jens Petter Egeland, the project manager for Sleipner West production. In mid year the project was running ahead of schedule and below budget, with costs cut from an original NKr 11 billion to NKr 9.8 billion, and further reductions still aimed for.
Contracted deliveries of Sleipner West gas, negotiated under the Troll agreements, start in October 1997 at some 9.6 MM cm/d, and will reach some 20 MM cm/d by the early 2000s. Before these deliveries start, the gas will replace current deliveries of Sleipner East gas, which will be re-injected to improve condensate recovery, Egeland says.
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