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The storage in underground geological formations is an attractive option for the removal, essentially permanently, of very large quantities of CO2 generated from a variety of industrial operations. One promising technological option is that of capturing CO2 and injecting it into deep underground saline aquifers, found in many parts of the world. One such formation is located above the Sleipner field, one of the larger natural gas producers in the North Sea. The natural gas produced from the field contains up to 9% CO2, however, in order to meet the required export specifications and the customers’ requirements, this has to be reduced to a maximum of 2.5%. At the present production rate, this equates to an extraction of nearly 1 million tonne CO2/year.
Using normal operational practices, this CO2 would have been released into the atmosphere, the result being an increase in Norway’s CO2 emissions of nearly 3%. In an era where efforts have been focused increasingly on minimising CO2 emissions, this clearly gave cause for concern. Thus, in order to help the country to meet national emission targets and avoid high CO2 taxes, Statoil, a major Norwegian oil and gas producer, adopted a saline aquifer storage strategy for the Sleipner West field. Natural gas production from the field started in 1996, with CO2 produced being injected into the Utsira aquifer formation (see below), some 800 metres below the bed of the North Sea. This process remains on-going, the project forming the world’s first commercial-scale storage of CO2 for mitigation of climate change. Had this process not been adopted, and the CO2 produced been allowed to escape to the atmosphere, the licensees of the Sleipner West field would have had to pay NOK 1 million/day in Norwegian CO2 taxes.
Background
Statoil operates the Sleipner field on behalf of a group of industrial partners, producing natural gas for a range of customers. However, before this gas can be sold on, it requires additional treatment in order to reduce its relatively high CO2 level of ~9%, to ~2.5%. This is achieved by stripping the CO2 from the gas stream using large absorption towers, each around 20 metres high. The CO2 removed by this process is then injected into a saline water-bearing structure known as the Utsira formation, which comprises sands well below seabed. Commercial production, based on this technology, from the Sleipner West gas and condensate field began in August 1996. Operating in this way, CO2 emissions to the atmosphere are negligible.
The Sleipner project marked a milestone in industrial history, as this technology had not previously been adopted for such a large-scale operation, accommodating ~1 mt CO2/year. In addition, this was the first occasion where carbon dioxide had been compressed and injected underground from an offshore platform.
The Sleipner CO2 Facility
The Sleipner West facility comprises two main installations, the Sleipner B (SLB) wellhead platform on the field and the Sleipner T (SLT) treatment platform, adjacent to the Sleipner East facilities. SLT is linked physically to the Sleipner A platform by a bridge. Other major component parts of the development include a 12.5 kilometre flow line from SLB to SLT. The wellhead platform is remotely operated from the Sleipner A (SLA) control room via an umbilical line.
The carbon dioxide removal process used to remove CO2 from the high pressure natural gas stream is based on amine scrubbing technology.
The natural gas flows into the bottom and out through the top of two contacting towers. The amine solutions (MDEA and water) treating the natural gas is flowing countercurrent and when the solution exits from the bottom of the towers it will have absorbed the major bulk of the carbon dioxide. Subsequent stages then remove the collected CO2, mainly by flash regeneration. The separation of the CO2 from the amine solution is carried out using equipment installed in one of the SLT modules, these comprising of heat exchangers, pressure vessels, storage tanks, pumps, Pelton turbines and filters. The amine is then recirculated to the two towers for further removal of CO2 from the natural gas.
Energy released by the amine treatment process runs two generators, yielding 6 MW of power, utilised on the platform itself. The carbon dioxide module weighs ~8200 tonnes and stands 35 metres high; overall costs amounted to over 350 M Euro.
The other SLT module is used for gas treatment. Following removal of the CO2, the natural gas is transferred to SLA for export to continental Europe the CO2 is also transferred to SLA, for injection into the Utsira aquifer. A percentage of the natural gas produced is also reinjected into the Sleipner East reservoir in order to improve overall condensate production.
Licensees for the Sleipner West field are Statoil (operator) with 49.50% (this includes 32.37% as the government's direct financial interest), Esso Norge with 32.24%, Norsk Hydro with 8.85%, TotalFinaElf Exploration Norge with 9.41%.
The Utsira Formation
The Utsira Formation is a 200-250m thick massive sandstone formation located at a depth of 800-1000m beneath the seabed. This was selected as the reservoir for the storage of CO2 produced from the Sleipner West Field.
The carbon dioxide extracted from natural gas production on Sleipner West is injected and stored into the Utsira formation 1,000 metres beneath the seabed, rather than released to the air. This is done by a four stage CO2 injection compression system. One million tonnes of carbon dioxide are stored in the sub-surface annually. It is estimated that the Utsira Formation is capable of storing up to 600 billion tonnes of CO2.
The CO2 separated is injected into a small "structural closure" northeast of the SLA platform. The CO2 (not dissolved in formation water) is captured within this closure. When the closure has been filled, the CO2 is expected to spill towards the north, and thereafter to the northwest. The CO2 gas is not expected to reach the SLA platform location so avoiding corrosion-related problems occurring as a result of the injected CO2.
3D seismic data covering the Sleipner East area has been used to map the Utsira Formation. The seismic response represents the changes in "impedance" in the water-filled reservoir. The impedance is affected by the CO2 injected into the reservoir. This change in seismic impedance is apparent on new seismic data produced. The presence of CO2 and the movement of the CO2 gas in the Utsira Formation reservoir can therefore be monitored by examination of later seismic data.
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