The Shearwater development is a major component in the Central North Sea infrastructure with its processing facilities designed for a 30-year life. The process facilities topsides lift, scheduled for next spring, will be the heaviest ever undertaken in the province - 11,100 tons. So far, Shearwater has been a model offshore project, despite the demands presented by the HP/HT nature of the reservoir.
The Shearwater Field lies some 207 km east of Aberdeen in 90 meters water depth. It is located in Block 22/30b within the Central Graben Area of the Central North Sea. The licence holders are Shell (28%), Esso (28%), Arco British (27.5%) and Mobil (16.5%). Shell is the operator. The field was first drilled in 1988 by Arco British. A second well in 1991 consolidated the discovery. Following a five-year evaluation period, development sanction was received in April 1997. With initial production targeted for July 2000, work on the project is moving ahead rapidly to meet a variety of deadlines.
Two blocks
The Shearwater Field consists of the Main Block and the North West Block. The Main Block has two strata of porous sandstone: the high connectivity Fulmar and the underlying poorer quality Pentland.
Located over 15,000 ft beneath the seabed, the reservoir contains condensate and gas in a ratio of 170 bbl/MMcf of gas. There is up to 30 ppm (parts per million) of hydrogen sulfide and 3% carbon dioxide present, in a high salinity aquifer. The reservoir is at high pressure and temperature (HP/HT) - 1,000 bar and 360°F.
Initial production will be from the Main Block Fulmar, which contains the bulk of the hydrocarbon. Estimated reserves are 159 million bbl of condensate and 844 bcf of gas. The facilities are designed to handle peak production rates of 82,000 b/d of condensate and 425 MMcf/ of gas.
Shearwater field has an estimated life of 12 years with a production plateau lasting for only the first three-four years. Nevertheless, the facilities have a 30-year life, designed so that they will form the hub for additional production from smaller accumulations known to exist nearby. Near field potential prospects - the North West Block and Main Block Pentland - are being evaluated with the intention of producing them via the new facilities.
Reducing well needs
Owing to the fact that reliable technology does not exist to drill into partially depleted HP/HT reservoirs, it was necessary to complete the drilling of all the development wells before production. At one point, 15 wells were being considered. This number soon came down to seven and finally a plan for five high-production rate wells with maximum deviation of up to about a mile from the facilities was drawn up. The need for an early start to drilling had a bearing on the development concept, which is based on a main platform with central processing facilities, utilities and quarters (PUQ), bridge-linked to a smaller wellhead platform positioned 80 meters away.
Drilling program
Maersk undertook the drilling of the five Shearwater development wells. The company acquired and refitted the JFP II jackup drilling rig specifically for this contract in Brownsville, Texas, at a cost of roughly £70 million. The rig was subsequently renamed the Maersk Endurer. The five wells were completed well ahead of schedule earlier this year and a significant saving recorded on the original budget. The Endurer has already gone on to drill a sixth well into the Pentland sand underlying the main reservoir.
During the design phase, it was determined that the number of wells could be reduced to five if 7-in. tubing was used. This involved the development of a suite of new completion tools and components, including a new 9 7/8-in. packer, tubing and wireline retrievable sub-surface safety valves, and an ABB Vetco Gray, 6 3/8-in. valve bore christmas tree rated for 15,000 psi at 350°F. Weighing 17 tons each, these trees were the first in the world to achieve such a pressure rating. Special consideration was also given to the design of the 4-in. choke valves needed to bring the pressure down from 12,500 psi to 1,300 psi.
The HP/HT, high flow rate, and highly corrosive production stream places enormous demands on the materials and construction of these items. Baker Oil Tools and Cameron have both worked closely with Shell on the well engineering side.
The 2,500-ton wellhead jacket was designed and its fabrication started at Odebrecht's Middlesbrough yard in northeast England well before sanction was obtained. As a result, it was completed and installed in August 1997, enabling the desired early start of the drilling program just a month later.
All processing will be performed on the main platform. Following separation - the primary separator will operate at 90 bar - the condensate will be exported via a new 24-in. diameter spur pipeline into the nearby Forties Production System and then onto the Cruden Bay terminal in north-east Scotland for further processing. Considerably more work will be done to the gas on board the platform. This must be treated to bring it up to sales quality, thereby enabling it to enter directly the UK's national transmission system or the Inter connector pipeline connection to Europe.
Carbon dioxide and hydrogen sulfide will be removed from the gas using an amine plant - the first time this has been carried out on a platform in the North Sea - with the regeneration tail gas being disposed of by incineration followed by seawater scrubbing. The gas will be dried using tri-ethylene glycol and a turbo-expander used for hydrocarbon dewpoint conditioning to minimize gas compression power and maximize NGL recovery. It will then undergo compression to 150 bar prior to export to the Bacton gas terminal in East Anglia via a new 34-in. diameter 300-mile pipeline which will be shared with the neighboring Elf-operated Elgin/Franklin development.
Environmental management
A very pro-active approach to safety and environmental issues has been adopted by the project with a series of environmental discharge objectives (EDOs) being laid down. Shell Expro believes that the early specification of these measures provides for cost minimization with a probable improvement in project economics.
The environmental discharge objectives designed for Shearwater include the following:
- Progressively introduce onshore cleanup and recycling of drill cuttings, resulting in a 50% reduction in mud discharge.
- Limit gas losses through flaring to the equivalent of 0.3 boe per day, by selection of suitable processing equipment for flare purging, the employment of valves with enhanced sealing arrangements, and recycle on startup.
- Flaring of the hydrogen sulfide removed from the export gas would have resulted in sulfur dioxide emissions. An alternative approach has been adopted involving incineration and seawater scrubbing. The resulting sulfates can be discharged to the sea with no harmful environmental impact. About two tons of hydrogen sulphide will be treated each day by this process.
- Water will be produced from Shearwater in only small quantities. Nevertheless, the aim is to reduce the oil in water content below the allowable limit before discharge - hydrocyclones will be used to deal with dispersed oil in the water.
- Minimize fuel gas usage. Energy efficiency will be in the region of 40% through the use of highly effective gas turbines and waste heat recovery. In addition, turbo-expanders have been provided as part of the gas processing facilities. A subsea cable to enable power sharing with the neighboring Elgin Franklin development will allow better power management.
Much of the emphasis now is on completing the integrated production facilities topsides at Amec's Wallsend, UK site. When sailaway starts next February, the giant structure will weigh approximately 11,500 tons. The incinerator, flare and bridge, being fabricated by Heerema in Hartlepool, and the living quarters, almost complete at Odebrecht's Middlesbrough yard, will be added offshore. The final operating weight of the topsides will be around 14,500 tons.
