RWE Dea begins major upgrade of Mittelplatte offshore field
Jeremy Beckman
Editor, Europe
Mittelplatte, Germany's sole active offshore oilfield, is undergoing a major face-lift. New drilling facili-ties will be installed at the field's production center in the Watten-meer tidelands, 7 km off the North Sea coast. These will allow operator RWE Dea to access greater quantities of oil via longer extended reach wells.
The resultant extra volumes should be sent to shore at Friedrichskoog, north of Brunsbüttel, via the field's first offshore pipeline system. This, however, is still subject to authority approval. Nothing can be left to chance when operating in a national park. In this regard, Mittelplatte is unique.
Following exploration drilling in the early 1980s, the local parliament in Schleswig-Holstein authorized development in 1985. The field's location was a Category 3 (since upgraded to 2) Protection Zone, where controlled production of hydrocarbons is permissible. Construction started the same year on an artificial drilling and production "island" on the sandy tidelands. When the tide comes in it becomes an offshore facility.
The structure covers an area of 70 x 95 m, with a leak-proof steel and concrete basin and sheet pile walls up to 11 m high. These protect operations on the platform against the elements and also prevent hazardous materials or waste seeping into the seabed. A 50-m wide anti-scouring rock fill also safeguards the island against wind and sea impacts.
Process equipment includes separation and injection water treatment units. The 52-m drilling rig is mounted on hydraulic skids, with 44 well slots spaced around the complex.
The new living quarters being installed on the Mittelplatte drilling/production island.
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Produced crude is stored on the island in a 2,000-cu-m-capacity double-walled steel tank, before being offloaded onto three specially designed, double-hulled tank barges in parcels of up to 1,000 cu m. These vessels are fully enclosed in the island's integrated harbor basin during transfer operations. Even when fully laden, they can withstand grounding, should tidal waters suddenly retreat. In addition, there are four supply vesels providing transportation of materials and the Mittelplate crew to the island.
Among other safety and environmental measures, fast-acting safety valves and pumps automatically close the well(s) down in the event of a production incident. The rig cellar and boreholes are positioned above the island's sealed concrete base.
Deviated wells
Mittelplatte contains an estimated 100 million tonnes of crude in multiple oil-bearing sandstone layers, at subsurface depths ranging from 2,000 to 3,000 m. Due to the complicated nature of the reservoir, deviated wells have been applied at all stages of the development, starting in 1986 on the western part of the reservoir. The early years of production were effectively an extended trial, designed to verify the production engineering, environmental protection, and transportation safety concepts.
Subsequent concerted drilling helped double production in 1995 to 1,750 million tonnes/d. The same year, the second dedicated tank barge was added to transport the additional output.
On the island complex, a second gas turbine was installed to increase production by using surplus gas converted for heat and power co-generation, eliminating flaring. A power cable was also laid to Friedrichskoog to feed the complex's unused electricity into the national grid.
During the six years that followed, the development focus switched to the deep basement in the Meldorf Bay area. Shallow water seismic was acquired, the aims being to improve understanding of the exact position and extent of the overlying Busum salt diapir; to determine the eastern extent of the oil-bearing rock; and to assess the potential for tapping more distant sections of the reservoir through extended reach wells from the shore.
In 1997, the first of these wells, Dieksand 1 was drilled from a well site on the coast at Friedrichskoog. Following the results of this well and analysis of 3D seismic over the local tidal flatlands, the much more ambitious Dieksand 2 was spudded from the same location, using T-6, a land rig designed and manufactured by Bentec. This is an electrically powered machine with an output of over 6.3 MW.
Dieksand 2 reached its target in Mittel-platte's oil-bearing Dogger Sands after a total drilling length of 7,727 m – qualifying at the time as one of the world's longest extended reach wells.
Successively longer wells were drilled by the rig into the eastern part of the reservoir. All were steered successfully through the salt diapir culminating in the 9,275-m-long Dieksand 6 in 2002. The latter established several records, including:
- Fifth largest deviation anywhere for an ERD well (8,404 m horizontal step-out)
- Deepest 8 3/8-in. section (from 7,247 m to 8,562 m)
- Deepest 6-in. section (from 8,562 m to 9,275 m).
The drilling program was concluded early in 2003, and the T-6 has since been stacked.
To process the crude from these shore-based wells, new oil treatment facilities were built at the Dieksand Land Station in Friedrichskoog. An overland pipeline was also laid, transporting oil, associated gas, and condensate southwest to the port of Bruns-buettel, from which the oil is piped due north to a Shell-owned refinery in Heide. Since 2001, combined production from the island and the Dieksand wells has totaled around 1.8t/yr.
Oilfields and processing centers near the coast of Schleswig-Holstein.
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New modules
Last August, RWE Dea took delivery of a new living quarters and helideck for the Mittelplatte island. The 830-tonne quarters, built in two sections by Aker Weft in Wismar, cost ¤22 million to build, transport, and install. It was shipped to Cuxhaven via the North Sea/Baltic Sea canal system. A helideck was also added, taken from the recently decommissioned German sector platform Schwedeneck-See.
As production experience has built up on both sides of the reservoir, RWE Dea has doubled its original recoverable estimate to nearly 50 million tonnes. So far around 13 million tonnes have been produced. Since the start of operations, 18 wells have been drilled from the island using the 52-m-high rig, built by Deutsche Schachtbau. The cuttings, conveyed to the surface with the drilling mud, are taken ashore in containers for disposal in a landfill.
In January 2006, a new rig will be installed and fully commissioned. The ¤38-million engineering, procurement, and construction contract was awarded to Bentec in March. Bentec had worked on the design on RWE Dea's behalf for over three years. According to a Bentec spokesman, it will feature conventional equipment. Wirth is supplying mud pumps and the drawworks and Maritime Hydraulics the top-drive system.
According to RWE Dea, the Dobber Beta reservoir is the target for the next batch of island-drilled wells. The planned distances of up to 7,000 m would have been beyond the reach of the T-345. Local officials of Germany's National Parks bureau have approved the new drilling scheme. The Mittelplatte partners also plan to convert six existing island-drilled oil producers to water injectors.
A more contentious aspect of the new development involves installing pipelines to export produced oil from the island to Brunsbuettel via the Dieksland Land Station and to bring in injection water from the opposite direction. The planned route would include a 7.5-km crossing of the surrounding Wadden Zee, classified as Protected Area 1. Farther west in the Dutch sector, this region has been off limits for years to drilling activity.
"The authorities' first reaction was, if you can drill that far out with your new rig, you don't need pipelines," says the RWE Dea spokesman. "With our existing offloading system, we are restricted to 900,000 tonnes/yr. Access is limited both by tidal movements and the local climate. In rough seas, for example, our barges don't go out for safety reasons. Then, we also have to shut our wells in. With a pipeline, however, we could increase production from current wells alone to 1.4 million tonnes/yr, and we could also keep our wells open in bad weather."
Currently, the consortium is seeking app-roval for two pipelines linking the Mittleplate island to Dieksand Land Station. The oil export line will have a diameter of 10 in., while the water injection flowline will have a diameter of 6 in. The partners plan to lay the lines in six sections to 1,400 m long, using horizontal directional drilling techniques. This should ensure completion of the work in a tight time frame of only three months in the Protected Area I of the national park.
Pipeline sections 1.4-km long would be pre-assembled mainly onshore and starting from a pontoon erected alongside the Mittelplate island, pulled to the respective end-stations. This procedure entails use of a route support system, comprising posts with roller bearings and mud flat protection mats on which the pipeline conduits would be transported. Welding is performed in special containers. First oil is scheduled to be exported to the shore in September 2005.
