Detailed, interdisciplinary planning of each operation based on its ability to deliver the overall aim can facilitate extended-reach wells.

The Mittelplate field, being developed by operator RWE Dea in 50% partnership with Wintershall, is Germany's largest oil field with an estimated 100 million tons of oil in place, of which 35 million tons should be recoverable using available technology.
The field lies under the Wattenmeer tidal mud flats about 4 miles (7 km) off the west coast of Schleswig-Holstein. Initial development was carried out from an artificial drilling and production island built on the mud flats directly above the reservoir. Stringent practices and controls are necessary because the surrounding area became a protected nature reserve shortly after field development approval was obtained.
The island is designed to resist the worst possible weather conditions and contain spillage of any liquids. Drilling waste is shipped ashore for treatment and disposal. The crude oil is loaded into double-skinned tank barges, twice daily at high tide. When the tides retreat, the island is rendered an "onshore" mud flat installation. This process limits oil production to about 800,000 tons annually.
The consortium wanted to accelerate the recovery rate and started the Dieksand project to exploit the field's eastern margins from the shore using extended-reach drilling (ERD) technology. The Dieksand onshore drilling location is about 4.3 miles (6.9 km) east of the field near the village of Friedrichskoog. Oil flows to the special gathering station nearby and is subsequently pumped to the refinery.
The ERD well challenge
ERD wells have a ratio of horizontal displacement to true vertical depth greater than 2.5. Standard equipment and techniques can be stretched to the extremes and the operational margins for error are usually greatly reduced.
The Dieksand consortium needed to drill the first onshore well (Dieksand 2) to a 24,600-ft (7,500-m) MD with a horizontal displacement that would put it in eighth place in the world ERD well rankings at that time. The top-hole geology and the requirement to drill 13,125 ft (4,000 m) horizontally through a salt dome further complicated the situation.
To minimize risk, the consortium decided to use the team concept BP had used so successfully at the world-record Wytch Farm project. By assembling an international drilling team that could draw on experience from other major projects, many of the generic ERD challenges were anticipated. The drilling team included operator and partner personnel, a drilling and completion fluids specialist from Halliburton, a directional drilling coordinator from Schlumberger and a drilling engineer/ rig manager from KCA/Deutag.
The group was based at RWE Dea's headquarters in an open plan environment, together with the project manager, the geologists and administrative personnel. This group expanded to include completion engineers, geophysicists and reservoir engineers as the requirements of the maturing project demanded.
Planning
The drilling team devoted the first 4 months of its existence to a detailed feasibility study. During that time, it finalized a casing program, analyzed potential operational risks and developed contingency plans. Once the first well was planned and approved, the team concept expanded to include rig-site personnel, who were encouraged to become involved in planning issues, query procedures, supply alternatives and actively participate as much as possible. Large-forum prespud meetings, prephase and post-phase discussions and daily rig-site morning meetings followed by a videoconference with the drilling team in Hamburg assisted this process. Information exchange is regarded as one of the keys to the operation's success.
The detail planning consisted of posing dozens of questions, answering them and trying to foresee the implications of the answers. For example:
• How fast should drilling fluid be pumped in 12 1/4-in. hole at 23,625 ft (7,200 m) MD?
• What are expected pressure losses with a fluid specific gravity of 1.4?
• If a fluid specific gravity of 2.0 is required to control the salt formation, what is the maximum pump rate?
This scenario was repeated for each aspect of the well construction process.
Rig modifications
To satisfy the expected requirements for drilling torque, pump rates and pressures, as well as moving the rig from one slot to the next, modifications included:
• installation of three new 2,000-hp, 12-in. stroke pumps, with a 1,600-hp unit as backup;
• upgrade of the surface system to 500 bar;
• placement of soft pump and soft torque control systems;
• modification of the flow line and cable-trays to allow the drilling unit to be moved 115 ft (35 m) to cover all slots, while the remainder of the rig system remained stationary. The contractor took the risk of modifying the drilling unit to be skiddable without lowering the derrick.
Drilling site
The team designed the site with storage of long strings of tubulars, large volumes of fluids and minimization of environmental impact in mind. The main area of 2.5 acres (10,000 sq m) is concrete, with specially strengthened areas designated for heavy storage. A concrete ditch that drains into a specially built pond surrounds the perimeter. Personnel monitor the pond constantly for pH and salinity changes so inflow can be stopped if necessary.
The inner area, comprising about 1.2 acres (4,900 sq m), includes the area around the drilling unit, the drilling fluid tanks and pumps and the area designated for casing storage, cleaning and greasing. A network of drains in this area flows to a concrete pit system of overflow and underflow chambers designed to separate water, oil and solids. The system is sized for expected rainfall, and all fluid collected is sent for controlled disposal. On one occasion, this amount approached 629 bbl during a 24-hour period.
The drilling slots were designed so completion trees are below ground level for purposes of operational safety, ease of drilling unit movement and aesthetics. Oil- and water-based cuttings are collected in concrete pits and sent for controlled disposal.
Directional drilling
When drilling an ERD well, directional drillers face considerable challenges during the planning and execution phases.
The build-angle phase should be designed to minimize torque and drag and maximize weight transfer downhole. This can be achieved using a pseudo-catenary well profile. However, excessive tortuosity in the top-hole phase can create so much torque and drag deeper down the well that it may be impossible to orient the drill bit, trip into the hole without rotation or even rotate the drillstring in extreme circumstances.
Can the critical build rates be achieved in the hole size dictated by the casing program? On the Dieksand wells, the build phase is in the 23-in. hole section, requiring a 171/2-in. pilot hole followed by reaming.
The tangent hole angle should be planned for between 75° and 85°. If less than 75°, the drillstring slides downhole too easily, which can cause tension in the build phase, pulling the string against the borehole wall, thereby increasing torque, the risk of key-seating or casing wear. If the angle is greater than 85°, friction on the low side of the hole can create too much torque and drag, and hole cleaning becomes more difficult.
Minimal downhole vibrations and shocks prolong the life of measurement-while-drilling tools. Because tripping in and out of the hole can take 3 days or more at depth, a slightly lower rate of penetration may deliver the well in a shorter time. For similar reasons, the use of rotary-steerable systems may be economic, despite the high utilization costs. The team used the Schlumberger PowerDrive system on the latest Dieksand well.
Drilling wells longer than 24,600 ft (7,500 m) may require a different configuration of logging-while-drilling tools compared to those used in shorter or shallower wells.
Drilling and completion fluids
Supplying fluids to satisfy the myriad requirements of drilling and completing the Dieksand ERD wells has been a challenge. Halliburton's Enviromul oil-based mud provided lubricity and inhibition during drilling. This particular situation also involved preventing mud contamination with salt from the salt dome and acidic gas from the reservoir.
Hole cleaning and controlling drilled solids in the mud can be major challenges in ERD wells. Modeling can optimize the fluid's rheological properties to maximize hole cleaning while minimizing the downhole equivalent circulating density.
Cuttings ground up by the rotating drillstring for several hours while traveling to the surface are difficult to remove even with the finest shaker screens possible. Although fine screens can remove the smaller particles, more adhered oil is lost as these fines are shipped to shore for disposal.
Handling the changing volume of mud also is a challenge. At times, more than 3,775 bbl of mud volume is circulating, about 70% of which is downhole.
Well cleanup and displacement to completion fluid was a major technical and logistical undertaking. Displacing 1,888 bbl of Enviromul, a low-toxicity, oil-based mud, out of the hole with a 440-bbl pill-train of cleaning fluids followed by 1,888 bbl of inhibited water required considerable planning and the involvement of the entire rig crew. It is essential that all personnel are fully involved and informed, bearing in mind the total mud tank volume is only about 1,000 bbl, with 1,260 bbl of silo storage space. The operation went smoothly on each well, with no excessive waste and no major problems.
Running casing
Geology and the well profile dictated the basic casing program. The team spent much effort choosing tubular grades and specifications and connection types for each section. Highlights include:
• 20-in. casing was run and cemented to a 5,085-ft (1,550-m) MD 83° from vertical;
• a combined casing string comprising 9,843 ft (3,000 m) of 103/4-in. casing above 14,765 ft (4,500 m) of 95/8-in. casing was floated in, with the flotation collar positioned in the 95/8-in. casing. (On the Dieksand 5 well, crews floated the 95/8-in. pipe in as a liner on 65/8-in. drill pipe.)
• a 7-in., 5,250-ft (1,600-m) liner was placed across reservoir, requiring rotation during cementation; and
• a 75/8-in. protection string was run inside the 95/8-in. casing as a liner through the salt dome. (On recent wells, about 13,780 ft, or 4,200 m, was run in one step and cemented.)
Achievements
Despite many challenges, the members of the Dieksand project believe the huge amount of planning has paid off, and implementing the plan was done in such a manner that the ability to adopt and apply new lessons and techniques was retained for use on subsequent wells. The Dieksand 7 well is being drilled at press time, and further wells are under discussion.
Finally and more importantly, the production rate from the reservoir has doubled (Figure 2). That was, after all, the aim of the project.

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