World's First Application of a Multilateral System Combining a Cased and Cemented Junction with Fullbore Access to Both Laterals
Authors: Hovda S. , Haugland T. , Waddell K. , Leknes R. 
Published: December 2007

Abstract
Norsk Hydro in conjunction with its service partners have completed the world's first multilateral well with a cased and cemented junction and fullbore access to both laterals.
Norsk Hydro's Oseberg C platform has a limited number of slots to drill new wells to recover considerable reserves that remain in the Ness Formation of the Brent Group. Multilateral wells were seen as a solution to provide additional cost effective drainage points to the reservoir unit. This highly visible and multidisciplined project demanded a dedicated team from Norsk Hydro and Halliburton during the planning and execution phases.
The C-12B horizontal section was drilled and completed as planned in the ORE (Oseberg, Rannoch & Etive) Formations. A window was cut in the 9 3/8-in. parent casing with a packer-based multilateral system. A new lateral was drilled and completed in the Ness Formation and the parent wellbore was regained. Commingled production is possible when the pressure differentials between the two formations have equalized.
The following list outlines the main objectives of the testing of multilateral technology.–install the whipstock and mill a window–install a hollow whipstock–install and cement the lateral liner–reestablish the parent wellbore–acquire fullbore access to the Ness and ORE laterals
Norsk Hydro and its service partners evaluated each operational phase according to success criteria mutually agreed upon at the outset of the project.

Introduction
The Oseberg Field is located in the central North Sea, 130 km northwest of Bergen, Norway, on the Norwegian Continental Shelf. The field, which was discovered in 1979, is one of the largest oil fields in the North Sea. Total recoverable reserves are 1.6 billion barrels. Two platforms, situated 13 km apart, operate the 27 × 5 km field. The southern part of the field is operated by the Field Center; which consists of a drilling/production platform and a process/accomodation platform connected by a 100 m long bridge. The northern part of the field is operated by the Oseberg C platform which is a fully integrated platform (Fig. 1). Until now, horizontal wells contributed greatly to the increase recovered reserves in the Oseberg Field. For the future, multilateral technology is seen as very promising.
All available well slots are currently allocated on the Oseberg C platform. Additional reserve potential is defined in the heterogeneous Ness channel sandstone, which would require additional wells to improve recovery. However, channel sandstone is difficult to map seismically because of its size and scattered recurrence. Because of the uncertain reserve potential in these sands, the operator required a solution that cost less than adding new slots. Multilateral wells offered the possibility of increasing the number of drainage points at a lower cost than drilling new wells. In addition, multilateral wells increased the possibility of recovering marginal reserves in other parts of the Oseberg Field.
The main reservoir unit is the Oseberg Formation which consists of a medium to coarse grained fan-delta sandstone within the Middle Jurassic Brent Group (Fig. 2). The main well 3016-C-12 is one of the oil producers completed during the predrilling phase in 1989. The well penetrated the Brent Group at approximately 30 inclination, high above the oil-water contact. Gas breakthrough will occur in these types of mature production wells and leave a considerable amount of oil downflanks between the wellbore and the oil-water contact.
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