The Impact of a Novel TCI Bit on Oseberg's Horizontal Drilling Program
Authors: Andresen S. , Syltoy S. , Jones R. , Wicker A.K. , Pessier R.C. , Olsen T.L. 
Published: December 2007

Abstract
As part of its objectives to increase recoverable reserves and reduce development costs in Norway's Oseberg field, Norsk Hydro has aggressively employed extended reach horizontal drilling over the past four years. Critical to the success realized in the Oseberg development program has been the use of an integrated steerable drilling assembly that features a near-bit sensor providing real-time measurements of the well path, thus enabling drilling in a corridor of one to two meters. Problems with orienting fixed cutter bits in the highly demanding sliding mode necessitated the use of tungsten carbide insert (TCI) roller cone bits to follow the required trajectory.
This paper describes the development of new IADC 437 and 447 Class TCI bits, which culminated in a unique gauge cutting structure with diamond-enhanced, chisel-shaped cutting elements. The authors will review the bearing, seal and cutting structure limitations of conventional roller cone bits used in earlier Oseberg wells, emphasizing the negative impact of excessive gauge wear and short bearing life to overall well costs. Laboratory and field data will be presented, with emphasis on the lessons learned during extensive cutting structure and bearing/seal examinations. The successful application of the new design in the Oseberg development drilling program will be discussed in detail.

Introduction
Norway's Oseberg field, located approximately 130 km north-west of Bergen, was discovered in 1979 and is presently being developed via two platforms - the Oseberg B and C - which are situated 15 km apart.
The Oseberg reservoir section is located in the Middle Jurassic comprising several sand units: the Tarbert Upper and Lower Ness, Etive and Oseberg formations (Fig. 1). The Tarbert shows subangular and subrounded sandstones moderately sorted with a thickness of up to 60 m TVD. Firm and blocky coal and silty claystones sequences interbed the sandstones in the Ness formation. The Etive shows angular to subrounded sandstones with silicate cementation and siltstones at the base. The Oseberg formation is the main reservoir, consisting of medium to coarse-grained fan delta sandstones of excellent reservoir quality. The vertical thickness is 20 to 60 m TVD. The Rannoch claystone sequence separates the Etive and Oseberg formations.
The first horizontal well was drilled on the field in 1992, and since then 20 wells have been successfully drilled and completed. Over that time, the lengths of both the horizontal section and the Total Measured Depth (TMD) have increased progressively. In 1995, Well C-26A established a then-world record with its 7,853 m horizontal displacement. In that well, the horizontal section was 2,100 m and the total depth 9,325 m. In early 1996, the first multi-lateral wells - C12A, B and C - were successfully drilled from the Oseberg C platform.
To maximize the recoverable hydrocarbons in the horizontal reservoir sections, the deviations from the trajectory have to be kept within a tight tolerance on vertical and tangential variations (often 1m). The tight tolerances force the operator to drill the reservoirs with a reservoir navigation tool (RNT).
Both fixed cutter (PDC) and roller cone tungsten carbide insert drill bits (TCI) are used, depending on the lithology and operating demands. Historically, dulled TCI bits exhibited severe abrasive wear in the gauge area, frequently resulting in under-gauge hole and early seal failures. Thus, to avoid the risk of losing cones, the operator was forced to pull bits after short times on bottom. Lost cones generally cause difficult and expensive fishing jobs.
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