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Abstract Since their introduction in 1997, rotary steerable drilling systems have delivered significant gains in drilling efficiency. In addition to this fundamental benefit, these systems have enabled more challenging wells to be drilled at low risk with a wide range of other advantages, including improved well placement, etc.
Continuous rotary drilling operations do however bring with them certain challenges which must be considered before rotary steering is selected for use. These include instantaneous penetration rate, specifications of rig rotary equipment, casing or drillpipe wear, stress on the drillstring, loss of drilling power through wellbore friction and drilling dynamics.
Using high powered drilling motors, traditional performance drilling has been applied since the early 1990's to improve penetration rates by applying high power and consistent operating parameters directly to the drillbit. This has however been limited to straight holes or the most basic of directional profiles.
A system has been developed which integrates a specially designed high power drilling motor within a high speed rotary steering assembly. By using this new system:many of the challenges of continuous rotary drilling are mitigated,
more complex wells benefit from the advantages of traditional performance drilling,
existing drilling envelopes can be extended to further improve field recovery.
This paper discusses the engineering design of the complete system, including the specially designed motor and high speed rotary steering system. The paper then goes on to discuss specific applications where the system should be considered for use, illustrated with results from real examples.
Introduction Mærsk Olie og Gas A/S operations include the development of several chalk reservoirs in the Danish sector of the North Sea. These reservoirs are located at approximately 6,700 ft True Vertical Depth (TVD) and are drained through the use of horizontal wells. A typical well design is shown in Figure 1. Productivity from these horizontal wells is largely dependent on the 3D precision with which the wellbores are positioned within the chalk and the length of reservoir exposed. Prior to the introduction of rotary steerable assemblies, conventional directional drilling techniques were employed on these fields. Using steerable motor assemblies, the limitations of steerability of these systems restricted horizontal reach to approximately 16,000 ft Measured Depth Rotary Table (MDRT) and TVD control to +/− 5 feet. To extend the reach of the wells and allow vertical wellpath position control, adjustable stabilizers were run in conjunction with the steerable motors. This gave an increase in the length of horizontal section past the sliding limit of the conventional system but wellpath position control was only possible in the vertical plane with no control in the lateral plane. Using adjustable stabilizers with motors allowed wells to be drilled with reach of 22,000 ft MDRT.
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