Extended Reach Drilling Limitations: A Shared Solution
Authors: Rodman D.W. , Swietlik G. 
Published: December 2007

Abstract
This paper describes the strategies and techniques employed to solve the problems associated with excessive torque during offshore drilling of the Miller Field template wells. A timely solution was required which would enable subsequent platform wells to be drilled economically. When available technology was judged inadequate to solve these problems, a torque reduction sub was designed, tested and made available for use in this series of critical, high angle, deep platform wells.
In addition to a description of the conceptual design of various solutions proposed, the paper details the concurrent engineering, testing and field application of the Drill String Torque Reduction (DSTR) sub, which was the preferred solution.

Introduction
It became apparent with the industry extending the range and departure of directional drilled wells that the service limits of present drilling equipment would soon be reached if associated problems were not addressed.
Higher torque inputs from the top drive, larger angular twist and fluctuations on drill pipe, together with tool joint fatigue and casing wear, are manifested as a consequence of the frictional forces between the rotating pipe and the low side of the hole.
Earlier attempts at addressing the torque problems included modifications to Bottom Hole Assembly (BHA) components and the deployment of tools such as Roller Reamers and Non-Rotating Stabilizers/Roller Stabilizers. The results from these field trials were questionable and did not cater for the concerns over casing wear. They did not take into account the fact that in all cases the BHA lengths were considerably less than the total length of drill string and the summation of torque losses due to frictional effects was greater in the interval over the drill pipe length.
Earlier mathematical analysis of torque profile clearly showed that the section of hole from the Kick-Off Point (KOP) to the End Of Build (EOB) provided the greatest opportunity for reductions in torque to be achieved. With this understanding, a new deviation plan was developed through the use of two build-up phases separated by an intermediate tangent section and also by lowering the KOP. These techniques were successful in easing the situation but they did not address the underlying mechanical problem or that of casing wear. Other work on casing wear using different mud systems and aluminium tool joints also met with limitations.
Several obstacles therefore had to be overcome in the design and development of the solution. The service industry responded with several potential solutions, however these failed to provide the minimum required objective of reducing off-bottom torque by 30%, combined with durability and reliability, to give a minimum of 200 rotating hours between service or laydown.
Given this situation and the impending requirement to drill the ERD wells on the Miller development, a team was pulled together consisting of the operator, service company and design consultant, with the objective of concurrently engineering the solution during the drilling of the Miller development wells.

Background
The sourcing and development of potential solutions came about due to ongoing problems experienced on the Miller project, North Sea location (Fig. 1).
The Miller Development Team recognized from the template drilling experience that the naturally high field torque values and top drive power limit would restrict Rate Of Penetration (ROP), well design and impact casing wear predictions. Initial investigations indicated that off-bottom torque losses accounted for a large percentage of deliverable torque.
It was requested that several service companies should look into finding a solution to these problems.
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