Development and Case Histories of an MWD Directional Drilling Package for 2-3/4" Openhole
Authors: Blount C.G. , Mooney M.B. , Smith B.E. , Quinn D. , Larson E.B. 
Published: December 2007

Abstract
This paper documents the development and first applications of extremely slim diameter mud-pulse-telemetry directional drilling tools for applications through 3-1/2" production tubing.
The highly successful Coiled Tubing Drilling (CTD) program at Prudhoe Bay has concentrated on equipment required to re-enter 4–1/2" and larger 'tubing' sizes. Typical CTD re-entry operations have sidetracked existing wells at a cost 25% to 75% of past conventional rotary operations. However, there are a significant number of wells at Prudhoe Bay, other North Slope fields, and worldwide that have been completed using 3-1/2" tubing. The development of smaller diameter directional tools allow through-tubing CTD re-entry techniques to be extended to a dramatically larger number of existing wells and also allows options for zonal isolation using small ID intermediate strings in larger tubing sizes.
Purpose built equipment needed to allow directional drilling through typical 3-1/2" tubulars has now been developed and utilized to drill the world's first directionally steered 2.75" bore holes at Prudhoe Bay. This newly developed equipment was used with CT to directionally extend the horizontal section below existing 3 1/2" tubing strings. Economics of the first 3 wells would indicate a level of performance and cost effectiveness consistent with the larger CTD equipment. This paper details the design criteria and covers the operational implementation of the 2–3/8" OD directional drilling assembly. The problems and solutions of the equipment, along with the execution of the first CTD jobs through 3-1/2" tubulars are also addressed.

Introduction
Coiled Tubing applications have contributed significantly to meeting the continued development and production goals at Prudhoe Bay. Since arrival at Prudhoe Bay in 1981, CT technical achievements and procedures have resulted in tremendous cost savings and revenue enhancement for the producing companies. The most significant CT developments include cement squeeze procedures, underreaming tools and techniques, inflatable through-tubing packers and bridge plug applications, CT with wireline for servicing horizontal wells, and most recently, CTD technology. Coiled tubing applications represent a critical cornerstone of activity in the continuing field development and production rate maintenance of Prudhoe Bay.
The latest CT applications to be exploited at Prudhoe Bay and rapidly expanding to international markets is CT Drilling - the most recent development being an ultra-slim Measurement While Drilling (MWD) directional package. Prior to this development, the CT re-entry drilling program at Prudhoe Bay had concentrated on equipment required to re-enter 4–1/2" and larger 'tubing' sizes. With a significant number of wells at Prudhoe Bay and other north slope fields completed using 3–1/2" tubing, a smaller directional drilling system, was required.
Prior to development of these ultra-slim tools, mud-pulse telemetry directional drilling was not thought possible in openholes smaller than about 3-3/4". A new approach to the problem of size limitations imposed by current generation of mud-pulse telemetry MWD equipment has resulted in tools that have successfully and cost effectively been applied in 3 CTD jobs through-3–1/2" tubing, at manuscript due date, with an additional 3 jobs slated for summer of 1997. This new drilling development enables directional drilling and gamma ray logging in 2–3/4" nominal diameter holes, allowing use through 3-1/2" tubulars In addition, these first 3 wells were drilled at a cost of only about 30% – 40% of a conventional rotary drilled sidetrack.
Re-entry drilling has and is expected to continue representing a highly cost effective and viable alternative to new wells at Prudhoe Bay. This development in the advancement of slim hole technology dramatically increases the potential candidate pool for worldwide re-entry drilling by orders of magnitude. This development achievement can be applied in both CTD wells and in rotary drilled wells.
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