The Evolution of Profitable Development Drilling in Prudhoe Bay: A Case of Adapting to Survive
Authors: Stanley M.J. , Stoltz D.S. 
Published: December 2007

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Abstract
The evolution of well designs that has taken place in Prudhoe Bay Unit (PBU) is described in this paper. The reasons for the differing well-types are explained, emphasizing how the completion profiles & designs are tailored for the particular reservoir offtake plan at the target location. The central driver of the evolving designs is always to retain development drilling as an attractive investment in an environment of a decreasing trend in incremental reserves recovered by new wells.
The major conclusion to be drawn from the review, is that to continue with a profitable development drilling program, the PBU operators have moved away from the simple 'off-the-shelf' PBU well designs of only a few years ago. The various drilling & completion methods employed today, recognize the value of the existing infrastructure available and the differing production opportunities and problems that are present in the PBU reservoir. The goal of the reservoir development drilling program is a sophisticated balance between minimizing development costs, while providing a quality completion that optimizes production rates, allows future well-work options, and provides appropriate well longevity.

Introduction
Prudhoe Bay Unit is currently undergoing the most active development drilling program in its twenty year production history. Over 100 reservoir penetrations have been achieved in the past year, with a similar rate of activity planned over the near-term future. This program is occurring despite the number of wells originally envisaged having been greatly exceeded and the field now being well into it's decline phase.
The Prudhoe Bay field was discovered in 1968. Original oil-in-place is estimated at 22 billion barrels, with ultimately recoverable oil reserves projected at around 13 billion barrels. To-date over 9 billion barrels has been produced. There has been an ongoing development drilling program since the mid-seventies. First production occurred in 1977, with a plateau production rate of 1.5 MMSTB/D being attained by end 1979 and lasting until 1988. The field has now been in decline eight years. Many incremental development projects are being undertaken in order to mitigate the production decline, and new reservoir penetrations are a key element of many of these projects.
As field-life has matured, the distribution of gas, oil and water in the reservoir has become far more complex and target locations have become smaller. Consequently drilling & completion practices have had to evolve to meet new challenges in order to keep development costs at an economically attractive level.
Unit development work is normally carried out in multi-company, multi-disciplinary teams. This practice has encouraged innovative well designs and reservoir access methods.
The main drilling & completion options currently available and utilized in different circumstances in PBU include: new wells vs. sidetracks; rig-type: conventional rotary vs. coil tubing drilling unit; completion profile: horizontal/high-angle vs. conventional; directional build profiles: short vs. medium radius; Liner type: slotted vs. solid & selectively perforated; completion tubing metallurgy: chrome vs. carbon-steel: New and developing options still in their infancy in PBU include, thru-tubing rotary drilling (TTRD) and multi-lateral well designs.
The field's history and that of it's development drilling program may be characterized into three distinct phases:discovery to plateau production;plateau production; andproduction decline.
Each segment required differing engineering emphasis. In the long post-plateau phase of PBU's drilling history, the key is, and will remain, that of cost effective incremental development by pursuing innovative techniques to access the field's remaining reserves.
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