The Heidrun Field -Pre-drilling: ACase Study in Designing forand Achieving Success
Authors: Deem D.B. , Eggemeyer J.C. , Estep F.E. , Hejnal D.M. 
Published: October 2008

Abstract
A 283 MM USD, 983 rig-day pre-drilling program was conducted inorder to accelerate oil production and initiate water injection as soon as possible from the Heidrun Field Tension Leg Platform (TLP). This paper describes the successful program with particular emphasis on: staffing & engineering, batch setting 62 slots, drilling and pre-completing 10 main template and 6 subsea satellite water injection wells, all from a semisubmersible in 345 meters of water, 165 km South of the Arctic Circle in the Norwegian Sea.

Introduction
A basic advantage of a TLP is the ability to have wells predrilled and completed during TLP construction, thereby greatly accelerating first oil, plateau production sod water injection for reservoir management. At Heidrun, pre-drilled wells accelerated the production of an estimated 20 MM STB, allowed reaching plateau of 220,000 BOPD in less than 2 1/2 months after first oil production, and allowed for full water injection prior to production start-up.
Planning of Heidrun pre-drilling started in January, 1991. Actual operations began from a semi submersible immediately after the installation of the 56 slot subsea template in September, 1992 and ended in August, 1995. The entire drilling effort was designed with considerable front-end loading'. A team of empowered operator and contractor personnel was put in place to develop detailed programs, identifi and manage risks, implement a continuous improvement program, and operate as aggressively as possible.
The operation was divided into 3 main phases: batch setting, pre-drilling and pre-completions, The scope of work was to set all 30" conductor and 20" or 22" surface casings in each of the 56 template slots, drill 9 (later increased to 10) wells to TD, complete 5 wells with gravel packs, and drill and complete 6 satellite subsea water injection wells. This program was designed to achieve two major additional benefits. Firstly, shallow gas was cased off, increasing safety and eliminating the need for a costly diverter system on the TLP. Secondly, well interference was managed by having the surface intervals directionally drilled with sufficient clearance to negate shutting in adjacent wells while drilling in the future, thus saving production.
The problem was that this defined scope of work was estimated to take 1250 rig days. Pre-drilling could not start until the template was set in September, 1992 and main-template work had to be completed by the time the TLP was to be installed in May, 1995, or roughly 975 days. Clearly either multiple rigs had to be used simultaneously, or significant reductions to the scope of work had to be endured, or major gains in efficiency had to be realized. Multiple rigs, although initially planned, were financially undesirable, a reduced scope of work was unacceptable, so major gains in efficiency became the driving focus.

Key Elements
In order to realize major gains in efficiency, each person working on Heidrun was empowered to question the status-quo and take reasonable risks. Front-end loading through proper staffing and engineering was critical, but only half of the story.