The Conversion of a Two-Well Modular Guidebase With Shared Posts to Subsea Production
Authors: Gomersall S.D. , Hopkinson J.M. 
Published: October 2008

Two exploration wells were drilled in U.K. North Sea block 16/7a in 1984/5 using modular guide base structures with the second well-head cantilevered from the first and the two wells sharing the centre two guideposts. The wells have subsequently been re-entered and completed for production sub sea, in such a way as to allow clearance to land a tree on each wellhead and so that with a tree installed on either well, access is available to run a BOP stack on the other well. This paper describes the wellhead modifications required, the unique sub sea tree design, and the associated equipment required to complete two wells in such close proximity.

Introduction

The Central Brae field was discovered in 1975 by well 16/7a-3 approximately 155 miles (249 kilometers) north east of Aberdeen in 350 ft (107 m) of water (Figure 1).

An appraisal well, 16/7a-27 was drilled in 1984 with a primary guide base of a modular wellhead system used as the permanent guide base for the well. A second appraisal well l6/7a-29 was drilled in 1985 using a cantilevered guide base hung from the primary guide structure of l6/7a-27 (Figure 2) In this configuration the wells are 8'6" (2.6m) between centers and share the two centre guideposts having only six posts between the two wells. The main reason for using the modular wellhead system was to allow the wells to be tied back to a jacket at a later date should a platform development prove viable. Due to the calculated size of the field and prevailing economics the decision was taken to develop the field using sub sea technology.

The Central Brae field is an Upper Jurassic black oil reservoir (37°API, 1400 scf/bbl GOR) containing approximately 70 million barrels of recoverable liquid hydrocarbons. Process Facilities on the Brae ' A' platform 4.3 miles (7 Km) away with spare capacity, allowed a sub sea development to utilize the existing infrastructure with minimum platform modifications.

In spring 1989 an eight slot sub sea template weighing 650 tones was placed adjacent to the two existing wells (Figure 3) The template structure extends to give protection for the two 'off template' wells and provides Mani folding for production, injection and service line tie-ins. In addition control line jumper bundles including electrical power and signal cables, and hydraulic lines were pre-installed on the template prior to launch. The template provides the guidance for drilling up to eight additional wells utilizing a modified permanent guide base to allow well completion.

Control is provided to the template via a 4.3 mile (7 KID) electro-hydraulic umbilical tied back to the Brae 'A' platform. On the template an electro-hydraulic distribution box provides signals and ontrol fluid to each well bay. In addition, a chemical injection umbilical capable of providing wax inhibitor, scale inhibitor, hydrate inhibitor and corrosion inhibitor is tied in to the template produced fluid lines.A

Two 6" flow lines connect Central Brae to the Brae 'A' platform along with a 10" water injection line and a 4" service line. The template has Mani folded piping allowing the future installation of a further two 6" flow lines.