Optimised Slot Utilization with a Geosteered Multilateral Well
Authors: Lodder Robert Jan , Winton Jack , Hamer Doug , Thackwray Ian 
Published: December 2007

In September 1998 Shell UK Exploration and Production, operating in the UK sector of the North Sea on behalf of Shell U.K. Limited and Esso Exploration and Production UK Limited, completed a fourth Multilateral well from the Tern Alpha platform situated in block 210/25a in the UK sector of the North Sea. This dual lateral well, TA-06, represents a major technical milestone in the process of constructing wells with both complex drainage architecture and highly specific reservoir targets from a slot valued platform. Moreover TA-06 also represents a major economic success in that the whole well was completed 7.5 days ahead of schedule with resultant savings in budgeted construction costs and the accelerated production of oil.

The primary objectives of the TA-06 well were to improve overall recovery of reserves within the Rannoch B and the Upper Ness reservoirs of the Brent sequence, in areas not adequately exploited by existing producers. The two horizontal lateral legs are effectively infill producers with the mainbore designed to improve recovery from the Rannoch in the crestal area of the field and the lateral designed to drain the Upper Ness as part of the existing flood strategy. Both legs also fulfilled an appraisal role in that they were drilled updip to assess the areal limits of the respective formations.

Due to tolerance constraints of target horizons, the requirement for real-time geosteering with 4 ¾" Gamma / Resistivity / Neutron Porosity and Density sensors was essential to ensure length of producing section was maximised. Due to the requirement to reduce mud weights through the reservoir and the use of 7.0" drilling liners to contain the formations above, both producing sections were drilled with 6.0" hole using 4 ¾" formation evaluation while drilling tools. The requirement for realtime Geosteering capability dictated the use of an advanced data acquisition system linked to a database in the operators office and enabled the office-based subsurface team to share information with and advise the rig site team with informed geosteering decisions.

The expected recoverable reserves per objective combined with the value of slots on the Tern platform warranted the use of a dual lateral well. A pre-cut window joint was installed and cemented as part of the 9⅝" casing string to allow simple lateral exit without the requirement for milling operations. The mainbore was constructed with an 8 ½" section and 7.0" drilling liner followed by 6.0" hole with 4 ½" cemented liner through the Rannoch. The upper lateral was drilled from the pre-cut window with the same hole and liner sizes. The 7.0" lateral liner was tied back to the mainbore and a washover effected to remove the liner top and complete a class 4 multilateral junction with full 8 ½" mainbore and 6.0" lateral access. The lateral was completed 11.5 days ahead of schedule.

This paper aims to show how a well co-ordinated and multifunctional team was used to effect the construction of the dual lateral well and how they optimised the use of a range of new technologies, techniques and prior experience to achieve this world-class performance. TA-06 was completed with minimum deviation from a carefully formulated programme and demonstrates how good planning, communication and teamwork contributed to overall success.