Drilling through Multiple Highly Depleted Zones in SNS

The Hewett –Rotliegendes A-10 appraisal well (Sept-Dec 2008) was designed and drilled as a one off well targeting the Leman sandstone reservoir.  To reach the target reservoir a high angle well path had to pass through the highly depleted Upper Bunter (at +/- 250psia) and Lower Bunter (at +/- 45psia) sandstones reservoirs in the SNS Hewett field. This presented potentially complex and hazardous well control and well stability challenges.

The offset wells encountered severe and uncontrollable lost circulation in the sandstones before reaching the target reservoir. It became evident during the planning for A-10 that some special practices would be required, including the use of technology new to the North Sea.

The scope of work included the process used to successfully plan, develop, and implement the depleted reservoir drilling technique for this well, with increased focus on casing design, drilling fluid selection in conjunction with a well bore stability study and BHA configuration challenges.

A steerable motor with a 12 ¼”x14 ¼” bi-centre bit was used to drill the depleted Upper Bunter Sand in conjunction with 7.5ppg OBM to minimise fluid losses, and an 11¾” drilling liner was utilised to isolate the Upper Bunter sandstone. The mud weight was then increased to 8.6ppg to drill the Lower Bunter Shale and Sandstone sequence for which an Rotary Steerable system combined with a hole opener and under-reamer was used to drill an 8-1/2”x12-1/4” hole in a single pass. A combined string of flush and coupled 9 5/8” casing was run and cemented to secure the hole section.

The successful application of the combined planning and implementation resulted in Hewett A-10 well being drilled without losses and completed successfully.