Pushing the ERD Envelope at Wytch Farm
Authors: Cocking D.A. , Bezant P.N. , Tooms P.J. 
Published: December 2007

BP Exploration, operator of the Wytch Farm Oil Field, has since April 1993 been developing the offshore extension of the Sherwood Reservoir using extended reach drilling (ERD) techniques. Over that time new drilling technology has become available, drilling strategies and methods modified, and many difficulties overcome. Wells are now being drilled to record breaking departures utilizing geosteering, casing floatation and torque reduction techniques. Horizontal reservoir sections in excess of 2.5km are being drilled, cased, successfully cemented and selectively perforated to boost accessible reserves. Increasingly, the conventional limits of directional drilling and completion are being pushed back.

This paper will summarize the many ERD achievements to date at Wytch Farm. It will describe the technology, tools, techniques and strategies currently being applied to drill and complete the more recent extended reach wells in Western Europe's largest onshore oilfield.

Introduction

BP's Wytch Farm oilfield, located on the south coast of Britain, is currently being developed through the drilling of extended reach (ER) wells into the offshore part of the Sherwood sandstone reservoir. As has been well documented in previous papers, the development drilling of the offshore extension of the reservoir was originally planned to be accomplished from an artificial island to be constructed in Poole Bay. In December 1991, based on advances being made within the industry, this plan was abandoned in favour of extended reach drilling from the mainland.

The cancellation of the Island project in favour of ERD assumed that 6km step-out wells could be drilled into the shallow (1,550m TVDss) Sherwood reservoir. In fact, 5 years on, over 8km step-out has been achieved, (Fig. 1).

SPE paper no. 28293 referred to four Wytch Farm wells (F18 to F21) which attained a maximum step-out of 5.5km. This paper deals initially with experience on wells M1, M2, M3 and M5, each of which have reached ever increasing departures (Fig. 2). M5 attained a step-out of 8,035m, claiming the world record departure at any vertical depth. The paper also discusses wells M6 to M9 where new equipment and techniques were employed to improve ERD efficiency and provide the means to reach targets which exist at even greater departure in the near future.

Trajectory Design

The last three wells on F site had been drilled with a tangent of 80 – 82. This was continued on the M site wells, utilizing higher tangent angles as the departure increased. On M2 the tangent was increased to 830 to achieve the largest departure at that time of 6.7km. Through the build the maximum planned build rate was 2.5 /30m.

The increasing tangent angle was minimizing torque during the drilling phase, but on M2 it became apparent that drag was becoming more of a limitation. The ability to slide a drillstring at depth was limited, and achievable reach with coiled tubing was being compromised. Comprehensive modeling indicated that the tangent angle should be reduced to 81 for M3 and M5 to improve sliding reach. This resulted in the build rate being increased to a maximum of 2.75 /30m on M5.

This all appears like very fine tuning, but a 1 change in angle over a 5km tangent section does have a significant effect on ultimate torque and drag at 8.5km total depth.

Once the reservoir was encountered, the inclination was immediately increased to 85 - 95 as required for the long geosteered 8 1/2" section.

Drilling Operations

Top Hole. Since moving to the newly constructed M-Site where near surface anti-collision problems were less severe, 24" hole was directionally drilled in one pass without the need for a pilot hole to 60 inclination by 220m. The 17.1/2" section attained 600 by casing point at circa 860m MDbrt. After running and cementing the 13.3/8" casing, the water based mud was displaced to oil based mud (OBM) before drilling the long,, 12.1/4" hole section. The build rate established in the 17.1/2 hole section was maintained at the start of the 12.1/4" hole section until the tangent angle of 800 to 830 was attained.

12.1/4" Hole Section. The length and tangent angle of the 12.1/4" hole section obviously depended on the step-out desired at the 9.5/8" shoe which was typically set at top reservoir. MS was drilled with an 81 12.1/4" hole section some 5,163m in length, 4,014m of which was drilled in one bit run. Steerable assemblies were used throughout, although the amount of sliding in the tangent section has with each well been steadily reduced to as little as 3%. Towards the end of the M5 12.1/4" section, an extended power section positive displacement motor (PDM) was run. P. 371^