Challenging environments such as unstable clay formation represent high operational risk when running tubular after drilling is completed. Use of Steerable Drilling Liner (SDL) system combines drilling and casing of the hole, thereby mitigating the risk of not being able to run the liner due to time dependent formation collapse.

This case study presents the difficulties encountered with a Semi-Submersible rig on the Åsgard / Midgard field in the North Sea during 7″ liner running operations in unstable clay formation using the conventional approach (drilling 8 ½″ section first and then running the 7″ liner), the planning of a SDL operation, and the execution of the SDL operation as well as results compared to similar operations.

The components of the SDL system will be reviewed and explained. Detailed planning is crucial for a successful SDL operation. The simulation work will be described with focus on ECD management and hole cleaning strategy. Torque and drag will also be covered. Actual drilling parameters will then be compared with the simulations.

Two attempts were made to complete the section using conventional technology. It was decided to plan and execute a sidetrack operation with the SDL system after numerous unsuccessful attempts to run the 7″ liner. Drilling operation with SDL was carried out as planned. No major drag was found over the whipstock despite the high local dogleg. Very smooth drilling operation with steady drilling parameters was experienced during the execution and the rate of penetration was comparable to drilling the section conventionally. ECD was slightly lower than simulated but around 2 points higher than with conventional drilling. Good hole cleaning was achieved despite a challenging well geometry and the liner was left in hole after drilling was completed. The liner hanger was then run and set before the cementing operation started.

The SDL system allowed drilling more than 1000m of the 8 ½″ section, making this operation a new world record and turning the well from an economic disaster to a complete success.

The SDL system is a relative new technology under continuous improvement. The primary application of this system is its use in unstable formation where it is anticipated problems for running the liner conventionally. Indeed, shocks and drilling energy transmitted to the formation is greatly reduced with this system, preventing time dependent phenomena such as hole collapse.

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