The Gullfaks field was discovered in 1979 and has been developed via three production platforms. To meet objectives the well designs require 20-in. and/or 17 ½-in. hole sections drilled to high angle (50° to 80° inclination), through the Utsira and Sotra formations and into the top of the Stord.

The Sotra member within the Hordaland group, contains intervals of claystone and sandstone with a significant strength contrast, some thin intervals of dolomite may also be present. Performance under-reaming-while-drilling the 20-in. section has been variable, typically requiring between two to four bit runs due to cutting structure, drillstring and downhole tool failures.

In an attempt to fully evaluate the downhole drilling environment, a new technology providing a comprehensive suite of advanced real-time and recorded-mode measurements of the drilling process was introduced. This information was used in the execution phase with remote interpretation support from drilling analysts. The higher resolution recorded data were analyzed in detail and used to calibrate models of the drilling process.

The information obtained enhanced understanding of the Sotra formation and the downhole behavior of the drilling system. This increased understanding resulted in changes to operational procedures and bottom hole assembly design, which reduced the risk of drillstring and downhole tool failures. For the first time this hole section was drilled in one bit run, validating the feasibility of this approach, which was then transferred into the hole opening while drilling operations within a satellite field.

Examples are included with novel insight into the response of under-reaming assemblies as the reamer encounters formation changes, an in-depth analysis of the static and dynamic loads placed on the bottom hole assemblies, and the impact of refinements to drilling procedures.

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