Statfjord field, one of the largest and oldest fields operating in the North Sea continental shelf, has been producing since early 1980s. It has three platforms, Statfjord A, B, and C, with more than 300 wellbores. Formation collapse (in the lower part of Shetland group) below casing/liner shoe in the overburden section on top of the reservoir has posed different challenges in the field. These challenges include problems in running screens and gravel packing in sand reservoirs. To respond to this challenge and focus on improving the drilling efficiency and rig cost savings, the drilling team evaluated drilling a 10 5/8 × 12 ¼-in. section reaching the top of reservoir—10 m true vertical depth (TVD) above the predicted top of the Mime formation—with minimum rathole. A standard underreaming bottomhole assembly (BHA) with one ball-drop reamer leaves a 35-m to 40-m rathole below the 9 5/8-in. casing shoe, which induces different issues related to casing reaching bottom, cementing, potential packoffs due to overburden formation collapse while drilling the 8 ½-in. section in reservoir, problems in running screens, and gravel-packing operations. On two previous wells, running screens faced some challenges, and gravel-pack efficiency was compromised. To cope with these technical challenges, an integrated dual-reamer rathole elimination (RHE) BHA was proposed; this would reduce the rathole to 9 m. The proposal was accepted by the operator after detailed risk assessment.

This section was planned to be drilled across formations in the Hordaland group building from 16° inclination to 30° inclination at landing on top of reservoir. To achieve the required directional control, a push-the-bit rotary steerable system (RSS) was selected. The dual-underreamer BHA with a ball-drop reamer above the surveying/logging tools and a hydraulic reamer on top of the RSS was selected to drill and underream the section in one run by minimizing rathole to 9-m. Dropping wireless communication across hydraulic underreamer made it possible to minimize the rathole to 9-m with some additional, but manageable, risks. Blind downlinks to the RSS, no real-time (RT) updates from the RSS and hydraulic underreamer, and compromising RSS steerability were the main risks identified at planning stage and agreed with the operator based on the controlling/mitigating measures in place.

The Statfjord drilling team successfully addressed these challenges with an innovative solution using the dual-reamers RSS BHA to drill the 10 5/8 × 12 1/4-in. section and remove the rathole in one run. Planned interval (∼900 m) was drilled and underreamed to target depth (TD), with the ball-drop activated upper reamer, and the lower hydraulically activated reamer remained redundant. At section TD, the string was pulled back to position the bit at the rathole shoulder, the upper ball-drop reamer was deactivated, and the lower hydraulic reamer was activated to remove the rathole. The 9 5/8-in. casing reached planned bottom with a successful cement job, and screens were run without any problem.

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