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A customer in a North Sea field needed to remove a FLEX-LOCK™ III liner hanger system and a 13 Cr 110 ksi 7-in. x 9 ⅞-in., 62.8-ppf (93.75 kg/m) Baker Hughes ZXP™ ultra liner top packer. The job presented an extra challenge because the ZXP packer had hold-down slips that restricted any upward movement. Additionally, there were concerns about the risk of skim-cutting the polished bore receptacle (PBR) or damaging the outer casing due to the thin-walled, decentralized liner system in the deviated wellbore. Efficient milling of the 13 Cr 110 ksi material was also key to keeping costs as low as possible.

A wellbore construction and fishing team from Baker Hughes, a GE company (BHGE), evaluated different options to address these concerns. BHGE suggested milling up the liner top packer and PBR with a specially designed casing mill dressed with industry leading METAL MUNCHER™ Advanced Milling Technology (AMT) carbide, the durable and effective cutting and swarf control tungsten-carbide carbide. The cutting inserts were affixed to specially designed blades to enable for good guiding and centralization throughout the operation. The ZXP ultra liner top packer was located at 15,748 ft (4799.9 m) in an 18.9° section of the deviated S-shaped well profile. Milling started out with the specially designed 8 ½-in. pilot milling assembly with initial favorable milling parameters and progress. After the slips of the liner top packer were milled away, progress slowed as packer components became free.

In one trip, the custom-made pilot mill with METAL MUNCHER AMT cutting knives drilled both the 13 Cr 110 ksi PBR and liner top packer. Wear on the mill indicated that skimming was minimized, proving the engineered step design on the blades was an effective solution. Furthermore, the AMT cutting inserts produced manageable cuttings, eliminating issues with swarf build-up or bird nesting of cuttings. In addition, only 20% wear was noted on the special pilot mill, providing the option of using the mill in subsequent pilot milling operations.

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Challenges & Results
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Challenges

  • Remove ultra liner top packer with hold-down slips restricting any upward movement
  • Eliminate risk of skim-cutting the polished bore receptacle
  • Prevent damaging outer casing with thin-walled decentralized liner system in deviated well

Results

  • Achieved one-trip milling operation on downhole equipment with no skimming of PBR
  • Experienced only a 20% wear rate on custom-made pilot mill, and swarf quality was good
  • Incurred zero NPT or extra expense associated with traditional methods