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A customer in the Gulf of Mexico identified a restriction in the wellbore while completing a deepwater well. The intended completion equipment required a 9.000-in. minimum inside diameter (ID) through the wellhead, but a 33-in. section of the dummy hanger retrieval sub had a smaller 8.569-in. ID. Leaving as is would require changes to the desired completion design while also complicating future operations. To avoid such measures, the operator reached out to Baker Hughes, a GE company (BHGE), for a solution to remove the restriction without damaging the casing.

Working closely with the operator, BHGE designed a customized step mill dressed with METAL MUNCHER™ Advanced Milling Technology (AMT) carbide to meet the required criteria. The final milled profile needed to be concentric with an ID between 9.000 to 9.356 in. to ensure completion equipment would drift while also preserving wellbore integrity. This was critical to the success of the operation as milling too much or milling an eccentric ID could result in recompletion or even well abandonment.

The AMT step mill featured a pilot stabilizer dressed with wear-resistant material to hold a tight tolerance gauge relevant to the hanger ID. The pilot guaranteed the mill was completely centralized as milling initiated; a crucial factor to maintain concentricity. To maximize performance, two steps were incorporated into the design. The lower step was sized to 8.785-in. and the upper to 9.000-in. to allow for minimal material removal per revolution. The mill was designed using a combination of shaped AMT carbide inserts so that the mill would remain concentric and not taper while boring the ID, while also leaving an optimal finished surface.

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Challenges & Results


  • Remove 33-in. long restriction in deepwater well without compromising well integrity
  • Avoid an eccentric ID that could result in recompletion or possible loss of well


  • Flawlessly removed restriction without compromising well integrity
  • Avoided risks related to future operations
  • Eliminated need for completion redesign
  • Efficiently milled out restriction in one hour