While running casing, the rig encountered a tight spot.
To pass it, the crew applied 90 klbf of slack-off.
The casing reached TD and was cemented.
During drill-out:
• Metal shavings appeared at surface
• The bit was severely damaged
• An unplanned trip followed to replace the bit
Post-analysis showed:
• The casing was properly designed for collapse, but ovalized while passing the tight spot
• After deformation, its collapse resistance dropped below the pressure applied during cementing
Takeaways:
• Consider higher collapse-resistant grades, especially in the lower part of the casing, where cement pressure is highest
• Remember: for the same ovality, higher-grade casing remains more robust
• If tight spots are expected, use a reaming shoe to pass intervals with minimal slack-off
Casing strength is not only a design parameter, and it can be compromised by operations.
What was your toughest casing run?