Downhole Vibration Measurement, Monitoring, and Modeling Reveal Stick/Slip as a Primary Cause of PDC-Bit Damage in Today

Abstract Since backward whirl was discovered as a severe cause of PDC bit failure, our industry has made great strides toward creating whirl-resistant bits and operating practices. But is whirl still the major cause of PDC bit damage in today's applications? This paper reports on a recent field study in which downhole vibrations were measured using a newly available in-bit vibration-monitoring device. The focus of this study was to understand today's North American vertical conventional rotary applications. In addition, four wells were also drilled using a research drill rig in Oklahoma. In these tests, PDC bits, BHAs, and operating parameters were varied to document their effect on downhole vibrations. In these four wells, vibration measurements from the new in-bit measuring device were validated against a commercially available and industry-proven MWD vibration monitoring service. Also, a computer model that accounts for the coupled response of bits and BHAs was run for selected tests in field wells and the research drilling rig. The models agreed well with the measured cases in both whirl and stick-slip. The model also allowed the authors to confirm that high-frequency torsional oscillations (4-9 Hz) which we observed were, as a previous industry paper suggests, due to BHA torsional resonance. The results of this study indicate that the most common field vibration today in hard rock PDC drilling is stick-slip, not whirl, in the target test region. In field tests, stick-slip was almost exclusively observed. For typical field applications with a surface rotary speed of about 70, we have measured peak downhole RPM during the slip phase as high as 500. This paper will report on these findings, document damage resulting from stick-slip, and suggest potential solutions to mitigate downhole vibrations.