Operators of Gulf of Mexico (GOM) wells frequently reported overtorque issue of bottomhole assembly (BHA) connections when drilling the 26-in. section through salt. Such overtorque often leads to costly tool damage beyond repair (DBR), additional trips, and high nonproductive time (NPT). The average DBR cost per BHA can be as high as USD 1 million. Combined with a complete BHA roundtrip, it can easily cost more than USD 3 million for operators if such failure happens. This has been a problem for several years and has caused significant damage: In 2014, of 15 26-in. PDC bit runs in salt, 40% had overtorque connections and 20% led to DBR. This paper discusses how an integrated multidisciplinary team identified the root cause of and the solution to the overtorque problem.

Torsional vibration was believed to be the cause of such failure. Comprehensive drilling dynamics simulation software that is based on empirical bit design knowledge was used to design a new bit to reduce the vibration. A newly developed high frequency downhole recording tool used in the 26-in. section recorded high-frequency torque, acceleration, and RPM fluctuation downhole. This dataset became the key to understanding the downhole vibration in detail because it provided information that cannot be acquired by a traditional MWD tool. Field-recorded data were fed into drilling dynamics simulations to accurately calibrate the drilling dynamics model. The simulations resembled downhole drilling conditions and clearly identified the root cause. The simulations precisely predicted the torque along the entire drillstring and identified why overtorque is present in only a certain part of the drillstring. The calibrated model was used to compare old and new bit designs. The newly designed bit showed much lower torque amplitude with similar torsional vibration frequencies. The simulation indicated that the newly designed bit can significantly alleviate the overtorque issue.

Implementation of the new bit mitigated the overtorque issue immediately. As of May 2016, there have been 18 runs with the new bit. Only one run had a slight overtorque issue whereas the rest showed no sign of overtorque connections. DBR and NPT related to overtorque were eliminated. As a byproduct, the average on-bottom rate of penetration increased by 9%. This case demonstrates the effectiveness of the integrated approach to solving drilling challenges.

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