| Popis: |
Stick-slip vibrations of drillstrings have been studied by researchers for several decades. The subject is gaining renewed interest as operating parameters for PDC bits have shifted to the stick-slip regime of higher bit weight and lower rotary speed for enhanced drilling performance. In Ledgerwood et al. (2010), stick-slip was identified as a primary cause of bit damage. The main objective of the current investigation is to answer the longstanding question: Do bit designs influence stick-slip behavior of the drilling system? Five prevailing industry perceptions reported in the literature are that anti-whirl bits, reduced exposure bits, and bits with bit-rock interaction number "β" > 1 are less prone to stick-slip, while highly aggressive bits and worn bits are more prone to stick-slip. Although the phenomenological basis of these theories has been provided, validation in most cases is based on anecdotal evidence from the field. Data with diagnosis based on downhole measurements in a controlled environment are scarce. Consequently, conflicting opinions continue to exist about whether any of these theories work in reality. To assess their validity, the five leading theories were reviewed and pairs of PDC bits were designed and manufactured. Each pair consisted of a bit with a standard design and a bit that embodied one of the theories. The bits were first tested in the laboratory to characterize their response. Full-scale wells were then drilled under controlled conditions using a research drill rig in Oklahoma. In these wells, only the operating parameters were varied while BHA, formation, and other variables were unchanged for a given bit pair. The downhole vibrations were measured with a new in-bit device and an industry-proven MWD vibration monitoring service. The most important conclusion emerging from this study is that PDC bit design has a significant effect on stick-slip vibrations. While some of the theories held true, evidence from this study did not support others. The details of test results are provided and various aspects of bit design are discussed in an attempt to enhance the understanding of stick-slip mitigation. |
