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High vibrational loads occur due to self-excited high-frequency torsional oscillations (HFTO) that are excited by the bit-rock interaction while drilling in hard and dense formations. These critical vibration loads with an associated acceleration above 100 g can result in premature failure of downhole components, leading to an increase of non-productive time and reduced reliability. Beside the formation, drilling parameters influence the occurrence of HFTO. This paper presents a novel method to determine stable and unstable combinations of operational parameters and an associated strategy to mitigate HFTO. The inputs for the derived algorithm are the drill string model, high-frequency downhole data that is related to HFTO and downhole rotary speed and downhole weight on bit (WOB) measurements representing the instantaneous operational parameters. The data is measured by a downhole tool for vibration and load measurements that collects data during triggered events. The high-frequency data is used to determine the energy input into critical HFTO modes regarding changes of drilling parameters. Herein, events are used that naturally occur during the drilling process, e.g. stick/slip. The amplitude change of HFTO for the current combination of bit rotary speed and WOB is used to determine the stability of operational parameters regarding HFTO. The resulting stability maps display operational parameters that are susceptible to HFTO and stable zones without HFTO. The utilization of downhole fluctuations allows a derivation of the stability for large operational parameter ranges and not just for the current set point. The method is investigated in a post well analysis of two runs where a superposition of high and low frequency torsional vibrations occurs. In contrast to conventional methods that are based on large data sets, the derived method requires only a few seconds of high frequency data to determine an accurate stability map. The study shows different scenarios of HFTO with and without stick/slip or low-frequency torsional oscillations and discusses the optimal HFTO-mitigation strategy within the given operational parameter envelope. For both runs, a similar threshold rotary speed is determined that needs to be exceeded to mitigate HFTO. This observation is also confirmed during drilling where stable drilling above the derived bit rotary speed threshold can be observed. Hence, increasing RPM above this threshold reduces HFTO occurrence while increasing ROP in contrast to the conservative way to reduce HFTO amplitude by reducing the bit rotary speed. The method enables the analysis of the stability of HFTO and gives a quantitative measure to investigate bit, rock and other influencing factors. The resulting stability maps are used to propose stable regimes for operational parameters without HFTO to the driller in the upcoming runs and in similar environments, and thus enable an increase in drilling efficiency and reliability. |
