The Nature of the Interaction Between Stick/Slip and High-Frequency Torsional Oscillations

Abstract

Abstract The drilling process is impacted by vibrations through limited drilling efficiency and rate of penetration, reduced reliability and increased non-productive time. The understanding of the mechanisms and physics that lead to high levels of vibrations is extremely important to elaborate vibration mitigation strategies. A typical vibration excitation mechanism is forced response excitation, e.g., caused by the imbalance of the mud motor that can lead to lateral resonance with severe impact on tool life. Self-excitation is prominently caused by the bit-rock interaction and mainly excites torsional oscillations if PDC bits are used. Representations are stick/slip with low frequencies (<1 Hz) and high-frequency torsional oscillations (HFTO) with frequencies up to 500 Hz. The large frequency gap between stick/slip and HFTO allows for different effects and excessively increasing loads. The nature of this interaction is diverse and requires different strategies to reduce the loads associated to HFTO and stick/slip to a minimum. The interaction between stick/slip and HFTO is analyzed and appropriate drilling optimization strategies are proposed. Several scenarios are discussed by examination of high-frequency downhole data (1000 Hz) measured in different field applications and physical modeling. It is shown that averaged statistical data or diagnostic data that are typically available can lead to misinterpretation of the drilling conditions. The first scenario is pure HFTO. The second scenario is stick-slip with superimposed HFTO that can lead to an amplification (up to factor two) or reduction of HFTO loads compared to the first scenario. Influencing parameters are discussed that determine either an amplification or a reduction of the loads in the second scenario. The third scenario shows the interaction in the context of stability measures that are determined by the operational parameters. The increased rotary speed in the slip phase of stick/slip can lead to a stabilization of HFTO and actually decreasing amplitudes. The observations in the field are further validated using theoretical drilling scenarios. For each scenario different strategies are presented to reduce the field loads associated to HFTO and the compromise of the strategies to the drilling efficiency and rate of penetration is discussed. The nature of the interaction between stick/slip and HFTO is analyzed and unveiled. Clearly, the necessary depth of understanding can only be achieved by analysis of high-frequency downhole data. The physics-based interpretation of the problem allows the development of very specific drilling optimization strategies. Depending on the scenario a complete mitigation of HFTO or at least a significant reduction of loads can be achieved. Ultimately, the drilling process can be optimized leading to a reduced cost of the well delivery since HFTO can be a major cause for non-productive time if not handled properly.