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The compound drilling plays an important role in improving borehole quality. The modal analysis model of drillstring is established to analyze the modal of longitudinal vibration, lateral vibration and torsional vibration, respectively. Compared with longitudinal and torsional vibrations, the intrinsic frequency range of lateral vibration are smaller, and it is harder to prevent resonance by adjusting critical rotation speed in case of lateral vibration. The intrinsic frequency of drillstring decreases as drillstring length increases. The longer drillstring is, the larger the increase and the smaller the interval range between the intrinsic frequencies of orders will be. It is easier to cause resonance when the frequency of exciting forces changes. As the number of order increases, the intrinsic frequency of the three vibrations increases and their increase amplitude is: longitudinal vibration &gt; torsional vibration &gt; lateral vibration. The results are benefit for the drillstring design and improving the wellbore quality.

The vibration modal characteristics of drillstring in compound drilling