The drillstring dynamic frequency domains modes and mode shapes characteristics of compound drilling for the high-quality slim borehole

Abstract

High-quality borehole is crucial to hydrogen underground storage (HUS) and oil & gas development, while compound drilling plays an important role in improving borehole quality. The modal model of compound drillstring in slim borehole is established by using finite element method to analyze the modal of longitudinal vibration, lateral vibration and torsional vibration. The results shows that as the number of order increases, the intrinsic frequency of the three vibrations increases. The amplitude of axial vibration is the largest, and the amplitude of lateral vibration is smallest. The natural frequency of drillstring in slim borehole decreases as drillstring length increasing. The longer drillstring is, the smaller the interval frequency between the intrinsic frequencies of orders will be, and it is easier to cause resonance. The axial vibration will cause minimal damage to the HUS quality. Lateral vibration has the greatest damage to wellbore quality, and is the most unfavorable for the establishment of HUS channels. The damage of torsional vibration to the HUS is less than the lateral vibration and larger than axial vibration. The higher the vibration frequency and amplitude, the harder it is to improve the HUS quality. In order to improve the quality of HUS, the weight on bit (WOB) and rotary per minutes (RPM) should be adjusted at the same time for lateral shock, and the WOB should be mainly reduced for controlling the axial and torsional vibration. The results are benefit for the drillstring design, improving the wellbore quality and HUS quality.