The drilling mud shows strong non-Newtonian rheology due to mixing a large number of cuttings at well bottom during drilling, which has great influences on the vibration behavior of the drill string. To investigate this phenomenon, an axial-lateral-torsion coupling (ALTC) nonlinear dynamic model based on Hamilton's principle was established, taking into account the effect of drilling fluid viscous damping, Herschel-Bulkley non-Newtonian rheological (HBNR) damping, bit-rock interaction and drillstring-borehole contact. In the model, a Rayleigh damping matrix calculation method was proposed to describe the influence of drilling fluid on the damping of ALTC vibration, and the Newmark-β method was used to solved the nonlinear discrete equations of the system. The validity of the model was verified through the simulation data and the field measurement data. Using the nonlinear model, the effect of drilling fluid rheology on ALTC vibration response of rotary drill string was investigated. It was found that compared with viscous damping, HBNR damping can significantly suppress the axial, lateral and torsional vibration of drill string, maintain the stability of wellbore diameter and improve ROP. Both damping models indicate that lateral vibration signals are difficult to transmit to the wellhead, while axial vibration and torsional vibration signals can be observed near the wellhead. The results can provide a theoretically guidance for suppressing the drill string vibration and reducing the risk of premature failure of drill string.
In deepwater riserless drilling operations, the vibration behavior of drill string may have a significant impact on wellbore pressure, which leads to serious risks of drilling accidents such as well leakage and well collapse in shallow risk areas. In order to solve this problem, based on Hamilton's principle, a three-dimensional nonlinear coupling dynamics model of drill string in deepwater riserless drilling is established to simulate the dynamic behavior of drill string under offshore drilling conditions, considering the influence of heave and offset motion of offshore platform, ocean current load, drillstring-borehole contact and bit-rock interaction. The Newmark-β method is used to solve the nonlinear discrete equations of the system, and the effectiveness of the model and calculation program is verified by the field test data. Meanwhile, a wellbore pressure field model under axial, lateral and torsional motion modes of drill string in deepwater riserless drilling is established. The three-dimensional nonlinear coupling vibration characteristic of drill string and its influence on wellbore pressure fluctuation are investigated. The results indicate that: excitation and natural frequency play a leading role in axial vibration response; the collision between the drill string and the borehole wall mostly occurs in the middle part of the formation section, and the lateral vibration at the bit is the most intense; and the closer to the upper platform, the more high-frequency components of torsional vibration response and the stronger the chaos. The larger the axial vibration amplitude of drill string is, the greater the influence on wellbore pressure field is; for the lateral vibration, the middle and bit position have great influence on the wellbore pressure field; for the torsional vibration, the bottom position has the greatest influence on wellbore pressure field. Compared with the axial and torsional vibration, the lateral vibration of drill string has the greatest influence on the wellbore pressure field.
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