Abstract: Drilling tools failure of occurs frequently in the process of ultra-deep well drilling. Therefore, the research on the dynamic characteristics of drilling string plays an important role in the safety of drilling tools. Considering the influence of the real well trajectory, the interaction between bit and formation, the contact between drilling string and borehole wall and the viscous effect of drilling fluid, a simulation model of the whole well drilling string with dynamic characteristics is established. Under different WOB and rotation speed, the changes of axial force, torque, displacement and equivalent stress over time are simulated. The fourth strength theory is used to calculate the assurance coefficient of the wellhead drilling tools and check the strength of the drilling tools in ultra-deep horizontal wells. The results show that the wellhead axial force and equivalent stress display a characteristics of low-frequency change, while the equivalent stress and acceleration at MWD are high-frequency vibration, and the lateral vibration is more severe than the axial vibration; when WOB and rotation speed are small, WOB and rotation speed have little influence on the wellhead axial load, torque, equivalent stress and assurance coefficient; the equivalent stress at MWD increases with the increase of WOB, and the amplitude of lateral acceleration increases significantly with the increase of rotation speed. When the well depth is more than 8 000 m, the hole size is φ120.65mm, and G105 φ114.3 mm drill pipe is used at the wellhead, the wellhead assurance coefficient obtained from dynamic analysis will fluctuate around 1.2, so generally the drilling tool is safe.

Key words: ultra-deep horizontal well, drilling tool failure, drilling string dynamics, strength check, assurance coefficient