Investigation on axial-lateral-torsion nonlinear coupling vibration model and stick-slip characteristics of drilling string in ultra-HPHT curved wells

In view of the vibration failure of drilling string system in ultra-high temperature and high pressure (ultra-HPHT) curved wells, an axial-lateral-torsion coupling (ALTC) nonlinear vibration model of drilling string system was established using energy method and Hamiltonian principle, in which, the influence of wellbore trajectory change, wellbore constraint, interaction between bit and rock and ultra-HPHT of wellbore on elastic modulus and viscosity of drilling fluid were taken into account. The finite element method (FEM) is used to realize the numerical solution of the nonlinear vibration model. The correctness and validity of the ALTC nonlinear vibration model was verified by comparing the measured data of four ultra-HPHT wells with the theoretical calculation results of the proposed model. Based on this, according to the parameters of M directional well in Ledong ultra-HPHT block, South China Sea, the influences of ground rotation speed, drilling string length, weight on bit (WOB), torsional impact tools and drill collar length on stick slip vibration characteristics were investigated, and the method of increasing drilling speed in ultra-HPHT curved wells was put forward. The results obtained demonstrate that, firstly, when other requirements were met, the rotary speed should be increased as much as possible, which can effectively improve the drilling efficiency. Secondly, with the increase of drilling depth on-site, the drilling efficiency will be reduced, the whirl phenomenon of drill string system will be intensified, and the service life of bottom hole assembly (BHA) will be reduced. Thirdly, the optimized parameters of WOB and the torsional impact tool depended on the well structure, downhole tool size, etc. and can be determined using the proposed analysis method. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving rate of penetration (ROP) and the service life of drilling string in ultra-HPHT curved wells.