Simulation Of The Buckling Characteristics Of The Drill String And Analysis Of Influencing Factors

Buckling behavior of string constrained by wellbore have adverse effects on many aspects of drilling. It will cause a significant increase in the drag and torque of drilling string and even cause the string "locked" which results the WOB in difficulty passing, drill string fatigue failure and other down hole problems. Especially with horizontal wells, extended reach wells, multi-branch wells application. The buckling problem constrained by wellbore is more prominent, and the drilling string buckling research has become a hot issue in the drill string design optimization.In this paper, the calculation equation of critical buckling load of drill stem in different sections are derived with using energy method approach. For ?215.9mm borehole, the influence of wellbore inclination, borehole curvature and different size of drill string on critical buckling load through example calculation. In deviated wells, the critical buckling load of drill string increases nonlinearly with the increment of inclination. The critical buckling load is smallest in vertical section. In horizontal section, the drill string lie on the low side of borehole and its buckling stability is higher than deviated section. The wellbore curvature is the most important factors influence buckling in curved section. The critical buckling load in build section increases with the borehole curvature increment, and for drop-off interval, the buckling of drill string as the borehole curvature increases decreases.Based on the static equilibrium method, the general buckling differential equation of drill string is established with consideration of the weight and torque effect. Buckling configuration assumption and galerkin method is applied to solve the buckling differential equation, and the torque effect on the buckling configuration is analyzed in different sections. After applying torque load, it has no influence on sinusoidal buckling of drill string configuration and little effect on the sinusoidal critical buckling load and the contact force. Torque will have a bigger influence on helical buckling configuration function, on the street of the drill string has a great influence on helical buckling load and contact force.Based on Nonlinear large deformation theory, the geometric nonlinear finite element model of drill string for buckling analysis is established and to simulate the buckling change process considering torque variation effect. Simulation results show that the drill string in deviated borehole begin to sinusoidal buckling on the low side of the borehole, and along with the load continues to increase, the bottom began to spiral buckling, the sinusoidal buckling section of the drill string was going on until the whole drill string helical buckling occurs, sinusoidal buckling to helical buckling is a gradual process; Build section starts to formate sinusoidal buckling from the bottom of the drill string, with the increase of axial load, starting from the bottom of the local helical buckling occurs, the top remained the initial state. Dropping section is opposite, the axial load pushed the string to wellbore high side firstly, then began buckling from the bottom, followed by the top began buckling, and last the middle section occurred buckling. The Dropping section is very easy to occur buckling. When torque load is added to the drill string of different borehole section, the Sin critical buckling load reduction of the drill string is very small and sinusoidal buckling deformation state change is not big. The presence of torque has s bigger influence on the helical critical buckling load of drill string, as the torque increases, the reduction of helical critical buckling load of drill string is at a lower degree.This paper makes a comparative analysis of the differences between buckling analytical solution and finite element simulation. The simulation results with the theoretical value of the relative change in less than 10%, and with consideration torque effect, the simulation results with the experimental results the error is within 15%. That proves the use of nonlinear finite element analysis method to determine the buckling critical load of the drill string is feasible.