Simulation And Analysis Of Drill String Buckling Characteristics

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.This thesis based on the theory of elasticity mechanics, consider the effect of the weight of the drilling string and the curved wellbore, established in different well segments buckling mechanical model (vertical wellbore, inclined wellbore, and curved wellbore), derived the calculation model of drill string in critical sinusoidal buckling load and critical helical buckling load and calculate contact force when the string has bucked. According to the different sizes of drill string, the influence of the inclination, and the wellbore curvature on critical buckling load and the contact forces were analyzed. In the slant straight wellbore, the critical buckling load is proportion to the inclination, and the string lying in horizontal section is more stable than straight section.The wellbore curvature is the most important factors influence buckling in curved section.The build-up rate in build section is proportion to the critical buckling load, but dropping section is opposite. With the increase of the axial force, contact force of the drill string and borehole wall also gradually increased.In this thesis, Nonlinear large deformation Theory was used to set up drill and geometrically nonlinear contact nonlinear finite element buckling analysis model, and simulate different sections buckling deformation process and the corresponding contact force vector. Because of its own weight, the drill string in inclined section lying in the wellbore lower side, with the increase of axial load will make the drill string along the lower wellbore side buck to sinusoidal. With further axial load the string will turn sinusoidal buckling to the helical buckling after the string has bucked.When reached the critical load, the buckling shape will become helical buckling from sinusoidal buckling, and that is a transient process. After the string became helical buckling, if the axial force continues to increase the contact force would surge. In inclined section, with the increase of the curvature of the wellbore the critical buckling load increases. The string starts to buck from bottom. 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.This thesis 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 than15%. That proves the use of FEM to determine the buckling critical load of the drill string is feasible.