Buckling Of Drill-strings With Connectors In Inclined Well-bores

The stability of drill-strings is important in drilling engineering. Drill-string in a real well is composed by many drill pipes connected by connectors, and the diameter of the connector may be greater than the diameter of the drill pipes for as much as 50%. For convenience, the drill-string was considered as a uniform beam with the same diameter in past researches, thus the effect of the connectors was neglected. This assumption is abandoned in the present research. The finite element method is used to analyze the sinusoidal buckling of drill-strings in inclined straight well-bores considering the effect of connectors. Three kinds of beam elements, namely, the general 3D beam element, the beam element with degree of deviated angle and the newly established transition element to connect these two different kinds of element, are adopted in the finite element model. Firstly, take a drill-string with equal diameter in an inclined well-bore as an example. The contact force between the drill-string and the well bore is computed to verify the assumption of that the drill-string is always contacted with the well-bore during deformation. The valid range of this assumption is given. Then the distance between the axes of the well bore and drill pipes is computed to show that the drill pipe lies within the well bore during the deformation before buckling. The sinusoidal buckling of drill-strings is calculated. The computed results of drill-strings, with and without considering the effect of connectors, are compared each other to study the influence of connectors on the buckling load. After that, the effect of boundary condition, inclination angles, and pipe's weight on the stability of drill-strings is also considered. The first buckling mode of the drill-string is given. The sinusoidal buckling load of drill-string with connectors in horizontal well is computed and compared with the result given by Mitchell. The possible reason to explain why the present results are on the safety side is discussed. Finally, the summary of the thesis is given.