Study On Coupled Transverse And Torsional Dynamics Of Drill String Based On Chaotic Theory

As a special structure in the procession of well drilling, there exists a three directional coupling vibration when the slender drill string by TDS driving in a well bore filled up with drilling fluid is rotating. Dynamic bifurcation of the drill string intensifies vibrations that the random contact and impact between drill string and well bore be occurred, causing damage of the well bore and drill tool. This irregular chaos motion law of drill string will lead to the force direction of drill bit uncertain as for the well trajectory is difficult to control. Due to the complexity of the drill string system structure and motion law, the field test can not be processed easily, yet it is difficult to solve by the general force analysis method, which makes it necessary to carry TDS, drill string, drill collar, drill fluid, well bole and rock, etc as a flexible multi-body system to conduct the studying on coupled transverse and torsional dynamics of drill string based on chaotic theory. It has great significance for technical developments, such as to understand the motion law of drill string, predict and control well trajectory, optimize BHA and avoiding accident of drilling. It also helps to drive and improve new drilling technologies and TDS techniques and so on.On the basis of summarizing previous studies, the auhtor makes researches on a fusion method and principle of the multi-body system dynamics and chaos dynamics based on flexible multi-body dynamics theory and chaos theory. The instability condition of drill string that strayes from the steady motion is obtained. The kinetic equation of the flexible multi-body drill string system is built with Hamilton principle, then the coupled axial and torsional vibration equations of upper of drill string are discussed by using perturbation method and numerical integration. Using FEM method, the auhtor derives the coupled transverse and torsional vibration non-linear dynamic model of flexible multi-body drill string in vertical wells based on vibration theory, dynamic gap element theory and non-linear structure-fluid theory, and then on the basis of (ui,p)form, the author analyzes the structure-fluid interaction between drill fluid, drill string and well bore.In vertical well, the coupled transverse and torsional transient dynamic characteristics of flexible multi-body drill string system within 1700m is simulated by using ANSYS software. The natural frequencies of drill string are obtained, and the results show that the interaction between drill fluid and drill string decreases natural frequencies of drill string transverse vibration; the axial load of drill string increases the natural frequencies of drill string transverse vibration; drill fluid and axial load have no effect on the natural frequencies of drill string torsional vibration; the natural frequencies of drill string torsional vibration decrease with the increase of length of drill string or drill collar. By applying axial load displacement extreme of drill string, transverse vibration moves to drill collar which aggravates drill collar vibration. It also leads drill string near drill bit happens buckling instability firstly, occurred the random contact and impact with the well bore, at the same time the drill string clinging to shaft wall to do interval or continuity revolution motion revolves around well bore's axis; drill string upper drill collar appears weak transverse vibration due to sustain tension. The transverse vibration trajectory of drill string about 50m from drill bit develops disorder, which has obvious phenomenon for impacting well bore; drill fluid can reduce the transverse vibration amplitude of drill string and impact probability between drill string and well bore effectively.