Studies Of Drilling Tool Combination's Performation For Slanting Drillstring

The drillstring is one of the most important tools in oil drilling engineering. Its operating state influences directly the working life of downhole drilling motors and the quality of wellbore. The dynamic bifurcation of the drillstring often causes strong vibrations and then leads to the damage of the bottom-hole-assembly. The irregularly periodic or chaotic motion of the drillstring often causes the indeterminate of the direction at which the bit pointing and that, in turn, induces the unpredictability of the trajectory of borehole. Based on the theory of modern dynamic system, the behaviors of complicated dynamics of the drillstring is studied theoretically and simulated digitally. The results obtained here may have some engineering significance in optimizing the working parameters and improving the operating state of the drillstring. The dissertation consists mainly of the following parts:(1)Considering the effect of the axial force on lateral deformation and the non-linearity in geometry of the drillstring, the author sets up the differential motion equation of the drillstring constrained in inclined wellbore. The vibration induced by the fluctuations of the weight-on-bit of the drillstring is expressed by a parametrically excited system. A Mathieu equation describing the system is established. Extending the solution into a Fourier series, the curves of dynamic bifurcation values and the dynamically unstable regions surrounded by the curves are shown.(2) Considering the extra axial force aroused by the lateral deformation, a parametrically excited system describing the drillstring leashed by the wellbore is established by the action of periodical weight-on bit. Then making use of Melnikov-Holmes method, the threshold value of the parameter excitation that can trigger chaotic motion of the drillstring is obtained. Digital simulation is made for an engineering case. By doing these researches, the complexity of the drillstring vibration is investigated primarily. It is found that there coexist many different attractors either in chaotic region or in non-chaotic region. Conditionally, the state of motion of the drillstring is sensitively dependent on the initial conditions and the degree of the excitation. (3) Finite element analysis has been utilized to investigate the behavior of the static bifurcation of the drillstring constrained in wellbore. A new method is put forward for describing the constrained action of the wall of the wellbore with the TRUSS element with variable stiffness. The digital simulating results of the buckling configuration and the critical weights-on bit inosculate the theoretical results with good agreement.