Study On Dynamic Characteristics Of Compound Drilling String

Nowadays the compound drilling technique with positive displacement motor combined with PDC bit has become a main current technology in drilling. Practical application shows that it can seriously improve the drilling rate, shorten drilling period and reduce drilling comprehensive cost. In this paper a large number of literatures of the compound drilling and drill string mechanics were collected. Through the analysis of these data and in combination with the practical situation of drilling, the development trend of this field had been understood and the research methods and ideas of this article had been confirmed.On the basis of previous studies and according to the characteristics of the compound drilling, the drill string’s finite element model of longitudinal vibration, lateral vibration and torsional vibration had been established, the dynamic response of the drill string had been calculated. The three dimensional finite element numerical model of BHA-bit-formation of compound drilling had been established by using the theory of nonlinear dynamics. Finally, combined the hole deviation control issues encountered by compound drilling, analyzed the WOB, bending angle and the stabilizer installed position on the influence law of hole deviation control. The research results of this article indicate a new way of compound drilling dynamics research and analysis of composite drilling hole deviation control, provides the theoretical basis and technical support for further development and application of the compound drilling.In this paper, the main research contents and achievements are as follows:(1) According to the nonlinear vibration theory, finite element model of compound drilling had been established, the drill string’s various order natural frequency and vibration mode of longitudinal vibration, lateral vibration, torsional vibration had been calculated and the harmonic response analysis was carried out. Through calculation and analysis, the relationship between the string’s mode and well depth had been obtained. Transverse vibration is the main vibration form of drill string, compared with the longitudinal vibration and torsional vibration, its natural frequency and the each order natural frequency interval smaller, is more likely to happen resonance.(2) In order to make the finite element model is more close to the real working condition of compound drilling, established a bottom drill string-bit-formation of the three-dimensional finite element model, by applying reverse speed to "formation" equivalent PDM’s speed. Using ABAQUS integral algorithm, the characteristics of contact force between BHA and wall of the well and the axial force produced by bit and bottom hole rock contact had been researched, the influence law of the BHA’s longitudinal and lateral acceleration had been analyzed.(3) The hole deviation control problem of compound drilling had been studied. Combining with drilling field data, the corresponding BHA finite element model was established. Analyzed the WOB, bending angle and the stabilizer installed position on the influence law of compound drilling hole deviation control, the simulation results agreed with the engineering practice, which provides the reliable theory basis to optimize the BHA structure of compound drilling, can effectively guide the on-site construction.