Modeling and simulation of vibration in deviated wells

During the engineering of deviated well, drillstring is in the complicated moving state, strong vibration is the main reason that induces drillstring failure. Drillstring vibrations usually have axial vibration, lateral vibration, torsional vibration and the drillstring near the bottom of well usually coupled vibrates strongly. A dynamic model to predict the effect of drillstring parameters on the type and severity of vibration is desired by the oil industry, to understand and prevent conditions that lead to costly downhole tool failures and expensive tripping or removal of the string from the wellbore. High-fidelity prediction of lateral vibrations is required due to its coupling with potentially destructive axial and torsional vibration. This research work analyses the dynamics of a horizontal oilwell drillstring. In this dynamics, the friction forces between the drillstring and the borehole are relevant and uncertain. Drillstring contact with its borehole, which can occur continuously over a line of contact for horizontal shafts such as drillstrings, generates normal forces using a user-definable stiff spring constitutive law. Tangential contact forces due to friction between the drillstring and borehole must be generated in order for whirl to occur. The potential for backward whirl and stick-slip requires the transition between static and dynamic Coulomb friction. The proposed model computes the relative velocity between sliding surfaces when contact occurs, and enforces a rolling-without-slip constraint as the velocity approaches zero. When the surfaces become ‘stuck’, a force larger than the maximum possible static friction force is required to break the surfaces loose, allowing sliding to resume. The drillstring bottom-hole-assembly has been modeled using a three-dimensional multibody dynamics approach implemented in vector bond graphs. Rigid lumped segments with 6 degrees of freedom are connected by axial, torsional, shear, and bending springs to approximate continuous system response. Parasitic springs and dampers are used to enforce boundary conditions. A complete deviated drillstring has been simulated by combining the bottom-hole-assembly model with a model of drill pipe and collars. The pipe and collars are modeled using a lumped-segment approach that predict axial and torsional motions. The proposed dynamic model has been incorporated the lumped segment approach which has been validated with finite element representation of shafts. Finally, the proposed contact and friction model have been validated using finite element LS-DYNA® commercial software. The model can predict how axial and torsional bit-rock reactions are propagated to the surface, and the role that lateral vibrations near the bit plays in exciting those vibrations and stressing components in the bottom-hole-assembly. The proposed model includes the mutual dependence of these vibrations, which arises due to bit-rock interaction and friction dynamics between drillstring and wellbore wall. The model can simulate the downhole axial vibration tool (or Agitator®). Simulation results show a better weight transfer to the bit, with a low frequency and high amplitude force excitation giving best performance but can increase the severity of lateral shock. The uniqueness of this proposed work lies in developing an efficient yet predictive dynamic model for a deviated drillstring.