Analysis of Postbucking Drillstring Vibrations in Rotary Drilling of Extended-Reach Wells

One of the most serious concerns of extended-reach drilling is the dynamic behavior of the drillstring and cleaning of well. Good cleaning requires an increased angular speed. However, at higher rotary speeds, the drill string sections lying on the borehole horizontal sections tend to buckle, first, in the form of “snake”, sliding up and down the borehole bottom wall, and then in the form of whirling as the angular velocity increases. This paper presents the 3D nonlinear dynamic model of drillstring in a wellbore of 3D profile. The model suggests the possible contact/lift-off of drill pipes with/from the wellbore wall. The interaction of lateral, torsion and axial vibrations is taken into account. The relation between the normal component of contact force and the deformation of the wellbore wall is taken as quadratic-elastic. The friction force is described based on a hysteretic dynamic model. The friction force model also takes into account the transition from a sliding to a whirling. The equations of the drillstring dynamics are solved numerically using the method of lines. The DYNTUB computer program is developed to analyze the drillstring time-varying processes under different loading. The program is used to study the effects of the angular velocity, compression load, torque, friction factor, well profile, and availability of connectors on the drillstring dynamic behavior.