A Study of Slip-Stick Motion of the Bit

Abstract

ABSTRACT This paper deals with torsional oscillations caused by slip-stick motion of the drillcollar section. This phenomenon is associated with a large amplitude, saw-tooth like variation in the applied torque. The term slip-stick motion refers to the belief that the amplitude of the torsional oscillations becomes so large that the drillcollar section periodically comes to a complete stop and does not come free until enough torque is built up in the drillstring to overcome the static friction. A mathematical model of slip-stick motion is presented. This model includes parameters describing downhole friction effects and a simplified description of the drillstring. The effects of damping, finite rotary table inertia, and the rotary speed control system are discussed. Theoretical predictions from this model are compared with measured torque signals recorded during field drilling. This kind of drilling performance is likely to be less effective than normal drilling and may also lead to fatigue problems. This paper discusses the possibilities for avoiding severe torsional oscillations by using a more sophisticated feedback system to control the rotary speed.