Lateral Drillstring Vibrations in Extended-Reach Wells

Abstract Drillstring dynamics and hole cleaning problems are some of the most important limiting factors in extended reach applications. Here, long sections of the drillstring lie on the low side of the wellbore while rotating. When the rotary speed exceeds a critical threshold the drillstring starts to "snake", sliding up and down the borehole wall. If rotated well beyond the threshold speed, the drillstring will eventually start to "whirl" which can cause severe damage to string components after only a short period of time. In this paper an analytical solution for the threshold rotary speed is derived and presented. It is shown to be in the range of the rotary speeds used in modern extended reach applications. The analytical results are verified using a versatile finite element formulation to model the drillstring in greater detail. Animated time domain simulations with this model provide deeper insight into the dynamic behavior of the drillstring. Conclusions on improved drilling practices in extended reach applications - especially with respect to hole cleaning problems - are drawn from the theoretical results.