A Model for Borehole Oscillations

Abstract

Abstract The presence of borehole rippling, spiraling, and hour-glassing have been known for many years in the drilling industry. Descriptions of these problems have become more precise with improved logging tools, yet the underlying mechanisms have not been mathematically formulated. Many directional models have been presented in the past that attempt to explain the steady state response of the bottom hole assembly, but little has been published on the non-equilibrium or dynamic response of the system. A theoretical/analytical dynamic model is proposed to explain the conditions required for borehole oscillations to occur. This model is presented along with correlating data from laboratory and full scale field tests for a steerable motor with an 8-1/2" hole size. For different parameters the model correctly predicts the presence and absence of oscillations, the oscillation wavelength, and the effect of bit geometry on borehole oscillations. It also makes testable predictions for the effects of bit speed, penetration rate, and rock strength. From these test cases, general guidelines are presented for selecting the appropriate bit, BHA, and operating parameters for an application.