Self-Excited Vibrations Induced by Drag Bits

Drillstring vibrations cause premature failure of downhole tools and the drillstring itself. Also, drill bits do not drill efficiently as a result of these vibrations and this inefficiency causes a significant reduction in penetration rate and a corresponding increase in footage cost. Drill bits are a primary source of excitation, but there has been no theory that adequately explains how drag bits excite drillstrings. The paper shows that these vibrations are self-excited by the interaction of drillstring, drill bit, and formation. The paper explains the mechanism for this self-excitation. Results show that drag bit design, shock absorber design and bit rotational speed dictate conditions of dynamic instability.