Abstract
High-frequency torsional oscillation (HFTO) is recognised as one of the more damaging drilling dysfunctional mechanisms. The HFTO can cause significant tool damage in addition to nonproductive time (NPT). This vibration has been shown to be more prolific in motor powered steering assembles where it is thought that the long stick-out resonates torsionally with the energy coming into the system at the bit and a reflection of torsional energy at the motor.
Using data derived from a next-generation measurement-while-drilling (MWD) tool, which includes high-speed vibration and internal pressure measurements, we will show that these dynamics are more complex than generally visualized.
HFTO is in fact three different families of vibration. The conventional HFTO, where the stickout below the motor is resonating, is considered to be Type 1. In the Type 2 group, energy is trapped in the lower bottomhole assembly (BHA). Typically, this vibration has a higher frequency, than the Type 1, and is independent of the tools in the upper BHA. The characteristics of this are determined by the contact points; thus, it is dominant in the curved section or where heterogeneities trigger microdoglegs. Finally, we will show that HFTO can develop when there is a steerable motor in the BHA, the Type 3 group. The torsional waves from the bit couple through the motor causing pressure perturbations; hence instabilities in the weight on bit.
This new understanding of HFTO explains some of the anomalies in the performance of different mitigation strategies for HFTO. In particular, why HFTO is much more prominent in heterogeneous formations and while drilling a curved section.
