Vibration from High Frequency Torsional Oscillation (HFTO) damages drilling tools and electronics. Destructive HFTO can occur in harsh drilling environments which reduces drilling performance and reliability and leads to non-productive time and associated costs. Because it is faster, cheaper, more precise, and more controllable compared to field testing, a laboratory test environment is optimal for developing HFTO countermeasures. However, until now, a full-scale test rig that reliably generates controllable HFTO did not exist. This paper will describe for the first time a laboratory drilling rig that generates HFTO and, therefore, can be used to develop and qualify anti-HFTO procedures and tools.
To study the HFTO susceptibility of bit-rock interactions, the full-scale laboratory drilling rig consists of a mud circulation system, hoisting system, bit, and BHA coupled with high-frequency instrumentation to measure torsional vibrations on a millisecond scale. Finite element models (FEM) built to characterize the drilling simulator are used to correctly interpret the results of drilling data. An experimental modal analysis (EMA) is used to validate and refine the FEM models. Next, PDC-bits are used to drill several rocks under varying pressures, RPMs, and weights on bit (WOB). The resulting high-frequency torque and tangential acceleration data are compared to a checklist of necessary criteria to prove that self-excited HFTO occur in the lab. These measurements, when considered with their axial sensor positions, are used to reliably identify HFTO and compare bit-rock combinations by their susceptibility to HFTO.
Results of the FEM-models and the EMA agree on the characteristic mode shapes and dominant frequencies which match dynamic measurements. Recorded data show that self-excited HFTO are reliably excited when the criteria for self-excitation are fulfilled. Vibration energy is concentrated in one dominant mode, the vibration amplitude is scaled by the RPM, and the frequency of torsional oscillations is independent of the rig RPM. HFTO-prone rocks are identified using segmented rock specimen tests. The excitation mechanism in the laboratory test rig corresponds to the mechanism in the field. Stability maps show that bits differ in excitability allowing a comparison based on bit features and subsequent bit improvements. Methods and tools tested in the lab environment form a framework for developing anti-HFTO field solutions and operational guidelines.
The upgraded full-scale drilling rig reliably generates HFTO in a laboratory environment under realistic drilling conditions. When coupled with extended research into the combination of bit, rock and BHA variables that lead to HFTO susceptibility, this rig will enable faster and cost-efficient product and procedure development cycles for proven and validated anti-HFTO tools and field guidelines. An HFTO suppressing bit or an HFTO suppressing damping device will have a significant impact on BHA reliability, drilling performance, and reduced NPT.
