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High-Frequency-Torsional Oscillations (HFTO) generate dynamic loads that can damage drilling tools, resulting in, for example cracks, twist-offs or broken electronics. They are triggered by the interaction of bits and rocks and force operators to reduce rotary speed (RPM) and weight on bit (WOB) losing drilling performance in the process. Recently, a full-scale drilling test rig was proven to generate verified HFTO behavior under laboratory conditions (Everhard et. al. 2023). This rig allows for a comprehensive study of the influences of bit characteristics on HFTO for the first time. This paper presents methods to qualify bit features to suppress HFTO. Effective HFTO influencing properties are identified and discussed. The full-scale laboratory test rig drills rocks in a pressurized rock chamber. ROP, WOB, RPM, pressure, bit type and rock type can be varied. High-frequency measurement instrumentation, including new in-bit sensing, record the tangential accelerations and dynamic torque at various positions in the laboratory rig. The type of excited torsional vibrations match vibrations in the field indicating that learnings in the lab translate to the field. To study the influence of bit and operating parameters on HFTO, PDC-bits of varying design are used to drill rocks under varying pressures, RPMs and WOB. The data are used to develop evaluation methods to rank bit-rock combinations with regards to the stability and severity of the generated vibrations. Stability maps relating RPM, WOB, and vibration proved to be a good measure to reliably identify HFTO and rank bit-rock combinations and applied operating parameters, by their susceptibility to HFTO. Bit properties, such as cutter shape, cutter placement or rock type control the energy intake per vibration cycle and, hence, the excitation of torsional vibrations. The operating parameter space indicating stable drilling states can be maximized by properly choosing bit features. Rock types triggering HFTO are identified using segmented core tests. When HFTO is present and fully developed, the severity of vibrations scales with the angular velocity of the bit (RPM) but not with the WOB. If HFTO is absent, WOB and RPM act as an "on-off" switch to HFTO. The threshold of WOB and RPM triggering HFTO is established for bit-rock combinations. The stable operating zone can be influenced by adding damping devices to the BHA. The findings also result in recommendations for operating BHAs in the field. Studying HFTO in a full-scale laboratory environment using the presented methods enables the development of robust and reliable HFTO countermeasures. Major influences on HFTO are identified and scientifically proven; understanding these characteristics will result in HFTO suppressing bits and tools. Ultimately, HFTO mitigation allows drilling engineers to optimize drilling parameters and reduce drilling time while simultaneously decreasing tool-failure probability and associated NPT and costs. |
