Integrating Bit-Rock Interaction and BHA Dynamics for the Mechanistic Modeling and Characterization of High-Frequency-Torsional Oscillations

Abstract With advances in sensing technology, high frequency torsional oscillations (HFTOs) have been validated to be a significant cause of bottom hole assembly (BHA) component damage. There is, however, a shortage of understanding on the nature of HFTO origination in the drill bit and its transference to the BHA. This study presents a novel approach to integrate bit-rock interaction (BRI) and BHA models to investigate the emergence, propagation, and dynamic impacts of HFTOs. A high-fidelity BRI model was first developed to simulate the cutting loads on the polycrystalline diamond cutters (PDC) during the penetration of heterogeneous formations. The frequency spectrum from the BRI simulation was validated against bit vibration data collected in lab tests. The output from the BRI model is then provided as input to the BHA finite element model for HFTO mode analysis. Dynamic responses of the BHA, especially at signature BRI and BHA resonance frequencies, are adopted as key metrics to evaluate the HFTO damage risks of different BHA components. For BRI modeling, each rotation of the bit is discretized into hundreds of steps and Fourier transformation is utilized to generate the spectrum of the cutting load (2000 Hz in this study). The BHA was axially discretized at intervals of 0.1 ft for torsional FE modeling, capturing changes in BHA radial dimensions, components, and mechanical properties. Then, the transfer function from the bit to different BHA locations (dynamic responses) is obtained for a frequency range up to 1000 Hz. Analysis indicates that torsional BHA resonance and mode shapes are critical to determining the instantaneous HFTO strength and long-term HFTO-related fatigue damage in different BHA components. Validation against lab data confirms that the BRI model captures the appropriate BRI load frequencies. Combining physics-based high-resolution BRI and BHA models, this work delivers a novel and robust dynamic characterization of HFTO origination and propagation. The analysis framework technically explains the HFTO risk differences between bit-BHA designs and provide guidance for bit selection, optimization of bit/BHA designs, and development of effective HFTO mitigation strategies. This paper thereby extends the industry's understanding of HFTO and provides a modeling framework for its mitigation.