Abstract
Vibrations have been identified as one of the most frequent and persistent performance limiters by limiting weight-on-bit, rate of penetration, or borehole quality even when they may be low enough not to cause damage to downhole tools and equipment.
A general purpose drillstring mechanics model has been developed to analyze axial and torsional vibrations in the frequency domain and provide vibration indices indicative of dysfunction in these modes. The model utilizes transfer matrices to solve for harmonic perturbations around a baseline solution obtained from a torque-and-drag type analysis, and accounts for effects of well path, tool joints, viscous damping due to the drilling fluid, surface boundary conditions, bit characteristics, and special vibration mitigation tools. The model supports workflows for real-time vibration surveillance as well as post-drill root cause analysis and well redesign. For example, real-time stick/slip severity monitoring is enabled using 1-second surface measurements. As a well redesign tool, a large number of design alternatives can be quickly evaluated to mitigate vibrations.
Case studies utilizing high-frequency surface/downhole drilling mechanics data validated the model and identified three types of torsional dysfunctions with distinct signatures and mitigators: Unstable Stick/Slip, an instability associated mostly with the lowest-frequency torsional resonance of the drillstring; Bit/Bottomhole Assembly (BHA) Stall, intermittent, sudden mechanical jamming at the bottom of the drillstring; and Synchronous Torsional Oscillation, the amplification of periodic excitations at torsional resonances of the drillstring. In one case study, the stick/slip surveillance tool was superior to realtime downhole measurements in lag time, bandwidth, and accuracy. In another case, the root cause of prevalent Unstable Stick/Slip was identified as velocity-weakening aggressiveness of the bit. Among redesign options using a topdrive controller tuned to damp out the lowest-frequency torsional resonance was deemed most effective. One such controller was evaluated in the field and was very effective at mitigating stick/slip in subsequent wells.
