Drillstring Mechanics Model for Surveillance, Root Cause Analysis, and Mitigation of Torsional Vibrations

Abstract

Summary 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 uses transfer matrices to solve for harmonic perturbations around a baseline solution obtained from a torque/drag type analysis, and accounts for effects of well path, tool joints, viscous damping caused by 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 by use of 1-second surface measurements. As a well redesign tool, a large number of design alternatives can be quickly evaluated to mitigate vibrations. Case studies using 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 stall, which is intermittent, sudden mechanical jamming at the bottom of the drillstring; and synchronous torsional oscillation, which is the amplification of periodic excitations at torsional resonances of the drillstring. In one case study, the stick/slip surveillance tool was superior to real-time 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. Of the redesign options, the use of 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.