Application of Stability Approach to Bit Dynamics
- V.A. Dunayevsky (IIT Research Institute) | F. Abbassian (BP Exploration)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1998
- Document Type
- Journal Paper
- 99 - 107
- 1998. Society of Petroleum Engineers
- 1.2.5 Drilling vibration management, 1.5.1 Bit Design, 1.6 Drilling Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.1 Drilling Operation Management, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.3.4 Scale, 1.4.4 Drill string dynamics, 1.5 Drill Bits, 1.2.2 Geomechanics, 1.10 Drilling Equipment
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This paper addresses the dynamic stability of poly-crystalline diamond (PDC) bits under induced torsional and lateral vibrations. Dynamic stability is investigated by considering three simple mechanical systems representing string torsional vibration, bit lateral dynamics, and coupled torsional-lateral vibration of the bit-string assembly. For each system, the RPM-WOB domain within which the system is dynamically stable is determined. The influence of various drillstring and bit parameters on the stability of bit motion is studied through their effect on the size of the stability zone on RPM-WOB plots. These illustrate the power of the stability approach in providing valuable qualitative information on the main parameters influencing bit stability.
With the identification of slip-stick and bit whirl as two major causes of premature PDC bit failure, bit dynamics has been the subject of extensive studies in the recent years. These studies have involved detailed analysis of surface and downhole vibration data assisted with theoretical investigation of various vibration events, notably string torsional (slip-stick) and bit lateral (whirl) vibration.
Most theoretical investigations have been based on time simulations of representative dynamic systems. These studies have been very valuable in increasing our understanding of the problem and the identification of the main influencing parameters. To gain an overall qualitative behaviour of a bit-drillstring assembly, however, often requires a parametric study which can be computationally intensive.
This paper illustrates the application of the stability approach to bit and drillstring dynamics via the investigation of three simple mechanical systems: the first system represents torsional vibration of the string; the second system corresponds to lateral whirl type dynamic behaviour of the bit, and the third system addresses the coupled torsional-lateral vibration of the bit-string assembly. This approach does not require intensive time simulations. Instead, it identifies the underlying relationship between various parameters such as rotary speed (RPM), weight-on-bit (WOB), string stiffness, bit and top-drive characteristics required for a stable bit motion.
The main purpose of the present study is to outline, via simple mechanical models, the potential application of linear stability analysis as a useful tool for understanding PDC bit behaviour. The success of the stability approach in achieving this objective is demonstrated through a number of examples, where the influence of various drillstring and bit design parameters on the stability condition is examined.
DYNAMIC STABILITY APPROACH
Numerical bit dynamics modelling is a rapidly maturing area. Recently developed 3-D numerical simulators present a predictive tool for PDC bit behaviour taking detailed account of bit-formation interaction and bit design parameters.
Our purpose is to enhance those studies by a qualitative rather than quantitative investigation of bit dynamics through a linear stability approach. The emphasis in this paper is not upon finding the solutions to the equations of bit motion, but rather upon describing the nature of the solution. The application of the stability approach to drillstring dynamics has been previously demonstrated for axial, and torsional vibration.
We have chosen three simple models (see Fig. 1): a torsional model in which the bit is rotating concentrically with any lateral motion fully suppressed;
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