oa Laboratory Scale Arrangement for Experimental Studies of Drill-String Motions

Background & Objectives

Drilling systems are used to identify geological reservoirs and carry out extraction of oil or natural gas from these reservoirs. Deep wells are drilled by using rock-cutting tools driven from the surface by a slender structure of pipes called a drill string. These slender rotating structures can experience undesired dynamics, which can be detrimental to the drilling system. As an example, it is mentioned that stick-slip oscillations can be an important cause for drilling inefficiency and failure of drill-string components, as violent torsional motions and large amplitude lateral motions can occur during these oscillations. As a step towards the development of appropriate vibration attenuation schemes, it is intended to study the nonlinear behavior of slender rotating structures representative of drill strings. In particular, efforts underway at Qatar University to develop a laboratory scale experimental arrangement to study these structures will be presented and discussed. Prior efforts undertaken in this area will also be briefly reviewed. Brief Review and Qatar University Experimental Arrangement Drill-string motions are important to understand the complex system behavior of a drilling system, in particular, as they relate to downhole vibration phenomena. As shown in Fig. 1, the drill string is a complex, flexible structure, which consists of hollow steel pipes screwed together to form a long continuous structure. Large diameter sections, which are referred to as stabilizers, are inserted between two drill pipes to help keep the drill string centered in the borehole. The base of the drill string is made up of two main components, namely, the drill collar and the drill bit. The drill bit, a tool which breaks down the rock and soil, is secured at the end of the drill-collar assembly. The composition of drill pipes, stabilizers, and drill collars is referred to as the bottom-hole assembly (BHA). The entire drill-string assembly is rotated at the surface by using a rotary table and a motor. This actuation is transmitted down the drill string and to the drill bit, which acts to crush the rock and soil. Throughout the drilling process, a hydraulic fluid, known as drill mud, is pumped down through the center of the drill string and collars. This drill mud serves two purposes. It not only keeps the drill bit cool and lubricated, but it is also used to wash away the soil and cut rock. After the mud flows through the drill-strings and the collars, it flows then in the annulus between the drill-string and borehole carrying the cuttings to the surface. During operations, a drill string can experience a whole range of vibrations, including axial, torsional, and lateral vibrations. Drill-string vibrations are sometimes further grouped together as vibrations without contact without the borehole, whirling motions (forward and backward) during which there can be rolling and sliding contact with the borehole, and snaking motions which are a form of lateral vibrations during which a part of the drill string rolls over a borehole contact point. Given that drill strings are long, slender structures, the first torsion natural frequency and the first bending natural frequency are typically in close proximity. In addition, the nature of the system allows for coupling and energy exchange between torsional and lateral motions. In order to focus on the behavior of drill strings in the BHA region, a number of studies have been conducted, with several including studies with scaled laboratory scale arrangements. A partial list of references is included at the end of this paper. Focusing on some of them from this list, in earlier work conducted at the University of Maryland, Liao (2011), Liao, Balachandran, Karkoub, and Abdel-Magid (2011), Liao, Vlajic, Karki, and Balachandran (2012), the focus was on stick-slip motions and whirling. Comparisons were made between experimental and numerical results. It was shown that the nonlinear nature of the contact force interactions is critical for capturing some of the associated phenomena. In a follow up study conducted by Vlajic (2014), forward and backward whirling motions were explored and the development of appropriate reduced-order models was continued. In the work carried out by Shyu (1989), which include validation with laboratory and field studies, a focus was on the coupling between lateral and axial vibrations. Later motion instabilities were experimentally investigated in the work of Berlioz, Der Hagopian, R. Dufour, and E. Draoui (1996). Other notable examples include the studies of Antunes, Axisa, and Hareux (1992), Kust (1998), Mihajlovic (2005), Gao and Miska (2008), and Khulief and Al-Sulaiman (2009). These studies will be reviewed during the conference presentation. In order to build further on previous experimental efforts and related analytical and numerical studies, efforts are underway at Qatar University to build a laboratory scale arrangement to focus on stick-slip oscillations and whirling. Some representative motions of interest are shown in Fig. 2. The proposed system, which is to be used to capture the dynamics in the BHA region, is also shown in Fig. 1 with dimensions. Details of this arrangement will be presented at the conference. It is expected that the proposed arrangement will help explore stick-slip interactions further and gain insights into different aspects including drilling mud that can be beneficial for drill-string vibration attenuation and realizing desired BHA dynamics.

Keywords

Rotor-stator interaction, Dry friction, Stick-slip motions, Torsional vibration, Whirling

Acknowledgment

The authors would like to gratefully acknowledge the support received from the Qatar National Research Fund for NPRP Project 7-083-2-041, to pursue this collaborative work between the University of Maryland, College Park, MD, USA and Qatar University, Doha, Qatar.

References

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Berlioz, A., J. Der Hagopian, R. Dufour, and E. Draoui. “Dynamic behavior of a drill-string: experimental investigation of lateral instabilities,” Journal of Vibration and Acoustics 118 (3), 1996, pp. 292-298.

Gao, G. and S. Miska. “Dynamic buckling and snaking motion of rotating drilling pipe in a horizontal well,” SPE Journal 15(3), 2010, pp. 867-877.

Khulief, Y. A., and F. A. Al-Sulaiman. “Laboratory investigation of drillstring vibrations,” Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 223 (10), 2009, pp. 2249-2262.

Kust, O. “Selbsterregte Drehschwingungen in Schlanken Torsionssta”- ben-Nichtlineare Dynamik und Regelung. PhD Dissertation, University of Hamburg-Harburg, Hamburg-Harburg, Germany, 1998.

Liao, C.-M. “Experimental and numerical studies of drill-string dynamics,” PhD Dissertation, University of Maryland, College Park, 2011.

Liao, C.-M., B. Balachandran, M. Karkoub, and Y. L. Abdel-Magid. “Drill-string dynamics: reduced-order models and experimental studies,” Journal of Vibration and Acoustics 133 (4), 2011, pp. 041008-1-041008-8.

Liao, C.-M., N. Vlajic, H. Karki, and B. Balachandran. “Parametric studies on drill-string motions,” International Journal of Mechanical Sciences 54 (1), 2012, pp. 260-268.

Mihajlovic, N. “Torsional and lateral vibrations in flexible rotor systems with friction.” PhD Dissertation, Technische Universiteit Eindhoven, 2005.

Shyu, R.-J. “Bending vibration of rotating drill strings,” PhD Dissertation, Massachusetts Institute of Technology, 1989. Vlajic, N. A. “Dynamics of slender, flexible structures,” PhD Dissertation, University of Maryland, College Park, 2014.