Development of an Innovative Model-Based Stick/Slip Control System
- Jens Rudat (Leibniz University Hannover) | Dmitriy Dashevskiy (INTEQ)
- Document ID
- Society of Petroleum Engineers
- SPE/IADC Drilling Conference and Exhibition, 1-3 March, Amsterdam, The Netherlands
- Publication Date
- Document Type
- Conference Paper
- 2011. SPE/IADC Drilling Conference and Exhibition
- 1.2.2 Drilling Optimisation, 1.6 Drilling Operations, 1.5 Drill Bits, 4.1.2 Separation and Treating, 1.7 Pressure Management, 1.6.1 Drilling Operation Management, 1.4.4 Drill string dynamics, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.3 Drilling Optimisation, 1.10 Drilling Equipment, 1.2.5 Drilling vibration management, 4.1.5 Processing Equipment, 1.7.5 Well Control
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Stick/slip is a severe type of torsional drillstring oscillation that affects the efficiency of the drilling process and can cause bit damage as well as drillstring failure. Different approaches to mitigate stick/slip oscillations are used in the field, ranging from procedures for optimizing drilling parameters, to passive downhole damping devices, and active surface controllers. To improve performance, some of these systems use mathematical models of the rotary system. Since the drilling process is subject to permanent changes, model-based control systems can only perform adequately when constantly updated with the actual conditions. To date, surface data has been used for this, but some key drilling process characteristics can only be identified with downhole data.
This paper presents an innovative model-based stick/slip control system. It is based on a model of the drilling process that predicts the intensity of downhole vibration with respect to drilling parameters. Using downhole measurements, the model is adapted to the actual drilling process and employed to find optimal drilling parameters. To overcome the limited bandwidth of the typically used mud-pulse telemetry systems, models are identified downhole in the dynamics measurement tool.
The paper presents field tests performed to evaluate the stick/slip control system and the investigation of how model parameters can be identified from measurements as well as how well the identified models represent the drilling process. Results show that this model can be used to predict the intensity of torsional oscillations and determine optimal drilling parameter values.
The presented stick/slip control system can provide quantitative recommendations on changing drilling control parameters to mitigate stick/slip and can be used in an automated mode by directly connecting to the rig control system for monitoring drilling conditions and adjusting drilling parameters. Proper handling of downhole vibrations can significantly increase reliability.
It is currently well recognized in the industry that automating the drilling process will increase its reliability and reduce drilling costs. Many efforts are documented on automation of drilling operations on the rig floor, mud handling, and pressure management (Eustes 2007). Vibration is also recognized as one of the critical factors contributing to the nonproductive time (NPT) and pure drilling performance. Monitoring and reduction of the bottomhole assembly (BHA) vibrations was a subject of many publications (Pavone and Desplans 1994, Heisig et al. 1998, Ledgerwood et al. 2010). But nevertheless, there is still no reliable system available to consistently handle BHA vibrations under changing drilling conditions. The complexity of the problem largely contributes to this fact.
Not all phenomena affecting drillstring dynamics are well understood and can be modeled with acceptable accuracy. And even if they are, information needed to describe the process is seldom available as to when these models can be utilized in realtime applications. For example, methods based on purely surface measurements have problems with robustness since some critical process characteristics can't be measured on the surface and need to be assumed.
The study presented in this paper focuses on stick/slip, a severe type of torsional oscillation. Stick/slip oscillations are characterized by a repeated alternation of sticking phases, where the bit comes to a complete standstill for a limited interval of time, and slipping phases, where its angular velocity ? increases up to two or three times the rotary speed O applied at the surface. The stick/slip effect has been the subject of numerous studies in the past and is still under investigation (Kyllingstad and Halsay 1988, Brett 1992, Jansen and van den Steen 1995, Richard et al. 2002, Detournay et al. 2008, Germay et al. 2009, Navarro-Lopez 2009).
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