Model-Based Stability Analysis of Torsional Drillstring Oscillations

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 systems use mathematical models of the rotary system. As the drilling process is subject to permanent changes, model-based control systems can only perform adequately when constantly updated with actual conditions. To date, surface data has been used for this, but some key drilling process characteristics can only be identified with downhole data. By using downhole measurements of drillstring dynamics it is possible to identify model parameters and estimate stick/slip instability regions for changing drilling conditions. A surface-based control system can use this information to maintain optimal weight on bit and rotary speed. This paper describes models to estimate the boundaries of stick/slip instability zones. To validate the approach a field test was performed. During this test, stick/slip instability zones were identified experimentally for various bits while drilling in different formations. Results of the test are summarized in the paper.