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Lateral Vibration of Oilwell Drillstring During Backreaming Operation Cristiano E. Agostini 1 , Rodrigo Nicoletti 1 1 Department of Mechanical Engineering, S ˜ ao Carlos School of Engineering, University of S˜ ao Paulo Trabalhador Sao-Carlense 400, 13566-590 S˜ ao Carlos, Brazil cr.ago [email protected], [email protected] ABSTRACT: Oilwell drillstring vibrations are renowned by their high damaging effects in the drillstring elements. In the case of backreaming operations, i.e. when the drillstring is pulled out of the wellbore, abnormal lateral vibrations have been reported by operators in the field. Such lateral vibrations of the drillstring can cause several problems during wellbore construction, e.g. BHA electronic equipment failure, falling rocks into the well, drillstring blockage, which result in high financial losses when they occur. This work focuses on the study of the dynamics of oilwell drillstrings during the withdrawing operation (backreaming). For that, a non-linear mathematical model is used for representing the drillstring vibrations considering fluid pumping and string rotation simultaneously. The proposed model focuses on the effects of lateral vibrations on the lower portion of the drill string, commonly known as Bottom Hole Assembly (BHA). The modelling approach is based on analytical, nonlinear and lumped parameters, which considers the effects of drilling fluid damping, stabilizer and drill collar contact with the borehole wall. The numerical results are compared to experimental results obtained in a scaled test rig. KEY WORDS: Rotor dynamics; Drillstring dynamics; Backreaming operation; Mechanical vibration. 1 INTR ODUCTI ON During the borehole drilling of oilwells, when the final depth is reached, there comes the task of pulling out the drillstring. There are two possible ways of pulling out the drillstring: dry operation (no rotation and no pumping of fluid) and backream- ing operati on (rotatio n and pumpi ng of fluid). The dry opera- tion represents a faster operation, but the friction between the dri lls tri ng and the bor eho le bec ome s so hig h tha t it usu all y jeo p- ardiz es the pullin g-out operatio n. The backreami ng opera tion is more commonly adopted, but their optimum conditions are still controversial in literature. In some applications, the back- reaming operation represents a much better friction condition for pulling out the drillstri ng but, in other applicati ons, the se- vere lateral vibration of the drillstring can ca use serious damage to the Bottom Hole Assembly (BHA), and even borehole col- lapse with drillstring imprisonment [1]. The challenge of modelling the drillstring during backream- ing operation is the correct representation of the coupling be- tween the torcional and lateral movements of the drillstring and its longitu dinal movement in conta ct with the boreh ole. The friction between the drillstring and the borehole during longitu- dinal movement, together with occasional impacts due to lateral vibration, lead the system to a strong nonlinear behaviour. In literature, one can find few studies on the subject, mainly focused on axial vibration. In [2,3], axial vibration of the drill- string is studied to improve the dynamic behaviour of the sys- tem during stick-slip phenomenon and to optimize the design of the stabilizers. Friction between the drillstring and the bore- hole is studied in [4], based on a low order mathematical model and exper ime nta l cor rel ati on to res ult s sho wn in [5] . The res ult s sho w tha t fric tion pla ys an imp ortant rol e in the dyn ami cs of the system and in the impa ct le ve l of the dr il lstr ing agai ns t the bore- hole walls . The control of the rotating spee d of the drillst ring has also been investigated to minimize lateral vibrations. In [6], the adopted mathematical model took into account the longitu- dinal movement of the drillstring and results showed that vi- bration is self-excited. Hence, in some cases, vibration was re- duced by increasing the rotating speed of the drillstring. How- ever, in other cases, the lateral vibration observed was severe. Despite the advances in the study of lateral vibrations of drill- strings, there are no related studies on the backreaming opera- tion, considering drillstring rotation and fluid pumping. In this work, one develops a mathematical model for the drillstring during backreaming. The modelling approach is based on ana- lytical, nonlinear and lumped parameters, which considers the effects of drilling fluid damping, stabilizer and drill collar con- tact with the borehole wal l. The results of numeric al simula- tions show the occurrence of abnormal lateral vibrations during the drillstring withdrawal, but this effect decreases with higher friction coefficients, and for rotating speeds near the natural fre- quenc y of the syste m. A comparis on betwee n numerica l and experimental results obtained in scaled test rig showed good agreement for conditions of low friction coefficients. 2 MA THEMA TICAL MODEL The followi ng mathemati cal model was based on the two degr ee-of -freedom model presente d by Jansen [7,8], which takes into account the pumping fluid, contact of the stabiliz- ers with the borehole walls, contact of the drillstring with the boreh ole walls, and excitat ion due to unbalanc e. The model is then complemented with additional degrees-of-freedom of tor- sion and longitudi nal movement. The basic hypothe ses of the Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014 Porto, Portugal, 30 June - 2 July 2014 A. Cunha, E. Caetano, P. Ribeiro, G. Müller (eds.) ISSN: 2311-9020; ISBN: 978-972-752 -165-4 893
