Study of Stick-slip torsional vibration in Drilling Operation

The purpose of this work is to analyze drill string torsional vibrations modes, and to advise on whether it is best to minimize, if not entirely eliminate these vibration or if they can be utilize to the benefit of the drilling performances. The work consists of two generic parts. One addressing resonance vibrations when employing roller cone bits, the other dealing with torsional vibrations when drilling using PDC bits. Furthermore , to equip the engineer with the knowledge to visualize and understand the shocks suffered by downhole tools, their distinction and causes to allows them to CORRECTLY interpret the shock data in Real Time, and subsequently take intelligent decisions to mitigate those vibrations and shocks.
The methodology implemented in this project involved Simulating Drillstring and find out natural frequency behavior of vibration and the friction force side effect. The results would be interpreted in term of external force frequency and the force frequency of the system which resulted always into resonance, this resonance affect the drillstring by either „Twist off, or Washout‟ Phenomena. Simulating would entirely be conducted in Matlab software, Drillstring Dynamic Calculator would have been far better to run the simulation.
Amodel describing the torsional behaviour of generic oil drillstring has been presented. This model is acombination of some previous model proposed in the literature. The problem of modeling is divided into two different problems, first, the problem of modeling of torsional behaviours of the drillstring. Second, the problem of modeling the rock bit interaction originating stick –slip self excited oscillations. This behavior are describes by the drilling parameters behaviour in the simulation. The result obtained shows that the most important of this modelingis to show that inserting RPM of more than 45m/h would endanger the drilling operation special when it hard formation as showing the drillstring disturbances.