Reduce Torsional Vibration and Improve Drilling Operations
Authors: Rajnauth Jerome J. 
Published: December 2007

Torsional vibration can be very damaging to drill string components during downhole drilling operations. Reducing torsional vibration as a result of monitoring and implementing corrective actions can be very useful in drilling optimization. While downhole vibrations cannot be totally eradicated, if properly monitored and kept to a minimum, they are harmless.

A software package was used during drilling operations on two wells off the East Coast of Trinidad. Results showed that severe slip stick vibrations were occurring and having major impact on downhole equipment such as bit, MWD and motor. The computer program provides real time display of the magnitude of torsional vibration by utilizing Fourier Spectral Analysis on the data from drilling sensors.

This paper will look at the results from these two wells and show that there are benefits from monitoring and reducing torsional vibrations. These include:reduction of the frequency of drill string/BHA failures;reduction in trip time and fishing time;increase in drilling rate and bit life

Introduction

Identification and control of drill string vibrations has become an area of considerable interest over the past 10 to 15 years. This is because drill string failures have increased tremendously during that time and have become a serious problem resulting in substantial financial losses. The financial impact of such failures can be significant both in terms of cost of replacing damaged components and extended operation time caused by unplanned events. Increasing drilling costs and the use of more complex and expensive downhole tools make vibration monitoring and control a key in drilling optimization. A number of oil companies have become aware of the benefits of controlling vibration and have implemented programs to raise awareness and create control strategies (1).

High levels of vibration during the drilling process frequently occur. A combination of technologies and methodologies can be useful in reduction of downhole vibration (2). The cost of unacceptable levels of vibration is high resulting indrill string failurepremature bit failurepoor directional tendencyhole enlargementstalling of the top drive or rotary tableMWD tool failureStabilizer / tool joint wear

Three modes of drill string vibration are known.Axial - motion along the length of the drill stringLateral - motion perpendicular to the drill stringTorsional - twisting motion of the drill string

Torque is a complex waveform created by cumulative resistance to rotation of the entire drill string. By analyzing the power input to the rotary drive, using Fourier analysis, the torque signal could be broken down into its constituent components and repetitive cycling waves and respective frequencies identified. These cyclic frequencies indicate the presence of torsional vibration. Determination of these frequencies and their magnitude (Kt) allows the driller to correct the problem by varying the associated drilling parameters of RPM and/or WOB.

The resulting benefits can bring substantial cost savings to a drilling program byreducing BHA and tubular failuresreducing bit damage and increasing bit lifedecreasing the number of bit tripsreducing tool failuresdetermining optimum rotational speed