A New Way of Using Surface Measurements to Detect Down Hole Vibrations
Authors: Mabile C. , Desplans J.P. , Pavone D. 
Published: December 2007

Abstract
Detecting drilling abnormal vibrations from the sole surface measurements is one of the major research goal today, but signal processing procedures are not efficient enough by themselves. This paper presents a new process that enhances the use of surface measurements with the help of a model representative of the dynamic phenomena of the whole drill string. We show how the model is transformed and reduced for real time application. One of the computed outputs of the reduced model is continuously fitted with the equivalent measured data to avoid its drifting away. Whenever a discrepancy is noticed between computed and measured data, the reduced model is automatically adapted. Therefore all the computed outputs of the reduced model and mainly the ones at the bit are close to the real movements of the bit which can be used for detecting abnormal vibrations of the drill string.

Introduction
Using surface measurements to detect drill string abnormal vibrations is not a recent idea. Whoever has been on a rig floor has noticed the vibrations of the Kelly cock or the noise generated in the drill stem. In 1971 Lutz et al. use an advance surface data acquisition and treatment system to produce a log called SNAP log. This log is built from surface measurements of the tension and vertical acceleration and can be read as a sonic. The same measurements were also used to detect bad functioning of the three cone bit.
The idea of using surface measurements to detect down hole vibrations was again raised in the second half of the eighties. The first step was to engineer and build an advanced surface data acquisition system. One can notice the ADAM system or the DYNAFOR project. Subs were built to record surface measurements relative to the mechanical behavior of the drill string such as drill string tension and torque but also measurements relative to vibrations as vertical and rotational accelerations. These subs are connected under the power swivel and therefore measure directly the vibrations of the drill string. These subs are now commercially available.
At the same time IFP has started a research program to understand the sources of abnormal vibrations in the drill string. One aspect of this program was the development of a data acquisition system named TRAFOR including a surface and a downhole measurements sub. An electric link between the two subs has been designed so that the system can be used during standard rotary drilling (Figure 1). This electric link allows synchronization between the downhole and surface data. TRAFOR provides a hill set of downhole and surface measurements and a high sampling rate equal to 360 data per second and per channel. The main characteristics of the TRAFOR system are listed in Table 1.
The problem was raised for interpreting the data recorded with these enhance data acquisition systems. Many theoretical works were performed to build models that can be used to simulate the main drill string vibrations. Very few take into account the relationship between the drill string and the borehole wall or the bit and the formations. Clayer and al. show that it is necessary to take into account the rig for the top end of the drill string and the relationship between the bit and the formation for its bottom end. Both ends have in fact an important effect on the behavior of the drill string dynamic regime.
Pavone et al. built a model simulating the dynamic torsion of the drill string. In this model the authors incorporate friction laws at the borehole walls and at the bit. These laws are derived from actual data recorded with the TRAFOR system. Through this model they show that the source of the stick slip phenomena is located at the bit level where reactive torque is decreasing when the rotation speed is increasing.
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