Abstract
In the last decades, drilling industry has been developing solutions to overcome challenges that impede drilling of long directional wells as excessive friction, poor directional control, narrow drilling windows, etc. One of the problems that is identified, yet not solved is the drillstring vibrations. Drillstring vibrations result in low ROP, fatigue and failure of the drillstring elements and weakened wellbore integrity. They are often detected through surface symptoms, as large surface WOB fluctuations (axial vibrations), torque and RPM fluctuations (torsional vibrations), reduced ROP, pressure fluctuations, rig/top drive shaking, etc.
To control vibrations, a good understanding of basic mechanisms of vibrations initiation and propagation is required. Therefore, this paper aims at the experimental investigation of the drillstring dynamics using a small-scale drilling rig constructed at the University of Stavanger. The experimental drilling rig is equipped with a WOB surface sensor, which works as a strain gauge, as well as RPM and torque encoder and other sensors. For a small-scale system, measurements of the surface sensors represent the combined responses of the whole rig structure, not just the drillstring. Therefore, more information to show drillstring dynamics (displacements and frequencies) is desired, besides surface measurement.
This paper presents experiments designed to detect vibrations of the drillstring using a high-speed camera, which in this study serves as an along-the-pipe downhole sensor. The paper also provides the image-processing algorithm that was developed to extract the signal from the images, digitize and normalize it. The high-speed camera has proved to be an accurate and practically noise-free displacement sensor.
As a part of this study, analysis of the captured frequencies and decaying amplitudes (damping) was performed for both the high-speed camera and the load cells data. It helped to evaluate whether the surface sensors are able to provide sufficient information about the downhole vibrations. We have seen that in the case where the drillstring interacts with the wellbore, the downhole vibrations are reflected to the surface. However, when the string hangs freely, surface and downhole sensors measure responses from different parts of the system. These conclusions are mostly valid for small-scale vertical systems, however, can also be considered for drilling shallow vertical wells/top sections.
