Dynamics and control of torsional vibrations in oilwell drillstrings

Abstract

A major concern in the oil industry is the high expense of oilwell drilling procedures, specially in the case of drillstring failures due to damage caused by vibrations. Even with several studies that aim to control these vibrations, the dynamics of the drilling procedure and the drillbit-rock interaction is not yet completely understood, mainly because it is a very complex problem. The mechanical system is basically composed of a torque applied to the surface on a drillstring column which, in turn, transmits the torque to the drillbit. The weight applied to the drill, damping, friction, rotational speed applied to the surface, the impact on the walls of the well and others are factors that cause vibrations in the system and also the stick-slip phenomenon. The occurrence of stick-slip is a major cause of the malfunction and failure of drillstrings and appears between the tip of the drill and the rock formation. This phenomenon causes a wide variation in speed on the drill tip at surface and creates oscillations that can develop speeds many times larger at the drillbit, as well as its complete halt. In order to minimize these problems, several control techniques were studied and, therefore, this project aims to compare them in case of a simplified model of the drill. Major aspects like the torsional flexibility of the drillstring and drillbit-rock interaction will be considered, but other effects such as lateral vibrations, impacts with the wall of the well and interaction with internal and external fluids will be neglected for simplicity. Among the control techniques to be studied are: i) the traditional speed control (PI, proportional to velocity and displacement), ii) soft-torque control which consists of PI control with a high-pass filter to mitigate the excitation of drillstring torsional vibrations, and iii) combined axial/torsional control aimed at complementing the PI speed control with a dynamic weight-on-bit to reduce the occurrence of stick-slip.