Abstract
Achieving optimal performance during drilling of complex well trajectories is often hindered by downhole drill-string vibrations and stick-slip. These can lead to drill bit and downhole tool damage, drill-string wear possibly leading to a twist-off, or formation damage. Recent advancements in drill-string vibration interpretation show that the sources of excitation are not only at the bit but anywhere along the string. Therefore, a solution that uses distributed along-string damping elements based on magnetic damping is investigated. This paper presents the design principles of a laboratory-scale setup to verify the concept and the accompanying test results.
Previously published numerical results show that stick-slip can be attenuated using the distributed damping elements. The elements attempt to reduce drill-string vibration by attenuating the sources of negative damping, and by increasing the sources of positive damping. Mechanical friction between the drill-string and the borehole, a major source of axial and torsional vibrations, is reduced, and its axial and tangential components are decoupled. Magnetic viscous damping is introduced by utilizing eddy current braking at the level of each element.
A laboratory-scale setup consisting of a 10-meter-long horizontal apparatus has been constructed to verify the damping effectiveness of an individual element. The setup was designed to mimic downhole drilling conditions such as drill-string elasticity, friction forces and inertial moments, and to recreate real-world adverse conditions such as vibrations, stick-slip, and twist-off. Sensors and actuators positioned along the experimental setup allow control of the rotational and axial velocities, contact forces at various locations, and adjustment of the magnetic braking force. Stick-slip was introduced in the system through an adjustable side force imposed on the drill-string as well as through a stepper motor operating in torque mode simulating the bit-rock interaction.
The first series of experiments in the laboratory-scale setup were aimed at evaluating the braking force obtained in different operating conditions. By controlling the strength of the eddy current effect, the magnitude of the braking force could be varied, and thus, the damping effectiveness of the element could be estimated. The braking force, measured by a load cell, was found to increase linearly with the rotational speed and with the strength of the magnetic field. The second round of experiments were focused on demonstrating how the magnetic braking effect helps damping out torsional vibrations and mitigating stick-slip.
A novel concept for damping stick-slip vibrations using magnetic damping elements distributed along the drill-string has been implemented and demonstrated at laboratory-scale. This concept aims to mitigate stick-slip vibration by addressing its root cause, the friction forces along the drill-string. The experimental setup can also be used to prototype and test new control strategies for damping of drill-string vibrations.
