Abnormal down hole pressure variation by axial stick-slip of drillstring

Downhole pressure variation is generally believed to be a kind of surge and swab pressure due to the axial movement of drillstring during the tripping in/out operation, and even wave loading induced heave motion. In order to predict downhole pressure variation, the Stribeck friction model has been used to analyze the friction-induced self-excitation vibration (stick-slip). This model describes the axial stick-slip motion of the drill pipe through four regimes of contact: sticking, boundary lubrication, partial fluid lubrication, and full lubrication. The results indicate a significant increase in the peak velocity of the drillstring compared to the velocity at the surface point. In addition, axial tension induced geometrical stiffness has been taken into account, which results in a time dependent normal force between the drillstring and the borehole in the curved section of borehole. Simulations show that the wave form of the downhole drillstring motion is different from the surface motion.