Abstract

Automated downhole rotary steering devices that were developed in the 1990's for directional control were initially d eployed on extended reach wells. They are currently more widely applied to improve both drilling performance and wellbore quality. Automated surface control systems have also been developed for safer pipe-handling, downhole pressure control and for mitigating stick-slip when induced by cumulative friction along the drill string. A novel self-adapting downhole drilling tool can now autonomously detect and suppress excessive downhole shock and vibration levels by adjusting the stiffness of its damping fluid which in turn instantaneously changes the vibration characteristics of the bottom-hole-assembly.

The downhole drilling environment is continuously and unpredictably changing as the drill string gets longer and as stabilizers navigate around micro-doglegs or past geological strata where the borehole has enlarged and no longer provides stabilization. Whenever the driller (or an automated control system) makes changes to the surface drilling parameters this also changes the downhole vibration characteristics. Each blade of the bit can additionally induce torsional shocks as it alternately cuts softer and then harder rock at a lithological transition in an inclined borehole when the formation on the upper side of the wellbore circumference is harder or softer than that on its lower side.

In order to respond to a frequently changing environment, vibration sensors in a shock-sub send data to its autonomous control system with an intelligent algorithm that iteratively adjusts the stiffness of a magneto-rheological damping fluid in response to the severity of the various modes of downhole vibrations. In this way the novel self-adapting shock-sub seeks out the optimal dynamic stiffness of the drilling assembly for each change in the downhole drilling environment.

This paper presents insights as to the nature of downhole vibrations encountered when drilling in the Middle East, describes the intelligent self-adapting shock-absorber, examines some of the characteristics of a magneto-rheological damping fluid and explains how the intelligent shock-absorber can be configured for various drilling scenarios.

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