Abstract

The state-of-the-art approach to downhole shock and vibration mitigation is to insert into the bottom-hole-assembly (BHA) a shock-sub or torque-limiter with response characteristics pre-configured prior to tripping into the borehole and then continuously monitor downhole vibrations with the driller making adjustments to surface control parameters whenever the downhole shock levels are excessive. Some top-drive control systems are also capable of mitigating the torsional stick-slip dysfunction. A novel approach is to use downhole shock and vibration measurements to actively adjust the damping characteristics of a downhole shock-absorber, thereby changing the axial (and optionally torsional) stiffness of the BHA itself, in response to transient downhole dysfunctions and without the inherent delay of transmitting data from downhole to the driller at the surface.

At the heart of this new adaptive shock-absorber is a magneto-rheological damping fluid whose viscosity is modified whenever excessive downhole vibrations are detected by adjusting the electro-magnetic field through which the damping fluid passes. This is similar technology to that used for protecting buildings during earthquakes and in high performance vehicles that have intelligent shock-absorbers that let the driver select a preferred stiffness for the vehicle's suspension.

This paper describes this innovative downhole self-adapting vibration damper and its autonomous control system that detects drilling dysfunctions and then iteratively adjusts the device's damping characteristics until the destructive downhole forces and motions are mitigated – potentially before the driller at the surface is even aware that there was a problem. Modeling of the magneto-rheological fluid response is presented together with some initial downhole test results demonstrating the tool's ability to control the stiffness and vibration characteristics of a bottom-hole-assembly. Comparisons of downhole shock levels and the amplitude of various types of vibrations at different frequencies illustrate how the self-adapting damper can suppress different modes of BHA dysfunction and thus also demonstrating the device's ability to extend bit life and drilling tool life and to improve overall drilling performance.

Automatically controlling drilling dysfunctions from within the BHA at their source is timelier and potentially more effective than the current approach of telemetering downhole vibration information to the driller at the surface after the bit and drill string components have already been damaged over some incremental time period. The more instantaneous response of a self-adapting tool has the ability to prevent drilling tool failures, reduce bit wear, sustain higher penetration rates with a sharper bit and extend the interval drilled during a single bit run. For certain drilling applications this can significantly reduce the number of bit runs per hole section as well as the related drilling time and cost.

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