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Abstract This paper presents a newly developed "intelligent" system designed to avoid resonance and to reduce vibrations. The system integrates real-time BHA dynamics software and real-time downhole vibration data to provide accurate modeling results and data analyses. Unlike conventional BHA dynamics software that is run for well planning or post-run analysis, this system uses real-time data (e.g., WOB, inclination, DLS) to produce real-time updates of critical rotary speeds. The updates are then displayed along with the rotary speed to show if the rotary speed is too close to one of the predicted critical rotary speeds. In addition, the modeling results can be compared with actual real-time downhole vibration data to corroborate the actual downhole condition. Field data have shown that the new system is effective in identifying the vibration mechanism and avoiding harmful vibrations.
Introduction Severe vibrations have been shown to be harmful to downhole equipment. Among them, lateral vibrations (particularly backward whirl) are commonly associated with drillstring fatigue failure (wash-outs, twist-offs), excessive bit wear, and MWD tool failure. Lateral vibrations are caused by one common reason - mass imbalance through a variety of sources: bit-formation interaction, mud motor, and drillstring mass imbalance, etc.
A rotating body is unbalanced when its center of gravity does not coincide with the axis of rotation. Due to the crookedness or mass imbalance, centrifugal forces are generated while rotating the unbalanced drillstring. The magnitude of the centrifugal force depends on its mass, the eccentricity and the rotary speed. In general, the higher the rotary speed, the larger the centrifugal force. Thus, the common practice is to lower the rotary speed when severe lateral vibration occurs. However, vibration will not be reduced if the lower rotary speed results in a resonant condition in the assembly. A resonant condition occurs when the frequency of any one of the excitation mechanisms matches the natural frequencies of the BHA (often called the critical rotary speeds). Under a resonant condition, the BHA has a tendency to vibrate laterally with continuously increasing amplitudes, resulting in severe vibration and causing drillstring and MWD failures.
Thus, it is important to identify and avoid critical rotary speeds during drilling operation. A number of finite element based computer programs have been developed to predict critical rotary speeds. However, the accuracy of their predictions is often limited due to the uncertainties in the input data and boundary conditions. Conventional BHA dynamics software is usually run during well planning or sometimes at the rig when the BHA is made up. And a set of predicted critical speeds, (CRPM), is provided to the driller to be avoided. Common operational difficulties with this approach are:complex BHA modeling and results;
inaccurate results due to incorrect input data;
modeling results not being used in conjunction with the real-time vibration data to optimize the drilling process.
To provide accurate modeling results on a "timely" basis that are "easy" to understand, an integrated drilling dynamics system has been developed. The system combines "real-time" modeling with downhole MWD vibration data. While running the "real-time" mode, real-time data (e.g., WOB, inclination, DLS, etc.) are used to produce real-time updates of critical rotary speeds. The updates are then displayed along with the rotary speed to show if the rotary speed is too close to one of the predicted critical rotary speeds. The modeling results are confirmed by actual real-time downhole vibration data for accurate vibration diagnosis. To integrate the real-time modeling and measurements, an integrated dynamics system has been developed for data acquisition, display, diagnosis, and optimization.
The Integrated Dynamics System This integrated dynamics system consists ofa real-time BHA dynamics software,
a MWD downhole vibration sensor, and
an integrated rigsite information system.
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