Drillstring Component Mass Imbalance: A Major Source of Downhole Vibrations

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Abstract
Summary: A study consisting of surface and downhole field experiments, theoretical analysis, and numerical modeling has shown that mass imbalance of drillstring components is a major source of downhole lateral vibrations. Factors that contribute to imbalance include bore misalignment, initial curvature, and gradual wear during service. Field experiments were conducted on the surface to quantify drill-string component mass imbalance for modeling purposes. Tests focused on drill collars. Lateral displacements of collars were measured while each was rotated in the derrick, and results were interpreted with simple models. All collars were unbalanced to some extent. A similar procedure can be used in the field to identify nearly balanced collars for use near the bit. Field experiments conducted downhole used a bull nose in place of a bit to evaluate drill-string vibrations without the bit as a source. A pendulum assembly with stabilizers 65 and 95 ft from the bull nose was used. Simultaneous surface and downhole measurements of accelerations were made. Lateral shocks caused by collar/wellbore collisions were measured at various locations in the drillstring at rotary speeds that caused the collars to whirl. Acceleration magnitudes were heavily influenced by local formation strength. Backward whirl of the drill-string was observed while rotating both on and off bottom. It was identified through downhole measurements, and was associated with sudden, dramatic increases in surface torque.

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