Underreamer Dynamics

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Abstract
In a hole-opening application, cutting forces are applied to the drillstring simultaneously at the bit and the underreamer. Cutting forces at the reamer are known widely in the industry to cause additional vibrations of the drillstring that could harm the bottomhole assembly (BHA). This causes unnecessary downtime at the rigsite and increases costs. Expandable underreamers have very limited design envelopes for the optimization of their cutter layout. Most have only three blades and high-profile angles. Common polycrystalline diamond compact (PDC) bits have five or more blades and generate most of the cutting forces at their inner cutters. Reamers, due to their resulting forces, have significantly different characteristics compared to bits. By using appropriate bit-dynamics simulation software it is possible to explain the reasons for these different bit and reamer characteristics. An in-depth study of the cutting forces enables a better understanding of the mechanics of vibration excitation, especially for underreamers. Dynamic cutting force models for bits and reamers have been developed based on the results of cutting force analyses. These models contain weight, torque, and residual side force. They were implemented into a highly sophisticated drillstring model using finite-element analysis. Dynamics simulations in time domain of complete drillstrings resulting from the cutting forces at the bit and reamer are shown. Interactions of the pilot bit and reamer coupled by the BHA are also investigated using this model. A field test was conducted using a hydraulically expandable underreamer. During this test, simultaneous dynamics measurements where taken at the bit and reamer in vertical and directional well sections. The measurements validate the simulation results.

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