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There exists a need in today's industry for a durable, yet versatile, underreamer which will withstand the tortures of hard rock drilling. This paper describes the design, field testing, and subsequent modifications of one such prototype underreamer. The underreamer was utilized, in conjunction with a bi-center polycrystalline diamond bit, to simultaneously drill and underream a fast moving salt formation. Due to the limited success of previously run underreamers, a new tool was designed and built to perform under the severe downhole conditions developed in a drilling/underreaming operation. Comparisons are made between the prototype underreamer and a similar tool based on body/arm strength and the cutter usage, placement, and durability. Modifications and improvements are documented in a summary of the field tests performed. Recommendations are given to further enhance the performance of the prototype underreamer and expand its applications.

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