This paper presents a review of the benefits and performance of downhole percussion hammers. The advancement of drilling technology and the extensive use of coiled tubing coupled with different downhole motors prompted another look at the percussion type of drills. Achieving very high extended reach wells also have necessitated a high axial driving mechanism. The potential benefits of the percussion type of drilling machine for future operations are given, supported by the performance of the tools. The use envelope and the benchmarks of the tools by different manufacturers down the years have been discussed. The hammers developed and operated have been supported by enough field runs to prove the sustainment of the tools. Different models of downhole hammers can be categorized as first generation tools which include the developments from early time to the development of Bassinger tools. The second generation tools from post-Bassinger to the recent developments are discussed broadly. The basic principle involved and the necessity to consider the correct stress waveform to simulate the performance of the downhole hammer energy transfer are reviewed.

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