A new percussion drilling system, combining a mud hammer with specific hammer drill bits, has been developed to increase drilling performances in hard formations. This system is based on a high-power down-the-hole hammer, capable of delivering high-energy impacts at high frequency. Weighted mud pressure is used as a power source, making it fully compatible with current drilling rigs and existing drilling procedures.

The hammer bits used are specifically designed depending on the formation properties and overburden pressure, using on a semi-empirical model developed from a large series of unitary and full-scale impact tests.

This system was tested extensively, using a full-scale drilling bench capable of accurately simulating deep downhole drilling conditions. Results showed significant performance gains over rotary techniques: ROP improvements from +100% to over +300% were obtained in different hard lithologies. The mud hammer and optimized 6″ bit were then used during a field trial to drill through a hard gneiss formation, increasing the ROP over roller cone bits fourfold, from 1.1 to 4.5 m/hr.

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