A brand new impact hammer–Axial & Torsional impact hammer (producing high-frequency axial and torsional impact forces simultaneously)–has been developed to improve the penetration rates and eliminate stick-slip vibrations in deep and ultra-deep wells. This paper presents its rock-breaking mechanism and evaluates its field application effects.

Based on the finite element method, a numerical model was first proposed to investigate the rock-breaking processes of a single PDC tooth under axial, torsional and axial & torsional impact forces respectively. The simulation results show that the axial & torsional impact drilling method can break rock with least mechanical specific energy and increase the rate of penetration (ROP) significantly. The mechanism is that the axial component of the compound impact force can increase the penetration depth of PDC teeth into rock and enlarge the rock-breaking volume, and that the torsional component facilitates to reduce stick-slip vibrations and prohibit premature failures of bits.

An Axial & Torsional impact hammer was then developed according to the principle of modulating hydraulic pulses. The hammer has been used in 20 deep and ultra-deep wells in western China, achieving 105.1%~163.4% higher ROP compared with the average ROP in the same block. Besides, the drilling process became more stable and the lives of PDC bits were greatly increased. This paper presents a comprehensive comparison of the penetration rate, torque fluctuation, bit wearing degrees and cuttings shape between this impact tool and other tools used in offset wells.

Finally, the structure of the new hammer is simple and reliable, and it is also convenient to maintain.

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