Use of Downhole Mud-Driven Hammer for Geothermal Applications

Abstract

Maintaining a good rate of penetration (ROP) in hard rock formation is a key challenge posed by deep geothermal wells to the drilling industry. This challenge is crucial, as well costs depend on the ROP and on the ability to keep the bit on bottom for an extended period of time. Hammer drilling is a drilling method that uses the energy of a moving hammer (hammering action, percussion drilling) to increase the weight on bit. This technique can increase the ROP in hard rock formations. Several down-hole hammer tools are commercially available for drilling deep wells. However, most of the tools reported in the literature are designed for air drilling, since a clean fluid is required for hammer driver. Hammer drilling has shown an improvement of the drilling rate by a factor of 2 to 3, depending on the formation. This paper describes the basic principle of a hammer drilling tool developed at the Institute of Petroleum Engineering, TU Clausthal, and focuses on the lessons learned through laboratory investigations. The tool was extensively tested and the results are promising. Field tests have shown the efficiency of the hammer, but also the intense vibrations induced during drilling through hard formations. A large number of computer simulations of the hammer’s performance were run, resulting in more than four PhD and MS theses. As a result, it was found that the vibrations are not produced by the hammer itself, but they result from the energy reflection during rock-bit interaction. The last testing campaign focused on high frequency sampling of acceleration and piston movement. The results led to a better characterization of the source of vibrations as well as of the hammering process itself.