Improvement of drilling performances in deep geothermal drilling in hard rocks, by using a novel Hydro-Mechanical Drilling
Résumé
A new drilling technique developed in the framework of the European H2020 ORCHYD project combines two mature technologies, high pressure water jetting (HPWJ) and hydraulic hammer, into a fully fluid driven system adapted to deep geothermal drilling in hard rock. This hybrid development requires the optimization of several processes including the release of high stress concentrations at the bottom of the hole during deep drilling, by playing on the geometric configuration of the bottom-hole. The preliminary results of this project, presented in this paper, concerns the study of the best geometrical configurations: bottom-hole profile (bit profile) and peripheral groove to release stresses in the immediate vicinity of the drilling bit action, in the context of hard rocks such as granites. It has been shown that the combination of a concave bit profile with a peripheral groove of a desired depth can significantly release stress concentrations in the rock undergoing the drilling action, thereby reducing its resistance to the drilling action and even generating tensile stresses highly beneficial to the drilling process. Initial laboratory tests with a flat hammer bit combined with a 208 MPa peripheral HPWJ, under moderate hydrostatic pressure, have shown promising performance in 150 MPa UCS granite. It is expected that under higher stress and pressure regimes, synonymous with deeper drilling, and for a slightly concave profile hammer, the creation of the peripheral groove of a desired depth should further reduce stresses in the immediate vicinity of the bit action, thereby reducing the resistance of the rock to the drilling action and improving the drilling performances.