A phenomenological model of percussive drilling: Review of experimental evidence

In this paper, we review and analyze published experimental findings in light of the hypothesis that the dependence of bit-rock interface (BRI) pseudo-stiffness on the weight-on-bit (WOB) is the key factor behind the existence of a sweet spot in percussive drilling, i.e., there is an optimal WOB for which the rate of penetration (ROP) is maximized. It has been conjectured previously that the presence of a sweet spot could be caused either by increased wear of the bit, or reduced indexing angles, or poor cleaning of the debris. Although perhaps applicable in particular cases, these explanations are debatable, however, as a body of contradictory experimental evidence exists. Instead, motivated by experimental results, we propose that the ROP in percussive drilling is directly related to the effective impact energy delivered to the rock at each impact and that the BRI pseudo-stiffness increases with WOB. Moreover, further experimental insights reveal that given a particular WOB, an optimal BRI pseudo-stiffness could be induced to maximize the energy transmission to the rock during the first impulsive wave. This suggests that the root cause of the existence of a sweet spot in the ROP-WOB relationship indeed lies in the nature of the BRI laws.