Chip-Hold-Down Effect on Rate-of-Penetration During Deep Percussive Drilling: A Theoretical Approach

Abstract

A key issue in deep hard rock drilling is the decrease of rate-of-penetration (ROP) with depth. For rotary drilling, extensively used in the oil and gas sector, well-recognized causes for ROP decrease are rock strengthening and chip-hold-down effects. For percussive drilling, mainly used in the mining sector, the field data and theoretical analyses for deep well drilling are more limited. By analogy, the combined effect of rock strengthening and chip-hold-down on ROP is investigated here. Firstly, a fluid dynamic study of the removal of one rock chip is treated as a 2D fluid-structure interaction problem with an impermeable rock and incompressible drilling fluid. In particular, the mechanisms are the fluid infiltration and the stiction under the chip. Secondly, a stochastic ROP evaluation is developed using (i) a chip shape and size distribution and (ii) a condition for chip regrinding based on the characteristic times from the chip removal study. The ROP predictions are in good agreement with field data from the literature and the influence of chip geometry and fluid parameters is evaluated. These results provide input on the importance of tool, drilling and fluid parameters on the ROP, and can support the innovation and technology developments in percussive drilling.