A bit-rock interaction model for rotary-percussive drilling

Rotary-percussive drilling is considered a promising approach to improve drilling performance in hard rock formations. Different from percussive drilling, it is a hybrid form of drilling, since the weight-on-bit and the angular velocity are still acting as in conventional rotary drilling. This paper deals with a bit-rock interaction model for bits under rotary and percussive actions and a methodology to evaluate rotary-percussive drilling performance. First, experimental drilling data from laboratory tests conducted with an in-house designed drilling rig are investigated. Next, a phenomenological model for the drilling action is proposed, assuming that the interface laws are rate-independent and that the state of variables are averaged over at least one revolution of the bit. Within the framework of the proposed model, quantitative information from drilling data related to rock properties, bit conditions and drilling efficiency can be extracted. Furthermore, the effectiveness of the percussive action is captured by a single number, lambda = epsilon/psi, which corresponds to the intrinsic specific energy ratio. The similarity between the experimental results and the theoretical predictions are encouraging in regard to the use of this model to investigate rotary-percussive drilling response. The results have also shown that rotary-percussive drilling technique has potential application as an alternative method for drilling highly deviated wells or horizontal wells, where the limitations of weight-on-bit can be compensated by percussive action. (C) 2011 Elsevier Ltd. All rights reserved.