Modelling of critical fragment size and rock breaking efficiency evaluation in drilling process

Understanding the rock fracture behaviors during drilling is crucial to evaluating the breaking efficiency. Since fragment generation is intrinsically related to drilling mechanism, the fragment size distribution is determined by the drilling response. In this paper, an analytical model is established to describe the fragment generation, from which a function of the cutting force incorporating critical fragment size, rock strength and drilling parameters is derived. A collapse index is defined to evaluate drilling efficiency. Digital drilling tests, coupled with laboratory tests, were conducted on four rock types to investigate the effect of confining pressures and lithologies on the fragment characteristics and drilling response. Results shown that the effect of confining pressure on rock fragmentation behavior during drilling becomes more significant, manifested as a segmented drilling response. The distribution characteristics of rock fragment was examined by the Rosin-Rammler (R-R) function to determine the critical size in drilling process. The fracture model prediction results are in good agreement with the experimental results. The proposed drilling efficiency index decreases with increasing confining pressure, due to the enhancing effect of confining pressure on rock toughness. A comparison among drilling efficiency, production capacity and specific cutting energy confirmed its practical applicability. The innovation of this study is to provide a method to evaluate rock breaking efficiency and to determine fragment size by considering drilling response.