Theoretical study on rock-breaking performances of PDC cutter by fractal characteristics of cutting size

Abstract

Given the technical problems in the process of deep oil exploration and exploitation, different shapes of special-shaped PDC cutters have emerged. The conical PDC cutter has been widely used in the design of hybrid-cutters PDC bit, leading to remarkable improvements both in the drilling speed and in the bit footage. However, the fundamental knowledge of the rock-breaking mechanisms of conical PDC cutters is still short of research in the deep hard formations. We hope to explore the rock-breaking characteristics of conical PDC cutters from different perspectives with the fractal theory widely applied in rock mechanics. Based on the fractal characteristics including the fractal dimension and the maximum cutting size of rock cuttings generated by single cutter cutting experiments, a theoretical model of mechanical specific energy is established for the PDC cutter in this study, and the validity can be verified by available experimental data. Meanwhile, comparing the rock-breaking performances of the conical PDC cutter with the conventional PDC cutter, the effects of the cutting parameters on the fractal dimension of cutting size, maximum cutting size and fractal characteristics on the transverse cutting force, friction work, mechanical specific energy are analyzed. Analysis results show that the predicted values of the model are highly fitting to the experimental values, and the prediction accuracy of the model is relatively high. The fractal dimension and the maximum cutting size of conical PDC cutters are larger than those of conventional PDC cutters. During the process of rock-breaking with conical PDC cutters, the lower the degree of rock-breaking, the larger the maximum cutting size, and the lower the mechanical specific energy of rock-breaking with conical PDC cutters. These also verify the stronger brittle failure ability of conical PDC cutters and the significant effect of conical PDC cutters’ plowing effect. Those findings can provide a theoretical understanding of the rock-breaking characteristics of conical PDC cutters and on for the cutter-layout method of the hybrid-cutters PDC bit in deep hard rocks.