Experimental study on the rock-breaking mechanism of annular-grooved structure and rotary direction for impregnated diamond bits

Abstract

Impregnated diamond (ID) bits have been widely used for drilling superior hardness, highly abrasive or non-homogeneous rocks. A reverse rotary dual ID bit provides better drilling performance than a conventional unidirectional rotary ID bit. However, the mechanism of rock-breaking by the annular-grooved structure and reverse rotary characteristics of ID bit is still unclear. In this study, a conventional ID bit, an annular-grooved ID bit, and a dual ID bit were prepared. Drilling experiments were carried out in hard, medium-hard, and soft rocks. The size distribution and surface topography of cuttings, the microscopic morphology of the rock ridges, the rate of penetration (ROP) and mechanical specific energy of three ID bits were comparatively analyzed. The results show that the annular-grooved structure can form a rock ridge at the borehole bottom, which will be broken volumetrically. Scanning electron microscope analysis shows that the dual ID bit has an effect of micro-shear breaking in addition to grinding and micro-volume breaking of the rock, which exacerbates crack propagation. Furthermore, the shear stresses on each side of rock ridge are superimposed under reverse rotation, which accelerates the breaking of rock ridge. As the rock hardness decreases, the ROP gain of dual ID bit is more obvious. ROP of the dual ID bit for drilling granite, limestone and sandstone is 2.1, 2.7 and 5.2 times higher than those of conventional ID bit. The key findings of this work will help to real the mechanism by which the annular-grooved structure and reverse rotary characteristics of the dual ID bit are responsible for improving rock-breaking efficiency.