Experimental and Numerical Investigation on Rock Breakage Mechanisms of a Conical Diamond Element

ABSTRACT The conical diamond element (CDE) is an innovative conical-shaped 3D cutter, which exhibits high rock-breaking efficiency in hard formation. But there are few researches on the rock breakage mechanism of CDE cutters. In this paper, a series of single cutter tests were conducted on granite. The rock breakage process, the initiation and propagation of micro-cracks, the surface topography and fracture morphology of the cutting groove and large-size debris were analyzed. A three-dimensional rock cutting model with a CDE cutter was established using the finite element method to study the stress distribution and rock damage evolution during the cutting process. The results show that the cutting process can be divided into two parts: crushing and chipping. There is a spherical stress concentration area formed at the tip of the CDE cutter. The rock breakage mechanism of the CDE cutter can be summarized as follows: the rock at the CDE cutter tip occurs shear-compression failure; the rock in front of the CDE is broken under the tensile action; tensile micro-cracks propagating to the rock inside can deteriorate rock strength. The key findings of this work will help to reveal the rock breakage mechanisms and provide guidelines for CDE bits design. INTRODUCTION Polycrystalline Diamond Compact (PDC) drill bits have been widely used for drilling wells in the exploitation of hydrocarbon and geothermal resources (Bellin et al. 2010) due to the high rock-breaking efficiency and long duration life. However, for deep and ultra-deep drilling, the conventional PDC cutter is facing the challenges of serious impact and wear damage (Brett et al. 2012). In order to improve the drilling performance of PDC drill bits in hard formation, Durrand et al. (2010) invented Conical Diamond Element (CDE), which has nearly twice as thick as polycrystalline diamond layer than the conventional PDC cutter, so the wear resistance and impact resistance is improved approximately 25% and 100% respectively (Azar et al. 2013). Since CDE was invented, the hybrid PDC drill bit, on which the CDE is strategically positioned at the bit center or behind the primary conventional PDC cutter, has drilled more than 26 million feet in hard and interbed formation. Based on the records, the hybrid PDC bits increased footage up to 77% with corresponding Rate of Penetration (ROP) increases up to 29% in the Permian Basin (Radhakrishnan et al. 2016).