Experimental investigation on the PDC cutter penetration efficiency in high-temperature granite

Hot dry rock (HDR) is a renewable and clean energy resource, its efficient exploration has attracted widespread attention from scholars. However, the rock encountered during drilling is characterized by poor drillability, high abrasiveness and great hardness, which aggravates the wear of bits and increases drilling costs. To address such an issue, much research has been conducted to investigate the failure mechanism of high-temperature rocks. But most of them focused on the rock cutting process by PDC cutters and the penetration of rocks was ignored. In the present study, a series of experiments were performed to determine the penetration efficiency of PDC cutters under different temperatures and back rake angles. The penetration force and 3D topography of penetration pits were measured to analyze rock failure characteristics. Besides, uniaxial compressive experiments were carried out to study the effects of temperature on rock properties. The results indicate that as the rock is heated from 25 to 240 degrees C, its properties change slightly. Analogous rock failure characteristics can be found in rock indentation process, where chipping and crushing of rocks, on following another, are presented, respectively. Moreover, the occurrence of chipping is the major contributor to the increase in penetration efficiency. The further increase in rock temperature (greater than 360 degrees C) results in a sharp decrease in the strength of rocks, which is accompanied by the enhancement of rock plasticity. At his moment, the plastic failure dominates rock indentation process under different back rake angles. The induced horizontal force cannot promote brittle failure and the penetration efficiency is hard to be improved, significantly affecting the performance of conventional PDC bits in high temperature formations. Based on the results, the low penetration efficiency at high temperatures is a major obstacle that restricts the exploration of HDR. Increasing the cutting depth should be an important target in optimizing drilling parameters and the arrangement of PDC bits.