Numerical Analysis on Characteristics of Cracks and ROP Enhancement Performance in Hard Rocks Subjected to Axial-Torsional Coupled Percussion

Abstract

ABSTRACT Axial-torsional coupled percussive drilling (ATCPD) is a practical technique for creating volumetric breaking patterns and increasing rate of penetration (ROP) of hard rocks. Our paper focus on numerical investigations on ATCPD process. We employed a continuous-discrete coupling method based on cohesive elements to characterize the rock failure subjected to different percussion (axial, torsional and coupled percussion). Based on this model, the penetration-cutting process of the single cutter at different percussion modes was simulated, respectively. The corresponding cracks characteristics, penetration/cutting force and torque were analyzed. Rock scratch experiments were further conducted on granites to validate the model. The results show that the complex cracks propagation zone and the larger tensile cracks around the cutter are developed subjected to coupled percussion, resulting in larger failure elements number and cracks volume. That is, more significant macro damage is thought to form in coupled percussion, thereby increasing the penetration and cutting force of single cutter. It is worth noting that the coupled percussion mode can also generate stable fluctuation torque, which improves the ROP enhancement performance. Our results are expected to provide some guidance for efficient drilling in hard rocks. INTRODUCTION Increasing oil, gas, and heat reserves in deep formations will be critical in addressing the worldwide energy supply (Santos et al., 2000; Moore and Simmons, 2013; Hu et al., 2013). The construction of wells to exploit deeper energy in more challenging environments requires large investments, which are mainly related to the involved drilling operations (Abdo and Haneef, 2013; Diaz et al., 2017). For example, the reliance on drilling costs is especially high in the development of enhanced geothermal system (EGS), primarily because this method of obtaining geothermal energy necessitates the challenge of drilling deep holes in hard crystalline basement rocks, such as granites (Song et al., 2018; Anderson and Rezaie, 2019). These limiting factors pose a major challenge to our traditional drilling technologies (e.g. rotary drilling), whose poor performance in hard rocks, in terms of wearing of the drill bits and low penetration rates (Diaz et al., 2017; Rossi et al., 2020). As a result, the development of drilling techniques to increase the ROP enhancement effect in deep reservoir hard rocks has attracted more and more interest in the engineering field.