A multi-dimensional percussion method for efficient drilling in HDR formations: Rock fragmentation mechanism, drilling energy analysis, and performance optimization

Abstract

Percussion drilling is a promising approach for hot dry rock (HDR) fragmentation. However, understanding of HDR fragmentation mechanism under multi-dimensional percussion remains limited and hinders the corresponding drilling performance. Herein, an innovative true triaxial multi-dimensional percussion device was developed for the study of HDR fragmentation mechanism under in-situ temperature and stress conditions. Multi-dimensional percussion, involving both axial and torsional components, was applied to drilling in granite and carbonatite rocks sampled from the typical HDR target areas. Multi-scale visualization techniques and a whale optimization-variational mode decomposition algorithm were employed to investigate the rock failure patterns and drilling energy characteristics. Results indicated that multi-dimensional percussion enhances brittle-ductile mixed failure in granite, characterized by transgranular, intergranular, and combined fracture patterns that promote rock cracking. In contrast, carbonatite drillhole displays enhanced brittle fragmentation and tortuous failure surface dominated by transgranular fracture pattern. Frequency-domain characteristics of penetration force signals for multi-dimensional percussion, especially the significant dominant frequency, amplitude, and high-frequency dissipation, indicate an increase in net energy for drilling into HDR and intensified rock fragmentation. Further, the effect of impact frequency on rock fragmentation performance was emphasized to maximize drilling efficiency. The optimal regulation schemes between axial and torsional impact frequencies are identified as 15 Hz + 15 Hz for granite and 30 Hz + 15 Hz for carbonatite. The reliability of the optimization approach was validated through a field test that employed a novel impactor in the geothermal well Fushen-1.