Dynamic fragmentation analysis of multi-dimensional percussion drilling into hot dry rock

Developing an efficient drilling approach for deep hot dry rock (HDR) formations is essential to reducing the commercial costs of enhanced geothermal system (EGS). A multi-dimensional percussion drilling (MDPD) method is proposed to improve HDR fragmentation efficiency. However, current laboratory drilling systems are insufficient in simulating in situ temperature and stress conditions, and limited visualization studies hinder an understanding of the HDR fragmentation mechanisms under the MDPD mode. To address these limitations, a true triaxial multi-dimensional percussion visualization drilling apparatus was developed. A series of drilling tests were performed on HDR samples under various modes, including rotary drilling (RD), single-dimensional percussion, and MDPD, considering in situ temperature and stress conditions. High-speed imaging, mechanical signal analysis, and mesoscopic-microscopic observations were employed to illuminate the rock fragmentation mechanism under the MDPD mode. Drilling rate and mechanical specific energy (MSE) were used as key indicators to evaluate MDPD drilling performance at depths of 1600-4000 m in the Gonghe Basin, China. Results indicated that the MDPD method promotes the joint development of large rock fragments, plastic flow zones, and ductile cuttings clusters, featuring a mixed brittle-ductile failure pattern. A complex crack system dominated by a combined tensile-shear stress mode is captured under the MDPD mode, leading to the formation of hill-shaped fragments and promoting volumetric rock fragmentation. The MDPD method shows the greatest influence on improving rock drilling rate and fragmentation efficiency, followed by the single-dimensional percussion and RD modes. Specifically, the average drilling rate in the MDPD mode is 1.09 times greater than that of the RD mode, while MSE is reduced by 42.31%. Among all the evaluated modes, the MDPD method demonstrates optimal drilling performance at depths of 1600-4000 m, highlighting its promising potential for efficient HDR drilling applications.