The Performance and Failure Mechanism of Drill Bit in Granite Formation Drilling

As a new energy resource with great development potential, the exploitation of hot dry rock geothermal resources is increasing around the world. However, the drilled formations, such as granite and other rocks with high hardness and wear resistance, seriously restrict the drilling efficiency. Polycrystalline diamond compact (PDC) bit, cone bit and PDC-cone composite bit are mainly used for geothermal drilling, but there is a lack of research on the drilling conditions of these bits to compare their advantages and disadvantages horizontally. This paper conducted rock-breaking experiment of granite under normal temperature and pressure using full-sized bit and analyzed weight on bit, torque and axial, radical and circumferential acceleration of three types of bits. This paper analyzed the cutting marks at the bottom of the rock and the wear of the bits, which can reflect the possible failure mode of bits in the process of drilling. Moreover, the monitored data from sandstone to granite in two wells numbered XX34-3-1 and XXB-1-1 and the wear and failure degree of three types of bits are discussed. Indoor experiments and field applications have proved that PDC bit has the highest rate of penetration (ROP) with the largest wear degree in granite formation. The wear of tri-cone bit is smallest but its drilling efficiency is low. The ROP and wear of PDC-cone composite bit are between the two kinds of bits mentioned above. This paper can be used to analyze the adaptability of drill bits and recommend drill bit types when drilling in granite formation.