## 2 Failure Modes of DTH Hammer Bit for Geothermal Drilling  
Shanxi Heshun (113.576\({}^{\circ}\) E, 37.334\({}^{\circ}\) N) hot spring well is located in the north of Malianqu village, Songyan Town, Heshun County, Jinzhong City, China, with a design well depth of 2500 m. The vertical distribution of strata is in the following order: the Changcheng System Chuanlinggou Formation, Changcheng System Changzhougou Formation, Proterozoic Hutuo Group, and Taiancient Zone Shijalan Formation. The formation lithology of the Changcheng System Chuanlinggou Formation is quartz-like sandstone, quartz sandstone, purplish red shale, and sandy shale. The bottom is coarse giant quartz sandstone with gravel and quartzite. The natural compressive strength of quartz sandstone is usually high, and the quartz content in the drilled cuttings is higher. Drilling into the stratum containing quartz sandstone and quartzite causes the edge buttons of the large-diameter DTH hammer bit to break, resulting in less footage and short bit life. Four DTH hammer bits were used within a 500 m well depth, i.e., the 1# bit for 20-250 m, the 2# bit for 250-296 m, and the 3# bit for 433-500 m. For observation and analysis, the failure modes of the 1#, 2#, and 3# bits are shown in Figure 3, and the relevant technique parameters are described in Table 1.  
From Figure 3, the bit buttons of the bits were all broken with different types of inner ring buttons wear. Broken buttons have a more significant impact on the normal work of the air DTH hammer; if there are too many broken buttons, the mechanical drilling rate decreases, and downhole vibration intensifies, which aggravates the wear of the other buttons of the bit and causes drill rigid-body wear. There is relatively serious wear of the inner ring buttons in the 1# bit, while only sporadic fracture is seen in the inner ring buttons of the 2# and 3# bits. Accordingly, the 1# bit has a relatively long lifespan with footage of 230 m, and the 2# and 3# bits have 46 m and 67 m, respectively. The reason for the long lifetime of the 1# bit can be explained by the fact that parts of the drilled formation are relatively weak (being of medium strength), which can make the inner ring buttons wear normally. The less cumulative footage of the 2# and 3# bits are mainly caused by the high strength of the drilled formation. The lifespan improvement of the 3# bit to the 2# bit is the result of the rotation speed from 60 to 30 rpm. There are many reasons for side tooth breakage, such as formation breakage, worker improper operation, and high formation strength. Drilling fracture development, broken \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mechanical Properties** & **1\#** & **2\#** & **3\#** \\ \hline Diameter, mm & 320 & 320 & 311 \\ Compressive strength of rock & medium to high & high & high \\ Rotation speed, rpm & 60 & 60 & 30 \\ Drilled interval, m & 20–250 & 250–296 & 433–500 \\ Footage, m & 230 & 46 & 67 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The drilling technique parameters of the DTH hammer bits of the hot spring well. Figure 3: The failure modes of the DTH hammer bits of the hot spring well drilling. The 1# bit failure is attributed to breakage of the edge buttons along with wear of the inner ring buttons, while the 2# and 3# bits are caused by breakage of the edge buttons without inner buttons wear. belts, and uneven texture layers cause the drill bottom hole to become stable and can easily cause the local column buttons instantaneous shear force to become too large and the force to become imbalanced, resulting in button collapse. Drilling in cracks and uneven soft and hard rock causes too much weight to be placed on the bit, and speed will make the torque and shear force on carbide buttons increase intermittently, resulting in broken buttons and buttons collapse.