## 2 Sensitivity Analysis of Structural Characteristics of Drill Bit
The Spearman rank correlation coefficient represents the sensitivity of the ROP and footage to each bit structural feature. The higher the rank correlation coefficient is, the greater the sensitivity of the ROP or footage to the bit structural feature, that is, the greater the influence of the structural feature on the objective function. The second spud in the Luzhou Block is about 80 m-1100 m long and stretches from the Shaximiao Formation to the Ziliujing Formation. The strata found during drilling are primarily mudstone mixed with sandstone, with good drillability. As illustrated in Figure 1, the blade number, crown top rotation radius, internal cone angle, cutting angle of cutters, and nozzle number have a significant influence on the ROP. The blade number, crown top blade thickness, equivalent diameter of the nozzle, and internal cone angle have a significant impact on the footage. The first stage of the third spud in Luzhou Block is around 1100-1600 m deep and belongs to the Xuijahe Formation. The drilling lithologies of Xuijahe Formation are mainly sandstone with shale and coal, with high quartz content, small grain size, and dense cement, strong formation heterogeneity, strong abrasiveness, and poor drillability. As illustrated \begin{table}
\begin{tabular}{c c c} \hline \hline
**The Stage** & **The Rock Strata** & **The Well Depth** \\ \hline \multirow{2}{*}{Second spud-1} & Shaximiao Formation–Ziliujing Formation & \multirow{2}{*}{80 m–900 \(\pm\) 50 m} \\  & (Lianggaoshan Formation) & \\ Second spud-2 & Ziliujing Formation–Xujahe Formation & 900 \(\pm\) 50 m–1100 \(\pm\) 50 m \\ Third spud-1 & Xuijahe Formation & 1100 \(\pm\) 50 m–1600 \(\pm\) 50 m \\ Third spud-2 & Jialinging Formation–Longtan Formation & 1600 \(\pm\) 50 m–2600 \(\pm\) 50 m \\ Third spud-3 & Moakou Formation–Qixia Formation & 2600 \(\pm\) 50 m–2950 \(\pm\) 50 m \\ Fourth spud-1 & Qixia Formation to Shiniulan Formation & 2950 \(\pm\) 50 m–3500 \(\pm\) 50 m \\ Fourth spud-2 & Shiniulan Formation–Longmaxi Formation & 3500 \(\pm\) 50 m–5900 \(\pm\) 50 m \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The specific stratification results for Luzhou Block in Sichuan Province. Figure 1: Rank correlation coefficient of structural characteristics from Shaximiao Formation to Ziliujing Formation in the second spud of Luzhou Block. in Figure 2, the cutting angle of cutters, crown top rotation radius, blade number, internal cone angle, and diameter of cutters have a significant influence on the ROP. The cutting angle of cutters, diameter of cutters, crown top rotation radius, equivalent diameter of the nozzle, internal cone angle, gauge diameter length, and gauge length have a significant impact on the footage. The first stage of the fourth spud in the Luzhou Block is around 2950-3500 m, with the Qixia Formation and the Shiniulan Formation. The lithology of the strata encountered while drilling is primarily abrasive limestone, shale, and siltstone. As illustrated in Figure 3, the blade number, cutting angle of cutters, diameter of cutters, crown top rotation radius, and internal cone angle have a significant influence on the ROP. The blade number, diameter of cutters, cutting angle of cutters, crown top blade thickness, gauge length, crown top rotation radius, and gauge diameter length have a significant impact on the footage. Figure 3: Rank correlation coefficient of structural characteristics from Qixia Formation to Shiniulan Formation in the fourth spud of Luzhou Block. Figure 2: Rank correlation coefficient of structural characteristics from Xujiahe Formation in the third spud of Luzhou Block. To summarize, the number of blades, cutting angle of cutters, crown top rotation radius, internal cone angle, and diameter of cutters are the main structural characteristics that affect the bit's ROP and footage. The cutter-arrangement method and the design of crown rotation radius and internal cone angle are the main optimization objectives to improve the bit's performance.