# Influence of Drilling Along Pilot Holes on Drilling Performance and Effective Rock Strength +
## 4 Analyzing the Impact of Pilot Hole Diameter on Drilling Efficiency
A thorough examination of the ROP and torque data from drilling into multidiameter pre-cored holes at various applied Weights on Bit (WOB) and constant rotating speed (RPM) is seen in Figures 9 and 10. Figure 9 presents the ROP achieved for four configurations: drilling with a full-face bit in pre-cored holes (HO) of 1.5, 1.0, and 0.5 inches in diameter, as well as drilling without a pre-cored hole (full-face). The results indicate a consistent trend of increasing ROP with increasing Weight on Bit (WOB) across all configurations. Notably, in both rock types, the ROP is higher for larger pre-cored holes (HO) (1.5 and 1.0 inches) than for smaller holes (0.5 inches) and the full-face configuration. This suggests that pre-coring can significantly enhance drilling efficiency. However, the Rate of Penetration (ROP) increase is not always consistent or proportional to the increase in hole diameter, suggesting that other factors might also play a role in multi-diameter hole drilling performance. The relationship between torque and the Rate of Penetration (ROP) observed during the drilling process is highlighted in the text excerpt. It states that in Figure 10, the torque data are proportionate to the ROP data. In particular, all pre-cored multi-diameter holes exhibit an increasing trend in torque and ROP as the Weight on Bit (WOB) increases. This  
Figure 9: Illustrates the ROP results (in N-m) for HO-1.5, HO-1.0, HO-0.5, and FF configurations for gabbro rock. finding implies a strong correlation between these drilling parameters by showing that variations in WOB consistently affect ROP and torque in gabbro rock. The idea is to replicate various downhole conditions and see how they affect torque and ROP. Therefore, the formation's diameter was changed while maintaining other parameters. This technique allows for investigating how drilling reactions alter when transitioning between different diameters by simulating various conditions in actual drilling operations. Those situations could involve drilling through formations already weakened by concurrent drilling or coring operations, which could cause micro-cracks and undermine the formation, or drilling through distinct lithology with differing strength properties. Additionally, this study aimed to assess the impact of worn teeth on the drill bit, which could lead to an increase in the bit-rock interaction (BRI) area and a decrease in the ROP. Surprisingly, this study found that torque did not increase as predicted by the BRI area, while ROP decreased as the bit-rock interaction (BRI) area increased. Further research is necessary to comprehend the relationship between torque, ROP, and BRI in light of this discrepancy. The Mechanical Specific Energy (MSE) calculation has become an essential technique for assessing drilling performance. It provides valuable information about bit performance enhancement and drilling efficiency. MSE values are widely used for pore pressure prediction, problem diagnosis, and drilling optimization in the drilling industry. Equation (2)'s real-time drilling parameters, used to calculate MSE, were used in this investigation. Figures 11 and 12 illustrate the Mechanical Specific Energy (MSE) results for various gabbro rock full-face (FF) and pilot hole diameters. The MSE values tend to rise for gabbro rocks as the diameter of the pilot hole decreases, indicating more difficult drilling conditions as smaller pilot holes require more energy to drill. Full-face (FF) has the highest MSE values, indicating that drilling without pilot holes uses the most power and emphasizes how challenging it is to penetrate the rock. Larger pilot holes (HO-1.5 and HO-1.0, for example) in gabbro rock produce lower MSE values, which denote more effective drilling. This pattern is more noticeable because the MSE decreases more noticeably as the diameter of the pilot hole grows. The diameter of the pilot hole and MSE values correlate negatively; larger pilot holes indicate lower MSE, which suggests higher drilling efficiency.