# Influence of Drilling Along Pilot Holes on Drilling Performance and Effective Rock Strength +
### Experiment Stage 2: Varied Hole Geometry Preparation  
In the second stage of this study, a thorough protocol was developed to track the drilling parameters' reactions amid diverse bit-rock interactions in a laboratory setting, as shown in Figure 3, which displays gabbro samples of various sizes and shapes chosen for the primary drilling tests. These samples (to the left of Figure 3) are cut from the same larger gabbro block to guarantee consistency in material properties and characteristics. During the drilling process, cuttings were collected using the chosen gabbro samples. Rock fragments, known as cuttings, are created during the drilling process. They are crucial for deciphering the bit-rock interactions and assessing the drilling performance. The gabbro samples are prepared for multiple diameter holes using three coring bits. The drilling locations for the ensuing examinations and experiments are these holes. The coring bits are chosen according to their outer diameters to create holes of varied sizes and depths. After the multi-diameter holes are prepared, drilling tests are conducted with a 5-cutter Polycrystalline Diamond Compact (PDC) bit (to the right of Figure 3). This procedure involved several key steps:  
Preparation of Seat: A shallow depth was drilled into the gabbro sample to create a stable base or seat using a 5-cutter Polycrystalline Diamond Compact (PDC) bit. This step aims to guarantee the appropriate centralization of the ensuing drilling operations, as seen in Figure 4. The PDC bit had a diameter of 2.25 inches (57.15 mm) and could provide stability and accuracy while drilling. Figure 2: Illustrates various aspects of the experimental setup. Drilling of Central Holes: Within the gabbro sample, central holes of various depths were subsequently prepared. This was accomplished with 0.5 inches (12.7 mm), 1.0 inches (25.4 mm), and 1.5 inches (38.1 mm) in diameter natural diamond-impregnated (NDI) coring bits. As seen in Figure 4, these coring bits were chosen to produce holes of many sizes, enabling the exploration of various hole geometries in the drilling experiments. Main Drilling Operation: After the central holes were prepared, the 5-cutter PDC bit with a diameter of 2.25 inches (57.15 mm) was used for the main drilling operation. Depths below the un-cored formation and through the pre-cored depths were reached during this drilling process. This step aimed to evaluate drilling performance in various conditions and simulate drilling operations in a range of hole geometries. Maintaining Constant Parameters: Weight on Bit (WOB), Revolution per Minute (RPM), and Flow Rate (FR) were precisely kept constant during the whole drilling process. Ensuring consistency and repeatability in the experimental results was crucial. Researchers could distinguish the impact of hole geometry on drilling performance and precisely evaluate the influence of other variables by holding these parameters constant. The experiments were conducted using the Large Drilling Simulator (LDS) at the Drilling Technology Laboratory, Memorial University of Newfoundland, which was used for all pre-cored holes and drilling experiments. This simulator allowed for the methodical  
Figure 4: Illustrates the process of preparing multi-diameter holes. Figure 3: Shows gabbro samples post-drilling (**left**). Displays one PDC bit and three impregnated natural diamond coring bits (**right**). testing of various drilling parameters, providing insights into the Rate of Penetration (ROP) and other performance metrics. Figure 5 shows the LDS setup, which consists of the multi-diameter geometry before drilling in the formation (bottom right), the Data Acquisition System (DAQ-Sys), the LDS drilling frame, and the sample holder containing gabbro formation (top right). The LDS is an advanced laboratory system replicating real-world drilling conditions under controlled parameters. It features a power system to drive a high-torque rotary motor capable of operating at speeds ranging from 0 to 1000 RPM, with a maximum torque capacity of 1100 Nm.