# A New Mechanical Specific Energy Model for Composite Impact Drilling
## 3 Model Field Application Results  
Lufeng Well X is an appraisal well-located in the Pearl River Mouth Basin. The formations of Zhuhai, Enping and Wenchang are mostly sandstone and mudstone, and the borehole wall is easy to collapse, and the bit wear is serious. Therefore, composite impactors are often used in the Lufeng block oil field, which has achieved a good speed-up effect. Taking Lufeng X well as an example, the interval of the well drilled with composite percussion was 2449.22-4100 m. The drilling tool assembly encountered the Paleogene Zhuhai Formation, Enping Formation and Wenchang Formation in one trip to the vertical depth, while the adjacent wells that were not drilled with a composite impactor required three trips to drill in this section on average, with a footage of 1650.78 m. The pure drilling time is 65.39 h, and the average ROP is 25.2 m/h. Its single bit footage and ROP have created records for the same horizon in the area, and the speed-up effect can reach up to 235.8% compared with adjacent wells The performance parameters of the composite impactor are shown in Table 1, the appearance dimensions are shown in Figure 2, the physical image is shown in Figure 3 and the dedicated PDC impactor bit used in conjunction is shown in Figure 4.  
\begin{table}
\begin{tabular}{c c} \hline
**Type** & **Drilling Speed up Tool** \\ \hline Size model & 8-1/2”186B \\ Nozzle & W20 \(\times\) 1 \\ Upper connection buckle type & NC50 Box \\ Bit connection type & 4-1/2”. REG Box \\ Largest diameter & 7-5/16’ (186 mm) \\ Displacement & 25-40 LPS (400-635 GPM) \\ Pressure drop & 2.0–2.5 MPa (290-363 Pis) \\ Impact frequency & 17-23 Hz \\ Maximum operating temperature & 270 \({}^{\circ}\)C \\ The biggest WOB & 160 kN (35 klbs) \\ The make-up torque & 16–20 kN.m (11,792–15,000 ft-lb) \\ \hline \end{tabular}
\end{table}
Table 1: Specifications and parameters of the compound impactor.  
Using the above MSE calculation model, and according to the relevant geological data of the Lufeng X well, the calculation results of the MSE are obtained, as shown in Figure 5. The 2355-2499.22 m interval was the interval without the composite impactor. It can be seen from the figure that the MSE suddenly increased and the ROP significantly decreased after the increase of weight on the bit. It can be seen that increasing the WOB may not effectively improve the ROP and may even reduce the influence of the bit speed and drilling fluid displacement on the WOB, leading to the decrease of the rock-breaking efficiency of the bit and the bottomhole cuttings cleaning effect. After the replacement of the composite impactor, the MSE was reduced to roughly equal to the CCS (Compressive Strength of rock under Confining pressure) of the rock, especially in the 2500-2900 m interval, where  
Figure 4: The PDC bit used in the Lufeng X well with a composite impactor.  
Figure 3: Composite impactor.  
Figure 2: Composite impactor appearance dimensions.  
the ROP increased most significantly. In the 2600-2650 m interval, the WOB decreased, MSE decreased and ROP increased and reached a peak, indicating that the rock-breaking efficiency of the bit was the highest. After 2900 m, drill into the Enping formation, the CCS of the rock group compared to the Zhuhai formation is increased, the energy for the broken rock is also increased; in the 2900 m\(\sim\)3650 m interval, MSE compared with the previous interval was increased, but the basic keep in the state of CCS is equal to or slightly greater than the rock WOB are stable fluctuations in this stage; the ROP decreased compared with before, but its stability will remain at a high speed, which shows that the rock fragmentation by this stage and drilling speed are efficient. After 3650 m, the MSE increases and is much higher than the CCS of the rock. As the weight on the bit increases, the ROP decreases and the rock-breaking efficiency of the bit decreases. To achieve a higher ROP, try to reduce the weight on the bit appropriately.