# Analysis of Application Parameters of Hydraulic Slotting Technology in Jointed Coal Reservoirs
## 3 Optimization of Slotting Parameters
### Effect of Slotting Parameters on Slot Shape  
#### 3.2.1 Effect of Pump Pressure on Slot Shape  
In the experimental investigation of the water jet pressure, the slotting speed was set to 0.12 m/s; the slot shape is shown in Figure 7. As can be seen, the slot width did not change significantly with different water jet pressures, and was approximately 15 mm. However, the slotting depth greatly varied with the change of water jet pressure. When the water jet pressure was 5 MPa, only shallow grooves were crushed on the specimen's surface. When the water jet pressure was 22 MPa, the specimen was slotted into two parts. Mainly because the impact force of the low-pressure jet was weak, the energy generated by the water jet could not reach the threshold required for coal destruction, and the slot depth was shallow. When the jet pressure exceeded the damage threshold, the depth of the slot increased significantly. Moreover, the transverse movement provided the jet with transverse velocity, and thus the water jet flow exerted a transverse shear force to the coal and aggravated the coal destruction. However, when the impact depth of the water jet reached a certain level, the water in the slot formed a cushion, which attenuated the jet energy and velocity, and thus rendered the water jet incapable of damaging the coal. In other words, the ultimate depth of the water jet slot was reached. Owing to the limited experimental conditions, a deeper slot depth could not be obtained in this experiment. The slot depth \(h\) and water jet pressure \(P_{w}\) are fitted in Figure 8. According to the data obtained, the slot depth and exposed area increased within a certain range with the water jet pressure, which indicates a quadratic positive correlation.  
Figure 6: Specim processing and equipment: (**a**) decomposed mold; (**b**) assembled mold; (**c**) specimen; (**d**) water jet test and control system.  
Figure 7: Specim under different water jet pressures.  
Moreover, it was observed that various fragments peeled off from the two edges of the slot when the water jet pressure was 22 MPa. This is mainly attributed to the fact that the impact effect increased with the water jet pressure. During the slotting process, stress waves are generated [37; 38]. The reflection and superposition of these stress waves generate tensile stress on the specimen. Additionally, the tensile stress increases with the increase of water jet pressure, and promotes the destruction of coal. Furthermore, the water in the slot is discharged outward and forms a water cushion under the water jet impact, which exerts an impact force on both sides of the slot. When the impact reaches a certain extent, various coal fragments peel off. The depth of the slot is one of the main factors affecting the slotting effect. As the water jet pressure increases, the slot depth becomes greater, the pressure relief area of the coal seam becomes larger, and the permeability-enhancing effect improves. However, high water jet pressure leads to large fragments being peeled off from the edge of the slot. This makes it difficult to discharge the slag, and causes the problems of boreholes being stuck and collapsing. Therefore, there exists an appropriate water jet pressure value, and this appropriate pressure should be determined according to the firmness coefficient of the coal seam.