Study and application of reasonable parameters for hydraulic punching of layer penetration boreholes in Changping Coal Mine

Aiming at the application of hydraulic punching technology in soft, low permeability, and high gas coal seams in Changping Coal Mine, there are problems such as large workload, poor permeability enhancement effect, and the formation of “blank zones” in the pressure relief and permeability enhancement areas caused by workers' blind construction. By using a combination of theoretical analysis, numerical simulation, and on‐site practice, the threshold pressure for punching and breaking coal was calculated. The simulation analysis determined the reasonable key parameters for hydraulic punching through layered drilling, providing theoretical support for engineering applications; and the application effect of key parameters was investigated in the 5302 bottom extraction roadway of Changping Coal Mine. The research results show that the threshold value of incident pressure for punching and breaking coal in the No. 3 coal seam of Changping Coal Mine is 4.26 MPa, the reasonable hydraulic punching water pressure is 15 MPa, the punching angle is 60°, the diameter of the punching hole is not more than 0.8 m, and the punching time is 120 min; when the punching water pressure is lower than 4.0 MPa, effective coal flushing cannot be achieved. When the water pressure is between 4.0 and 15.0 MPa, the total amount of coal flushing is positively correlated with the punching water pressure; under the determined reasonable parameter punching conditions, the extraction influence radius is 8.9 m, the effective extraction radius is 5 m, and the average pure gas extraction volume is 0.089 m3/min. Under this parameter, hydraulic punching technology effectively improves the permeability characteristics of coal seams, which is conducive to long‐term gas extraction and has a significant effect on pressure relief and drainage promotion.


| INTRODUCTION
For a long time, coal has accounted for about 70% of China's disposable energy production and consumption, serving as an energy booster for China's rapid economic development and a ballast for ensuring China's energy security. Although the country has promoted energy diversification in recent years, the dominant position of coal in primary energy production and consumption will not change for a long time. Soft and low permeability coal seams have the characteristics of low solidity coefficient, low permeability, high gas content and pressure, and are widely distributed in China. Currently, hydraulic measures are often used to relieve pressure and increase permeability in soft coal seams to achieve efficient extraction. The hydraulic pressure relief and transparency enhancement method has the advantages of simple operation, good transparency enhancement effect, and low cost. [1][2][3][4][5] It mainly includes measures such as hydraulic extrusion, hydraulic fracturing, hydraulic cutting, hydraulic punching, and hydraulic expansion. Hydraulic punching is a widely used hydraulic permeability enhancement measure in soft and low permeability coal seams. [6][7][8] For a long time, many scholars have conducted extensive research on hydraulic pressure relief and transparency enhancement technologies and processes. Wu et al. 9 analyzed the technical principle of hydraulic antireflection measures, combined with 3DEC to simulate the fracturing effects under different water pressures, different fracturing times and different in situ stresses, and verified that hydraulic fracturing has a positive impact on gas drainage. Gu et al. 10 studied the field implementation and application of hydraulic punching in the three soft coal seams. Through field construction, combined with FLAC3D software, the hydraulic punching pressure relief and reflection reduction model was established, and concluded that hydraulic punching can effectively increase the permeability of the three soft coal seams and improve the gas drainage efficiency. Wang et al. 11 used theoretical analysis, numerical simulation and engineering tests to study the influence of different coal washings on the effective radius of hydraulic punching, and proved that when the extraction time is fixed, the effective extraction radius gradually increases with the increase of unit coal washings, but the increasing trend gradually weakens. Qin et al. 12 proposed a new cross-borehole hydraulic caving technique in the coal seam with a soft layer for preventing coal and gas outbursts during coal roadway excavation. Research has shown that hydraulic caving techniques can effectively improve permeability and rapidly eliminate coal and gas outbursts in the coal seam with a soft layer. Xie et al. 13 used high-pressure water jet drilling in the Shihuatian Coal Mine in Guizhou, which alleviated the gas pressure inside the coal seam, increased the permeability of the coal body, improved the gas drainage efficiency, and eliminated the risk of outburst. Based on theoretical analysis, numerical simulation and field experiments, Wang et al. 14 proposed a new method for enhancing the permeability of soft coal seams-hydraulic punching cavitation. The study found that compared with drilling cavitation, hydraulic punching cavitation has a wider plastic zone and a more significant pressure relief effect. The above research is based on hydraulic measures for pressure relief and transparency enhancement technology, but there is no comprehensive study on the specific key parameters of hydraulic punching. The study of reasonable key parameters of hydraulic punching technology not only has guiding significance for on-site application, but also has important significance for ensuring the safety production of mines.
Since 2011, Changping Coal Industry has been facing problems such as low gas extraction rate, short drilling extraction life, and difficulty in drilling holes. The ordinary drilling pre drainage method for coal seam gas cannot achieve good results, which seriously restricts the safe and efficient production of mines. There is an urgent need for enhanced drainage and rapid and efficient implementation of enhanced permeability technology. Therefore, the mine proposes to use hydraulic punching technology to increase the permeability of coal seams, to improve the effectiveness of coal seam gas extraction. However, when applying hydraulic punching technology in Coal Seam 3, there is a lack of theoretical guidance for the punching process parameters. The blind construction by workers has caused problems such as heavy workload, poor transparency enhancement effect, and the formation of "blank zones" in the pressure relief and transparency enhancement area. It is necessary to have a comprehensive set of key parameters to guide the application of hydraulic punching engineering and improve the application effect of this technology.
In summary, based on the theoretical calculation of the minimum hydraulic pressure for rock breaking, this article adopts numerical simulation methods to study and determine the comprehensive and reasonable key parameters for hydraulic punching applicable to the No. 3 coal seam of Changping Coal Mine, providing theoretical support for engineering applications; and key parameter effects were investigated in the second roadway of 5302 bottom drainage in Changping Coal Mine, with the aim of achieving long-term stability and efficiency in gas extraction through the application of hydraulic punching and permeability enhancement technology.

| Numerical simulation of reasonable parameters of hydraulic punching in cross-layer drilling
Before the engineering application, given the complex fluid-solid coupling nonlinear problems involved in coal breaking by hydraulic punching, ABAQUS software was used for numerical simulation to determine the punching water pressure, drilling angle, punching borehole diameter, and other parameters of hydraulic punching measures. Changping Coal Mine has four fully or partially minable coal seams, from top to bottom, including Coal Seams 2 and 3 of the Shanxi Formation and Coal Seams 8 and 15 of the Taiyuan Formation. The thickness of Coal Seam 3# is 2.50-10.60 m, with an average thickness of 5.58 m. The coal seam contains 0-3 layers of mudstone and carbonaceous mudstone with dirt bands, and the structure is simple relatively simple. The roof of the coal seam mainly consists of mudstone, sandy mudstone, siltstone, and medium and fine sandstone or siltstone locally. The floor is black mudstone, sandy mudstone and dark gray siltstone. The basic gas parameters of the coal seam show that the measured relative gas pressure of the coal seam is 0.28-0.45 MPa, the original gas content is 6.00-12.50 m 3 /t, the permeability coefficient is 0.012-0.052 m 2 /(MPa 2 d), and the firmness coefficient of the coal is 0.65. The gas occurrence in Changping Coal Mine is greatly affected by geological conditions. Among them, the gas content in the synclinal axis of Panel 4 and Panel 5 and its surrounding areas is increased, up to 12.5 m 3 /t. The coal mining method mainly adopts the retreating mining method, the top coal caving mining, and the roof management adopts the full caving method. See Table 1 for the details of the geological and physical and mechanical parameters of 5302 working face coal and its adjacent strata in Changping Coal Mine No. 3 Coal Seam.
According to the main rock mechanics parameters of Coal Seam 3# and its roof and floor in Table 1, the initial model of Coal Seam 3# and its roof and floor slate is determined, as shown in Figure Table 1.
Determining the coal-breaking pressure of hydraulic punching is of great significance for improving the coal- breaking efficiency of hydraulic punching. [15][16][17] According to the relevant data collected and measured on-site, the threshold value of coal-breaking water pressure, theoretical threshold speed of coal-breaking water pressure, the threshold value of coal-breaking incident pressure and other relevant parameters in the process of hydraulic punching test are determined. The crushing strength of coal under the impact of high-pressure water jet can be calculated according to the following formula: The compression shear coefficient depends on the shear stress σ θ with compressive strength Y, the numerical relationship between the three is: The theoretical threshold speed of coal breaking by high-pressure water jet is: where v is threshold speed of coal breaking by water jet (m/s), R t is the crushing strength of coal (Pa), β is the reflected water jet area factor, ρ w is the density of water (kg/m 3 ). The incident pressure of high-pressure water is: where P is the incident pressure of high-pressure water (Pa), ρ w is the water density (kg/m 3 ), v is the threshold speed of coal breaking by water jet (m/s). The specific mechanical parameters of Coal Seam 3 in Changping Coal Mine are shown in Table 1. Shear stress can be obtained according to the internal friction angle and cohesion coefficient of No. 3 coal σ θ is 1.13 MPa, then the compression shear coefficient k is 0.228 (k is the dimensionless coefficient) calculated by Equation (2). According to Formula (1), the calculation coefficient can be further calculated η the value of 1.35. Substituting the specific mechanical parameters of Coal Seam 3 in Table 1 into Formula (1), we can get the theoretical critical crushing strength of the coal body of Coal Seam 3 in Changping Coal Mine under the action of the high-pressure water jet, that is, the water pressure threshold of high-pressure water jet coal breaking R t is 4.73 MPa. According to Formula (3), the theoretical threshold velocity v of the coal-breaking water pressure of a high-pressure water jet is 92.27 m/s. Then according to Formula (4), the threshold value of the incident pressure of hydraulic punching coal breaking is 4.26 MPa.

| Engineering practice of hydraulic punching through layer drilling
To investigate the implementation effect of the measures of pressure relief and permeability enhancement by strip hydraulic punching in the bottom extraction roadway, the first extraction unit of strip pre-extraction drilling on the east side of 53,023 roadway in 5302 bottom extraction roadway in Changping Coal Mine was selected as the investigation site for hydraulic punching test. Boreholes are selected on the east side of the first extraction unit of 5302 bottom extraction roadway 2 to implement hydraulic punching pressure relief and permeability enhancement measures. The plan of the third borehole in the sixth group of the first extraction unit is shown in Figure 2. The blue point in the figure is the location of hydraulic punching boreholes, while the red dots represent the observation holes 7-3#, 6-4#, and 8-3#.
The technical parameters of hydraulic punching construction are very important to the coal-breaking effect and the permeability-increasing effect and are also an important basis for optimizing the hydraulic punching equipment and drilling design. When applying hydraulic punching on site, when setting the punching pressure, it is necessary to consider the local resistance factors such as frictional resistance, diameter change, bending and bending of the high-pressure rubber hose, as well as the leakage water pressure loss caused by poor sealing effect between drill pipes. The actual pump pressure setting should be much greater than the theoretical coal breaking water pressure. Finally, the punching water pressure should be set at 15 MPa during the hydraulic punching on-site test, the punching angle is 60°, the punching time is 120 min and the punching diameter is not more than 0.8 m. It can be seen from the distribution changes of the plastic zone in Figures 3-7 that under the same conditions, the range of plastic zone formed after punching increases with the increase of punching water pressure. According to the relationship curve between the plastic zone area and water pressure, as shown in Figure 8, with the increase of punching water pressure, the plastic zone area increases synchronously, and the punching water pressure and the plastic zone area meet a positive correlation.
To analyze the degree of pressure relief under different water pressure of hydraulic punching, the vertical stress and horizontal stress within the range of 0-10 m around five groups of different water pressure punching holes from 5 to 25 MPa are plotted, as shown in Figure 9. Figure 9 shows that vertical stress relief is the most significant when punching at 25 MPa water pressure, with a maximum relief rate of 25.57%. The horizontal stress relief is also the most significant when punching at 25 MPa water pressure, with a maximum relief rate of F I G U R E 2 Effective radius observation hole of hydraulic punching in Unit 1.  21.38%. If the principle of "last relief first" is adopted, horizontal stress shall be considered the standard principal stress for relief judgment. From the perspective of the pressure relief effect, the degree of pressure relief of vertical stress and horizontal stress increases with the increase of punching water pressure, but the pressure relief curve of vertical stress and horizontal stress has become stable after the punching water pressure reaches 15 MPa, and the slope of the curve does not change dramatically with the increase of punching water pressure. At this time, it is difficult to significantly improve the pressure relief effect by simply increasing the punching water pressure. Therefore, the horizontal stress shall be selected as the pressure relief sensitive index, and the reasonable punching water pressure is 15 MPa.  F I G U R E 8 Impact of punching water pressure on the area of the hydraulic punching plastic zone.

F I G U R E 9
Relief rate under different water pressure. Figures 10-14, respectively, simulate the impact of punching angle on the plastic zone range at 20°-60°. It can be seen from Figure 15 that the angle and the plastic zone area meet a positive correlation. Therefore, the effect is better when the drilling angle is about 60°. From the perspective of the degree of pressure relief, the curve below shows the vertical stress and horizontal stress of the 20°-60°h ole angle node within the range of 0-10 m around the borehole.
It can be seen from Figure 16 that with the increase of angle, the pressure relief rate of vertical stress and horizontal stress increases. With the increase of angle, the pressure relief peak value continues to increase, and the absolute value of the slope around the peak value also increases. The vertical stress reaches the maximum pressure relief rate of 19.45% at the angle of 60°, and the horizontal stress reaches the maximum pressure relief rate of 14.6% at the angle of 60°. Therefore, the horizontal stress shall be selected as the sensitive relief index, and the reasonable drilling angle is 60°.

| Influence of hole diameter of punching hole on pressure relief effect
To analyze the influence of punching hole diameter on pressure relief, under the condition of a punching angle  The change of plastic zone under different punching diameters is shown in Figures 17-21. It can be seen that the plastic zone range increases with the increase of punching hole diameter, so the hole diameter and the pressure relief influence range meet a positive correlation.
It can be seen from Figure 22 that the hole diameter and the plastic zone area meet a positive correlation, but after the hole, diameter reaches 1 m, the plastic zone area increases by 2%. To further analyze the pressure relief degree at different hole diameters, the vertical stress and horizontal stress changes of five groups of holes with different diameters from 0.4 to 1.2 m within the range of 0-10 m around the holes are analyzed, as shown in Figure 22.
It can be seen from Figure 23 that with the increase of punching hole diameter, the degree of pressure relief of vertical and horizontal stress shows an increasing trend, but when the hole diameter increases from 0.8 to 1.0 m, the pressure relief rate of horizontal stress does not increase but decreases, from 16.22% to 14.98%. At this time, increasing the diameter of the punching hole does not achieve a better pressure relief effect, but reduces the pressure relief effect. Therefore, the pressure relief effect is the best when the diameter of the punched hole is 0.8 m, and the coal volume is about 0.7 t. At the same time, the vertical stress reaches the maximum pressure relief rate of 25.24% at the 1.2 m hole diameter, and the horizontal stress reaches the maximum pressure relief rate of 18.14% at the 1.2 m hole diameter. From the perspective of the pressure relief effect, horizontal stress should be selected as the pressure relief sensitive index. Therefore, from the perspective of the pressure relief effect, it is reasonable that the diameter of the punched hole is not more than 0.8 m.
3.4 | The influence of punching time on pressure relief effect in layered drilling The model parameters of this group of examples are uniformly set as: punching angle 60°, punching hole diameter 0.8 m. Simulate the influence range of plastic zone under five different punching water pressures for 30, 60, 90, 120, and 150 min, analyze the disturbance and failure range under the same punching angle, punching hole diameter, and boundary conditions, and study the pressure relief effect of punching time on coal and rock mass.
From the distribution changes of plastic zone in Figures 24-28, it can be seen that under the same  conditions, the range of plastic zone formed after punching increases with the increase of punching time. From the relationship curve between plastic zone area and time, as shown in Figure 29, it can be seen that as the punching time increases, the plastic zone area increases synchronously, and the punching time and plastic zone area value satisfy a positive correlation.
To analyze the degree of coal pressure relief under different hydraulic punching times, five sets of vertical and horizontal stresses within the range of 0-10 m around the holes with different punching times ranging from 30 to 150 min were plotted, as shown in Figure 30. Figure 30 shows that vertical stress has the most significant pressure relief effect at the water pressure of 150 min of punching, with a maximum pressure relief rate of 25.20%. The horizontal stress also has the most significant pressure relief effect at the water pressure of 150 min of punching, with a maximum pressure relief rate of 21.01%. If the principle of "final pressure relief priority" is followed, horizontal stress should be considered as the main stress for pressure relief judgment. From the perspective of pressure relief effect, the degree of vertical and horizontal stress relief increases with the increase of punching water pressure. However, after the punching time reaches 120 min, the relief curve of vertical and horizontal stress has stabilized, and the slope of the curve has not changed significantly with the increase of punching water pressure. At this time, it is difficult to significantly improve the relief effect by increasing punching water pressure alone. Therefore, the sensitive indicator for pressure relief should be horizontal stress, with a reasonable punching water pressure of 120 min.  Figure 31 that the amount of coal washed out meets the positive correlation with the water pressure. When the water pressure is lower than 4.0 MPa, the punching water pressure cannot damage the coal body and cannot achieve effective coal washing; when the water pressure is between 4.0 and 15.0 MPa, the total amount of coal washing fluctuates greatly and increases with the increase of water pressure. The changing trend of coal yield below 15 MPa with water pressure proves that even under the action of low-pressure water punching, the increase of punching water pressure has a positive impact on coal yield, and when the water pressure is stable at 15 MPa, the coal yield reaches the maximum value of 0.69 t/m. The punching time and coal output are shown in Figure 32. It can be seen that the fitting line of the scatter plot of punching time and coal output is a power function. Under the condition of 15 MPa punching water pressure, the total coal output of hydraulic punching increases in the first 120 min, and then changes gently. The slope of the curve between punching time and coal output decreases sharply, and the increasing range of the total coal output decreases with the increase of time. When the punching time is 90 min, the coal output is 0.58 t; When the punching time is 120 min, the coal output is 0.66 t; when the punching time is 180 min, the coal output is 0.68 t. It can be seen that when the punching time is increased from 120 to 180 min, the increase of coal yield is unchanged. Therefore, the reasonable punching time of hydraulic punching measures for Coal Seam 3# is 120 min.
3.6 | Investigation on the effect of hydraulic punching to promote pumping in cross-layer drilling The three groups of drilling holes 7-3#, 6-4#, and 8-3# around the 3# punching measurement hole in Unit 1 are hydraulic punching inspection holes. The normal distances between each inspection hole are 7-3#, 6-4#, and 8-3# and the measurement hole are 5, 8.9, and 10 m, respectively, the location of the inspection hole is shown in Figure 2 to analyze the implementation effect of various pumping measures, the tracking inspection of pumping purity was carried out. Before and after hydraulic punching, the change of gas purity in the hole shall be recorded and investigated in detail every 2 h, as shown in Figure 33. It can be seen from Figure 33 that the implementation effect of hydraulic punching on site is slightly different according to the distance of the observation holes, but the gas flow of the three groups of observation holes has little change before and after the hydraulic punching. The net amount of gas drainage of 7-3# inspection hole (5 m normal distance) have an obvious influence. The net amount of gas drainage increases from 0.035 m 3 /min to a maximum of 0.12 m 3 /min, with an average of 0.089 m 3 /min. The influence of the 6-4# inspection hole (8.9 m normal distance) is reduced, and the average extraction purity are 0.074 m 3 /min. The influence of the 8-3# inspection hole (normal distance: 10 m) is very weak, and the average extraction purity are 0.063 m 3 /min. 7-3# and 6-4# inspection holes were affected by hydraulic punching, but the pure extraction volume of the 8-3# hole, which is 10 m away from the measured hole, changed slightly, while the pure extraction volume of 6-4# hole, which is 8.9 m away from the measured hole, experienced a process of pure increase and decline within 18 h after the implementation of hydraulic punching measures. Therefore, the effective extraction radius of the first unit of hydraulic punching is 5 m, and the extraction influence radius is 8.9 m.
The test results show that under the condition of 15 MPa water pressure, the reasonable punching time of hydraulic punching is 120 min, and the coal flushing amount is 0.66 t. When the punching water pressure is 15 MPa, the punching angle is 60°, the diameter of the punching hole is 0.8 m, and the punching time is 120 min, the average pure amount of extraction within 5 m of the extraction radius is 0.089 m 3 /min, the effective extraction radius is 8.9 m, and the extraction influence radius is 10 m. The relationship between the amount of coal washed out and the water pressure is discrete, but the overall relationship is positive.

| CONCLUSIONS
Based on the numerical simulation and field test analysis of hydraulic punching in the No. 3 coal seam of Changping Coal Mine, the following conclusions are obtained: 1. Through theoretical calculation, it is determined that the threshold value of the incident pressure of punching coal breaking in the No. 3 Coal Seam of Changping Coal Mine is 4.26 MPa; 2. Using numerical simulation, the reasonable water pressure for hydraulic punching of No. 3 Coal Seam in Changping Coal Mine is determined to be 15 MPa, the reasonable punching time is 120 min, the reasonable punching angle is 60°, and the reasonable diameter of the punching hole is not more than 0.8 m from the perspective of pressure relief; 3. Engineering practice shows that when the punching water pressure is lower than 4.0 MPa, effective coal flushing cannot be realized; When the water pressure is between 4.0 and 15.0 MPa, there is a positive correlation between the total amount of coal washing and the water pressure of punching, but there is a certain fluctuation; 4. Engineering practice shows that when the punching water pressure of the cross-layer borehole is 15 MPa, the punching angle is 60°, the amount of coal washed out is 0.66 t, the punching time is about 120 min, the impact radius of extraction is 8.9 m, the effective extraction radius is 5 m, the average pure gas extraction volume is 0.089 m 3 /min, and the effect of pressure relief and drainage promotion is good.