Experimental Study Via Dynamic Prediction Model on High Pressure Hydraulic Slotting and High Efficiency Gas Drainage Technology under Complex Geological Conditions

In view of the difficult problem of dynamic disaster prevention and control in deep high gas, high stress and low permeability coal seam, the change law of drilling stress after seam cutting through drilling and bedding drilling is revealed by using numerical simulation method. The high-pressure hydraulic slotting method is used in Xinji Mining Area to carry out pressure relief and permeability increase test on coal seams. The test results are as follows: the slotting radius reaches 2.5m, and the net amount of gas extraction is 4.5 times higher than that of ordinary drilling holes, so as to realize efficient gas extraction in coal seams and reduce the risk of coal seam outburst. The technology of high-pressure hydraulic slotting, pressure relief and permeability enhancement can be used as a supplementary means for the prevention and control of coal seam dynamic disasters in deep mines.


Introduction
Under the condition of deep high ground stress, high gas and low permeability coal seam, the difficulty of gas drainage is increased, and the coal and gas outburst disaster still occurs [1][2][3] after the pre drainage of coal seam gas. The pressure of the existing reaming and slotting equipment in China is relatively low, and the pressure is generally 40-60mpa, which can not realize the function of drilling while drilling and slotting without withdrawing the drill pipe, and there are defects in technology and equipment, In the low permeability coal seam, the permeability increasing effect and construction efficiency are low, and the influence of hole expanding depth and seam cutting on fracture is small (300 mm ~ 800 mm) [2]. The existing hydraulic punching technology and equipment can not meet the prevention and control needs of coal and gas outburst [4][5][6][7][8][9][10][11][12][13][14]. Therefore, it is urgent to develop highpressure hydraulic slotting technology and technology for pressure relief and permeability enhancement, so as to realize rapid pressure relief and permeability enhancement under high stress, high gas and low permeability conditions. By adopting high-pressure hydraulic slotting method (pressure 90-100mpa), coal seam pressure relief and permeability enhancement are realized, gas

Numerical analysis of stress variation in slotted drilling
Combined with the actual situation of the mining area, the Mohr Coulomb constitutive model is established by FLAC3D numerical simulation software. The stress distribution law of cross seam drilling and bedding hole slotting under different coal seam hardness (0.5, 0.8, 1.0) conditions were simulated. The slotting pressures of simulated through layer drilling and bedding drilling are 60MPa, 80MPa and 100MPa.  For bedding drilling, when the coal firmness coefficient is 0.5, 0.8, 1.0, the horizontal stress changes between adjacent seams after cutting with different cutting pressure. With the increase of firmness coefficient, the minimum horizontal stress decreases. To sum up, when the coal seam hardness is constant, the greater the slotting pressure is, the greater the vertical stress relief degree around the slot is, and the better the slotting pressure relief effect is; when the slotting pressure is constant, the greater the coal seam hardness is, the smaller the vertical stress relief degree around the seam slot is, and the worse the slotting pressure relief effect is. For bedding drilling and slotting, when the seam hardness is constant, the greater the slotting pressure is, the greater the horizontal stress relief degree around the slot is, and the better the slotting pressure relief effect is; when the slotting pressure is constant, the greater the coal seam hardness is, the smaller the horizontal stress relief degree around the slot is, and the worse the slotting pressure relief effect is.

Overview of test area
The test area is 220108 working face in 2201 mining area. The coal seam 1 in 2201 mining area consists of 1 upper coal and 1 coal. The average thickness of 1 upper coal is 3.4m, and the average thickness of 1 coal is 3.9m. The average distance between 1 coal and 1 coal is 0.9m, and the average inclination of coal seam is 5 °. In the test area of 220108 working face, the measured maximum gas pressure is 1.65mpa, the measured gas content is 7.0m3/t, the soundness coefficient of coal seam group 1 in the test area is f = 0.48, and the initial gas release speed is 8. In 220108 working face, 50 slotting holes were drilled along the seam to carry out the slotting test. The spacing between boreholes is 10m, and the design plan of boreholes is shown in Figure 4. According to the research and determination of ultra-high pressure hydraulic slotting parameters of typical low permeability and medium hardness coal seam in Xinji Mining Area, the back slotting process is adopted during the slotting period, with nozzle diameter of 2.0mm, slotting interval of 3M, slotting pressure of 90-100mpa, maximum pressure slotting time of 5min, slotting rotation speed of 40R/min and cross slotting..

Analysis of slag removal effect
During drilling and slotting, the pressure is 90-100mpa, the slotting time of single hole is 1-2.5h, the average slotting time of single hole is 1.2h, and the slotting time of single knife is 10min. During the seam cutting process, the coal body is cut into small particles with a particle size of 0.5-1.5cm. See Figure 5. Under the joint action of water and spiral drill pipe, the coal body is smoothly discharged to the orifice, and there is no hole plugging in the seam cutting process. There are 50 seam cutting holes in machine lane of 220108 working face, each of which has 19-20 cutters, with an average drainage of 58-72l / min per minute, and an average discharge of 0.43t per cutter. According to the calculation, the groove radius formed after cutting is 2.2-2.5m. During the slotting test, see Figure 6 for the average slag discharge of each slotted hole.

Analysis of coal seam permeability
Before the ultra-high pressure hydraulic slotting in the machine lane of the 220108 working face, drill holes in the middle of any two adjacent boreholes, test the natural gas emission of the 1 coal seam boreholes, and use the simple drainage method to measure.
According to the measured data of natural gas emission, measured gas pressure and drilling construction parameters of 1 coal seam, the original permeability coefficient of 1 coal seam in machine lane of 220108 working face is 0.04582m 2 /mpa 2 ·d.
The permeability coefficient of coal seam 1 is 1.191 m 2 /mpa 2 ·d, which is 26 times higher than the original coal seam.

Comparative analysis of pumping capacity
After the completion of drilling and slotting in the machine lane of 220108 working face, the gas drainage investigation (a total of 50 drill holes along the seam) is carried out. In addition, the data of 500m long drilling and mining in the original organic lane of coal seam group 1 in the mine is selected, and 100 effective drainage are compared and investigated. The change curve of average extraction purity of slotted drilling and effective extraction single hole within 60 days is shown in Figure 7. It can be seen from Figure 4 that the maximum average extraction purity of single hole of slotted drilling is 0.255m 3 /min, the minimum is 0.128m 3 /min, and the average is 0.178m 3 /min; the maximum average extraction purity of single hole of common drilling is 0.108m 3 /min, the minimum is 0.012m 3 /min, and the average is 0.040m 3 /min. The average single hole extraction pure quantity slotted drilling is 4.5 times that of ordinary drilling.

Comparison and analysis of drilling quantities and standard time of extraction
The comparison between slotted drilling and ordinary drilling is shown in Figure 8.

Figure 8. Comparison between slotted drilling and ordinary drilling
From the figure, we can see that within the same 500m range, the number of slotted holes is 50, the number of ordinary holes is 100, and the amount of drilling work after ultra-high pressure hydraulic slotting is reduced by 50%. On the premise of reducing the amount of drilling work, due to the improvement of single hole extraction effect, the standard time of extraction is shortened, from 80 days when the seam is not cut to 50 days after the seam is cut, and the standard time of extraction is shortened by about 40%.

Conclusion
(1) The high-pressure hydraulic slotting increases the equivalent radius of gas drainage borehole. The average amount of chip removal per cutting seam is 0.43t, and the radius of slot formed after cutting seam is calculated to be 2.2 ~ 2.5m.
(2) The high-pressure hydraulic cutting improved the fracture network of coal seam and significantly improved the gas drainage. The permeability of the seam after cutting is 26 times higher than that of the original seam, and the average extraction volume of single hole is 4.5 times higher than that of the common hole.
(3) High pressure hydraulic slotting technology is adopted in the coal seam with complex geological conditions, which can reduce 50% of the drilling work quantity of disaster prevention and reduce the time of reaching the standard of extraction by about 40%.
(4) The technology of high-pressure hydraulic slotting, pressure relief and permeability enhancement can be used as a supplementary means to prevent and control the coal seam dynamic disaster under complex geological conditions in deep mines.