Gas outburst prevention and extraction treatment method for coal roadway tunneling working face

Abstract

The present application belongs to the field of gas prevention and control in coal mining, and particularly relates to a method for gas outburst elimination and pre-drainage management in a coal roadway driving working face, wherein advanced geological drilling is carried out at the coal roadway driving working face to detect the change of the coal seam inclination and the distribution of the geological structure, and the inclination of the coal roadway in subsequent construction is adjusted accordingly; the radius test of liquid CO2 phase change cracking is carried out; the layout scheme of the outburst elimination and pre-drainage borehole in the coal roadway driving working face is formulated; the construction and extraction scheme of the outburst elimination and pre-drainage borehole in the coal roadway driving working face is formulated, and the gas extraction parameters are monitored and recorded; the coal seam gas extraction rate and the residual gas quantity are calculated, and after it is determined that the gas extraction rate of the coal roadway driving working face meets the standard and there is no gas outburst risk, the coal roadway working face starts to drive; and the cycle is repeated. The present application can minimize the gas outburst risk during the driving of the coal roadway, and is also beneficial to improving the coal seam gas extraction rate and extraction speed, reducing the gas overrun frequency of the driving working face, and fully utilizing the coal seam gas resources.

Description

Technical Field

[0001] This invention belongs to the field of coal mining gas control, specifically relating to a method for pre-drainage and control of gas outbursts in coal roadway excavation faces. Background Technology

[0002] With the long-term and large-scale development of coal resources, deep mining at depths exceeding 800 meters has become the new normal in coal resource development. Affected by factors such as burial depth, high ground stress, and high ground temperature, deep coal mining faces a series of problems, including increased gas emissions, increased gas pressure, and sudden abnormal gas outbursts in certain areas, directly leading to a greater threat of gas disasters to the mine. Methane is a typical greenhouse gas, with its greenhouse effect and ozone layer depletion being 21 times and 7 times that of CO2, respectively. At the same time, methane is also a clean and efficient energy resource and chemical raw material. Increasing the intensity of coal seam gas extraction and minimizing its content can not only ensure the safe and efficient mining of deep coal mines and promote atmospheric environmental protection, but also help to fully utilize coal seam gas resources and improve the economic benefits of the mine.

[0003] Coal mine gas outbursts primarily occur at coal roadway excavation faces, accounting for over 70% of all outbursts. This is mainly because excavation faces are generally less affected by mining activities, leaving the surrounding coal seams in their original, high-pressure state. Furthermore, the limited space at excavation faces compared to coal mining faces leads to more concentrated gas emissions. High-gas coal seams typically possess inherent properties such as low permeability, high adsorption, and slow desorption rates, resulting in difficult, inefficient, and limited-area gas extraction, long recovery times, and rapid decay rates. Therefore, to eliminate the risk of coal seam gas outbursts, artificial permeability enhancement is necessary to improve gas extraction rates and shorten pre-extraction time. Currently, methods for enhancing the permeability of high-gas, low-permeability coal seams mainly include hydraulic fracturing, hydraulic slotting, explosive blasting, CO2 phase change blasting, and shock wave fracturing. Among these, CO2 phase change blasting combines fracturing and permeability enhancement with displacement and replacement to strengthen gas extraction. On the one hand, the instantaneous gasification and expansion of liquid CO2 creates new fractures in the coal seam, improving its permeability. On the other hand, it fully utilizes the stronger adsorption capacity of CO2 on coal than CH4 to displace gas within the coal seam. Therefore, researching a CO2-based fracturing and permeability enhancement method for pre-extraction and gas outburst prevention in coal roadway excavation faces is of great significance for improving coal seam gas extraction rates and speeds, reducing coal seam gas content and pressure, eliminating the risk of gas outbursts during coal roadway excavation, and achieving safe, efficient, and green mining in deep coal mines. Summary of the Invention

[0004] The present application is aimed at the gas outburst disaster problem faced by deep high-gas coal seam tunneling working face, and proposes a coal roadway tunneling working face gas outburst pre-extraction treatment method based on CO2 induced fracturing and permeability improvement, which helps to improve the gas extraction effect of high-gas coal seam, reduce the gas overrun frequency of the tunneling working face, eliminate the gas outburst risk during the tunneling, fully utilize the coal seam gas resources, and ensure the safe, efficient and green mining of deep coal mines.

[0005] The specific steps are as follows:

[0006] a. Advance geological drilling is carried out at the coal roadway tunneling working face to detect the coal seam inclination change and the geological structure distribution, and to measure the original gas content of the coal seam; the coal roadway inclination during subsequent construction is adjusted to be the same as the coal seam inclination change value;

[0007] Preferably, in step a, the drilling depth is designed to be 120m, and the coal roadway is designed to be axially excavated along the coal seam development direction.

[0008] Preferably, in step a, when the advance geological drilling hole enters the coal seam roof or floor, the coal seam inclination change value is determined, and the advance geological drilling hole inclination is adjusted based on the coal seam inclination change value to make the advance geological drilling hole parallel to the coal seam, and the drilling depth is still designed to be 120m.

[0009] Preferably, in step a, when the advance geological drilling hole enters the geological structure, the drilling is immediately withdrawn, and the advance outburst pre-extraction distance is changed from 120m to the actual drilling depth.

[0010] b. Liquid CO2 phase change induced fracturing radius R test is carried out on the coal wall of the coal roadway tunneling working face;

[0011] Preferably, in step b, extraction holes are arranged at different intervals on both sides of the induced fracturing hole, a total of 19 holes including 1 induced fracturing hole and 18 extraction holes, and the induced fracturing hole and the extraction holes have a hole depth of 15m; the 1# induced fracturing hole is arranged in the middle, the 1# induced fracturing hole and the 2# to 8# extraction holes on the left side are arranged in a three-flower eye pattern with an interval of 1.0m, and the vertical interval of adjacent holes is 0.5m; the 1# induced fracturing hole and the 9# to 19# extraction holes on the right side are arranged in a variable interval, the interval value gradually decreases from 1.3m to 0.5m and then remains unchanged, and the vertical interval of adjacent holes is 0.5m.

[0012] Further, after the arrangement of each extraction hole is completed, seal the hole in time and install a gas pressure gauge respectively, record the gas pressure of each extraction hole after the pressure of the gas pressure gauge is stable, then remove the gas pressure gauge and install a gas comprehensive parameter tester to determine the gas natural flow of each extraction hole, monitor for 10 days, and obtain the average value of the gas extraction concentration and the extraction amount in each extraction hole; implement liquid CO2 phase change fracturing in the fracturing hole, continue to collect the relevant data after the coal body fracturing is completed, and determine the liquid CO2 phase change fracturing radius by comparing the average values of the gas extraction concentration and the extraction amount of each extraction hole before and after fracturing.

[0013] c. Formulate the coal roadway tunneling working face outburst elimination pre-extraction borehole arrangement scheme: ① Arrange four rows of boreholes according to equal interval row spacing, boreholes in two extraction rows are all extraction holes, and the fracturing holes and extraction holes in the two fracturing extraction rows are arranged alternately; ② The fracturing extraction rows and the extraction rows are arranged at intervals, and the total number of boreholes in the fracturing extraction rows is one more than that in the extraction rows; ③ The two adjacent rows of boreholes are arranged in three flower eyes, and the fracturing holes in the two fracturing extraction rows are all arranged alternately; ④ The number of fracturing holes in each fracturing extraction row is not less than three; ⑤ The interval between adjacent boreholes and the row spacing r are R / 8~R / 4, and the total number of boreholes to be arranged is determined according to the actual size of the coal roadway cross section; ⑥ Each row of boreholes is arranged in a diverging manner parallel to the coal seam inclination, and the boreholes in the rows are staggered with each other in the top view plane.

[0014] Preferably, in step c, the uppermost part is the fracturing extraction row.

[0015] Preferably, in ⑥ of step c, the final hole position of the middle hole is located in a vertical plane that is 120m away from the tunneling working face head along the coal roadway axial direction or the vertical plane of the retreat drilling depth, the left and right boundaries of the vertical plane are respectively 15m away from the corresponding coal roadway side, the horizontal interval of the middle hole at the final hole position is (b+30) / (n-1), wherein b is the width of the coal roadway and n is the number of middle holes, the depth of the middle hole and the included angle with the coal roadway axial direction can be determined according to the hole opening position and the final hole position of the middle hole; the final hole position of the boundary hole is also controlled to be 15m outside the profile line of the coal roadway sides, the hole position parameters of the boundary hole with the maximum included angle with the coal roadway axial direction can be determined according to the minimum distance between the boundary hole and the tunneling working face head along the coal roadway axial direction and the designed hole opening position of the tunneling working face head, the minimum included angle between the hole position of the boundary hole with the maximum included angle and the hole position of the middle hole in the top view plane is equally divided by m, which is 0.5 times the number of boundary holes, and the intersection point of the angle equal division line and the 15m boundary profile line away from the coal roadway sides is the projection point of the other boundary holes in the top view plane, and the specific hole position parameters of the other boundary holes can be determined according to the designed hole opening position of the other boundary holes.

[0016] d. Formulate the coal roadway tunneling working face outburst elimination pre-extraction borehole construction and extraction scheme, monitor and record the gas pressure, flow, flow rate and concentration parameters of each borehole and the network;

[0017] Preferably, the extraction pipe is placed in the hole after the extraction hole is formed, and the hole is sealed and extracted as soon as possible. The liquid CO2 phase change cracking is carried out in time after the cracking hole is formed.

[0018] Preferably, the extraction hole is first constructed around the cracking hole, and the cracking hole is used as the extraction hole after the CO2 phase change cracking is completed.

[0019] e. Calculate the coal seam gas extraction rate and residual gas quantity, and determine that the coal roadway excavation face gas extraction rate meets the standard and there is no gas outburst risk, and then the coal roadway working face starts to excavate;

[0020] f. The coal roadway excavation face stops construction after excavating to the preset next cycle outburst prevention and pre-extraction position, and the previous outburst prevention and pre-extraction work is repeated according to steps a-e.

[0021] Preferably, the specific range of the advanced outburst prevention and pre-extraction area of the coal roadway excavation face is a cuboid area with a length of 120m or the depth of the advanced geological drilling hole, a width of 15+b+15m, and a height of the coal roadway, wherein b is the width of the coal roadway, and the left and right sides each increase by 15m. The coal roadway excavation face is excavated every 100m, and the depth of the drilling hole is reduced by 20m when the drilling hole is withdrawn. One outburst prevention and pre-extraction drilling field is constructed, and the overlapping length of two adjacent advanced outburst prevention and pre-extraction areas is 20m.

[0022] Beneficial effects: The present application proposes a coal roadway excavation face gas outburst prevention and pre-extraction treatment method based on CO2 cracking and permeability enhancement. The method can not only minimize the gas outburst risk during coal roadway excavation, but also improve the coal seam gas extraction rate and extraction speed, reduce the gas over-limit frequency of the excavation face, and fully utilize the coal seam gas resources. The method has important practical significance for ensuring safe, efficient and green mining in deep coal mines. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a schematic diagram of the implementation process of the coal roadway excavation face gas outburst prevention and pre-extraction treatment method;

[0024] Figure 2 is a schematic diagram of the drilling hole arrangement for liquid CO2 phase change cracking radius field test;

[0025] Figure 3 is a schematic diagram of the drilling hole arrangement principle for the coal roadway excavation face head-on outburst prevention and pre-extraction;

[0026] Figure 4 is a schematic diagram of the drilling hole arrangement for the coal roadway excavation face head-on outburst prevention and pre-extraction in the embodiment;

[0027] Figure 5 is one of the drilling hole arrangement plan views (top view) for the coal roadway excavation face outburst prevention and pre-extraction in the embodiment;

[0028] Figure 6 is the second (top view) of the layout plane of the outburst-preventing pre-drainage borehole in the coal roadway tunneling working face in the embodiment;

[0029] Figure 7 is the specific layout parameter of the fracturing drainage borehole in the coal roadway tunneling working face in the embodiment. DETAILED DESCRIPTION

[0030] As shown in Figure 1 , a gas outburst-preventing pre-drainage treatment method for a coal roadway tunneling working face, comprising the following steps:

[0031] a. performing advanced geological drilling in the coal roadway tunneling working face to detect the coal seam inclination change and the geological structure distribution, and to measure the original content of the coal seam gas; based on the coal seam inclination change value, adjusting the coal roadway inclination in subsequent construction;

[0032] Specifically, a Φ94mm drill bit is selected to perform advanced geological drilling in the coal roadway tunneling working face head-on along the coal roadway axial direction (i.e. the design tunneling direction of the coal roadway, which has been specifically determined when the design mining working face layout scheme is arranged), and the design drilling depth is 120m.

[0033] When it is determined that the borehole enters the coal seam roof or floor according to the out-of-hole slagging, coring and other conditions, immediately retreat the drill, measure the coal seam inclination change value, and determine the method as follows: the retreat depth and the drilling angle are known, the height difference and the horizontal distance between the hole opening position and the retreat position can be calculated in sequence through the sine and cosine functions, the height difference between the tunneling working face head-on roof / floor and the retreat position roof / floor can be determined according to the vertical distance between the hole opening position and the roof / floor, and the tangent value of the coal seam inclination = (the vertical height difference of the roof / floor) / the horizontal distance, so that the coal seam inclination change value can be finally determined. Based on the coal seam inclination change value, the advanced geological drilling borehole inclination (the angle between the advanced geological drilling borehole and the horizontal plane) is adjusted to ensure that the advanced geological drilling borehole is parallel to the coal seam, and the design drilling depth is still 120m. After completing the advanced geological drilling, the inclination of the outburst-preventing pre-drainage borehole (including the fracturing hole and the drainage hole) in subsequent construction is also adjusted according to the coal seam inclination, i.e. the angle between the outburst-preventing pre-drainage borehole and the horizontal plane is adjusted. The outburst-preventing pre-drainage borehole is a bedding borehole, i.e. parallel to the coal seam, and the inclination of the outburst-preventing pre-drainage borehole, the coal roadway inclination and the coal seam inclination are equal.

[0034] When it is determined that the borehole enters the geological structure such as fault, fold and the like according to the out-of-hole slagging, coring and other conditions, immediately retreat the drill, measure the distance to the front geological structure, change the advanced outburst-preventing pre-drainage distance from 120m to the actual drilling depth, and adjust the drilling depth of the outburst-preventing pre-drainage borehole (including the fracturing hole and the drainage hole) in subsequent construction based on the actual drilling depth.

[0035] When the depth of the advanced geological drilling borehole reaches 120m or after drilling is withdrawn, fresh coal samples are obtained by coring machine, sealed and preserved, and sent to the laboratory to determine the original content of coal seam gas using a direct gas content determination device and a gas composition analyzer. After the advanced geological drilling borehole is cored, it is sealed in time and can be used as a fracture hole or extraction hole in the future.

[0036] b. Conduct tests on the radius of cracking caused by liquid CO2 phase transformation at the coal roadway excavation face;

[0037] Liquid CO2 phase change fracturing technology is a coal seam permeability enhancement technology based on physical blasting. This technology utilizes the instantaneous high-pressure gas generated by the phase change of liquid CO2 to fracture the coal seam surrounding the borehole, thereby depressurizing the gas in the coal seam, reducing gas content, and eliminating the risk of gas outbursts. The CO2 phase change fracturing radius should be determined comprehensively based on actual factors such as coal seam properties (coal seam hardness and brittleness-plasticity), liquid CO2 phase change fracturing process parameters, and the skill level of the operators. This provides a reasonable parameter selection basis for the on-site layout of gas outburst prevention and pre-drainage boreholes (including fracturing boreholes and drainage boreholes), ensuring the effectiveness of gas outburst prevention and pre-drainage at the coal roadway face, reducing drilling workload, shortening coal seam gas pre-drainage time, and ensuring safe and efficient mine mining.

[0038] Specifically, to enhance the measured accuracy of the fracturing radius caused by liquid CO2 phase change in the coal seam, and to verify the gas extraction effect after liquid CO2 phase change fracturing, the fracturing radius test was conducted on the coal wall of the face face during tunneling. After determining the specific location of the fracturing hole, extraction holes were arranged at different intervals on both sides of the fracturing hole, such as... Figure 2 As shown, a total of 19 boreholes are arranged, including 1 fracturing borehole and 18 extraction boreholes. The depth of both the fracturing borehole and the extraction boreholes is 15m. The #1 fracturing borehole is centrally located. The #1 fracturing borehole and the #2 to #8 extraction boreholes on the left are arranged in a three-hole pattern with a spacing of 1.0m. The vertical spacing between adjacent boreholes is 0.5m. Based on past construction experience, the fracturing radius of liquid CO2 phase change is generally 2.0 to 6.5m. Considering that if the spacing between adjacent extraction boreholes is too small, it is easy for the borehole to collapse during drilling, it is determined that the #1 fracturing borehole and the #9 to #19 extraction boreholes on the right are arranged in a variable spacing pattern. The spacing value gradually decreases from 1.3m to 0.5m and then remains unchanged, while the vertical spacing between adjacent boreholes remains 0.5m.

[0039] After the arrangement of each extraction hole is completed, seal the hole in time and install a gas pressure gauge respectively, record the gas pressure of each extraction hole after the pressure of the gas pressure gauge is stable, then remove the gas pressure gauge and install a coal gas meter (or a gas comprehensive parameter tester) to measure the natural flow of each extraction hole, the monitoring time is 10 days, and the stable average value of the gas extraction concentration and quantity in each extraction hole is obtained; after the above test work is completed, place the liquid CO2 phase change fracturing device in the fracturing hole by using a push rod, seal the hole, and implement liquid CO2 phase change fracturing by the operator; after the coal body fracturing is completed, continue to collect the relevant data, and determine the liquid CO2 phase change fracturing radius by comparing the average values of the gas extraction concentration and quantity before and after the fracturing of each extraction hole.

[0040] c. Based on the test results of the liquid CO2 phase change fracturing radius, a coal roadway tunneling working face outburst elimination pre-extraction drill hole (including a fracturing hole and an extraction hole) arrangement scheme is formulated;

[0041] Reference Figure 3, the layout principle of the pre-drainage boreholes (including the fracturing boreholes and the drainage boreholes) of the outburst elimination at the coal roadway tunneling face is as follows: (1) four rows of boreholes are arranged according to the equal interval, among which two rows of boreholes are all the drainage boreholes, namely the drainage rows, and the fracturing boreholes and the drainage boreholes are staggered arranged in the other two rows, namely the fracturing and drainage rows; (2) the drainage rows and the fracturing and drainage rows are arranged at intervals, and the total number of the boreholes in the fracturing and drainage rows is one more than that in the drainage rows, so as to ensure that each drainage borehole is adjacent to a fracturing borehole, and the uppermost part is preferably the fracturing and drainage row; (3) the two adjacent rows of boreholes are arranged in a three-flower eye mode, and the fracturing boreholes in the two fracturing and drainage rows are staggered arranged; (4) the number of the fracturing boreholes in each fracturing and drainage row is not less than three; (5) if the liquid CO2 phase change fracturing radius R is determined by the site measurement, the interval between the adjacent boreholes and the row interval r can be taken as R / 8~R / 4 by comprehensively considering the factors such as the improvement of the gas drainage volume, the shortening of the coal seam gas pre-drainage time, the reduction of the borehole construction workload and the engineering experience, and the total number of the boreholes to be arranged can be determined according to the actual size of the coal roadway section after the value of r is determined; (6) each row of boreholes is arranged in a divergent mode parallel to the coal seam inclination, and the boreholes in the adjacent rows are staggered in the plane view, the terminal hole position of the middle borehole is located in the vertical plane which is 120m away from the tunneling face along the coal roadway axial direction or the vertical plane of the retreat drilling depth, the boundary of the left and right of the vertical plane is respectively 15m away from the corresponding coal roadway rib, and the horizontal interval of the middle borehole at the terminal hole position is (b+30) / (n-1)m (wherein b is the width of the coal roadway, and n is the number of the middle boreholes, the depth of the middle borehole and the included angle between the middle borehole and the coal roadway axial direction can be determined according to the opening position and the terminal hole position of the middle borehole; the terminal hole position of the boundary borehole is also controlled at 15m outside the contour line of the coal roadway two ribs, according to the engineering experience, the minimum interval between the boundary borehole and the tunneling face along the coal roadway axial direction is 29.5m, based on this, the borehole position parameters of the boundary borehole with the maximum included angle with the coal roadway axial direction can be determined according to the opening position of the boundary borehole which has been designed at the tunneling face, the minimum included angle between the borehole position of the boundary borehole and the borehole position of the middle borehole in the plane view is equally divided by m (m is 0.5 times the number of the boundary boreholes), and the intersection point of the angle equal division line and the 15m boundary contour line away from the coal roadway two ribs is the projection point of the other boundary boreholes in the plane view, and then the specific borehole position parameters of the other boundary boreholes can be determined according to the opening position of the other boundary boreholes which have been designed.

[0042] In the embodiment, as shown in Figures 4-6As shown, according to the coal seam properties (hardness and brittle plasticity of the coal seam) and the coal roadway size, a reasonable outburst-preventing pre-drainage scheme is formulated, thereby improving the coal seam permeability, reducing the coal seam gas content and pressure, shortening the gas pre-drainage time, eliminating the gas outburst risk during the tunneling, and ensuring the safe and efficient mining of the coal mine. The coal roadway cross section size is 4.8m x 3.0m, and the test results in step b show that the liquid CO2 phase change fracturing radius is 4.5m-5.0m. According to the outburst-preventing pre-drainage borehole arrangement principle for the coal roadway tunneling face, the adjacent borehole row spacing r is determined as 0.7m, and the total number of the boreholes is determined as 26, of which 7 are fracturing boreholes and 19 are drainage boreholes, i.e. 2#, 6#, 10#, 14#, 18#, 21# and 25# boreholes are the fracturing boreholes, and 1#, 3#, 4#, 5#, 7#, 8#, 9#, 11#, 12#, 13#, 15#, 16#, 17#, 19#, 20#, 22#, 23#, 24# and 26# boreholes are the drainage boreholes. The specific arrangement parameters of the fracturing boreholes and the drainage boreholes are shown in Table 1 and Table 2. Figure 7 , Figure 7 The positive and negative discrimination method of the angle between each borehole and the coal roadway axial direction is that the borehole position is rotated from the coal roadway axial direction, and the counterclockwise rotation is positive and the clockwise rotation is negative.

[0043] d. Based on the outburst-preventing pre-drainage borehole arrangement scheme (including the fracturing boreholes and the drainage boreholes), the corresponding construction and drainage schemes of each borehole are designed, and the gas pressure, flow, flow rate and concentration parameters of each borehole and the collection network are monitored and recorded;

[0044] During the drilling, the drilling machine position needs to be fixed, a Φ94mm drill bit is selected, and the construction is strictly conducted according to the fracturing borehole and drainage borehole arrangement scheme. Slow pushing is adopted to ensure that the borehole body is straight, the inner wall is smooth, and the borehole is clean. Before the drilling is withdrawn, the residue in the borehole needs to be blown clean. If the borehole stringing phenomenon occurs, the borehole needs to be re-drilled. In order to prevent the adverse effects of the borehole collapse on the subsequent fracturing and drainage, the drainage pipe can be pre-installed in the borehole after the drainage borehole is completed, and the borehole sealing and drainage are conducted as soon as possible. The fracturing borehole needs to be timely informed to the operating personnel for the liquid CO2 phase change fracturing after the borehole is completed.

[0045] After the construction of each drainage borehole is completed, the "two plugging and one grouting" method is used for the borehole sealing, i.e. the two ends of the sealing section are first plugged by the bagged polyurethane, and then the drilling section between the two polyurethane plugging sections is grouted through the grouting pipe. Under the action of the grouting pressure, the slurry penetrates and fills the fractures around the borehole wall to improve the sealing property of the borehole.

[0046] Before the fracturing borehole is constructed, the adjacent drainage borehole around the fracturing borehole is first constructed to be used as a pressure relief borehole. Before the liquid CO2 phase change fracturing is conducted, the gas and CO2 concentrations in the coal roadway need to be monitored. When the gas concentration is lower than 0.8% and the CO2 concentration is lower than 0.5%, the phase change fracturing can be conducted.

[0047] In this embodiment, the construction and extraction scheme of the outburst-preventing pre-extraction drill hole at the coal roadway excavation face are as follows: (1) the construction sequence of each drill hole is as follows: 3#, 4# and 5# extraction holes → 2# cracking hole → 1#, 8# and 9# extraction holes → 6# cracking hole → 7#, 11#, 12#, 13# extraction holes → 10# cracking hole → 16# and 17# extraction holes → 14# cracking hole → 15#, 19#, 20#, 22# extraction holes → 18# cracking hole → 24# and 26# extraction holes → 21# cracking hole → 23# extraction hole → 25# cracking hole; (2) after the construction of each group of extraction holes (extraction holes constructed at the same time are regarded as one group, for example, 3#, 4# and 5# extraction holes are the first group of extraction holes, 1#, 8# and 9# extraction holes are the second group of extraction holes, and so on) is completed, the holes are promptly sealed and connected to the negative pressure pre-extraction network, and after the gas flow tends to be stable, the corresponding cracking hole is constructed; (3) after the construction of each cracking hole is completed, the relevant cracking device is installed in the hole, and after the installation is completed, the liquid CO2 phase change cracking is implemented, and after the CO2 phase change cracking is completed, the cracking hole is used as an extraction hole and connected to the gas extraction network for negative pressure pre-extraction; (4) after the gas flow of the extraction network tends to be stable, the next group of extraction holes is constructed according to the designed sequence; (5) during the whole gas extraction process, the gas pressure, flow, flow rate and concentration of each drill hole and the extraction network are monitored and recorded.

[0048] e.Calculating the coal seam gas extraction rate and residual gas quantity, determining that the coal roadway excavation face gas extraction rate meets the standard and there is no gas outburst risk, and then starting the excavation of the coal roadway face;

[0049] According to the measured data of the gas extraction site, the coal seam gas extraction rate and residual gas quantity are calculated. If the calculation result meets the standard requirement of the gas extraction rate of the excavation face and meets the determination standard of no gas outburst risk, three inspection drill holes are arranged at the head of the excavation face in reference to the spatial positions of the middle hole and the boundary hole to verify the residual gas content of the coal seam. After it is finally determined that the gas extraction rate of the excavation face meets the standard and there is no gas outburst risk within the cyclic advancing distance, the coal roadway face starts to excavate.

[0050] The calculation formula of the gas extraction rate η is as follows:

[0051]

[0052] In the formula, Q mj is the average gas extraction quantity of the excavation face during extraction, m 3 / min; Q mf is the average air gas exhaust quantity of the excavation face during extraction, m 3 / min.

[0053] The calculation formula of the residual gas quantity W cy of the coal seam after extraction is as follows:

[0054]

[0055] In the formula, W0 is the original gas content of the coal body, m 3 G is the coal reserves involved in the calculation of the evaluation unit, t; Q is the total amount of gas extracted by drilling in the evaluation unit, m 3 .

[0056] The coal reserves involved in the calculation of the evaluation unit G are calculated by the following formula:

[0057] G = (L + 2R) (1 + R) m γ

[0058] In the formula, L is the length of the coal seam in the evaluation unit, m; l is the average inclination length of the coal seam within the control range of the extraction hole, m; R is the effective influence radius of the extraction hole, m; m is the average coal seam thickness of the evaluation unit, m; γ is the density of the coal body of the evaluation unit, t / m 3 .

[0059] The gas extraction rate of the coal roadway excavation face meets the following requirements: when the original gas content of the coal seam is <10 m 3 / t, the gas extraction rate η should not be less than 35%; when the original gas content of the coal seam is 10 m 3 / t ~ 15 m 3 / t, the gas extraction rate η should not be less than 45%; when the original gas content of the coal seam is 15 m 3 / t or more, the gas extraction rate η should not be less than 60%.

[0060] According to the "Provisions for the Prevention and Control of Coal and Gas Outburst", "Coal Mine Gas Extraction Specification", "Coal Mine Gas Extraction Basic Index", the pre-extraction area with residual gas pressure less than 0.74 MPa or residual gas content less than 8 m 3 / t is a non-gas outburst danger zone.

[0061] f. The coal roadway excavation face is stopped after excavating to the preset lower cycle outburst elimination and pre-extraction position, and the previous outburst elimination and pre-extraction work is repeated according to steps a-e.

[0062] Specifically, the specific range of the advanced outburst elimination and pre-extraction area of the coal roadway excavation face is a cuboid area with a length of 120 m or the depth and width of the advanced geological drilling hole, a width of (15 + b + 15) m (where b is the width of the coal roadway, and 15 m is added on both sides), and a height of the height of the coal roadway in front of the excavation face. The depth of the drilling hole is reduced by 20 m every 100 m interval of the coal roadway excavation face, and one outburst elimination and pre-extraction drilling field is constructed, i.e. the overlapping length of two adjacent advanced outburst elimination and pre-extraction areas is 20 m.

Claims

1. A method for pre-drainage and control of gas outburst at a coal roadway excavation face, characterized in that, The specific steps are as follows: a. Conduct advanced geological drilling at the coal seam excavation face to detect changes in coal seam dip angle and the distribution of geological structures, and determine the original content of coal seam gas. Based on the change in coal seam dip angle, adjust the coal seam dip angle during subsequent construction to be the same. The method for determining the change in coal seam dip angle is as follows: Given the drilling depth and borehole angle, calculate the height difference and horizontal distance between the drilling position and the drilling position using sine and cosine functions. Then, determine the height difference between the roof and floor of the excavation face and the roof and floor of the drilling position based on the vertical distance between the drilling position and the roof and floor. Finally, determine the change in coal seam dip angle by using the tangent of the coal seam dip angle = (vertical height difference between roof and floor) / horizontal distance. b. Conduct tests on the radius R of liquid CO2 phase transformation-induced cracking in the coal face of the coal roadway excavation face; c. Develop a borehole layout plan for pre-drainage drilling at the coal roadway excavation face: ① Arrange four rows of boreholes at equal intervals, with all boreholes in the two drainage rows being drainage holes, and fracturing holes and drainage holes in the two fracturing drainage rows arranged alternately; ② Drainage rows and fracturing drainage rows are arranged alternately, with the uppermost row being the fracturing drainage row, and the total number of boreholes in the fracturing drainage row being one more than the total number of boreholes in the drainage row; ③ Adjacent rows of boreholes are arranged in a three-hole pattern, and fracturing holes in the two fracturing drainage rows are staggered; ④ Each fracturing drainage row... The number of fracture-causing holes in the fracture extraction row shall not be less than three; ⑤ The spacing between adjacent boreholes and the row spacing r shall be R / 8 to R / 4, and the total number of boreholes to be arranged shall be determined according to the actual dimensions of the coal roadway cross-section; ⑥ The boreholes in each row shall be arranged in a divergent pattern parallel to the dip angle of the coal seam, and the boreholes between rows shall be staggered in the top view; the final position of the intermediate hole shall be located in a vertical plane 120m away from the face of the working face or the retraction depth along the axial direction of the coal roadway, and the left and right boundaries of the vertical plane shall be respectively far from the corresponding coal roadway. The horizontal spacing between intermediate holes at the end of the coal roadway is (b+30) / (n-1)m, where b is the width of the coal roadway and n is the number of intermediate holes. The depth of the intermediate holes and their angle with the coal roadway axis can be determined based on the opening and closing positions of the intermediate holes. The closing position of the boundary holes is also controlled at 15m outside the outline of the two sides of the coal roadway. Based on the minimum distance between the boundary holes and the face of the working face along the coal roadway axis and the designed opening position of the face of the working face, the hole position parameters of the boundary holes with the maximum angle with the coal roadway axis can be determined. The minimum angle between the boundary hole position with the maximum angle and the intermediate hole position in the top view plane is divided into m equal parts, where m is 0.5 times the number of boundary holes. The intersection of the angle dividing line and the boundary outline 15m away from the two sides of the coal roadway is the projection point of the other boundary holes in the top view plane. The specific hole position parameters of the other boundary holes are determined based on the designed opening positions of the other boundary holes. d. Develop a plan for the construction and extraction of pre-drainage boreholes for gas outburst suppression in coal roadway excavation faces, and monitor and record the gas pressure, flow rate, velocity and concentration parameters of each borehole and the overall network; e. Calculate the coal seam gas extraction rate and residual gas quantity. After confirming that the gas extraction rate of the coal roadway excavation face meets the standard and there is no risk of gas outburst, the coal roadway excavation face will begin to be excavated. f. After the coal roadway excavation face reaches the preset lower circulation pre-drainage position, construction is stopped, and the aforementioned pre-drainage work is repeated according to steps a to e. The specific scope of the pre-gas elimination and pre-drainage area in the coal roadway excavation face is a rectangular area with a length of 120m directly in front of the excavation face or the depth of the pre-geological drilling hole withdrawal, a width of 15+b+15m, where b is the width of the coal roadway, and an additional 15m on each side, with a height equal to the height of the coal roadway. Every 100m interval between the coal roadway excavation faces, when the drilling is withdrawn, the withdrawal depth is reduced by 20m, and a pre-gas elimination and pre-drainage drilling site is constructed, that is, the overlap length of two adjacent pre-gas elimination and pre-drainage areas is 20m.

2. The gas outburst suppression and pre-drainage treatment method according to claim 1, characterized in that, In step a, the designed drilling depth is 120m, and the tunnel is excavated along the designed axis of the coal roadway, which is the estimated direction of coal seam development.

3. The gas outburst suppression and pre-drainage treatment method according to claim 2, characterized in that, In step a, when the advanced geological drilling borehole enters the top or bottom of the coal seam, the change value of the coal seam dip angle is determined. Based on the change value of the coal seam dip angle, the dip angle of the advanced geological drilling borehole is adjusted so that the advanced geological drilling borehole is arranged parallel to the coal seam, and the designed drilling depth is still 120m.

4. The gas outburst suppression and pre-drainage treatment method according to claim 3, characterized in that, In step a, when the advanced geological drilling borehole enters the geological structure, the drill is immediately withdrawn, and the advanced anti-outburst pre-extraction distance is changed from 120m to the actual drilling depth.

5. The gas outburst suppression and pre-drainage treatment method according to claim 1, characterized in that, In step b, extraction holes are arranged at different intervals on both sides of the fracture hole, with a total of 19 boreholes, including 1 fracture hole and 18 extraction holes. The depth of both the fracture hole and the extraction holes is 15m. The fracture hole #1 is arranged in the center. The fracture hole #1 and the extraction holes #2 to #8 on the left are arranged in a three-hole pattern with a 1.0m interval. The vertical distance between adjacent boreholes is 0.5m. The fracture hole #1 and the extraction holes #9 to #19 on the right are arranged in a variable interval pattern. The interval value gradually decreases from 1.3m to 0.5m and then remains unchanged. The vertical distance between adjacent boreholes is 0.5m.

6. The gas outburst suppression and pre-drainage treatment method according to claim 5, characterized in that, After the arrangement of each extraction hole is completed, the holes are sealed in time and gas pressure gauges are installed. After the gas pressure gauges stabilize, the gas pressure of each extraction hole is recorded. Then the gas pressure gauges are removed, and a gas comprehensive parameter measuring instrument is installed to measure the natural gas flow rate of each extraction hole. The monitoring is carried out for 10 days to obtain the average gas extraction concentration and extraction purity of each extraction hole. Liquid CO2 phase change fracturing is carried out in the fracturing hole. After the coal body fracturing is completed, the aforementioned relevant data are collected again. By comparing the average gas extraction concentration and extraction purity before and after fracturing of each extraction hole, the radius of liquid CO2 phase change fracturing is determined.

7. The gas outburst suppression and pre-drainage method according to claim 1, characterized in that, After the extraction hole is formed, the extraction pipe is placed in the hole in advance, and the hole is sealed and extracted as soon as possible. After the fracture hole is formed, liquid CO2 phase change is carried out in time to induce fracture.

8. The gas outburst suppression and pre-drainage treatment method according to claim 7, characterized in that, Before constructing the fracturing hole, the adjacent extraction hole is constructed first. After the CO2 phase change fracturing is completed, the fracturing hole is used as an extraction hole.

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