Study on time–space characteristics of gas drainage in advanced self‐pressure‐relief area of the stope

Gas drainage is an important measure to ensure energy safety in coal mines. However, for mines with high gas content and low permeability, the critical question is how to enhance the drainage effect of drilling. By using the self‐unloading pressure effect on the coal body in front of the mine, the permeability of the coal body can be improved, which facilitates gas drainage. Therefore, establishing the fluid–solid model of gas‐bearing coal mass under the influence of mining, this paper simulates the advanced self‐pressure‐relief range in front of the coal working face and the permeability of coal seam at different heights. Through simulations and field tests, the reasonableness of the advanced self‐pressure‐relief gas drainage was verified, and the spatial and temporal characteristics of the gas drainage from the borehole were quantified and analyzed. The results show that the width of the advanced self‐pressure‐relief zone after simulated mining is 25 m, the vertical permeability of the coal seam increases first and then decreases with the increase of the height from the floor, and the maximum permeability of the layer is 2.6 m from the floor, which is 8.62 times higher than the initial permeability; the gas drainage effect of test boreholes has a periodic law with the coal seam recovery, which, divided into the initial drainage stage, the scalar volume of gas drainage increase stage, and the scalar volume of gas drainage decay stage. At a distance from the working face 20–40 m, the boreholes' scalar volume of gas drainage increased by 3.89 times compared with the initial drainage stage, and the scalar drainage volume of the boreholes through the seam increased by 8.01 times compared with the initial drainage stage; in the section of increasing scalar volume of drainage, the gas drainage volume of the boreholes at different heights of the coal seam had obvious “stratification” phenomenon. The test boreholes were optimally placed in the middle of the seam at the lower end. The study provides a reference for self‐pressure‐relief gas drainage technology and optimization of borehole placement.


| INTRODUCTION
The most common accident in underground coal mines is the gas catastrophe. Gas drainage is crucial to reduce the gas pressure in coal seams and ensure personnel safety. 1 China's coalfields have undergone multiple phases of geological formation during their development and evolution, resulting in high coal seam gas content, strong adsorption capacity, complex plastic structure, and low coal seam permeability, resulting in poor gas drainage measures and restricting safe coal mining.
High-density drainage boreholes are usually deployed to reduce the gas content of coal seams. However, there are still problems of high engineering volume, high construction costs, and inefficient gas drainage. After gas drainage in some low-permeability coal seams, the residual gas content in the coal seams is still very high, which is easy to cause the gas concentration to exceed the limit during the mining process. To reach the gas drainage standard, it is also necessary to develop the self-pressure-relief gas drainage technology and optimize the drilling hole layout parameters.
The key to enhancing the effectiveness of gas drainage lies in increasing the permeability of the coal body. In addition to being related to its pore and fracture structure, coal-rock body permeability can also cause deformation of the coal-rock skeleton and change in pore volume under different stress states. 2 Under the influence of mining, the permeability evolution of gasbearing coal-rock bodies is a complex process involving mining stress-fracture-seepage coupling, which is closely related to geological structure, rock properties, mining parameters, and ground stress. [3][4][5] Using the pressurerelieving effect generated by coal seam mining, the surrounding coal permeability can be influenced, resulting in a significant increase in gas drainage from nearby boreholes over time. 6,7 As the upper and lower rock seams are gradually expanded and deformed during cyclic incoming pressure after coal seam mining, the surrounding rock is subjected to layered cyclic unloading, which continuously produces plastic deformation and the corresponding permeability gradually increases with stress unloading. 8 Gas transport in the surrounding coal seam also creates different flow partitions with the advancement of the working face. 9 Therefore, protected seam mining is the preferred technique for unloading gas drainage. Wang et al. 10 established a numerical model for unloading gas drainage in the coal seam group of the Baode coal mine and investigated the evolution of the plastic zone of the protected coal seam under mining disturbance. 11 Huang et al. carried out numerical simulations of the evolution of the permeability of its underlying coal seams during the mining of the upper coal seam using UDEC software. 12 In contrast, cost-effective means of unloading gas drainage are limited for single-thick coal seams mined without a protective layer. Xu et al. put forward the cooperative gas drainage technology via high-and lowlevel roadways (HLLR), and the optimal technical parameters of the cooperative technology via HLLR are determined and applied. 13 Li et al. using active water with different concentrations of PAM prepared using polyacrylamide (PAM). To explore the effects of slippery water fracturing on coal seam pores and to develop a gel breaker that can promote the degradation of PAM polymer molecules and the reduction of residual liquid viscosity after fracking, and improve the gas drainage effect. 14,15 In the mining process of the coal face, the original stress in the coal body of the coal face is also destroyed, and the stress in the overlying strata is redistributed; In the direction of horizontal stress, it experiences the loading process and unloading process of initial rock stress, overburden stress. 16 In the initial stress state, the gas drainage volume is small due to the low initial permeability of coal; when the stress in the coal body is concentrated, the pores in the coal body gradually close under the action of oversupport pressure, which is not conducive to gas drainage; when the over propping pressure exceeds the strength limit of the coal body and breaks and deforms, the permeability increases, which is manifested by the effect of unloading to increase flow and unloading to increase permeability. Unloading areas to enhance gas emissions can achieve the effect of improving emissions. 17 The current study discusses the determination of the width of the self-pressure-relief zone at the working face and analyzes the change law of gas drainage in a different time and distance range of drilling holes. However, more quantitative studies must be conducted on the effect of drilling gas drainage in the self-pressure-relief zone. Therefore, according to the field test, it is determined that under single coal seam mining conditions, the coal body in front of the mining face is in different spatial positions-the drilling gas extraction changes with the surface propulsion and time. Furthermore, under unprotected layer mining, the spatial and temporal characteristics of pre-self-unloading gas extraction are studied, which provides the basis for coal and gas co-mining technology.
The coal mining operation destroys the original rock stress of the coal body around the mining face. The overlying load transfers the concentrated stress to the coal body adjacent to it after the coal body is mined so that the stress of the coal body in the adjacent area is continuously concentrated and eventually reaches the strength limit of the coal body and plastic damage occurs. 18 The bearing capacity of the damaged coal body is reduced, and the concentrated stresses in the overlying rock seam continue to transfer to the deeper part of the coal seam, eventually reaching stress equilibrium and forming an unloading zone, stress concentration zone, and original stress zone in front of the recovery. The coal body is broken and deformed in the pressure-relief area, resulting in many fissure channels. Coal seam mining will significantly reduce the bearing capacity of the edge coal body, and overburden migration will transfer the action point of stress concentration from the edge to the interior when it reaches a stable state.
After stress balance, a pressure-relief area of a certain width will form in front of the mining face. In the stress concentration area, the concentrated stress carried by the coal body gradually transfers to the deep with the destruction of the coal body strength, forming a stress concentration zone between the pressure-relief area and the original stress area. The initial stress area is far from the working face and is not affected by coal mining.
The coal mining operation destroys the original rock stress of the coal body around the mining face, which makes the coal body deformed and broken. Also, it eliminates the dynamic transformation balance of the gas stored in the coal body. The adsorbed gas will be converted into a free state when the pressure is reduced, and the free gas will migrate to the surrounding area through the pore fracture channel formed by coal crushing under a pressure gradient. The permeability will also change with the mining stress in the migration process. It is necessary to establish a permeability change model in the process of coal deformation in front of the working face and build a fluid-solid coupling model of gas-bearing coal and rock. Utilize COMSOL Multiphysics software to simulate the change process of coal deformation and gas migration in front of the working face.

| Model assumptions
(1) Gas-bearing coal rocks are regarded as isotropic and homogeneous media. (2) The coal seam temperature is constant, and the law of coal seam gas desorption follows the Langmuir equation. (3) The relationship between the strain generated by the deformation of the gas-bearing coal rock and the effective stress follows the generalized Hooke's law.
(4) The theoretical model does not exchange energy and material with the outside world.

| Deformation field control equations for gas-bearing coal rocks
Without considering the gravity of the gas and the inertial forces on the coal skeleton during seepage, any square unit in the gas-bearing coal rock is in hydrostatic equilibrium due to the interaction of the internal forces in the coal rock. At the same time, the gas-bearing coal body, as a deformable pore-fissure dual medium, has a strong adsorption capacity for gas and produces a specific adsorption expansion stress, which changes the force distribution of the coal rock. The coal body's permeability is related to the development of its pore and fissure structure and the different stress states that can cause changes in the skeleton deformation and pore volume of the coal rock. 19 Therefore, the equation for the change in the stress field of gas-bearing coal, taking into account the ground stress, the adsorption of a gas by coal-rock particles, and the change in gas pressure is 3,12,20 .
There, φ 0 is the initial coal-rock porosity; e is the coal-rock volumetric strain; Ks is the bulk modulus of the coal body, MPa; T is the thermodynamic temperature of the coal seam, K; α is the Bio coefficient.

| Controlling equations for the seepage field of gas-bearing coal bodies
The seepage transport of gas in a coal body follows the flow equation, the Langmuir adsorption equilibrium equation, and the equation of state for coal seam gas. 21,22 When gas is transported in a low-permeability gasbearing coal seam, the gas molecules near the surface of the coal wall exhibit a nonzero flow velocity, which is known as the slippage effect. 23 On the basis of the coal seam gas flow equation taking into account the Klinkenberg effect and the modified Langmuir adsorption equilibrium equation, the total coal seam gas content equation can be found as follows, and the usual coal seam gas state equation is expressed as follows 10,24,25 : where q is the gas seepage velocity vector, m/s; μ the gas dynamic viscosity, taken as 1.08 × 10 −5 Pa s; m the Klinkenberg coefficient, MPa; ∇P the gas pressure gradient within the coal seam, Pa/m; Q is the total gas content per unit volume of coal, kg/m 3 ; c the calibration parameter for coal quality, kg/m 3 ; A the ash content of coal, %; M the moisture of coal, %; M g the molar mass of gas, kg/mol; T the thermodynamic temperature of coal seam, K; ρ g the gas density at gas pressure at P, kg/m 3 ; Z the gas compression factor, approximated as 1 for small temperature differences; P n the gas pressure at standard conditions, P n = 0.10325 MPa; ρ n the density of coal seam gas at standard conditions, kg/m 3 . Assuming the model is isolated from the outside world, and no exchange of material and energy occurs, the flow of gas through the coal seam conforms to the law of conservation of mass, expressed in the form of a differential equation as follows: where Q is the coal-rock gas content, kg/m 3 ; I the source-sink term.
On the basis of the gas flow equation in the coal seam, the gas content equation, and the gas state equation. They are calculated and collated in conjunction with Equation (3) to obtain the gas seepage field equation as follows 20 :

| COMSOL MULTIPHYSICS FIELD SIMULATION
COMSOL can solve a system of partial differential equations (PDEs) by the finite element method to achieve coupled multiphysical field phenomena simulation calculations. Here the numerical simulation of fluid-solid coupling in porous media is carried out using COMSOL, using the PDE interface of the mathematics module to customize the permeability evolution equations and the gas seepage field control equations to couple the gas transport patterns under the mining stress field. In the solid mechanic's module, the upper part of the top plate of the geometric model is set as the load boundary, and the load is 12 MPa. After the coal seam is mined, the interface between the top and floor of the goaf, the coal wall, and the test borehole wall are set as free boundaries. The other boundaries are fixed constraints in the gas seepage field of gas-bearing coal; when t = 0, the gas pressure P 0 of the coal seam subregion in the model is 0.74 MPa. The negative pressure of drilling hole drainage in the model coal seam subregion is 17 kPa, and the gas flow at the given model boundary is 0.

| Assignment of material parameters
On the basis of the actual data measured at the test site, the basic parameters of the top and bottom rock and coal seams involved in the simulation process were assigned values, as shown in Table 1.
On the basis of the test mine foundation information, the coal seam gas foundation parameters are shown in Table 2.

| Analysis of numerical simulation results
3.3.1 | Change in permeability in the vertical direction of the coal body in front after mining As the workings advance 15, 30, and 40 m forward and reach a steady state, the coal permeability distribution in the vertical direction of the seam in front of the recovery workings is shown in Figure 2.
When the coal mining distance is different, the permeability change pattern in the vertical direction of the coal seam has consistency. The coal seam has the lowest gas permeability from the base plate at 0-1 m. As the height of the coal seam increases from the base plate, the gas permeability of the coal seam also increases, reaching a peak at a position where the seam is 2.6 m high and low from the base plate. The F I G U R E 1 Geometric model for coal and gas co-mining.

| Simulation results for overrun unloading width
After the coal seam is mined, the top plate of the mining hollow area transports and settles, forming the vertical three zones, the rock seam of the riser zone breaks and collapses periodically, which will have an impact on the stress-strain of the coal body in front and cause the redistribution of stress-strain around the working face. 26,27 The influence of mined area and overlay rock collapse is fully considered, and the width of the discharge area is simulated by COMSOL software. The excavation direction is from left to right, and the excavation distances are 15, 20, 25, and 30 m, respectively. The simulated stress cloud diagram and the stress change curve of the coal seam floor are shown in Figure 3. According to the stress cloud diagram analysis, after 15 m of coal mining, due to the short distance of mining, the roof plate in the mining area is not entirely settled, and the influence of mining is relatively small. According to its stress change curve, after 15 m of coal mining, the width of the overrun unloading zone in front of the coal body is only 4 m. After 20 m of coal mining, the width of the invaded unloading zone is about 9.7 m. After 25 m of coal mining, the stress change curve shows the width of the overrun unloading zone is about 15.2 m.
As the distance of coal mining increases, the bubble fall zone in the mining area is fully developed, and the stress distribution in front of the mining area is more fully developed.
When the coal seam is advanced 30 m, the width of the overburden zone is about 23 m. As the excavation width increases, the width of the overrunning pressure-relief zone in front of the coal body no longer increases, and the stress distribution around the working face has reached equilibrium at this time, reaching the limit of the working face conditions. The simulation results show that the coal body in front of the test working face has the effect of overrunning pressure relief, and the simulated width of the overrunning pressure-relief zone is 23-25 m.

| Basic information of the field test site
The Guhanshan Coal Mine is located in Jiaozuo City, Henan Province. This mine is a coal and gas outburst mine, single mining No.

| Test borehole layout
In front of the 1604 working face, select Test Areas A and B. Test Area A was located in the 793-844.9 m in the backwind lane of the 1604 working face, and five groups of down-layer boreholes were arranged, a total of 66 investigation holes. The test inseam borehole spacing was 1.6 m, the drilling angle was perpendicular to the coal wall, the borehole depth was 75-80 m, and the borehole height was 1.5 m above the bottom plate; every 12-15 boreholes were arranged as a group, and each group was installed with an orifice flow meter to measure the gas drainage concentration and flow rate regularly. Each group of 12-15 boreholes is installed with an orifice flow meter to measure the gas drainage concentration and flow rate at regular intervals. Test Area B is located in the range of 546-565 m in the middle bottom pumping lane of the 1604 working face. Four inspection penetration boreholes are arranged, with the borehole inclination set at 85°-90°, approximately perpendicular to the coal seam. The absolute depths of the boreholes in the coal seam are 1.5, 2.5, 3.5, and 5.3 m, respectively. The boreholes are numbered 1#, 2#, 3#, and 4#. The drill hole arrangement is shown in Figure 4.
When the borehole is in the stress relief zone, the drainage parameters will change significantly, and the distance of the test borehole from the working face can be considered as the width of the self-pressure-relief zone. When the borehole is in the stress relief zone, the permeability of the mining-disturbed coal seam is appeared permeability increasing moderately zone and permeability increasing-substantially zone. 11 Due to the influence of the coal seam and the sealing effect, the width of the overburden zone measured in the test borehole may fluctuate. The drainage parameters of the    The gas drainage data from the inseam borehole of Test Area A was observed, and the results of the representative down-seam test boreholes A, B, C, D, and E were selected, as shown in Figure 5. As the borehole advances from the working face, the mix volume of gas drainage and scalar volume of gas drainage show a small to large pattern and then gradually decrease.
The mix volume of gas drainage and scalar volume of gas drainage decreased when the drill hole was 40-50 m away from the working face; the mix volume of gas drainage and scalar volume of gas drainage gradually increased when the drill hole was less than 40 m away from the working face; the mix volume of gas drainage and scalar volume of gas drainage reached the peak when the drill hole was about 30 m away from the working face. The analysis concluded that the internal stress release reduced strength, and increased fracturing of the coal body in the self-pressure-relief zone increased gas drainage. In the process of gas drainage and coal seam mining, the rock mass around the borehole is broken, there is a sound of gas leakage in the drainage pipeline, the sealing capacity of the borehole is reduced, and the gas drainage effect begins to decline.
In the test borehole's initial state, the gas drainage's scalar volume is small. As coal mining advances, the borehole is in the self-pressure-relief zone where the scalar volume of gas drainage reaches its peak, and the distance of the borehole from the mining face is taken as the width of the self-pressure-relief zone. The width of the self-pressure-relief zone for the statistical single-hole test is shown in Table 3. On the basis of the data analysis, the average width of the overrun relief zone at the 1604 working face was 26.9 m.
(2) Gas drainage test results for each group of boreholes Further quantitative comparisons of the variation in the scalar volume of gas drainage and concentration of gas extracted from each group of cascade boreholes at different locations from the working face are shown in Figure 6.
From Figure 6, the scalar volume of gas drainage from the five groups of hole plates showed the same regular trend. From 90 to 60 m from the working face, the scalar volume of gas drainage remained at low values with slight fluctuations, indicating that the test boreholes were far away from the working face and had not yet been disturbed by mining pressure relief; from 60 to 40 m from the working face, the scalar volume of gas drainage increased slightly, indicating that the permeability of the coal body gradually became better.
The pore space of the coal body began to grow, leading to an increase in the volume of gas drainage and a gradual transition of the coal body from a concentrated stress zone to a self-pressure-relief zone; at a distance of 40-20 m from the working face, the scalar volume of gas drainage rises significantly, reaching a peak near 30 m distance from the working face. It is because the mining disturbance puts the coal seam in an unloading state, which effectively reduces the coal seam gas pressure, expands and deforms the coal body internally, microporous fissure channels develop, and the gas adsorbed in the coal body is desorbed in large quantities, which increases the gas drainage amount from the borehole. This area is the best zone for unloading gas drainage. At a distance of 20-0 m from the working face, the scalar volume of gas drainage from the borehole gradually decreases because the closer the working face is, the looser the coal-rock block is, the more fissure pores are developed, the sealing effect of drainage borehole at this position is affected by this, and the gas in the coal seam cannot be extracted effectively, instead, the air in the tunnel is sucked along the fissure of the coal wall under negative pressure, which reduces the concentration of gas drainage.
T A B L E 3 Overrunning pressure-relief zone width test results statistics. As shown in Table 4, the average scalar volume of gas drainage in the range of 20-40 m from the working face is 3.08 times that in the range of 0-20 m, 1.85 times that in the range of 40-60 m, and 3.89 times that in the range of 60-90 m. The average scalar volume of gas drainage in the range of 20-40 m from the working face is 3.08 times that in the range of 0-20 m, 1.85 times that in the range of 40-60 m, and 3.89 times that in the range of 60-90 m. The width of the self-pressure-relief zone was determined to be 28-30 m through the variation of drainage concentration in the down-layer test borehole; 20-40 m in front of the working face is the gas-efficient drainage zone. 4.3.2 | Testing of drainage levels within the self-pressure-relief zone (1) The scalar volume analysis of gas drainage from boreholes through layers The variation of the mix volume of gas drainage and scalar volume of gas drainage of Test Area B penetration test boreholes 1#, 2#, 3#, and 4# concerning the distance from the working face is shown in Figure 7. We can see that the mix volume of gas drainage and scalar volume of gas drainage in the test boreholes concerning the distance from the working face can be divided into drainage scalar volume stable section, scalar drainage volume increasing section, and scalar drainage volume decreasing section. The comparison of gas drainage parameters at different stages is shown in Table 5.

Single-hole number for down-layer drilling
Drainage scalar volume stable section (working face advanced beyond 40 m). In this stage, pressure-relief's gas drainage from the borehole is less affected, and the gas drainage volume is lower, with an average gas drainage volume of 0.0061 m 3 /min and a stable gas drainage concentration of 0.5%-1%, with an average gas drainage concentration of 0.77%.
Scalar drainage volume increasing section (working face pushed to 20-40 m). During this stage, the final hole position of the drainage drill hole gradually entered the overpressure-relief zone with the recovery of the working face, and the gas drainage concentration and pure drainage volume of the test drill hole both showed an increasing trend.
Scalar drainage volume decreasing section (working face pushed to 0-20 m). The unloading effect caused by the mining of this seam also destroyed the integrity of the surrounding coal body. The sealing effect and hole formation quality of the test boreholes in this area were affected. The drainage concentration and pure drainage volume started to decline, with an average gas drainage volume of 0.0098 m 3 /min and a stable gas drainage concentration of 1%-2%, with an average gas drainage concentration of 1.49%.
(2) Scalar volume of gas drainage analysis from boreholes through layers According to the analysis in Figure 7, the gas drainage concentration of test boreholes 1#, 2#, 3#, and 4# showed the same trend with the relative distance of the boreholes from the coal mining face. Still, there were differences in the scalar volume of gas drainage in each test borehole. In the distance of 0-20 m from the working | 1321 face and outside the distance of 40 m from the working face, the scalar volume of gas drainage of the test boreholes remained in a small flow range; in the distance of 20-40 m from the working face, the variation of the scalar volume of gas drainage of the test boreholes was "∧"-shaped, with obvious "stratification" phenomenon, from high to low: 4#, 3#, 2#, and 1#. The longer the length of the drill hole in the coal seam section, the greater the scalar volume of gas drainage. A new test of extraction effectiveness needs to be introduced to evaluate the specificity of the effect of gas extraction at different heights in the coal body. In practice, for drilling boreholes arranged in coal seams where the gas content is relatively stable, the main factors affecting drilling gas extraction are the length of the borehole. It is unreasonable to compare the effect of drilling gas extraction only on the amount of gas extracted from drilling without considering the effect of the borehole's length. Therefore, taking into account the influence of the length of the borehole, the scalar volume of gas drainage from the borehole is converted into the scalar volume of gas removed per minute per meter of borehole for a given gas drainage condition. The average scalar volume of gas drainage per meter of the borehole at different heights within a single-thick coal seam is obtained, as shown in Table 6.
where q is the 100 m borehole flow rate, m 3 /min; q′ is the actual measured borehole flow rate m 3 /min; L is the actual depth of the borehole, m.
Comparing the scalar volume of gas drainage in different height sections of the coal body in the selfpressure-relief area, from large to small, 2# > 3# > 4# > 1#, the scalar volume of gas drainage in the coal seam height 0-2.5 m is better than that in the coal seam height of 0-3.5 and 0-5.3 m. The drainage effect in the coal seam height of 0-1.5 m is the worst.
Analysis of the reasons for gas with the formation of the coal seam continues to accumulate, the coal seam top and the bottom plate have the effect of sealing gas, but there is always a tiny amount of gas escaping to the nearby surrounding rock because the mass fraction of gas is less than air, in the coal body transport has an upward effect, resulting in differences in the gas content of the upper and lower boundaries of the coal seam; when the coal body is affected by mining, first of all, the coal body above the mining surface is subjected to oversupporting stress, more than after the strength limit of the coal body is exceeded, the coal body is broken and decompressed, and the oversupporting stress continues to be transferred to the deeper part of the coal seam. This transfer to the deeper part of the seam is both along the horizontal direction at the same height of the coal body and along the vertical path of the coal body, resulting in the variability of the degree of unloading of the same coal seam at different thickness interfaces, that is, the degree T A B L E 5 Comparison of gas drainage parameters at different positions from cut off.

| CONCLUSIONS
Considering the unloading effect caused by coal seam mining on the coal body in front of the working face, a flow-solid coupling model of coal mining deformation and coal seam gas seepage was established to elucidate the drainage characteristics of gas drainage at different times in different seam spaces. Combined with the field tests, we can draw the following conclusions.
(1) The width of the self-pressure-relief zone after stabilization by COMSOL simulation is 25 m. When affected by mining, the permeability of the coal body of this seam, with the height from the bottom plate, shows a trend of rising and then falling. The permeability of the coal body is most incredible at 2.6 m from the base plate, which is 8.62 times higher than that of the initial permeability. (2) The scalar volume of gas drainage from different forms of test boreholes varies in stages with the distance of workings retrieved. They are divided into the initial drainage stage, the increasing pure drainage stage, and the decreasing pure drainage stage. In the area where the boreholes are 20-40 m away from the working face, the self-pressure-relief effect of the coal body is gradually enhanced, which is the highly efficient gas drainage zone of the test borehole. (3) The width of the self-pressure-relief zone in the field test 1604 working face is about 29.1 m. The average scalar volume of gas drainage in the range of 20-40 m from the working face in the down-seam test borehole is 3.08 times that in the range of 0-20 m, 1.85 times that in the range of 40-60 m, and 3.89 times that in the range of 60-90 m. The average scalar volume of gas drainage of the penetration borehole in the self-pressure-relief zone is 0.0439 m 3 / min, which is 8.01 times greater than the initial drainage stage of the penetration borehole. (4) The test holes are arranged 20-40 m from the working face. The change of pure gas drainage volume is "∧"-shaped and has an apparent stratification phenomenon. By examining the scalar volume of gas drainage even meters, it is determined that the best drainage effect is achieved by 2#. In the coal seam height, 2.5 m is a reasonable level for control.

AUTHOR CONTRIBUTIONS
Xi Jie developed the analytical models, analyzed the data, and wrote the paper. Wang Zhaofeng, Chen Dongdong, and Chen Jinsheng gave valuable advice and contributed to the manuscript editing. All authors have read and agreed to the published version of the manuscript.