The base map is sourced from the Standard Map Service of the Ministry of Natural Resources of China (http://bzdt.ch.mnr.gov.cn/browse.html?picId=%224o28b0625501ad13015501ad2bfc0288%22), Approval No. GS(2019)1673. The map has been modified only for visualization purposes (coloring and annotation).

Figure 1. Schematic diagram showing the location of the mining area.

Xi’an Coal Mine, Beiliu Area is located within Liaoyuan City, Jilin Province, at the northwestern end of the Liaoyuan Coalfield, extending 6.31 km from east to west and 1.66 km from north to south. The location of the mining area is shown in Figure 1.

The Beiliu Area is located within the Tianshan-Xing’an Geosynclinal Fold System, the Jihei Fold System (Grade I), the Jilin Eugeosynclinal Fold Belt (Grade II), and the Shiling Uplift (Grade III) in the northwestern part of the Liaoyuan Coalfield.

Figure 2. Structural outline of the mine.

The Beiliu Area is an asymmetric syncline opening to the northwest. The axis of the syncline trends northwest and plunges to the northwest. In this section, the entire southwestern limb in the southeastern part and most of the southwestern limb in the northwestern part are missing, leaving only the northeastern limb in the southeastern part of the syncline and most of the syncline at the northwestern end. Most of the rock strata dip at 297˚ with a dip angle of 10˚ - 25˚, forming the main structural framework of the Beiliu area. Additionally, ten strike or oblique faults have developed in this syncline. Most of the faults trend northwest, with a few trending northeast and nearly east-west. Based on the cross-cutting relationships among the faults, the earliest formed faults are the northwest- and nearly east-west-trending faults (F₂, F₄, F₉, F₁₀), followed by the northwest-trending faults (F₅, F₆, F₈), while the latest formed is the northeast-trending fault (F₁). The faults were generated after the Late Jurassic and before the Tertiary. The structural outline of the coal mine is shown in Figure 2.

The main faults characteristics are shown in Table 1.

Table 1.Summary of faults in the Beiliu Area.

Danyi Coal Seam is the single minable coal seam in the Beiliu Area, which occurs at the bottom of the mudstone sub-member of the lower coal-bearing section ( J 3 2 − 1 ) of the Liaoyuan coal-bearing group ( J 3 2 ). Danyi Coal Seam is mostly minable in the mining area. The coal seam is relatively well-developed and its strata horizon is stable. The controlled strike length is about 1.5 km, and the controlled dip extension is about 2.4 km. The dip of the coal seam is 297˚, the dip angle is 10˚ - 25˚. The coal seam thickness is 0.70 - 6.06 m, with an average thickness of 2.68 m. The variation coefficient of the coal seam thickness is 63%, and the occurrence elevation is from −130 m to −1000 m. The coal seam structure is simple, generally containing 1 - 3 layers of parting. The thickness of the parting is generally about 0.2 m, with a maximum thickness of 0.48 m. The parting is mostly mudstone and carbonaceous mudstone. The coal seam is relatively thin and the thickness variation is not large. From the limb to the axis of the syncline, the coal seam thickness gradually increases, and the number and thickness of the parting in the coal seam also tend to increase.

The roof of the coal seam is a black mudstone marker layer, with a thickness of 2.70 - 75.00 m and an average thickness of 35.00 m. The floor of the coal seam is thin-layer mudstone, carbonaceous mudstone, tuffaceous sandstone, and tuff, with a thickness of 0 - 11.20 m.

The absolute gas emission of the mine is 13.53 m³/min, and the relative gas emission is 3.79 m³/t. The maximum absolute gas emission of the coal-mining face is 7.18 m³/min, and the maximum absolute gas emission of the tunneling face is 1.45 m³/min. It is a high-gas mine. No gas dynamic phenomena such as hole-spraying and drill-jamming have occurred during the daily mining process.

Coal failure type refers to the category formed due to different degrees of destruction of the coal body structure under the action of tectonic stress, resulting in different physical and mechanical properties and characteristics of the coal. The more severe the coal failure, the greater the outburst risk. When determining the coal failure type, the following aspects are generally considered: coal luster, coal structure and structural characteristics, coal joint properties, coal joint surface properties, coal strength, and coal fracture properties. According to the “Regulations on Prevention and Control of Coal and Gas Outbursts,” coal failure types are classified into Types I, II, III, IV, and V. The outburst risk of coal increases gradually from Type I to Type V. Generally, Types I and II belong to the non-outburst-prone type, Type III belongs to the outburst-prone type (tending to be dangerous), and Types IV and V belong to the outburst-prone type.

On-site observations were carried out on newly exposed coal seams at locations such as the Belt Decline Roadway (BDR) in the Danyi Coal Seam of the Beiliu Area. The observation points are arranged as shown in Figure 3, numbered TCJD20170174, TCJD20170175, TCJD20170176, TCJD20170177, and TCJD20170178. Coal samples at these points were mainly collected from the soft sub-layer within the observed coal seam, representing the portion of the entire seam most prone to outburst. Through methods such as visual inspection and hand manipulation, it was found that the Danyi Coal Seam exhibits a vitreous to greasy luster, belonging to the semi-bright to semi-dark type. In terms of structure and texture, the coal seam shows relatively obvious horizontal bedding and is basically layered in structure. Regarding joint properties, secondary joint surfaces are relatively developed and irregular. In terms of joint surface properties, the joint surfaces are slippery and can be easily broken apart by hands. As for fracture properties, the fractures are conchoidal and irregular. A comprehensive judgment indicates that the Danyi Coal Seam belongs to Type II failure type.

The initial velocity of gas emission (Δp) is one of the indicators for predicting the risk of coal and gas outburst. This indicator reflects the coal’s ability to adsorb gas under atmospheric pressure and the speed of gas emission (the transition of gas from adsorbed state to free state) when the gas-bearing coal body is exposed. The initial velocity of gas emission can reflect the pore structure and microscopic degree of failure of the coal. The performance of coal in emitting gas is determined by the physical and mechanical properties of the coal. It is a single indicator reflecting the risk of outburst in a coal seam area. In the development process of coal and gas outburst, the movement and destructive force of gas largely depend on the desorption and emission capacity of gas when the gas-bearing coal body is destroyed. Under the same gas content conditions, the greater the initial velocity of gas emission of the coal, the more favorable it is for the occurrence and development of outbursts.

The firmness coefficient of coal (f) is a comprehensive indicator representing the ability of coal to resist external destructive forces. In essence, it reflects the amount of energy consumed per unit mass of coal when destroyed. The firmness coefficient of coal is mainly determined by the physical and mechanical properties of the coal. The greater the strength of the coal body, the higher the firmness coefficient, the greater the resistance encountered during coal and gas outburst, and the lower the possibility of outburst occurrence.

Coal samples of the Danyi Coal Seam were collected at five sampling points from exposed coal seams or through core drilling, and the sampling and tested results are detailed in Figure 4 and Table 2.

Table 2.Tested results of initial velocity of gas emission (Δp) and firmness coefficient (f).

(a) TCJD20170174 (b) TCJD20170175

(c) TCJD20170176 (d) TCJD20170177

(e) TCJD20170178

Figure 4. Gas emission curve of Danyi coal seam coal samples.

Coal seam gas pressure is one of the main indicators for judging the risk of coal and gas outburst. Gas pressure is also an important indicator of the level of gas compression energy contained in the coal body, serves as the necessary driving force for the occurrence of coal and gas outburst, and is a necessary condition for the occurrence of coal and gas outburst.

Based on the existing roadway layout and coal seam exposure conditions in the mine within the identification zone, and after comprehensive analysis of rock fracture development, coal seam exposure time, roadway cross-section size, and other factors, two rock roadways—the Extension of the Second-stage Blind Return Air Shaft (ESBRAS) and the Belt Decline Roadway (BDR)—were adopted to carry out a total of 5 measurement points (10 cross-layer boreholes) to determine the coal seam gas pressure, among which measurement point No. 5 is located at the deepest part of this identification zone. The starting point of upward boreholes is 1.5 m to 2.0 m from the roadway floor, while that of downward boreholes is about 0.5 m from the roadway floor. The completion parameters of the boreholes are listed in Table 3. The borehole diameter is 113 mm, and the cross-layer pressure-measuring boreholes penetrate the full thickness of the coal seam. During the construction of all boreholes, no gas dynamic phenomena such as bit pulling or blowout were encountered in the Danyi Coal Seam. The passive pressure measurement method was adopted for this identification to measure the gas pressure of the Danyi Coal Seam.

Table 3.Completion parameters of boreholes for gas outburst identification.

a. Coal Intersection Point; b. Starting Point Elevation; c. Coal Seam Floor Elevation; d. Coal Seam Burial Depth; e. Coal Seam Burial Depth.

(a) Cross-section of Upward Borehole (b) Cross-section of Downward Borehole

Figure 5. Typical borehole cross-section.

Cement slurry was used as the sealing material. The sealing length of cross-layer boreholes depends on the lithology of the coal seam roof (or floor) and the borehole length. The pressure measuring pipe is connected using 4-point seamless steel pipes with threads at both ends. At the bottom section of the borehole, within 0.5 m from the end, sieve holes are made to serve as the gas inlet channel. Each section of the pressure measuring steel pipe is 2 m in length. To ensure the airtightness, the pipe joints are wrapped with PTFE tape. The end of the pressure measuring tail pipe is wrapped with fine copper gauze to prevent coal chips and debris from entering and clogging the pipeline. The cross-section of the completed boreholes is shown in Figure 5.

The gas pressure observation results were recorded on a coal seam gas pressure measurement record sheet, and plotted on a coordinate graph with time (T/d) as the horizontal axis and gas pressure (P/MPa) as the vertical axis. If the pressure observation time meets the above principles and the pressure change is less than 0.015 MPa within 3 days, the pressure measurement work can be concluded, and the current pressure is the final gas pressure (relative pressure) of that pressure-measuring borehole. The general pressure observation period should be no less than 30 days.

Table 4. Summary of coal seam gas pressure determination results.

Figure 6. Gas pressure rise curve of pressure-reading boreholes in the Danyi coal seam.

Ultimately, the readings of pressure gauges at Borehole 1-1#, 1-2#, 2-1#, 3-1#, 3-2#, 4-1#, and 4-2# were 0 MPa; 5-1# was 0.040 MPa, 5-2# was 0.105 MPa, and 2-2# was 0.140 MPa, as shown in Table 4. The observation results were plotted, as shown in Figure 6.

For gas content determination, coal samples were taken using a fixed-point sampling device at a sampling depth of 25 m. The determination results of the coal seam gas content are shown in Table 5.

Table 5.Summary of gas content determination results.

It can be seen that the gas content of the Danyi Coal Seam ranges from 1.19 m³/t to 2.92 m³/t, with a maximum gas content of 2.92 m³/t and an average gas content of 1.85 m³/t. Gas is generated in and stored in the coal seam. The magnitude of gas content is also closely related to the burial depth of the coal seam: the deeper the burial depth, the less likely the gas is to escape, so more gas is stored in the coal seam, resulting in a higher gas content, and vice versa.

In Table 5, the gas content at the −270 m level return air crosscut significantly deviates from the overall gas distribution in the area, indicating that this point is influenced by local geological structures and should only be used as a reference for analyzing gas distribution patterns. The analysis of gas distribution patterns was conducted using the other three measurement points.

Figure 7.Relationship between gas content and burial depth.

Using the Adopted data of the Single Coal Seam gas content in Table 5, the relationship curve between gas content and burial depth for the single coal seam is shown in Figure 7.

As can be seen from Figure 7, the gas content of the Danyi Coal Seam in the Beiliu Area of Xi’an Coal Mine increases with the increase of coal seam burial depth. There is a good linear statistical relationship between the gas content and burial depth of this coal seam (correlation coefficient R² = 0.9958), as shown in Equation (1).

W = 0.004H − 0.8824 (1)

where:

W—coal seam gas content, m³/t;

H—coal seam burial depth, m.

Gas content gradient refers to the average increase in gas content for every 100 m increase in coal seam burial depth. From Equation (1), it can be seen that the gas content growth gradient of the Danyi Coal Seam is 0.4 (m³/t)/(100 m), that is, for every 100 m increase in depth, the coal seam gas content increases by 0.4 m³/t.