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Article

Study on Coal Seepage Characteristics and Secondary Enhanced Gas Extraction Technology under Dual Stress Disturbance

by
Xiong Ding
1,
Cheng Zhai
1,2,*,
Jizhao Xu
1,2,
Xu Yu
1,2 and
Yong Sun
3
1
School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China
2
Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou 221116, China
3
School of Low-Carbon Energy and Power Engineering, China University of Mining and Technology, Xuzhou 221116, China
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(22), 15118; https://doi.org/10.3390/su142215118
Submission received: 20 October 2022 / Revised: 2 November 2022 / Accepted: 11 November 2022 / Published: 15 November 2022
(This article belongs to the Special Issue Prevention and Control of Coal Mine Gas Disasters)
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Abstract

:
During the mining of coal seams with outburst hazard, abnormal gas emissions in front of the coal mining working face (CMWF) may induce gas overrun. To address this technical problem, this study analyzed the permeability variation of coal in front of the CMWF at different stress paths through physical experiments, numerical simulation and on-site tests. The spatial-temporal evolution law of the unloading area of the working face under dual stress disturbance caused by hydraulic punching (HP) and coal seam mining was explored; next, a secondary enhanced extraction technology was proposed and applied in the Shoushan No. 1 Coal Mine, Henan Province, China. The results reveal the following: (1) the coal permeability decreases linearly with increasing confining pressure (CP) and axial pressure (AP) under Stress Paths 1 and 2 (that is, fixed AP and CP). (2) The coal permeability is negatively related to the distance from the stress peak point under Stress Paths 3 and 4 (that is, AP and CP are, respectively, the vertical stress and horizontal stress before the stress peak). (3) As the distance from the peak stress declines, the reduction amplitude of coal permeability in the test area first decreases, and then increases, under Stress Paths 5 and 6 (that is, the vertical stress as CP and the horizontal stress as AP). The plastic damage range of coal around the HP cavities expands due to the dual impact of HP and coal seam mining, which can realize both regional unloading and provide channels for gas extraction within 60 m in front of the CMWF. According to the gas extraction concentration of boreholes, the coal body in front of the CMWF is divided into three zones: efficient, effective and original extraction zones. The efficient extraction zone is within 20 m in front of the CMWF, with an average gas extraction concentration of over 30%. In the effective extraction zone, the gas extraction concentration falls with the increase in the distance from the CMWF. The original extraction zone is beyond 50–60 m, and the borehole gas concentration stabilizes below 10%. The number of extraction boreholes in the stress disturbance area of the middle-floor gas extraction roadway accounts for 5–10% of the total number of boreholes, but its maximum monthly extraction volume can reach 38.5% of the total volume.

1. Introduction

As shallow coal resources become depleted, the mining depth of coal seams is increasing [1]. Deep high-gas-content coal seams feature low permeability, high adsorption and a high outburst hazard, which causes great difficulty in gas control [2,3]. For high-gas-content coal seams with an outburst hazard, it must be ensured that gas pre-extraction is up to the standard before the working face mining, hence preventing coal and gas outbursts [4,5,6]. However, engineering practice shows that even if coal seam gas has been pre-extracted, coal seam gas still flows into the mining space during working face mining, causing the gas concentration to exceed the limit [7,8,9]. The secondary enhanced gas extraction, after pre-pumping in the stress disturbance area (the stress loading and unloading area) in front of the CMWF, can effectively reduce the amount of desorbed gas gushing out under the mining and hydraulic punching disturbance, reduce the gas concentration in the mining space, and improve the mining efficiency.
The variations of stress distribution and coal permeability are the focus of research concerned with the permeability enhancement and mining of coal seams. Influenced by mining, gas in the pre-peak stress loading area is accumulated under a low permeability, which forms a high gas pressure. A great difference in gas pressure exists between the pre-peak loading area and the post-peak stress unloading area, which induces large quantities of gas to gush out into the working space, and even causes gas outbursts [10,11]. Wang et al. [12,13] studied the coal permeability characteristics and gas extraction effect in the unloading area, demonstrating the necessity of unloading extraction during protective layer mining. Duan et al. [14] analyzed how periodic stress disturbance influenced gas extraction by studying the acoustic emission signals and damage variables of coal samples in tri-axial loading tests. The mining disturbance changes the original stress and gas pressure, and it also exerts a great impact on the coal permeability [15,16,17]. Cui et al. [18] quantitatively explored the influence of coal reservoir pressure and adsorption volumetric strain on coal permeability during coal seam mining, and proposed a permeability model related to stress. Liu et al. [19] obtained the seepage evolution law of coal under different stress paths using the transient measurement method, and established a gas diffusion model under high stress. Zhao et al. [20] established a gas seepage model based on the coupling of theoretical equations, simulated the variations of coal gas seepage and pressure in boreholes, and determined the reasonable spacing for boreholes. In recent years, scholars tested the permeability of coal samples under high stress and yielded fruitful results by triaxial loading devices [21,22,23,24]. Jiang et al. [25] recorded the seepage characteristics of coal after the yielding point, during triaxial loading and unloading of raw coal, with the aid of X-ray tomography. Xue et al. [26] put forward a post-peak stress permeability model of coal by monitoring the coal fracturing process and permeability variation through a 3D acoustic emission system.
The efficiency of gas extraction can be improved through artificial fracturing and permeability enhancement measures (hydraulic punching (HP), hydraulic fracturing, loose blasting, etc.) or gas injection displacement in coal seams [27,28,29,30,31]. HP is a popular permeability enhancement method at present. Based on the permeability evolution of coal around HP cavities, Gao et al. [32] classified three different seepage zones. Zhang et al. [33] designed a reasonable gas extraction scheme by analyzing the permeability enhancement mechanism and gas flow characteristics in HP cavities. However, research on HP is faced with a common problem: HP has been studied and analyzed in isolation, without considering the influence of coal seam mining. In addition to improving gas extraction rates through coal seam permeability enhancement and gas injection displacement, gas extraction in goaf during coal seam mining also plays an important role in reducing the gas concentration in the mining space [34,35]. However, gas extraction in goaf cannot lower the gas concentration of coal in the stress disturbance area, and thus fails to reduce the hazard of coal and gas outbursts in the CMWF.
In summary, current research on gas extraction in coal mining space under the influence of mining mainly focused on the unloading area in front of the CMWF and the goaf area behind the CMWF. There is a lack of research on gas extraction in the stress disturbance area. Research on the influence of mining and HP on gas extraction in the working face are relatively independent, and have ignored enhanced gas extraction in the working face under dual stress disturbance. Relevant experiments on the permeability of unloaded coal has primarily applied CP and AP with constant gradients, and the stress loading and unloading paths of coal have all been based on experience, which failed to reflect the actual stress variation of coal in front of the CMWF. In addition, few scholars have applied the research results to the practice of gas extraction in the disturbance area.
Deep high-gas-content mines are of a poor coal seam gas extraction effect, leading to massive gas emission from coal seam, thus inducing the gas concentration to frequently exceed the limit. This study took a typical outburst coal seam with a high gas content in the Shoushan No. 1 Coal Mine as its background and employed coal sample permeability measurement experiments under in-situ stress loading, numerical simulation of a large stope and on-site gas extraction concentration detection. The characteristics of coal seepage in the stress disturbance area and the effect of gas extraction were studied, and a technology of gas secondary enhanced unloading extraction under the dual stress disturbance of HP and coal seam mining is proposed to solve the problem of gas concentration exceeding the limit in the mining space.

2. Research Methods

2.1. The Materials

The F15–17-12110 working face in the Shoushan No. 1 Mine is of a single structure, with an elevation of −728 m to −771 m, and a thickness of 4.01–6.5 m (5.3 m in average). A large burial depth, good sealing and great gas pressure result in complex geological conditions for gas. The F15–17 coal seam has a permeability coefficient of 0.038–0.871 m2/(MPa2•d), belonging to low-permeability extractable coal seams. Furthermore, it is prone to outburst and is unsuitable for protective layer mining. Gas in the mining area was pre-extracted through cross-measure drilling (HP) and along-measure drilling. Coal seam mining destroys the dynamic equilibrium of gas adsorption-desorption of the coal body in the stress disturbance area and large volumes of gas in the coal flows into the working space, along the penetrated fractures. In areas with abnormal gas occurrences, the gas concentration is likely to exceed the limit, which seriously threatens production safety.

2.2. Establishment of Numerical Models

According to the working face layout and geological conditions of the Shoushan No. 1 Coal Mine, the 12,110 working face mining was numerically simulated. Numerical simulations were carried out in two parts. First, by deriving stress distribution curves as the stress loading path for the permeability experiment. Next, the seepage characteristics of the coal body in the stress disturbance area during mining were explored. The continuum mechanics analysis software, FLAC3D, was used in the simulation while the Mohr-Coulomb criterion served as the failure criterion [36].
Two numerical models were established on the basis of the data regarding the 12,110 working face (Figure 1). Model 1 contained no cross-measure HP cavities, while Model 2 included HP cavities. The parameters of coal and rock mass are shown in Table 1. The sizes of both models were 200 m × 300 m × 50 m. Normal displacement was constrained at the sides; omnidirectional displacement was constrained at the bottom [37]; and a load of 15 MPa was applied to the top. The working face was 130 m wide and was mined along the Y-axis by 180 m. This study assumes the HP cavities to be cylinders [38], with a cavity radius of 0.8 m, a cavity height of 5 m, and a borehole spacing of 6 m.

2.3. Sample Preparation and Experimental Instruments

Coal samples were taken from the F15–17 coal seam of the Shoushan No. 1 Coal Mine. Large coal samples were processed into 6 cylinders (P1–P6, ϕ50 mm × 100 mm) by wire cutting for the seepage experiments under the in-situ stress state.
The experimental device, which is a mechanics and seepage experiment platform for in-situ development of coalbed methane [39] (Figure 2), is composed of: a data acquisition module (a); a computer (b); an ultrasonic acquisition system (c); a gas control system (d); a core holder (e); and a stress loading system (f). Data of gas inlet pressure, gas outlet pressure, AP, CP, gas flow rate and ultrasonic signal can be collected in the seepage experiment.

2.4. Experimental Procedure

2.4.1. Extraction of Stress Loading Path

In the simulation, the coal seam mining length was 60 m, 120 m and 180 m. The variation of the ground stress value within 50 m of the working face, at 120 m coal seam mining length, was extracted. According to Figure 3, Mσ represents the stress curves without cross-measure HP cavities, while MHσ represents the stress curves under the dual influence of mining and HP cavities.
The coal body in front of the CMWF can be divided into the pre-peak stress loading area and post-peak stress unloading area, with the stress peak point as the boundary. Influenced by the unloading of coal roadway and goaf, the horizontal stress in the middle of the working face also experiences stress concentration. The peak values of the vertical stress under the two conditions appear 7.5–8 m in front of the CMWF, which are 2.99 and 3.76 times greater than the initial vertical stress σz. The monitoring curves were arranged in the center of the working face, i.e., in the middle of two rows of HP cavities. The coal around the cavity is unloaded, and the vertical stress transfers to the center of the HP cavities. Under the joint action of stress transfer of the working face and the HP cavities, the peak value of the vertical stress rises. The peak values of horizontal stress in the X-axis direction are 23.6 MPa and 23.3 Mpa, respectively, located within 7–8 m in front of the CMWF. Due to the influence of HP cavities and coal roadway, the stress of coal among the two rows of boreholes is superposed, and the vertical stress MHσxis greater than Mσx, which is influenced by the coal roadway alone. The peak horizontal stresses are located within 10–11 m in front of the CMWF, at 24.6 MPa and 29.9 MPa, respectively.

2.4.2. Permeability Tests

Three groups of contrastive permeability tests were conducted (Table 2 and Figure 4). For the first group (Paths 1 and 2), the sensitivity of coal permeability to the variations of vertical and horizontal stress in front of the CMWF was explored by loading CP (or AP) with a constant gradient, within the range of 11–20 MPa under a fixed AP (or CP) of 15 MPa. In the post-peak unloading stage, the coal body is severely damaged, and the permeability coefficient of coal surges; resultantly, it is pointless to conduct permeability tests. In addition, for the purpose of preventing the coal sample from fracturing and the pulverized coal from blocking the instrument outlet, the second and third groups of permeability tests merely used the pre-peak stress as the stress loading path. The range of stress values is from approximately 48 m in front of the CMWF to the peak stress area, and the stress value is derived every 4 m as the stress loading path. In the second group (Paths 3 and 4), according to the stress distribution of the two numerical models in Figure 3, the vertical stress σz and horizontal stress σx serve as AP and CP, respectively, for the tests. Path 3 refers to the distribution of stress in front of the CMWF (48–12 m) without HP cavities in the coal seam, while Path 4 refers to the distribution of stress in front of the working face (48–16 m) with HP cavities. To study the effect of different principal stress directions on the permeability of the samples, considering the location direction of the coal samples in the original seam, in the third group, the permeabilities of coal samples under Paths 5 and 6 were tested with the vertical stress in the stress disturbance area as the CP and the horizontal stress as the AP.
To ensure the safety of the tests, gas adsorption by coal was ignored, and methane was replaced with helium to test the sample permeability [40,41]. The experimental procedure is described as follows:
(1)
Preparation: The samples were dried in an incubator at 40 °C, for 12 h, to reduce the influence of moisture on permeability. After the diameter and height of the samples were measured (P1–P6), the side of the coal samples were wrapped with heat shrink tubes to improve the experimental accuracy because coal particles might fall from the sample surface during the tests.
(2)
Tests: Sample P1 was placed into a core holder; then, the AP and CP rose to the initial stress boundary at the same time. The air inlet valve was opened after the stress stabilized, and the air inlet pressure was controlled at 1.0 MPa. When the internal pressure of coal reached an equilibrium and the gas flow stabilized, the inlet pressure, outlet pressure, and gas outlet flow were recorded. The AP and CP were adjusted according to the designed stress loading paths and the experimental parameters were recorded.
(3)
Cleaning after the tests: The air inlet valve was closed when the pre-set end stress was reached in order to reduce the air inlet pressure to 0 MPa. Subsequently, the AP and CP decreased, slowly, at the same time. After the stress decreased to 0 MPa, the pump with constant flow and constant pressure was turned off; Sample P1 was taken out and the coal chips in the core holder were cleaned.
The above Steps (2) and (3) were repeated, and the permeability of coal samples (P1–P6) was tested using the steady state method. When helium reaches a steady flow under the effect of pressure difference, the permeability of coal can be calculated by Darcy’s law [42]:
k = 2 Q P 0 μ L 0 F ( p 1 2 − p 2 2 )
where k is coal permeability, m2; F is the cross-sectional area of the sample, m2; Q is the gas outlet flow, m3/s; P0 is the standard atmospheric pressure, Pa; P1 is the gas inlet pressure, Pa; P2 is the gas outlet pressure, Pa; μ is the helium dynamic viscosity, Pa⋅s; L0 is the sample height, m.

3. Results and Analysis

3.1. Numerical Simulation

3.1.1. Distribution of Plastic Zone

As shown in Figure 5, status A refers to the distribution of the plastic failure of coal in the stress disturbance area under dual stress disturbance, when the coal seam is mined by 120 m. Status B shows the distribution of the plastic zone without the disturbance of coal seam mining. According to the damage range of coal around the HP cavities in Status A, the 10 groups of boreholes can be divided into two zones. Zone I is approximately 20 m in front of the CMWF, where the coal around the HP cavities dilates, causing the plastic damage range to expand. Zone II lies 20~60 m from the CMWF, the cavity is less affected by the mining, and the distribution state of the plastic zone of the coal body is less variable. Under the dual stress disturbance of HP and coal seam mining, the coal around the HP cavities, within approximately 20 m in front of the CMWF, dilates; the plastic damage range expands, and the pores and fractures develop, which effectively facilitates enhanced gas extraction.

3.1.2. Stress Distribution

Six rows of boreholes were arranged in the simulation. The stress distribution among the fourth row of boreholes (x = 103 m) in front of the CMWF is shown in Figure 6, where “MH” is the stress distribution curves of coal under the dual influence of mining and HP, and “M” is the initial stress distribution curves of coal influenced only by mining. The peak values of vertical stress σz and horizontal stress σx among cavities are both higher than the initial stress, while the horizontal stress σy is lower than the initial stress. This is because, for coal around HP cavities, σz is superposed with the σx stress concentration area and σy stress unloading area. The distributions of stress of the second, fourth, sixth and eighth groups of boreholes, when y = 132 m, 144 m, 156 m and 168 m in front of the CMWF, are shown in Figure 7. Compared with the initial stress, σx experiences stress reduction in boreholes due to HP cavities. Vertical stress σz and horizontal stress σy display stress reduction and concentration area, gradually, from the HP cavities, outward. Even if influenced by the high abutment stress of the CMWF, the coal around the HP cavities also displays an annular unloading zone.
In general, under the dual disturbance, the differential stress of coal around cavities within 50 m in front of the CMWF increases, which expands the plastic damage range of the coal around the HP cavities and the macro-cracks further expand, providing channels for gas extraction. In addition, stress loading and unloading areas exist in the coal around the HP cavities, within 50 m in front of the CMWF. As the ground stress in the unloading area falls, the effective stress decreases and the pore fracture opening increases.

3.2. Tests for Coal Permeability

3.2.1. Variation of Coal Permeability under Conventional Paths

To analyze the sensitivity of coal permeability in the pre-peak stress loading area of the working face to the variation of vertical stress and horizontal stress, permeability tests were conducted on coal under fixed AP and CP of 15 MPa, respectively, according to Paths 1 and 2. The stress loading gradient is 1 MPa, and the loading range is 11–20 MPa. The variation of coal permeability is shown in Figure 8. As the volume stress rises, the coal pores are compressed, and the permeability decreases linearly in the stress loading area. The fitting curve of permeability ka decreases at 0.00919 mD/MPa under a fixed AP. The CP rises from 11 MPa to 20 MPa and the permeability drops from 0.144 mD to 0.063 mD, by 56.5%. The fitting curve of permeability kc decreases at a gradient of 0.00425 mD/MPa under a fixed CP. The permeability falls by 19.9% when the AP rises from 11 MPa to 20 MPa. Compared with the permeability under a fixed AP and varying CP, the permeability varies slowly under a fixed CP and small differential stress, indicating that the CP plays a leading role in the permeability test under a low differential stress. According to Figure 6, under the mining disturbance, the coal seam beyond 30–50 m in front of the CMWF is of a relatively stable volumetric stress, small differential stress and small variation of permeability, which does not change the state that the gas extraction efficiency plummets after pre-extraction.

3.2.2. Variation of Coal Permeability under In-Situ Stress Paths

In the experiment, helium is a non-adsorption gas which does not involve coal adsorption and matrix shrinkage and expansion. The relationship between stress and permeability can be analyzed according to the permeability evolution under the stress loading path of coal. As the permeability ranges of the samples differ, the permeability k0 of the first test point in the stress loading path, serving as the initial value; then, the permeability ratio k/k0 was obtained by proportionally treating the permeability k [39] (Figure 9).
When the coal in the stress disturbance area is only affected by mining without HP cavities, the variation of the k/k0 of coal under Path 3 is shown in Figure 9a, the initial permeability k0 being 0.166 mD. The closer it is to the peak stress, the more the AP and CP rise, the smaller the pores and fractures in the coal body are, and the lower the permeability is. During stress unloading along Path 3, the stress of the test points within 30–50 m from the CMWF varies slightly, and the permeability decreases slowly. The permeability k at the test point 32 m decreases by approximately 9.3% compared to the k0. The stress loading amplitude of the test point within 10–30 m from the CMWF increases, and the permeability decreases rapidly. The permeability k of the test point at 12 m is 0.057 mD, 17.9 MPa higher than the initial AP; the CP rises by 8.2 Mpa, and the k/k0 declines by 65.9%. In the stress loading area, the coal permeability and the length of coal from the peak stress point are displayed in a logarithmic distribution; the permeability is close to 0 around the peak stress. During stress unloading along Path 3, the permeability fails to be restored to the original value, indicating an irrecoverable compression effect of high ground stress on the coal body. When the stress returns to the initial value, the permeability k of the coal is 0.111 mD, and the k/k0 is 66.8%. When the coal body is affected by the dual disturbance, the superposed stress between two rows of boreholes is derived as Path 4. In order to prevent coal from being damaged by excessive differential stress, the stress extraction range is 16–48 m in front of the CMWF. The variation of coal permeability is shown in Figure 9b. Similar to Path 3, the permeability at the stress extraction point, within 30–50 m in front of the working face, varies slightly and the k/k0 at the test point at 32 m is approximately 89.3%. Within 15–30 m in front of the CMWF, the permeability of the stress extraction point plunges due to an effective rise of stress. The AP at the test point 16 m away from the CMWF is 15.7 Mpa higher than the initial AP; the CP rises by 5.8 Mpa; the coal permeability k decreases from 0.358 mD to 0.175 mD, and the k/k0 decreases by 51.1%. When the applied stress of the sample recovers to the initial stress value, the permeability k of the coal sample is 0.298 mD and the k/k0 is 83.2%.
The vertical stress of the coal body in the stress disturbance area served as the CP, and the horizontal stress served as the AP. Hence, Paths 5 and 6 were formed to test the coal permeability. The variation of the permeability ratio k/k0 is shown in Figure 10. The k/k0 of Paths 5 and 6 essentially vary in the same way: as the distance from the peak stress declines, the permeability of the sample gradually decreases and the reduction amplitude of k/k0 in the test area first rises, and then declines. When the reduction amplitude of permeability decreases, the CPs of Paths 5 and 6 are 27.5 Mpa and 25.3 Mpa, respectively. The reduction amplitude of permeability decreases under 25 Mpa CP. The coal permeability k in Path 5 decreases from the initial 0.161 mD to 0.018 mD, and the permeability ratio k/k0 decreases by 88.9%; these two parameters of Path 6 decreases from 0.072 mD to 0.015 mD and fall by 79.5%, respectively. This is because, compared with Paths 3 and 4, Paths 5 and 6 correspond to a larger CP and larger effective stress, which directly leads to a greater reduction amplitude of coal permeability.
According to the permeability test results of the coal samples under the in-situ stress loading path, as the effective stress rises, the permeability of coal declines, and the minimum value appears near the peak stress. When the vertical stress is loaded as the AP, the permeability of the pre-peak stress loading area in the working face gradually decreases at a rising rate. When the vertical stress is loaded as the CP, the permeability reduction amplitude of the pre-peak stress loading area in the working face increases first, yet it decreases under approximately 25 Mpa CP. Under the influence of coal mining, the coal body behind the peak stress is destroyed, and the permeability is greatly enhanced. On the contrary, the coal permeability in the stress loading area in front of the peak stress decreases, which further weakens the gas extraction effect. As a result, a large gas pressure gradient is formed in front of the CMWF. If an abnormal gas occurrence area is encountered, massive coal seam gas flows into the working face and even induces gas outburst.

4. Secondary Enhanced Extraction (SEE) in Coal Working Face

At present, for the main minable coal seam of the Shoushan No. 1 Coal Mine, coal seam gas is pre-extracted by constructing bedding boreholes and drilling cross-measure boreholes (HP) in the absence of protective layer mining. The working face is mined after the pre-extraction has reached standard. However, due to mining-induced disturbances, large volumes of unloaded gas still flow into the mining space, threatening safe production in the CMWF. When the gas concentration in the mining space exceeds the limit, it is necessary to conduct SEE.
According to the results of the numerical simulation and permeability experiments, under the dual influence of HP and mining, the coal body around the cavity within 50 m in front of the CMWF forms an unloading area, and the differential stress increases; the plastic damage range expands, and the macro-cracks further develop, providing channels for gas extraction. When coal gas in front of the CMWF is extracted through HP cavities, it not only affects the mining progress, but also improves the utilization efficiency of the floor rock roadway, hence facilitating effectively SEE of the working face. On-site extraction was conducted in the 12,110 working face of the Shoushan No. 1 Coal Mine.

4.1. Experimental Plan for the SEE in the Stress Disturbance Area

To compare the effect of gas extraction in different areas in front of the CMWF, the range of the SEE test in the stress disturbance area was initially set as 0–100 m in front of the CMWF, including the original rock stress area, the pre-peak stress loading area, and the post-peak stress unloading area. The test site is the middle floor gas extraction roadway (MFGER) of the 12,110 working face. Gas can be pre-extracted by intensive cross-measure drilling at the coal seam floor. However, under high ground stress, stress concentration circles are formed around the boreholes, resulting in the closure of radial cracks around the boreholes. This is unfavorable for the radial flow of gas towards the borehole and may restrict the sustainability of gas extraction [43]. Therefore, HP is required after the formation of cross-measure boreholes to promote the development of coal fractures and create dominant channels for gas flow around boreholes. The specific extraction experiment scheme is as follows:
Two extraction pipelines were laid in the MFGER. In addition to the original pre-extraction pipelines, special extraction pipelines were laid to increase the negative extraction pressure. With the aid of the 12,110 MFGER, SEE was conducted in the stress disturbance area by cross-measure drilling (HP) in the strata beyond 100 m from the open-off cut of the CMWF to be mined. Cross-measure drilling is designed as follows: from 20 m outside the stopping line, cross-measure drilling is conducted from the 12,110 MFGER to the coal seam. Each group contains 14 boreholes, of which odd-numbered and even-numbered boreholes are constructed separately. In the MFGER, the initial group spacing of the boreholes is 6.4 m, and the row spacing is 3.2 m. Subsequently, five densified boreholes are constructed in the middle of each group of boreholes to enhance gas extraction. The borehole layout and gas extraction method are shown in Figure 11.

4.2. On-Site Extraction Effect

4.2.1. Gas Concentration Detection of SEE in the Stress Disturbance area

The gas concentration of SEE in the stress disturbance area was detected in the 12,110 MFGER. The gas concentration was measured by the infrared principle, which can monitor the gas concentration of 0–100% VOL. As different gases have different absorption spectra, the infrared detection is free from the influence of other gases. The method used to measure the gas concentration is shown in Figure 12. The measurement process is as follows: First, the rubber tube at one end of the mining high negative pressure gas collector was connected to the gas extraction borehole; afterwards, gas mixture in the borehole was pressed into the rubber tube through the collector. After the gas was filtered by gas desiccant, the gas concentration meter was connected. The mixed gas was continuously extracted until the indication of the gas concentration meter stabilized and the value was recorded.
As the gas concentrations of 15 groups of boreholes within 100 m in front of the work should be detected within the 8 h of equipment maintenance, only four boreholes (numbered 5#-8#) in each group were selected for gas concentration detection.
As shown in Figure 13a, the 5#-8# boreholes of Groups 85–99 were detected first. The closer it is to the working face borehole, the higher the gas concentration is. The 5# and 6# boreholes in Group 98 correspond to the maximum gas extraction concentrations of 55.3% and 50.1%, respectively. The boreholes in Group 98 are located 6.4–12.8 m in front of the CMWF. Coal in this area corresponds to the peak stress. With reference to the distribution range of the plastic failure area, the coal body around the HP cavities is seriously damaged; the cracks are connected, and the coal permeability is enhanced. However, the boreholes in Group 99 are the closest to the working face. They may leak air and thus result in a decline in gas concentration. The average gas concentration of the three groups of boreholes within 20 m in front of the CMWF exceeds 30%. As the distance from the CMWF rises, the gas concentration of the boreholes generally falls. The gas extraction concentration in the original stress zone stabilizes at a low value, and the average gas concentrations in Groups 85–88 are approximately 4.39%.
As shown in Figure 13b, the 5#-8# boreholes in Groups 76–90 were detected next. Compared with the first detection, the CMWF advanced for approximately 100 m. The average gas concentration of the boreholes in Group 90, which is the closest to the CMWF, is 44.5%, which is higher than that of the first detection. Such a result may be attributed to weak air leakage of boreholes. The 5# borehole in Group 89, which is 6.4–12.8 m away from the CMWF, has the maximum gas extraction concentration of 62.5%. The average gas concentration of the boreholes within 20 m in front of the CMWF reaches 36.76%, without taking into consideration the boreholes with abnormal gas concentrations. Within the range of 20–60 m, the gas concentration declines in fluctuations and tends to stabilize. The boreholes in Groups 76–80, which are far from the CMWF, are less affected by the mining, and their average gas concentration is below 5%.
The stress disturbance area can be divided into three zones along the coal seam mining trend: (1) The first zone is an efficient gas extraction zone, within 20 m in front of the CMWF, where the coal body dilates and fractures; the fractures develop and connect, therefore, gas can easily flow. The gas in the coal body close to the CMWF escapes through fractures under the influence of air flow in the working face, therefore, the gas concentration in the borehole is relatively low. (2) The second zone is an effective gas extraction zone. The coal body is in the pre-peak stress loading area, and the gas extraction concentration fluctuates and decreases with the increase in distance from the CMWF. (3) The third zone is an original extraction zone, beyond 50–60 m from the CMWF. The borehole gas concentration in this zone is below 10%. The coal body is slightly affected by mining and is in the original stress area or corresponds to a small stress concentration.

4.2.2. Monthly Gas Extraction Volume

Gas in the stress disturbance area was extracted from February 2021. The monthly gas extraction volume of the MFGER of the 12,110 working face is shown in Figure 14. The number of extraction boreholes in the stress disturbance area of the MFGER accounts for 5–10% of the total number of boreholes. In February, 65,768 m3 gas was extracted through cross-measure boreholes in the stress disturbance area, accounting for 35.5% of the gas extracted through cross-measure boreholes in the MFGER in February. In March, these two figures were 55,763 m3 and 28.5%, respectively. In April, the amount of gas extracted from cross-measure drilling in the stress disturbance area was 38.5% of that in the MFGER in April, with a total of 155,690 m3 of gas extracted. The SEE in the mining area using the cross-measure drilling of the MFGER achieves a significant effect. The boreholes have a high gas concentration, which leads to a large amount of gas extraction, hence effectively reducing the gas content of coal in front of the CMWF and the gas concentration in the mining space.

5. Conclusions

Large quantities of gas pour into the working space during the mining of high-gas-content coal seams that have an outburst hazard. To solve this problem, the permeability characteristics of the coal seam in the stress disturbance area and the SEE technology were explored by experimental analysis, numerical simulation and on-site tests. The conclusions are as follows:
(1)
Under a fixed AP (CP) of 15 MPa, as the effective stress rises, the coal pores are compressed; the gas flow channels narrow; and the sample permeability decreases linearly, within the stress loading range of 10–20 MPa. Under a fixed CP and small differential stress, the permeability varies slowly.
(2)
In the stress loading area, when the vertical stress and the horizontal stress of the coal body in front of the CMWF serve as the CP and AP, respectively, the coal permeability and the distance of the coal body to the stress peak point show a negative exponential relationship, and the minimum value appears near the stress peak. When the vertical and horizontal stress of the coal body in the stress disturbance area serve as the CP and AP, respectively, the coal permeability in the experimental area first decreases, and then increases.
(3)
Under the dual influence of mining and hydraulic disturbance, an annular unloading zone exists in the coal body around the cavity within 30–50 m in front of the CMWF and the differential stress increases. Therefore, the plastic damage range of the coal body around the HP cavities expands; the permeability of the coal body is enhanced; and the macro-fractures further expand, providing channels for gas extraction.
(4)
According to the variation of gas concentration in the boreholes, the coal seam in front of the CMWF can be divided into the efficient, effective, and original gas extraction zone, respectively. The efficient gas extraction zone is located approximately 20 m away from the CMWF, belonging to an efficient gas extraction area. Under the influence of mining, the coal mass around the HP cavities in this area dilates; the plastic damage range expands the pores, and fractures develop and connect; and the average gas extraction concentration exceeds 30%. In the effective gas extraction zone, the coal around some of the cavities changes from the yielding state to the shear failure state, and the gas extraction concentration decreases, in fluctuations, with the increase in the distance from the working face. The original extraction zone is located beyond 50–60 m from the CMWF. The distribution of the plastic area of the coal body around the cavities is similar to that without mining disturbance. The average borehole gas concentration stabilizes at below 10%. The number of extraction boreholes in the stress disturbance area of MFGER is 5–10% of the total number of boreholes, but the maximum monthly extraction volume can reach 38.5% of the total monthly extraction volume.

Author Contributions

Conceptualization, X.D. and C.Z.; Software, X.D.; Validation, J.X., X.Y. and Y.S.; Resources, C.Z.; Data Curation, X.D., X.Y. and J.X.; Writing—Original Draft Preparation, X.D. and C.Z.; Writing—Review & Editing, C.Z. and J.X.; Visualization, X.D. and Y.S.; Supervision, C.Z.; Funding Acquisition, C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Fund for Distinguished Young Scholars (51925404), the National Natural Science Foundation of China (51774278, 52104228), China Postdoctoral Science Foundation (2021M693409).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The author thank the editor and anonymous reviewers for their useful advice.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Numerical simulation models.
Figure 1. Numerical simulation models.
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Figure 2. Platform for seepage experiments.
Figure 2. Platform for seepage experiments.
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Figure 3. Coal seam stress distribution in front of the working face.
Figure 3. Coal seam stress distribution in front of the working face.
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Figure 4. Stress loading paths of coal samples.
Figure 4. Stress loading paths of coal samples.
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Figure 5. Distribution of plastic zone around the HP cavities.
Figure 5. Distribution of plastic zone around the HP cavities.
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Figure 6. Stress distribution of HP cavities when x = 103 m.
Figure 6. Stress distribution of HP cavities when x = 103 m.
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Figure 7. Stress distributions of HP cavities in the Y-axis direction.
Figure 7. Stress distributions of HP cavities in the Y-axis direction.
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Figure 8. Variation of sample permeability under Stress Loading Paths 1 and 2.
Figure 8. Variation of sample permeability under Stress Loading Paths 1 and 2.
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Figure 9. Variation of sample permeability under Loading Paths 3 and 4. (a) Path 3; (b) Path 4.
Figure 9. Variation of sample permeability under Loading Paths 3 and 4. (a) Path 3; (b) Path 4.
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Figure 10. Variation of sample permeability under Paths 5 and 6.
Figure 10. Variation of sample permeability under Paths 5 and 6.
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Figure 11. SEE scheme in the stress disturbance area.
Figure 11. SEE scheme in the stress disturbance area.
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Figure 12. Method to determine gas concentration.
Figure 12. Method to determine gas concentration.
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Figure 13. Gas concentrations of the boreholes in front of the CMWF.
Figure 13. Gas concentrations of the boreholes in front of the CMWF.
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Figure 14. Monthly gas extraction volume of MFGER.
Figure 14. Monthly gas extraction volume of MFGER.
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Table
Table 1. The parameters of coal and rock mass.
Table 1. The parameters of coal and rock mass.
LithologyThickness
(m)		Density
(kg/m3)		Bulk Modulus
(GPa)		Shear Modulus
(GPa)		Cohesion
(MPa)		Tensile Strength
(MPa)		Internal Friction
Angle (°)
Medium-grained sandstone1726115.243.614.23.538
Mudstone524033.571.741.71.229
Coal seam513802.691.101.10.827
Fine-grained
sandstone		625635.233.764.6430
Marl1724883.481.802.83.233
Table
Table 2. Stress loading paths.
Table 2. Stress loading paths.
Stress
Path		Stress
Direction		Stress (MPa)Note
12345678910
1AP15151515151515151515Fixed AP, loading CP step by step
CP11121314151617181920
2AP11121314151617181920Fixed CP, loading AP step by step
CP15151515151515151515
3AP32.825.821.518.817.216.315.915.515.214.9Coal stress σz (AP) and σx (CP) in front of the working face without HP cavities
CP20.51816.114.713.813.212.912.712.512.3
4AP32.325.321.92018.417.917.616.816.6-Coal stress σz (AP) and σx (CP) in front of the working face with HP cavities
CP16.11412.511.911.310.810.810.610.3-
5AP20.51816.114.713.813.212.912.712.512.3Coal stress σz (CP) and σx (AP) in front of the working face without HP cavities
CP32.825.821.518.817.216.315.915.515.214.9
6AP16.11412.511.911.310.810.810.610.3-Coal stress σz (CP) and σx (AP) in front of the working face with HP cavities
CP32.325.321.92018.417.917.616.816.6-
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Ding, X.; Zhai, C.; Xu, J.; Yu, X.; Sun, Y. Study on Coal Seepage Characteristics and Secondary Enhanced Gas Extraction Technology under Dual Stress Disturbance. Sustainability 2022, 14, 15118. https://doi.org/10.3390/su142215118

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Ding X, Zhai C, Xu J, Yu X, Sun Y. Study on Coal Seepage Characteristics and Secondary Enhanced Gas Extraction Technology under Dual Stress Disturbance. Sustainability. 2022; 14(22):15118. https://doi.org/10.3390/su142215118

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Ding, Xiong, Cheng Zhai, Jizhao Xu, Xu Yu, and Yong Sun. 2022. "Study on Coal Seepage Characteristics and Secondary Enhanced Gas Extraction Technology under Dual Stress Disturbance" Sustainability 14, no. 22: 15118. https://doi.org/10.3390/su142215118

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