Introduction

China is one of the world’s major coal producers and consumers. As coal resources eventually deplete, the adoption of efficient coal mining methods for the rational use of coal resources and sustainable development becomes essentialbelow1,2,3,4 In recent years, scholars have proposed the technology of double roof cutting and retaining roadway without coal pillar mining, which significantly improves the coal recovery rate of and presents an important direction for sustainable coal development5,6,7,8,9,10,11. However, the mechanisms of gas migration in goaf remain unclear, leading to gaps in effective gas control strategies. Therefore, it is crucial to study the gas migration patterns in goaf with double roof cutting and retaining roadway and to propose scientific and effective control methods to achieve safe and efficient gas extraction.

Extensive research has been conducted on the mechanisms of gas migration. Early studies indicated that gas migration follows the linear gas seepage theory, nonlinear gas seepage theory, and gas fluid-solid coupling theory. Ren observed that gas in the goaf gradually migrates to deeper and higher areas12. Chen et al. examined the fracture evolution in overlying strata and gas migration patterns in goaf13.Ma et al. simulated the characteristics of gas migration and accumulation in goaf and its influencing factors14. Shao et al. studied gas migration under two ventilation modes, U + L and Y + L15.Cao et al. investigated gas migration in fractures induced by mining16. Si et al. analyzed the performance of gas extraction boreholes in goaf based on gas flow rate, extraction negative pressure, and gas composition17. Guo proposed using method horizontal drilling for gas extraction in goaf under longwall mining18. Zhou examined the negative pressure of gas extraction and optimized borehole parameters in goaf19. Liu et al. explored the evolution of gas migration channels during composite roof mining20.

Research on gas migration under traditional gob-side entry retaining mining technology is well established. However, the roof cutting gob-side entry retaining technology is a new method that requires further investigation. Zhou studied the air leakage characteristics and gas accumulation patterns of ‘Y’ type ventilation in roof cutting gob-side entry retaining21. Zhang proposed an innovative roadway layout scheme for high-gas long-wall fully-mechanized caving goaf22. Ma suggested combining non-pillar gob-side entry retaining technology with high drainage roadways for gas control23. Zhou simulated coal seam failure, gas pressure changes, and stress concentration in inclined coal seam goaf24. Zhang investigated the impact of empty roadways as a return air roadway and air leakage prevention on gas migration in goaf25. Feng examined airflow movement in the working face and goaf during roof-cutting roadway mining26.

Currently, research gas migration in goaf under traditional coal pillar mining conditions is extensive, but the mechanisms under double-cut roof retaining roadway conditions are not well understood, and gas enrichment areas cannot be accurately identified. Consequently, blind spots in gas extraction cannot hinder effective gas disaster control. At the same time, the permeability characteristics of goaf and the porosity of mining surrounding rock under the condition of double roof cutting are not clear, which will lead to the inaccuracy of gas control. This paper aims to elucidate the gas migration mechanisms in goaf under double roof cutting and retaining roadway conditions and proposes scientific and effective gas control methods. This provides a theoretical basis for efficient gas extraction and has significant theoretical and practical implications for promoting the green, high-quality, and sustainable development of coal resources.

Test scheme and result analysis of permeability characteristics in goaf

Test scheme and result analysis of surrounding rock permeability in goaf

Test scheme

In the 4502 working face of a mine in China, the double roof cutting and retaining roadway technique was implemented. The belt roadway and track roadway of the working face were simultaneously designed for cutting and retaining. During mining, the track roadway serves as the main air intake route, while the belt roadway functions as an auxiliary air intake route. The track roadway in the working face measures 1107 m in length, the belt roadway measures 1095 m, the cut length is 220 m, the coal seam has an average thickness of 4.2 m, and the coal seam dip angle is approximately 5°. A 40-meter coal pillar on the east side of the 4502 working face is designated as a supplementary roadway for construction, serving as a return air passage for the 4502 track roadway before the 4501 working face is connected. Figure 1 provides a schematic diagram of the working face of the double roof-cutting roadway.

Fig. 1
figure 1

Schematic diagram of the working face with double-cut roof retaining roadway.

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A three-dimensional numerical model was established based on the comprehensive bar chart of the 4502 working face in a mine in China. The model advances along the x-axis, parallel to the working face, for a length of 300 m. The y-axis is perpendicular to the working face, also extending 300 m, and the z-axis is vertical with a height of 100 m. The inclined length of the 4502 working face is 220 m, with 40 m of protective coal pillars left on each side; the excavation length is 200 m. The Fix command was used to restrict displacement and velocity to zero on the model’s left, right, front, and back, and bottom sides. The model’s upper boundary was treated as a free boundary. Based on the buried depth, a uniform load \(\sigma \)= 8.2 MPa was applied to the top surface to simulate the weight of the overlying rock, as shown in Fig. 2.

Fig. 2
figure 2

FLAC3D numerical model.

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The mechanical parameters of the coal and rock mass were obtained from mine data, summarized in Table 1.

Table 1 Mechanical parameters of coal and rock strata.
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To eliminate the influence of boundary effects, excavation was conducted inside the model. After the open-off cut, the roadway was excavated first. The belt and track roadways were set to a height of 4 m each. The roof cutting angle of the 4502 working face was 15°, with an inclined seam length of 13.5 m. Pre-splitting blasting was used to achieve double roadway roof cutting. The model adopted mechanical parameters for coal and rock mass around the weakening blasting holes to simulate pre-splitting blasting roof cutting. The internal friction angle was set to half that of mudstone, while the remaining parameters were set to 1/20 of the corresponding mechanical parameters of mudstone. The weakening range is shown in Fig. 3.

After completing the roof cutting of the double roadway, the working face excavation was carried out. Starting from a position 50 m away from the model’s left, excavation was performed from left to right in 10 m increments. After each excavation step, the model was balanced, and the double roof cutting was performed ahead of the working face within a certain distance, continuing until the working face was excavated to 200 m.

Fig. 3
figure 3

Stress distribution after double roof cutting.

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Permeability distribution law of surrounding rock in goaf

The permeability distribution characteristics of the surrounding rock in the goaf are shown in Fig. 4. As the working face advances, the roof and floor of the goaf undergo pressure relief, leading to an increase in permeability. This permeability increase exhibits an arched distribution pattern. The larger the advancing distance, the broader the high permeability area becomes, with permeability in the pressure relief area of the overburden reaching approximately 1.75 × 10− 16 m2. Due to the compaction of the overlying rock, the stress in the middle of the goaf is restored, and the permeability subsequently decreases. Generally, permeability in the goaf increases gradually from the inside to the outside, forming high permeability channels on the sides of the open-off cut, the working face, and the two lanes. The distribution of these high permeability areas aligns with the pressure relief zones, where permeability can reach up to 6.4 × 10− 16 m2. Additionally, Fig. 4 demonstrates the obvious layered distribution characteristics of permeability within the goaf, attributable to the differing strength characteristics and permeability-stress function relationships of each coal rock layer.

Fig. 4
figure 4

Distribution characteristics of permeability in the goaf at different mining distances.

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To quantitatively characterize the spatial distribution of permeability within the goaf, permeability data were extracted from the simulation results. The permeability of the coal and rock strata was regarded as a function of the strike distance (x) and the dip distance (y), and these data were fitted accordingly. The spatial distribution of permeability in the goaf is shown in Fig. 5, which reveals a high degree of symmetry. Taking the center of the goaf as the origin, the x-axis and y-axis divide the goaf into four regions: A, B, C, and D. For example, as shown in Fig. 6, once the spatial distribution formula of permeability is fitted for any region, the fitting formula for the other three regions can be obtained through function transformation.

Fig. 5
figure 5

Distribution characteristics of permeability in the goaf.

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Fig. 6
figure 6

Division of fitting regions.

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The permeability data for area A were fitted using the Parabola2D function to model the nonlinear surface of permeability based on the distribution characteristics of the data points. The fitted surface is shown in Fig. 7, with the fitting results demonstrating a good agreement with the original data (deterministic coefficient R2 = 0.914). The fitting formula can thus represent the spatial distribution of permeability in area A as follows:

$$k=0.90788 - 0.02644x+8.34403 \times {10^{ - 4}}y+6.76996 \times {10^{ - 4}}{x^2}+4.60132 \times {10^{ - 4}}{y^2}$$
(1)
Fig. 7
figure 7

Fitting surface diagram.

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Applying the parity law of the function, a function transformation was used to derive the permeability fitting formulas for the other regions, summarized in Eq. (2).

$$ k = \left\{ {\begin{array}{llllll} 0.90788-0.02644 x + 8.34403 \times 10^{-4}+\\ 6.76996 \times 10^{-4}x^{2}+ 4.60132 \times 10^{-4}y^{2},\quad (0 \leq x \leq 100,\; 0 \leq y \leq 110)\\ \\ 0.90788+0.02644x + 8.34403 \times 10^{-4}y+\\ 6.76996 \times 10^{-4} x^{2} + 4.60132 \times 10^{-4}y^{2}\quad (-100 \leq x < 0,\; 0 \leq y \leq 110)\\ \\0.90788 + 0.02644x - 8.34403 \times 10^{-4}y+ \\ 6.76996 \times 10^{-4} x^{2} + 4.60132 \times 10^{-4}y^{2}, \quad (-100 \leq x \leq 0,\; -110 \leq y < 0)\\ \\0.90788-0.02644x -8.34403 \times 10^{-4}y+\\ 6.76996\times 10^{-4}x^{2}+4.60132\times 10^{-4}y^{2} \quad (0 < x < 100,\; -110 \leq y < 0) \end{array}}\right.$$
(2)

Test scheme and result analysis of surrounding rock porosity in goaf

Test scheme

PFC2D was used to establish a two-dimensional model of the 4502 double-cut roof retaining roadway working face. The model dimensions were 300 m×100 m (inclined length × height), and the track and belt roadways each 4 m × 4 m. The radius of the model particles ranged from 0.2 to 0.3 m, and the original void ratio of the generated model was 0.01. According to Table 1, the parallel bond model was selected as the constitutive model. The established model is depicted in Fig. 8.

Fig. 8
figure 8

Fracture development model.

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Porosity distribution law of surrounding rock in goaf

As the working face progresses, a large area of goaf composed of residual coal and caving rock is left behind, and its porosity becomes a crucial parameter for studying the gas migration flow field within the goaf. Different seepage characteristics lead to varying gas migration trends and enrichment areas. Current simulations of the flow field in goafs often rely on custom porosity models, resulting in discrepancies between simulation outcomes and field observations. To address this issue, porosity data collected from PFC2D discrete element numerical simulations is used for porosity distribution.

Figure 9 shows that after coal seam mining, the porosity within the separation range is significantly higher than in other areas. The region with notable porosity changes corresponds to the caving zone, whereas the porosity in the fracture zone shows minimal variation. PFC2D numerical simulation results indicate that the void ratio of overburden rock in the vertical direction within the caving zone does not change significantly and lacks regularity. Therefore, the height (z) influence on the results is ignored when fitting the caving zone, and the extracted data are fitted to the tendency to obtain:

$$\varepsilon =2.20844 \times {10^{ - 4}} \times {e^{((220 - y)/35.04021)}}+0.13 \quad 0 \leqslant y<220$$
(3)

In the fracture zone of the overlying strata, fractures are well-developed, while gaps are less so. Consequently, the direction, size, and density of cracks in porous media predominantly influence permeability changes. Compared to the caving zone, permeability and porosity values in the fracture zone, along with their rates of change, are smaller. This results in a reduced impact on gas migration and distribution in the goaf. Therefore, a constant value is assigned to porosity in the fracture zone, with an average porosity of 0.189, representing the middle value for the fracture zone in the goaf.

Fig. 9
figure 9

Distribution of voidage in goaf.

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Test and result analysis of gas migration law in goaf

Test scheme

The physical model of the stope is established using Fluent Fluid Mechanics numerical calculation software. The model dimensions are 200 m in length and 220 m in inclination. The intake lane, return lane, and supplement lane are all modeled as regular cuboids with a cross-section of 4 m × 4 m, as shown in Fig. 10.

Fig. 10
figure 10

Physical model of gas migration in goaf.

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The working face of the double roof cutting roadway uses the ‘Y’ type ventilation mode, where the belt roadway and the track roadway serve as air intakes and the gob side entry returns the air. The ventilation rate is 1620 m3/min, with an inlet air speed set to 1.69 m/s and a gas concentration of 0%. The return air lane is set to outflow, the outlet pressure is static, and the remaining solid boundaries are set to non-flux walls.

The distribution functions of permeability and porosity in the goaf under double roof cutting conditions, as derived previously (Eqs. 2 and 3), are compiled into a UDF file and imported into Fluent numerical simulation software.

Within the model, gas mass sources are present in the goaf, the coal wall of the working face, and the adjacent layer, with absolute gas emissions of 8.82 m3/min, 5.88 m3/min, and 2.2 m3/min, respectively. The calculation formula for the gas quality source term is:

$$ Q_{m}= \frac{Q.p}{V}$$
(4)

Qm—gas quality source term, kg/(m3/s);

Q—absolute gas emission quantity, m3/s;

\(\rho \)—gas density, 0.716 kg/m3;

V—volume of gas source term, m3.

Therefore, the gas quality sources for the goaf, the coal wall of the working face, and the adjacent coal and rock strata in the model are 3.98 × 10− 7 kg/(m3/s), 1.99 × 10− 5 kg/(m3/s), and 6.63 × 10− 8kg/(m3/s), respectively.

Gas migration law in goaf

To study the gas flow trend and enrichment areas in goaf under the condition of double roof cutting, the simulation results are analyzed along the strike, tendency, and horizontal planes of the goaf.

Figure 11 shows the distribution of gas concentration at different distances from the mining-induced fracture field to the working face. The figure shows that without extraction, both the belt roadway and the track roadway serve as air intake roadways, resulting in a symmetrical gas distribution in the goaf on both sides, with minimal changes in gas concentration changes along the tendency. However, in the vertical direction, mining generates numerous cross-layer cracks in the overlying strata of the goaf, which become gas migration channels. Gas accumulates in the upper part of the goaf due to its density difference from the surrounding gas.

As the goaf extends behind the working face, the gas concentration continues to be higher in the upper part and lower in the lower part, vertically. Along the strike, both in the caving zone and fracture zones, gas concentration increases closer to the open-off cut. This occurs because, under ‘Y’ type ventilation, fresh air flow blows the coal wall of the working face and pushes the gas in the goaf toward the return airway. Gas that is not expelled through the return airway accumulates deeper in the goaf.

Fig. 11
figure 11

Distribution map of gas concentration at different strike distances of the mining-induced fracture field.

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Figure 12 shows the distribution of gas concentration at different distances from the mining-induced fracture field to the track roadway. The figure indicates higher gas concentrations near the open-off cut and the upper part of the fracture zone. This results from the combined effects of the ventilation mode and gas buoyancy, leading to gas accumulation on the side of the open-off cut. The higher the position within the goaf, the greater the gas concentration. Moving towards the belt roadway, gas concentration patterns remain consistent across different inclination distances, with the highest concentration in the upper goaf near the open-off cut.

Fig. 12
figure 12

Gas concentration distribution at different inclination distances in the mining fracture field.

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Figure 13 illustrates the gas concentration distribution at different heights within the goaf. The figure shows that the gas concentration on the side of the working face is approximately 15% on the horizontal section. In the deeper goaf, gas concentration ranges from 30 to 50%, reaching 65% at the cut hole. This occurs because gas from the goaf continuously escapes to the gob-side entry retaining area and migrates backward with airflow, making the end of the gob-side entry retaining section a high concentration gas enrichment area.

With increased section height, gas concentration continues to rise from the working face to the open-off cut, with significant value differences. As height increases, the high gas concentration area expands and moves closer to the working face space. At the top of the fracture zone, gas concentration exceeds 50%, and above the working face space, it is about 40%, preventing normal mining operations.

Fig. 13
figure 13

Gas concentration distribution at different heights in the mining-induced fracture field.

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Based on the above analysis, desorbed gas from the coal wall and gas emitted from the goaf migrate to the rear of the goaf and the upper part of the fracture zone under the combined effect of ‘Y’ type ventilation and density differences. This causes the goaf on the side of the open-off cut to become a high concentration gas enrichment area, with the high concentration area expanding with height. Without extraction, gas concentration at the working face exceeds 2%, significantly affecting normal mining operations. Moreover, during subsequent mining, enriched gas influenced by mining activities surges into the working space of the working face and the gob-side entry retaining area, leading to gas concentrations that exceed safety limits.

To address this, a scientific extraction method is proposed to control gas disasters in the goaf based on the gas migration law in goaf and the distribution characteristics of the enrichment area.

Gas collaborative control method in goaf

Gas extraction in the goaf is an effective method to mitigate gas overruns in the working face and reduce gas outburst accidents. Based on the identified gas enrichment areas and flow trends, a gas control method is proposed. The effectiveness of this method is evaluated through numerical simulation and verified in actual field conditions. Figure 14 presents a three-dimensional representation of three different gas collaborative control methods in the goaf.

Fig. 14
figure 14

Three-dimensional representation of gas collaborative control methods in goaf.

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Numerical simulation of high-level directional long borehole drainage

Numerical model

To address the gas emission from the goaf to the return airway of the gob-side entry retaining, a high-level directional long borehole is designed along the belt roadway to intercept and extract gas from the goaf. The borehole has a depth of 180 m, a diameter of 100 mm, and operates under a negative pressure of 15 KPa. The numerical model is shown in Fig. 15.

Fig. 15
figure 15

Numerical model of high-directional long borehole drainage.

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Effect analysis

To evaluate the impact of high-level directional long borehole extraction on gas drainage in the goaf, the simulation results were analyzed along the strike, dip, and horizontal planes.

Figure 16 illustrates the gas concentration distribution in the goaf at different distances from the working face after 30 days of high-level directional long borehole extraction. From Fig. 16(a), it can be observed that, under high-level directional long borehole extraction, the gas concentration in the goaf on the working face side still shows a gradual increase from bottom to top, but the overall values have significantly decreased, from 2 to 40% before extraction to 0.5-10%.

From Fig. 16(b)-(e), as the distance behind the working face increases, the gas concentration in the goaf generally rises, but the rate of increase is not uniform on both sides. Due to the high-level directional long boreholes in the goaf on the belt roadway side, the gas volume fraction remains below 12.8%, and the concentration at the open-off cut decreases by 52.2% compared to pre-extraction levels. On the track roadway side, the gas concentration in the goaf is higher, with a concentration of 16.7% at the open-off cut, a 48.3% reduction from pre-extraction levels. This comparison indicates that the high-level directional long borehole is effective in extracting gas from the goaf.

Fig. 16
figure 16

Gas concentration distribution at different strike distances under high-level directional long borehole drainage.

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Figure 17 shows the gas concentration distribution in the goaf at different distances from the track roadway after 30 days of high-level directional long borehole extraction. From Fig. 17(a), it is evident that, compared to pre-extraction levels, the gas volume fraction in the goaf still shows a trend of gradually increasing from the working face to the open-off cut, but with significantly reduced values. The gas concentration in the working face ultimately remains below 2.6%. The gas concentration in the upper fracture zone on the working face side of the goaf ranges from 7.7 to 12.8%. The highest gas concentration, approximately 16%, is found in the upper part of the goaf on the track roadway side at the open-off cut.

From Fig. 17(b)-(e), as the profile moves toward the belt roadway, the gas concentration in the goaf maintains a similar distribution trend under high-level directional long borehole extraction, gradually increasing from the working face to the upper part of the open-off cut. However, the values show significant differences. The gas concentration on the open-off cut side gradually decreases, and the extraction effect reduces the gas concentration in most of the goaf near the belt roadway, maintaining it below 9%.

Fig. 17
figure 17

Gas concentration distribution at different inclination distances under high-level directional long borehole.

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Figure 18 shows that the high-level directional long borehole creates negative pressure in the fracture zone, causing low-level gas to rise due to density and pressure differences. This results in progressively lower gas concentrations at the bottom of the goaf. Approximately half of the area near the working face exhibits low gas concentration, reducing the gas concentration in the working face space to less than 0.5%. The areas with gas concentrations exceeding 15% are limited to the height of the fracture zone in the goaf at the intersection between the belt roadway and the open-off cut, reducing the risk of gas overrun.

Based on this analysis, arranging high-level directional long boreholes along the belt roadway significantly reduces the gas concentration in the goaf, particularly on the belt roadway side and the working face. However, under this single extraction method, gas concentration in much of the goaf space remains above the safety limit, and the risk of gas gushes to the working persists. Additional extraction methods are required to address the limitations of high-level directional long borehole extraction.

Fig. 18
figure 18

Gas concentration distribution at different heights under high-level directional long borehole extraction.

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Numerical Simulation of Pipe jacking extraction in retained Roadway

Numerical model

To address the enrichment of gas in the fracture zone within the goaf, a pipe jacking system was designed along the belt roadway to drain and extract gas from this zone. The pipe jacking system consists of pipes 2 m in length and 200 mm in diameter, operating under a negative pressure of 20 KPa. The numerical model is depicted in Fig. 19.

Fig. 19
figure 19

Numerical model of pipe jacking extraction in retained roadway.

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Effect analysis

To evaluate the effectiveness of pipe jacking extraction in the goaf gas retaining roadway, the simulation results were analyzed along the strike, dip and horizontal planes.

Figure 20 shows the gas concentration distribution in the goaf at different distances from the working face after pipe jacking extraction in the retained roadway. From Fig. 20(a), it is evident that post-extraction, the gas concentration in the working face is significantly reduced to about 0.3%, which is within the permissible range for safe operation. From Fig. 20(b)-(e), as we move further into the goaf, the overall gas concentration increases, particularly within the caving zone. However, due to the negative pressure created by the pipe jacking, the gas extraction effect is more pronounced in the fracture zone. Gas near the track and belt roadways follows the dominant migration channels around the ‘O’ ring and is effectively drawn out by the pipe jacking system, thereby significantly reducing the gas concentration near the gob side of the two troughs.

Fig. 20
figure 20

Gas concentration distribution map at different strike distances under gob-side entry retaining pipe jacking drainage.

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Figure 21 presents the gas concentration distribution in the goaf at different distances from the track roadway following pipe jacking extraction in the retained roadway. From Fig. 21(a), it is clear that the gas concentration in the goaf is notably reduced by the pipe jacking system, particularly on the working face side, with gas concentrations on the open-off cut side also falling below 5%. From Fig. 21(b)-(e), as the profile moves towards the belt roadway, the gas concentration initially rises before decreasing, indicating a more effective extraction of gas near the roadways, while the gas concentration in the middle of the goaf remains above safe levels.

Fig. 21
figure 21

Gas concentration distribution at different dip distances under pipe jacking drainage.

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Figure 22 demonstrates that pipe jacking extraction effectively reduces gas concentrations on the working face side and near the two roadways, with the extraction effect being more pronounced closer to the height of the pipe jacking installation. The gas concentration on the working face side remains consistently below 0.3%. However, the gas concentration on the cut-hole side of the goaf remains high, with regional concentrations reaching 12.5% at the top of the caving and fracture zones. This indicates that while the pipe jacking extraction method in the retained roadway reduces gas concentration effectively, it does not fully resolve the issue of gas accumulation on the cut-hole side of the goaf. Therefore, further design and implementation of effective extraction methods are required to control gas on the cut-hole side.

Fig. 22
figure 22

Gas concentration distribution at different heights under pipe jacking extraction.

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Numerical simulation of drilling extraction in a rear cut hole

Numerical model

To address the limitation of pipe jacking extraction in the retained roadway, which cannot effectively reduce the gas concentration on the open-off cut side, boreholes were arranged in the 4502 filling roadway to extract gas from the goaf on the open-off cut side. Given the favorable extraction effect in high permeability areas, boreholes with depths of 30 m, 40 m, and 50 m were positioned based on permeability and porosity distribution characteristics with an aperture of 140 mm. The model is shown in Fig. 23.

Fig. 23
figure 23

Numerical model of drilling extraction in a rear cut hole.

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Effect analysis

To evaluate the effectiveness of borehole extraction in the rear cut hole of goaf gas, the simulation results were analyzed along the strike, dip, and horizontal planes.

Figure 24 shows the gas concentration distribution in the goaf at different distances from the working face after drilling in the rear cut hole. From Fig. 24(a), it is evident that drilling in the rear cut hole reduces the gas concentration in the working face to 1-2%, achieving a better extraction effect than high-level directional long drilling and slightly inferior to pipe jacking extraction in the retained roadway. From Fig. 24(b)-(e), as we move deeper into the goaf, the gas concentration in the fracture zone increases slightly, while it increases significantly in the caving zone. Near the open-off cut, the goaf becomes a low-gas area due to gas migration to the rear open-off cut drilling, with a gas concentration reaching 12%.

Fig. 24
figure 24

Gas concentration distribution map at different strike distances rear-cut hole drilling extraction.

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Figure 25 shows the gas concentration distribution in the goaf at different distances from the track roadway after drilling and extraction in the rear cut hole. From Fig. 25(a), it is clear that gas concentrations on the working face side and open-off cut sides decrease significantly after rear open-off cut drilling, although the gas concentration on the open-off cut side is 1-3% higher than that on the working face side. As the profile moves towards the belt roadway, the gas concentration initially increases before decreasing, indicating that rear cut hole drilling is more effective at extracting gas near the roadways on both sides, with slightly poorer extraction in the middle of the goaf, where gas concentration can reach up to 12%. From Fig. 25(b) to 25(d), it is evident that rear-cut hole drilling creates a distinct low gas concentration band in the fracture zone in the middle of the goaf, with the area being smaller in the middle and larger on both sides. This is due to stress recovery in the middle of the goaf, resulting in low permeability and porosity, which hinder gas flow and enrichment.

Fig. 25
figure 25

Gas concentration distribution map at different dip distances under rear-cut hole drilling extraction.

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Figure 26 shows the gas concentration distribution at different heights in the goaf after the drilling and extraction in the rear-cut hole. It can be seen that the gas concentration in the working face space is below 1%, and near the roadway, it ranges is between 1% and 5%. This indicates that rear-cut hole drilling achieves the best gas extraction effect on the goaf on the working face side and near the two crossheadings. The closer to the height of the borehole, the larger the range of the low gas area. In the fracture zone, the gas concentration is below 4%, indicating that rear cut hole drilling effectively solves the problem of gas accumulation in the fracture zone on the cut hole side.

Fig. 26
figure 26

Gas concentration distribution map at different heights under rear-cut hole drilling extraction.

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On-site verification

Field test program

High-level directional long borehole

To control gas emissions from the goaf to the gob-side entry retaining, high-level directional long boreholes were constructed along the belt roadway to intercept and extract gas in the goaf. Each borehole has a depth of 180 m and a diameter of 100 mm. The spacing between adjacent boreholes is initially 1 m, with a final spacing of 5 m, as shown in Fig. 27.

Fig. 27
figure 27

Layout of high-level directional long boreholes in the 4502 working face.

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Retained roadway pipe jacking

During normal mining operations of the 4502 working face, in the loose gangue area of the caving zone behind the support at the end of the working face, a special diameter DN200 and 2 m long spiral welded steel pipe are pushed into the goaf every 10 to 20 m using pipe jacking equipment. The front end of the steel pipe is conical, with a 1.5-meter section of the pipeline fitted with a sieve.

Rear cut hole drilling

Rear cut hole drilling was conducted in the 4502 roadway to extract gas from the mining fissure zone and goaf during and after mining.As shown in Fig. 28, the specific arrangement is as follows:

In the 9–33 m section on the west side of the 4502 supplementary roadway, vertical coal walls were placed every 6 m to penetrate the 4502 cut hole and the goaf of the coal pillar. In the 45–93 m section on the west side of the 4502 supplementary roadway, vertical coal walls were placed every 12 m to penetrate the 4502 cut hole and the goaf of the coal pillar. A total of 10 boreholes were arranged, with an azimuth of 258°. The boreholes had inclination angles of 9°, 15°, and 21°, with design depths of 30 m, 40 m, and 50 m, respectively.

Fig. 28
figure 28

Layout of drilling holes in high- and low-level fracture zones and goaf of the 4502 supplementary roadway.

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On-site monitoring of gas concentration

The 4502 working face utilized high-level directional long-hole drainage, retained roadway pipe jacking drainage, and goaf rear-cut hole drilling drainage to control gas. Figure 29 shows the borehole layout. Throughout the extraction period, there was no gas overrun in the upper corner or return air flow. The gas concentration in the upper corner and return airflow of the 4502 working face remained below 0.3% and showed a downward trend, as shown in Figs. 30 and 31. Additionally, during the two extraction periods from August 28 to September 1 and from November 8 to November 17, the extraction concentration of all boreholes decreased significantly, and the concentration of the upper corner and return airflow remained below 0.2%, demonstrating effective gas control in the 4502 working face goaf.

Fig. 29
figure 29

Borehole layout site of the 4502 working face.

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Fig. 30
figure 30

Gas concentration change in the upper corner of the working face.

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Fig. 31
figure 31

Gas concentration variation in the return airflow of the working face.

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Conclusion

  1. (1)

    Under the conditions of double roof cutting and retaining roadway, the permeability of the goaf gradually increases from inside to outside. High-permeability channels are formed on the side of the open-off cut, the working face, and both roadways. The distribution of high-permeability areas aligns with that of pressure relief zones. The results from the nonlinear surface fitting of permeability data are reliable.

  2. (2)

    In the fracture zone of the overlying strata in the goaf, fractures are fully developed while gaps are not. Therefore, the direction, size, and density of cracks in the porous medium predominantly influence permeability changes. Compared to the caving zone, the fracture zones show smaller values and rates in permeability and porosity, resulting in a significantly lesser impact on gas migration and distribution. Therefore, a constant is used to assign the porosity in the fracture zone, with an average porosity value of 0.189, reflective of the middle of the fracture zone in the goaf.

  3. (3)

    High-level directional long borehole drainage effectively reduces gas consideration in the lower part of the goaf on the working face side and belt roadway side, bringing it down to less than 0.5% in the working face space. The areas where gas concentration exceeds 15% are confined to the height of the fracture zone at the angle between the belt roadway and the cut hole.

  4. (4)

    The gas concentration on the side of the working face side and both roadway sides can be effectively reduced by retained roadway pipe jacking drainage. The closer the location to the top pipe arrangement height, the better the extraction effect. The gas concentration on the working face side consistently remains below 0.3%, although the gas concentration in the goaf on the cut hole side remains high; due to the existence of negative pressure of pipe jacking in fracture zone, the effect of gas extraction is better, with a regional gas concentration of 12.5% at the top of the caving and fracture zones.

  5. (5)

    The gas extraction effect is most effective in the goaf on the working face side and near both roadways. The closer to the borehole height, the larger the low gas concentration area; there is an obvious banded low gas concentration area in the fracture zone in the middle of the goaf, and the area is smaller in the middle of the goaf and larger on both sides. In the fracture zone, gas concentration is reduced by 61%, indicating that rear cut hole drilling effectively addresses gas accumulation in the fracture zone on the cut hole side.

  6. (6)

    The 4502 working face successfully employs a combination of high-level directional long-hole drainage, retained roadway pipe jacking drainage, and rear-cut hole drilling drainage to control gas in the goaf. Throughout the drainage period, there were no gas overruns in the upper corner or return air flow. The gas concentration in both the upper corner and return airflow of the 4502 working face remained below 0.3% and showed a downward trend, demonstrating effective gas control in the 4502 working face goaf.

  7. (7)

    With the wide application of roof cutting gob-side entry retaining technology in the field of coal mining, the lack of relevant research will lead to the inability to accurately identify the location and flow trend of gas enrichment when carrying out gas extraction work, which will affect the effect of gas extraction and cannot effectively prevent and control gas disasters. The non-pillar mining technology of double roof cutting and retaining roadway is a special case of the non-pillar mining technology of roof cutting and retaining roadway. Therefore, this paper takes the 4502 double roof cutting and retaining roadway working face of Huajin coking coal as the engineering background to carry out the research on the fracture development and gas migration law of mining overburden rock under the condition of double roof cutting and retaining roadway, which provides some reference significance for other similar mines to carry out the technology of roof cutting and retaining roadway, in order to provide theoretical support for the comprehensive gas control project in goaf of roof cutting working face.