Four-Zone Distribution of Coal Seam Stress in Hard-Roof Working Faces and Its Influence on Gas Flow

Abstract

To investigate the distribution of coal seam stress ahead of the working face under hard-roof conditions and analyze its impact on gas flow, this study focused on the 16,021 working face in Wu Hua No. 1 Mine. First, we established a mining model using UDEC to analyze stress distribution at different coal seam extraction distances. Second, we used COMSOL Multiphysics 6.3 to simulate the influence of stress on the permeability and gas pressure of coal seams during extraction, thereby exploring how stress distribution affects gas flow. Finally, we deployed gas extraction boreholes to validate the gas flow characteristics associated with the stress zones. The results indicate that the coal seam stress ahead of the working face forms four distinct zones, influenced by the main roof hanging: stress reduction zone I, stress concentration zone, stress reduction zone II, and original stress zone. When extraction days are equal, under high-stress conditions, the extracted coal seam exhibits low permeability and a small decrease in gas pressure, making gas extraction difficult; in contrast, under low-stress conditions, it exhibits high permeability and a large decrease in gas pressure, making gas extraction relatively easier. Field measurements show that the gas extraction flow rate initially increases and then decreases with distance from the coal wall, exhibiting a noticeable rise within the 47–62 m range before stabilizing. This trend aligns well with the characteristics of stress zoning.

1. Introduction

China is rich in coal resources but relatively deficient in oil and natural gas, presenting an energy structure characterized by “abundant coal, scarce oil, and limited gas”. Due to prolonged and extensive mining activities, shallow coal resources are being progressively depleted, leading to a continuous increase in mining depth at a rate of 10–50 m per year [1,2,3]. As mining activities extend into deeper coal seams, the influence of abutment stress on coal becomes more significant [4]. This is particularly pronounced when hard roofs are present in the overlying rock, which not only poses a serious threat to mine safety but also elevates the risk of coal and gas outbursts. Therefore, a thorough study into the distribution of coal seam stress under hard-roof conditions and its impact on gas flow is essential. This work provides a scientific basis for developing effective gas extraction plans, which is important for guiding safe mining operations and reducing the risk of coal and gas outbursts.

The stress distribution characteristics of coal seams are significantly influenced by mining activities. Research generally indicates that the stress distribution in the coal seam ahead of the working face can be divided into three typical zones: stress reduction zone, stress concentration zone, and original stress zone [5,6,7,8]. However, under hard-roof conditions, relevant studies have revealed more complex stress distribution characteristics through theoretical analysis and field measurements, providing new insights for a deeper understanding of stress distribution. Based on the elastic foundation theory, Pan et al. [9] used MATLAB to analyze the stress distribution of coal seams under different foundation constants. The results indicate that an obvious stress reduction zone occurs between the front of the abutment pressure peak and the original rock stress. This finding corroborates Hui [10]’s research, which derived an analytical expression for the abutment stress distribution in longwall faces and found that the abutment stress distribution of the working face presents a peak–valley pattern under hard-roof conditions. Through underground field measurements, Yin et al. [11] also observed a decrease in abutment pressure ahead of the original stress zone, providing empirical evidence for the existence of the stress reduction zone.

Coal seam abutment stress is closely related to gas extraction behavior. Nian et al. [12] established the coupled anisotropic dual-porosity model and analyzed the effect of working face stress on gas pressure, gas extraction volume, and effective drainage area, finding that stress is a key factor affecting coal seam permeability and gas extraction volume. Zhang et al. [13] further indicated through experiments that the stress path is also closely related to coal seam permeability. Peng et al. [14] revealed the relationship between stress and extraction flow rate through physical simulation tests and concluded that gas extraction flow rate is negatively correlated with stress. Yang et al. [15] further revealed through a fluid–solid coupling model that increased stress intensifies stress concentration around boreholes, which explains the intrinsic mechanism underlying the difficulty of gas extraction in the stress concentration zone. Based on the above understanding, researchers have begun investigating methods to enhance extraction efficiency through active decompression. Zhang [16] analyzed the distribution of vertical stress in the protected seam during protective seam mining using the FLAC3D model, providing theoretical guidance for the layout of gas extraction boreholes. Hou et al. [17] and Liu et al. [18] employed numerical simulation to analyze gas migration characteristics and permeability changes before and after protective seam mining. Their findings indicate that decompression can increase permeability and improve gas extraction efficiency. On this basis, Ding et al. [19] used the evolution laws of abutment stress and permeability to propose a secondary enhanced pressure relief and gas extraction technology. Li et al. [20] and Guo et al. [21] adopted a combined method of numerical simulation and field measurement to reveal the coupling effect between the stress field and gas field at the working face, as well as its influence on the migration and distribution of gas, proposing an effective gas drainage technology in the fractured zone which has enriched the theoretical system of multi-field-coupled gas migration at the working face.

While existing studies have noted the new characteristics of coal seam stress distribution under hard roofs and confirmed the close correlation between stress and gas extraction, they have not conducted detailed exploration of these new characteristics. Furthermore, the fine zoning laws of stress distribution and their differentiated guidance for gas extraction remain unclear. This study takes the 16,021 working face of Wu Hua No. 1 Mine with a typical hard roof as the research background, First, we employed UDEC numerical simulation to analyze the evolution of coal seam stress. Subsequently, to elucidate the impact of stress on gas extraction behavior, we selected static characteristic stress values from UDEC as boundary conditions, and using COMSOL Multiphysics, we established a gas–solid coupling model to simulate how different stress conditions govern coal seam permeability and gas pressure, thereby exploring the influence of stress zoning on gas extraction. Finally, we conducted field tests for validation and proposed optimized gas extraction strategies. This is expected to provide useful references for gas disaster prevention and control at mine working faces under similar geological conditions.

2. Numerical Simulation of Overlying Strata Collapse and Stress Distribution

2.1. Numerical Model Establishment

Wu Hua No. 1 Mine is located in Inner Mongolia, and it is characterized by thick and hard roof strata that tend to form large-scale suspended roofs after mining. To reveal the stress evolution law of the coal seam ahead of the working face under hard-roof conditions, this study takes the 16,021 working face of Wu Hua No. 1 Mine as the research background. The physical and mechanical parameters of the coal and rock mass at the working face are presented in

Table 1. Rock mechanical parameters.

No.	Lithology	Density (kg/m3)	Bulk (GPa)	Tensile (MPa)	Cohesion (MPa)	Friction (°)
1	quartz sandstone	2559	7.36	5.56	3.87	6.90
2	limestone	2516	5.38	4.12	5.32	5.80
3	coal seam	1413	1.26	1.14	0.7	1.16
4	quartz sandstone	2519	5.38	4.22	5.64	4.80
5	sandstone	2528	7.45	5.85	6.58	6.80
6	fine sandstone	2421	9.06	6.88	5.68	5.80
7	overburden	2236	3.16	2.05	2.23	3.18

.

According to the conditions of the working face 16,021, the numerical model as shown in Figure 1 is established using UDEC. The length and width of the model are 250 m, 63 m, respectively. In the model, deformable block elements adopt the Mohr–Coulomb constitutive model, while discontinuous structural planes use the Coulomb slip model to describe potential tensile or shear failures of the contact surfaces under stress. Displacement constraints are applied at the model boundaries of x = 0 m, x = 250 m, and y = 0 m. To simulate the deep mining environment, a stress load of 10 MPa is applied to the upper boundary (y = 63 m) of the model, as illustrated in Figure 1.

2.2. Analysis of Numerical Simulation Results

2.2.1. Roof Collapse Characteristics at Different Coal Seam Mining Distances

Excavation commenced from x = 0 m with a 10 m step length, advancing the face by 40 m over 4 steps. The collapse of overlying strata during this process of the working face is shown in Figure 2. When the mining length reached 10 m, as shown in Figure 2a, the immediate roof was exposed but did not collapse. After 20 m of mining, as shown in Figure 2b, the exposure distance of the immediate roof reached its limit caving span, leading to the first collapse, while the suspension distance of the main roof extended synchronously with mining. Following 30 m of mining, as shown in Figure 2c, the suspension distance of the main roof continued to increase but did not exceed its limit caving span. At 40 m of mining, as shown in Figure 2d, the main roof collapses and the overlying strata exhibit significant deformation.

2.2.2. Coal Seam Stress Distribution at Different Mining Distances

Figure 3 presents the curves of coal seam stress distribution. At a mining distance of 10 m, as shown in Figure 3a, the zone within 0–6 m is identified as stress reduction zone I, where the abutment stress is lower than the original stress. The stress concentration zone extends from 6 m to 50 m, with a peak stress of 28.1 MPa occurring 16 m from the coal wall and a concentration factor of 2.5. Beyond 50 m lies the original stress zone, where the stress stabilizes at 11.1 MPa. When the mining reaches 20 m, as shown in Figure 3b stress reduction zone I spans 0–5 m. The stress concentration zone extends from 5 m to 55 m, with a peak stress of 32.9 MPa at 26 m and a concentration factor of 2.9. Stress reduction zone II (where stress is more than 5% below the original stress) is observed between 65 m and 85 m, with a minimum stress of 9.6 MPa. The original stress zone is restored beyond 85 m. At 30 m of mining, as shown in Figure 3c, stress reduction zone I covers 0–4 m. The stress concentration zone ranges from 4 m to 45 m, exhibiting a peak stress of 35.3 MPa and a concentration factor of 3.1. Stress reduction zone II is located between 55 m and 85 m, with the stress dropping to a minimum of 8.9 MPa. Beyond 85 m, the stress stabilizes at 11.3 MPa. After 40 m of mining, as shown in Figure 3d, stress reduction zone I is found within 0–6 m. The stress concentration zone extends from 6 m to 60 m, with a peak stress of 27.1 MPa and a concentration factor of 2.3. Stress reduction zone II lies between 75 m and 90 m, where the stress decreases to a minimum of 9.4 MPa. The original stress zone is re-established beyond 90 m.

It is noteworthy that after 10 m mining, the coal seam stress exhibited the conventional three-zone distribution pattern. However, with the increase in the roof suspension distance, a stress reduction phenomenon emerged approximately 50 m ahead of the face, ultimately characterizing a four-zone distribution: stress reduction zone I, stress concentration zone, stress reduction zone II, and the original rock stress zone, as illustrated in Figure 4. Stress reduction zone II is mainly affected by the hanging and fracturing behaviors of the main roof. As the hanging distance of the main roof increases, both the scope and pressure relief magnitude of stress reduction zone II expand; after the main roof fractures, the scope and pressure relief magnitude of stress reduction zone II decrease.

3. Numerical Simulation of Stress Effects on Gas Extraction Flow

3.1. Gas–Solid Coupling Model

Coal seam gas extraction is a complex process involving fluid dynamics and solid mechanics. To simplify the computation, the following fundamental assumptions are introduced: (1) Coal is an elastic continuum with a dual pore-fracture structure but a single permeability, where gas transport obeys Fick’s Law in the pores and Darcy’s Law in the fractures. (2) The gas within the coal seam is an ideal gas, conforming to the Ideal Gas Law. (3) The coal seam is considered dry, containing no moisture. (4) The gas flow process is isothermal.

3.1.1. Stress Field Equation

Based on the fundamental assumption of a dual pore-fracture medium and considering the gas pressure in the matrix and gas adsorption/desorption effects, the governing equation for the deformation of gas-containing coal is given as follows [22]:

$$\begin{array}{c}G{u}_{i,jj}+{\displaystyle \frac{G{u}_{j,ji}}{1-2v}}-{\alpha }_m{p}_{m,i}-{\alpha }_f{p}_{f,i}-K{\mathsf{\epsilon }}_{s,i}+F_i=0\end{array}$$

(1)

where ${\mathrm{u}}_{\mathrm{i},\mathrm{j}\mathrm{j}}$ is in tensor form, the first subscript denotes the $i$-direction tensor of variable u, the second subscript indicates the partial derivative of $u_i$ with respect to the j-direction, and the third subscript signifies the partial derivative of $u_{i,j}$ with respect to the j-direction. Similarly, $u_{j,ji}$ follows the same convention. G is the shear modulus (MPa), E is the elastic modulus of coal (MPa), ${\alpha }_m$ and ${\alpha }_f$ are the Biot coefficients corresponding to the matrix pores and fractures, respectively. $p_m$ and $p_f$ represent the gas pressures in the matrix and fractures (MPa), while $p_{m,i}$ and $p_{f,i}$ denote the i-direction components of $p_m$ and $p_f$, respectively. K is the bulk modulus (Pa) ${\mathsf{\epsilon }}_s$ is the gas adsorption-induced swelling strain, with ${\mathsf{\epsilon }}_{s,i}$ being its i-direction component. $F_i$ denotes the volume force (MPa).

3.1.2. Seepage Field Equation

Coal possesses a dual pore-fracture structure, with its void spaces containing significant amounts of adsorbed and free-state gas. Initially, the gas is in an adsorption–desorption equilibrium state, where the gas pressure in the matrix pores equals that in the fractures. When extraction disrupts this equilibrium, adsorbed gas desorbs and, driven by the concentration gradient, migrates into the fracture system primarily via diffusion. According to Fick’s law of diffusion, the mass conservation equation for methane in the matrix can be expressed as follows [23]:

$$\begin{array}{c} {\displaystyle \frac{\partial m_m}{\partial t}}=-{\displaystyle \frac{M_g}{\tau RT}}\left(p_m-p_{fg}\right)\end{array}$$

(2)

$$\begin{array}{c} m_m={V_{sg}\rho }_c{\displaystyle \frac{M_g}{R{T}_n}}{p}_n+{\varphi }_m{\displaystyle \frac{M_g}{RT}}{p}_m\end{array}$$

(3)

where $m_m$ is the gas content in the matrix of unit volume (kg/m³), $M_g$ is the molar mass of gas (kg/mol), R = 8.314 (J/molo/K) is the gas molar constant, T is the temperature in the coal seam, and $\tau$ is the time of desorption and diffusion; $V_{sg}$ is the content of adsorbed gas, ${\rho }_c$ is the skeleton density (kg/m³), $p_n$ = 101 (kPa) is the standard atmospheric pressure, $T_n$ = 273.15 (K) is the temperature under standard conditions, and ${\varphi }_m$ is the matric porosity.

By substituting Equation (3) into Equation (2), the matrix gas transport equation can be obtained as follows:

$$\begin{array}{c} {\displaystyle \frac{\partial }{\partial t}}{(V}_{sg}{\rho }_c{\displaystyle \frac{M_g}{R{T}_n}}{p}_n+{\varphi }_m{\displaystyle \frac{M_g}{RT}}{p}_m)=-{\displaystyle \frac{M_g}{\tau RT}}\left(p_m-p_{fg}\right)\end{array}$$

(4)

When the equilibrium state is disturbed, gas diffuses from the matrix into the fractures. The gas migration within the fractures is governed by the mass conservation equation [24].

$${\displaystyle \frac{\partial \left({\varphi }_f{\rho }_g\right)}{\partial t}}+\nabla ·\left({\rho }_g{\upsilon }_g\right)=\left(1-{\varphi }_f\right){\displaystyle \frac{M_g\left(p_m-p_{fg}\right)}{\tau RT}}$$

(5)

where ${\rho }_g$ is the gas density (kg/m³) and ${\upsilon }_g$ is the gas flow velocity within the fractures (m/s).

3.1.3. Cross Coupling

The coal matrix pores serve as the primary storage space for gas, while the fracture network acts as the main migration pathway. The gas transport processes within the matrix pores and the fractures are distinct yet interconnected. The matrix porosity can be defined as follows [12]:

$${\varphi }_m={\displaystyle \frac{1}{\left(1+S\right)}}\left({\varphi }_{m0}\left(1+S_0\right)+{\alpha }_m\left(S-S_0\right)\right)$$

(6)

where $S$=${\epsilon }_v+{\displaystyle \frac{P_m}{K_s}}-{\epsilon }_L{\displaystyle \frac{P_m}{P_L+P_m}}$; $S_0$=${\epsilon }_{v0}+{\displaystyle \frac{P_{m0}}{K_s}}-{\epsilon }_L{\displaystyle \frac{P_{m0}}{P_L+P_{m0}}}$; ${\varphi }_{m0}$ is initial matrix porosity; ${\epsilon }_v$ is the volumetric strain; $K_s$ is the bulk modulus of the coal matrix (Pa); ${\epsilon }_L$ is the Langmuir strain constant; and $P_L$ is the Langmuir pressure constant.

The fracture porosity can be defined as follows [25]:

$${\displaystyle \frac{{\varphi }_f}{{\varphi }_{f0}}}=1+{\displaystyle \frac{∆b}{b}}=1-{\displaystyle \frac{3}{{\varphi }_{f0}+\frac{3K_f}{K}}}\left({\displaystyle \frac{{\epsilon }_L∆p_m}{P_L+∆p_m}}-{\epsilon }_v\right)$$

(7)

where ${\varphi }_{f0}$ is the initial fracture porosity and b is the fracture aperture (m).

3.2. Numerical Model Establishment

The aforementioned gas–solid coupling model was implemented in COMSOL Multiphysics using the Solid Mechanics and PDE modules. Based on the physical parameters of the coal and rock from Wu Hua No. 1 Mine, key parameter settings are listed in

Table 2. Key parameters in the model.

Parameter	Variable	Value	Unit
Coal elastic modulus	E	2723	MPa
Elastic modulus of coal matrix	$K_S$	8459	MPa
Poisson’s ration	υ	0.36
Coal skeleton density	${\rho }_c$	1470	kg/m3
Coal matrix porosity	${\varphi }_m$	0.045
Coal fracture porosity	${\varphi }_f$	0.011
Gas dynamic viscosity	${\mu }_g$	1.34 × 10−15	Pa·s
Initial horizontal permeability	$k_0$	5.14 × 10−16	m2
Langmuir volume constant	$V_L$	0.0256	m3/kg
Langmuir pressure constant	$P_L$	2.07	MPa
Adsorption time	$\tau$	0.221	d
Initial porosity of the coal matrix	$p_m$	1.25	MPa
Initial porosity of the coal fracture	$p_f$	1.25	MPa

. Since precise measurement methods were unavailable for a limited number of physical parameters, their values were assigned with reference to recently published studies [15,26,27,28,29], ensuring all parameters are within reasonable ranges.

A simplified 2D rectangular coal seam model with dimensions of 40 m × 3.5 m (length × width) was constructed. Three 120 mm diameter gas extraction boreholes were incorporated into the model at 5 m spacing. The model’s bottom boundary was assigned a fixed constraint, its sides were given roller supports, and different vertical stress loads were applied to the top boundary, as shown in Figure 5. The initial coal seam gas pressure was set at 1.25 MPa, and the extraction negative pressure was set at 20 kPa.

Affected by hard-roof hanging, the coal seam ahead of the working face exhibits a four-zone stress distribution. This variation in stress leads to significant differences in gas drainage efficiency. To further investigate the impact of stress on gas flow in drainage boreholes, based on UDEC simulations, this study selected five static characteristic stress values (before main roof fracturing) as boundary conditions for analysis. These values are as follows: 5.7 MPa (minimum value in stress reduction zone I), 8.9 MPa (minimum value in stress reduction zone II), 11.3 MPa (original stress value), 23.3 MPa, and 35.3 MPa (peak value in the stress concentration zone).

3.3. Analysis of Simulation Results

3.3.1. Impact of Stress on Coal Seam Permeability

Coal permeability is one of the key factors governing the efficiency of coal seam gas extraction. The variation curves of coal permeability around borehole under different stress conditions are shown in Figure 6.

Under the same extraction duration, a negative correlation exists between coal seam permeability and stress: permeability decreases as stress increases and rises as stress decreases. This mechanism occurs because elevated stress compresses coal pores and fractures, reducing permeability, while stress reduction promotes the expansion of voids, thereby enhancing permeability. When the gas extraction duration reaches 50 days, the corresponding results are as shown in Figure 6a. Increasing the coal seam stress from 11.3 MPa to 23.3 MPa reduced the minimum permeability from 73.1% to 55.5% of its initial value, representing a decrease of 17.6 percentage points. A further increase in stress from 23.3 MPa to 35.3 MPa lowered the minimum permeability from 55.5% to 41.1% of the initial value, a reduction of 14.4 percentage points. These results indicate that permeability decreases nonlinearly with increasing stress, and the magnitude of permeability reduction for a given stress increment diminishes as the stress level rises. Conversely, when the coal seam stress decreased from the original stress of 11.3 MPa to 8.9 MPa, the minimum permeability increased from 73.1% to 79% of the initial value—a gain of 5.9 percentage points. When the stress was reduced from 11.3 MPa to 5.7 MPa, the minimum permeability rose from 73.1% to 89.6% of the initial value, an increase of 16.5 percentage points. This demonstrates that greater stress relief leads to a more significant improvement in coal permeability.

3.3.2. Impact of Stress on Gas Pressure

Coal seam gas pressure is not only the primary driving force for gas transport within the coal matrix but also a key indicator of the difficulty of gas extraction. The contour plots of gas pressure evolution under different stress conditions are presented in Figure 7. The results show a distinct low-pressure zone around the boreholes and a significant gas pressure gradient extending from the boreholes to the model boundary. However, despite identical initial gas pressures, notable differences in the gas pressure distribution around the boreholes are observed under different applied stresses. For quantitative analysis, data along a horizontal 2D profile intersecting all three boreholes were extracted at extraction times of 50, 100, and 150 days. To focus on the core area of influence, the analysis was concentrated on the region between the horizontal coordinates of 10 m and 30 m. The variation curves of coal seam gas pressure under different stress conditions within this region are shown in Figure 8.

Given the symmetry of the geometric model and the influence of the gas drainage radius around the boreholes, the analysis focuses on the gas pressure distribution between two adjacent boreholes (15–20 m). After 50 days of extraction, the corresponding results are as shown in Figure 8a; the peak gas pressure between the boreholes decreased to 0.84 MPa under a stress of 5.7 MPa, representing a reduction of 32.8%. At 8.9 MPa, the peak pressure dropped to 0.87 MPa (30.4% reduction); at 11.3 MPa, it fell to 0.89 MPa (28.8% reduction); at 23.3 MPa, it was 0.94 MPa (24.8% reduction); and at 35.3 MPa, it decreased to 1.0 MPa (20% reduction). As the applied stress increases, the peak gas pressure between the boreholes rises correspondingly, while the magnitude of the pressure reduction gradually diminishes. This indicates that under identical drainage conditions, higher stress inhibits gas flow, making gas extraction more difficult and resulting in a smaller decrease in gas pressure.

A comparison of Figure 8a–c reveals that, regardless of whether the extraction duration was 50, 100, or 150 days, the gas pressure in the high-stress coal seam remained consistently higher than that in the low-stress seam under identical extraction conditions and initial gas pressures. The primary reason for this is the lower permeability of the coal seam under high stress, which significantly impedes gas extraction. Furthermore, the gas pressure curves at different extraction times show that the disparity in gas pressure between the high-stress and low-stress coal seams widens as extraction continues. This indicates that the inhibitory effect of high stress on gas extraction becomes progressively more pronounced over time.

The analysis above demonstrates that stress governs coal seam permeability during extraction, which in turn dictates the difficulty of gas drainage: higher stress leads to lower permeability, a smaller reduction in gas pressure, and more challenging extraction; conversely, lower stress results in higher permeability, a greater pressure drop, and more efficient gas recovery. Based on the four-zone distribution characteristics of coal seam stress, it can be inferred that stress reduction zone II, located farther ahead from the working face, provides favorable conditions for gas extraction and should be considered a preferential zone for drainage operations.

4. Field Measurement of Gas Extraction Flow Rate at the Working Face

To validate the aforementioned analysis, gas extraction boreholes were arranged in the air return tunnel and transport tunnel of the 16,021 working face in Wu Hua No. 1 Mine. The boreholes were aligned parallel to the open-off cut direction, forming a 90° angle with the air return tunnel and transport tunnel. To ensure rational borehole spacing while minimizing construction, the layout was designed as follows: in the 0–40 m section ahead of the face, nine boreholes were installed with a 5 m spacing; in the 40–100 m section, 21 boreholes were deployed with a tighter 3 m spacing. All boreholes were drilled to a depth of 45 m. We arranged a total of 30 boreholes. The boreholes were sealed using a mixture of cement mortar, synthetic resin, and polyurethane to a sealing depth of 12.5 m. The extraction negative pressure was set at 20 kPa. The specific layout of the gas extraction boreholes is detailed in Figure 9.

As shown in Figure 10, the variation in the field-measured gas extraction flow rate with distance from the working face exhibits four distinct stages, which is highly consistent with the stress zoning results obtained from numerical simulations. In stress reduction zone I near the working face, the coal is subjected to intense abutment stress disturbance leading to the full development and connectivity of fractures. This causes a sharp increase in permeability, driving the gas extraction flow rate to its peak and exhibiting a typical “pressure relief-enhanced flow” effect. As the distance increases into the stress concentration zone, the coal seam permeability decreases. Pores and fractures tend to close under pressure, leading to a corresponding reduction in drainage flow rate. Within the range of 47–62 m, affected by the suspended roof, the coal seam stress decreases locally, forming stress reduction zone II. The gas flow conditions are improved, and the drainage flow rate shows a slight recovery—this phenomenon is consistent with stress reduction zone II as identified in the numerical simulation. With a further increase in distance, the mining disturbance gradually weakens. The coal seam stress returns to its original state, and the gas extraction flow rate accordingly enters a relatively stable stage.

5. Discussion

This study proposes a four-zone distribution model of coal seam stress under hard-roof conditions and puts forward optimization suggestions for gas extraction in stress reduction zone II. However, there are still some limitations:

(1) The four-zone distribution model is proposed based on the case study of Wu Hua No. 1 Mine. This model has potential applicability to mining areas with “hard roof” and “large span” conditions. Theoretically, similar stress zoning may occur in analogous geological environments, but its general applicability needs to be verified by more field measurements under different geological backgrounds in future research.

(2) The gas–solid coupling model adopted in this study does not consider the effects of temperature and moisture. In reality, elevated temperature promotes gas desorption, increases the diffusion kinetic energy of gas molecules, and accelerates gas migration [30]. Conversely, moisture reduces permeability due to pore-filling effects [31,32]. Therefore, the comprehensive influence of stress, temperature, and moisture on gas extraction requires further investigation.

6. Conclusions

This study investigates the distribution characteristics of coal seam stress under conditions of a hard roof and analyzes its impact on gas drainage flow. The primary conclusions are as follows:

(1) When hard roofs are present in the overlying strata of the working face, the coal seam stress distribution deviates from the conventional three-zone model. Proceeding from the coal wall into the deep seam, a typical four-zone pattern emerges: stress reduction zone I, stress concentration zone, stress reduction zone II, and original stress zone.

(2) Under identical extraction conditions, elevated stress reduces coal seam permeability, thereby inhibiting the decline in gas pressure and increasing the difficulty of gas drainage. Conversely, stress reduction enhances permeability and facilitates gas extraction. The lower stress within the far-field stress reduction zone II improves coal seam permeability, reduces drainage difficulty, and manifests as an increased gas extraction flow rate.

(3) Although pre-drainage measures mitigate the risk of coal and gas outbursts before mining, abutment stress disturbances continuously promote gas desorption and migration toward the working face, leading to persistent gas emissions. Therefore, continuous gas extraction must be implemented during mining operations. Particularly under hard-roof conditions, extraction efforts should strategically target stress reduction zone II to ensure mining safety.

Future research will further explore permeability enhancement technologies for stress reduction zone II, such as creating seepage channels in this zone using directional hydraulic fracturing to achieve precise gas control in deep mines.