Investigation of Uneven Gas Emission Mechanisms with Hard Roofs and Control Strategies by Ground Fracturing

Abstract

The permeability of a coal seam is a crucial factor in coal seam gas extraction. Poor permeability of coal seams can lead to difficulties in over-pumping as well as high gas emissions after mining. This issue is particularly prominent when mining extra-thick coal seams with hard roofs, and it is the major problem that restricts the safe and efficient mining of coal seams. In the context of extra-thick coal seam mining in the Datong mine area, field investigation into the gas emission patterns of the working face reveals that the volume of gas emissions correlates closely with variations in working face pressure, demonstrating a high degree of consistency. The mechanism of irregular gas emission was analyzed, and the influence law of roof breakage on gas emission was obtained. It was found that roof breakage will aggravate gas emission. As a result, an integrated control technology involving “ground fracturing + gas extraction” was innovatively proposed. Based on the characteristics of ground fracture network, the mechanism of pressure relief and permeability enhancement of fractured wells and the characteristics of full time and space extraction were analyzed. Using the 8101 and 8204 working faces of the Tashan Coal Mine as a case study, the results demonstrated that vertical well fracturing of the 8101 working face enabled gas extraction 150 m ahead, with an accelerated increase in gas concentration within a 40 m range. Similarly, the horizontal well of the 8204 working face served as a drainage well after fracturing. Gas concentration at the mining position 50 m away from the horizontal well increased rapidly, and the gas extraction rate stabilized at approximately 30 m³/min. The approach effectively mitigated the problem of uneven gas emission caused by gas accumulation and roof fractures in the working face. Ground fracturing not only reduced the area and intensity of stress concentration in the advanced coal body but also enhanced gas emission. Furthermore, the fracturing well served as a gas drainage well, improving the control and achieving positive application results.

1. Introduction

Gas in coal seams is a clean energy source, and the efficient extraction and utilization of gas is a crucial aspect of coal resource development. However, it is also a primary cause of coal mining accidents. Therefore, achieving efficient gas extraction from coal seams not only enables the utilization of gas as a clean energy source but also helps prevent gas disaster accidents in coal mines. Currently, the main methods for gas extraction are advanced extraction and delayed extraction. Advanced extraction primarily targets the gas within the coal seam, which is closely related to the permeability of the coal seam [1,2]. In contrast, delayed extraction focuses on extracting residual gas from the goaf after coal mining. This approach not only aims to improve the gas extraction rate but also helps prevent sudden large-scale gas outbursts in the goaf caused by roof collapses. When the coal seam is covered by hard roofs, the stress concentration in the hard roofs is high, resulting in significant stress concentration in the advanced coal body, making advanced gas extraction difficult. On the other hand, the large breaking step distance of the hard roofs can lead to sudden and extensive roof failures, which may cause a sudden large-scale outburst of gas in the goaf, increasing the risk of gas disaster accidents, as shown in Figure 1.

In some mining areas with extremely thick coal seams, for example, the Datong mining area in China, the low permeability of the coal seam with hard roofs in the overlying, combined with a relative gas content of 2.92 m³/t and a 20 m thickness, results in absolute gas emissions during mining of 50 m³/min or more [3]. These conditions hinder safe and efficient mining due to difficulties in pre-draining gas before mining and high gas emissions during mining. Moreover, the Datong mining area is characterized by hard roof conditions, with overlying rock comprising multiple hard rock layers. The gradual upward breaking and destabilization of the roof during mining leads to gas extrusion in the gob, resulting in frequent gas over-limit situations in the working face that are challenging to manage.

Researchers have analyzed the compression effect of hard roof breaking on the gob and the distribution characteristics of the fracture network in the overburden fall belt, crack belt, and bending subsidence belt [4]. Pan Jicheng studied the relationship between the gas outflow and the mining state of the working face. The gas emissions during initial weighting and periodic pressure were clarified [5]. Based on microseismic monitoring, Meng Xiangjun recorded and analyzed the overlying rock fracture characteristics, and the morphology of overlying rock fissure development and its effect on gas gathering were obtained, and accordingly, the optimal high-level extraction hole arrangement parameters were determined [6]. Additionally, studies have explored the relationship between gas outflow and the breaking of multiple hard rock strata under hard roof conditions in nearly 20 m-thick coal seams. However, further research is needed to fully address this relationship.

In managing gas in low-permeability coal seams with hard roofs, advanced pre-draining is crucial for reducing gas concentration and preventing secondary disasters such as uneven gas outbursts caused by roof breaking. Improving gas permeability in such conditions presents a technical challenge, and extensive discussions and studies have been conducted on this aspect [7,8,9]. By measuring the coalbed methane content and monitoring the pore pressure changes before and after mining, Qingdong Qu et al. [10] concluded that monitoring the pore pressure helps to characterize the gas emission, which provides a basis for optimizing gas extraction and emission reduction. Chunshan Zheng et al. [11] studied the effect of coal damage on permeability and found that tensile damage is on the upper and lower sides of the roadway, shear damage is on the left and right sides, and the increase in the friction angle reduces the damage area, and the results of the research can provide scientific guidance for the design of gas extraction drilling. Dong Chen et al. [12] improved the traditional relative-permeability model of porous media for coupled simulation and concluded that changes in porosity have a significant impact on gas-water saturation and gas production rate, highlighting the importance of further research on the relative permeability characteristics of coal. The above study proved that the poorer the permeability of the coal seam, the more difficult it is for gas to seep through the seam, resulting in lower extraction efficiency. Pressure relief and permeability enhancement of coal have been identified as effective ways to improve coal permeability and enhance gas extraction. Hydraulic fracturing techniques, such as pulse hydraulic fracturing, variable displacement hydraulic fracturing, and staged fracturing, have been developed and applied downhole, yielding positive results in enhancing permeability [13,14,15]. In addition, overburden rock early pressure relief is also a key technical means to improve the permeability of the coal body, and liberation layer mining is the most typical pressure relief mining technology widely used in coal mine gas control and it can achieve very good application results [16,17,18,19]. Furthermore, mature gas extraction technologies, including continuous vertical holes on the ground, long boreholes on the ground, high alley pumping, and long boreholes downhole, have been developed domestically and internationally for post-mining gas management. These technologies have formed a spatial and three-dimensional extraction system, enabling the efficient extraction of gas from caving zones and cracking belts [20,21,22].

From the perspective of improving the stress environment of coal seams to enhance the effect of over-pre-pumping and increase the amount of gas extraction after mining, many useful technical means have been put forward, and good application results have been achieved. A synergistic method of formation-gas control by “ground fracturing of the formation + synergistic gas extraction” is proposed in the paper. This method involves using ground fracturing wells to fracture and weaken the cover rock hard roofs, reduce the strength of the rock body, and extract mining field gas in real time and space. Ground fracturing technology, originally used to control the dynamic disaster of mining pressure in hard rock formations, has been widely applied in mining areas due to its ability to create a large-scale fracture network in a rock formation, modify the strength and permeability of the formation, and change the stress environment of the coal body within the fracturing area. The fracturing wells can also be used as gas extraction wells, achieving dual functionality and positive application results.

2. Gas Emission in Extra-Thick Coal Seam Working Faces with Hard Roofs

2.1. Gas Occurrence Characteristics

The Tashan Coal Mine in the Datong Mining District primarily extracts coal from the Carboniferous Permian 3–5# coal seams, which have a thickness of 14 to 20 m. The original gas pressure within the coal seam ranges from 0.14 to 0.17 MPa (absolute pressure), with a gas content of 1.6 to 1.97 m³/t, averaging 1.78 m³/t. Furthermore, the Tashan Mine’s coal seam is covered by multiple layers of thick and hard rock strata, making it a typical hard roof mine. The compressive strength of the overlying rock strata falls within the range of 60 to 120 MPa. The permeability of the coal seam is determined by drilling a borehole, using the downhole method to determine the permeability coefficient. Two gas pressure measurement boreholes, #1 and #2, with a diameter of 65 mm, were selected for measurement. The permeability coefficients of the extra-thick 3–5# coal seams at the Tashan Coal Mine are presented in

Table 1. Determination of coal seam permeability coefficient.

Borehole Number	Absolute Gas Pressure/MPa	Gas Content Coefficient/m3/(m3·MPa0.5)	Flow Rate of Gas in Boreholes/m3/d	Air Pressure/MPa	Gas Permeability Coefficient/(m2/MPa2·d)
1#	0.22	6.78	0.003	0.10	1.108 × 10−4
2#	0.19	6.70	0.002	0.10	1.328 × 10−4

.

The extra-thick coal seam at the Tashan Coal Mine falls into the low permeability coefficient class, making it a challenging coal seam for gas extraction, with poor results from advanced pre-pumping efforts. Additionally, the gas flow attenuation coefficient from boreholes is a crucial indicator of the ease of gas extraction from coal seams. The measurement of this coefficient involves drilling holes in the coal seam for gas flow monitoring. The borehole diameter was 65 mm, and the flow measurement records for boreholes 3# and 4# are detailed in

Table 2. Borehole flow monitoring.

Time of Emission/d	Borehole Flow Rate (m3/d)
	3#	4#
1	0.0080	0.0069
2	0.0023	0.0021
3	0	0

.

Based on the data in Table 2, it is evident that the average gas flow rate from the monitoring borehole was 0.0075 m³ on the first day, which then decreased to 0.0022 m³ on the second day. Furthermore, the gas flow rate could not be monitored on the third day, indicating a significant gas attenuation rate. This suggests that it is currently challenging to use the borehole for advanced gas extraction.

2.2. Gas Emission Law

During the mining of the Tashan Coal Mine, the gas emission from the roadway and the working face is monitored. The gas emission from the roadway is obtained by arranging sensors in the return-air lane of the shaft and transmitting the data in real time to the monitoring center on the ground. The gas emission from the working face is then temporarily detected by a portable infrared detector to obtain the amount of gas emission during the mining process. On-site statistics on gas emissions from the working faces in the first and second panel areas revealed significant variations. In the first panel area, the absolute gas emission quantity during normal mining ranged from 25–30 m³/min, while during abnormal gas emission, it could reach 35–45 m³/min or higher. Similarly, in the second panel area, the absolute gas emission quantity during normal mining was between 30–35 m³/min, and during abnormal gas emission, it could escalate to 40–50 m³/min or higher. Additionally, the gas emission quantity from the cutting face in both panel areas was relatively low, with approximately 2 m³/min during normal digging and around 10 m³/min during abnormal gas emission.

The current main management method utilized at Tashan Mine to address the technical challenges of low accumulation and high emission of gas involves the integrated working face gas management technology system. This system is primarily based on air distribution in the working face and closed extraction in the high alley pumping lane of the roof plate. The extraction capacity of the roof-high pumping alleys is leveraged to mitigate gas overlimit at the working face.

On-site statistics regarding the correlation between the pressure from the hard roofs breaking and the gas extraction concentration in the high pumping alley during the working face mining process are depicted in Figure 2. The figure illustrates that the gas concentration in the high pumping alley fluctuates in tandem with the variation in support resistance, demonstrating a notable consistency. The large cycle of gas-concentration change spans 11–13 days, corresponding to the large cyclic weighting interval of 61.6–72.8 m in the working face, with the maximum concentration of the large cycle reaching 7.9%. Additionally, the small cycle exhibits gas-concentration changes over a 6–7 day period, corresponding to a working face to weighting interval of 33.6–39.2 m, and reaching a maximum concentration of 6.1%.

The comparative analysis data of gas emission and its sources before and after the pressure event from one of the working faces are presented in Figure 3. Before the ground pressure event, the gas concentration was 0.18% in the return air stream and 2.8% in the high pumping alley, resulting in a total emission of 56.61 m³/min. Specifically, the gas emission from the coal wall, the gob, and the fallen coal were 2 m³/min, 35.61 m³/min, and 19 m³/min, respectively. Following the ground pressure event, the gas concentration in the return airflow, the high pumping alley, and the total emission increased to 0.28 m³/min, 4.2 m³/min, and 83.73 m³/min, respectively. This indicates a significant increase in both concentration and total amount of gas emission after the pressure event. Notably, the gas emission from the gob surged to 52.74 m³/min, marking it as the primary source of the increased gas emission during the pressure event period.

The monitoring results indicate a strong correlation between the gas extraction concentration in the high alley pumping and the characteristics of the roof breaking cycle. It is observed that the free gas in the gob is extruded and disturbed by the breaking roof slewing, leading to gas emission. Additionally, there exists a direct correlation between the amount of abnormal gas emission and the size of the breaking roof slewing area. Furthermore, the size of the roof breaking cycle’s pressure leads to varying degrees of gas emission. The substantial absolute gas emission from the extra-thick coal seam, combined with the wide range of overburden rupture and the high degree of fissure development, contributes to the complex gas enrichment law. As the overburden breaks and destabilizes gradually upwards, it induces complex gas emission characteristics, ultimately leading to accidents of gas exceeding the limit.

3. Mechanism of Unbalanced Gas Emission Due to Roof Breaking

3.1. Stress Evolution on Hard Roofs Affecting Gas Desorption

The hard roof exhibits high strength and noticeable stress-concentration characteristics. Under the influence of mining stress in the working face, the periodic breaking of the hard roof is characterized by a specific breaking law. Preceding the hard roof breaking is a high stress concentration in the roof plate. When the roof limit span is reached, roof breaking instability occurs, leading to a change in the stress distribution of the overlying rock. The abutment stress of the overhanging coal body in the working face is simultaneously affected. Following roof breaking, a large-scale breaking structure is formed, causing rotary subsidence of the underlying rock layer, which exerts a compression effect on the overlying coal body of the working face. This induces a change in the stress environment of the overlying coal body, further impacting gas resolution and transportation.

To characterize the rock breaking and instability on the overtopping coal body, a numerical simulation model was established with dimensions of 492 m × 600 m. Horizontal constraints were set on both sides of the model, and fixed constraints were applied at the bottom. Multiple key strata were included in the model at distances of 17.6 m, 45.7 m, 75.3 m, 107.2 m, 146.8 m, and 184.7 m from the coal beds, with respective thicknesses of 9.44 m, 9.1 m, 10.12 m, 12.2 m, 12.9 m, and 12.1 m [23]. The mechanical parameters of the modeled coal and rock bodies were obtained from the field sampling of 8101 and 8204 workings in Tashan Mine and then tested in the laboratory. Initially, the influence of roof breaking and destabilization on the overtopping coal body abutment stresses in two different layers of hard roof breaking at 17.6 m and 107.2 m from the seam was investigated, as shown in Figure 4.

Based on Figure 4, it is evident that the abutment stress of the advanced coal body experienced significant effects both before and after the hard roof breaking. Immediately following the breaking, the abutment stress of the advanced coal body exhibited subsequent increases. Prior to the breakage of KS1, the peak advanced abutment stress of the coal body was 26 MPa, located 24 m from the working face. After the breakage of KS1, the peak advanced abutment stress of the coal body increased to 27.2 MPa, positioned 18 m from the working face. Before the breakage of KS4, the peak advanced abutment stress of the coal body measured 28.3 MPa, situated 29 m away from the working face. After the breakage of KS4, the peak advanced abutment stress of the coal body rose to 30.5 MPa, located 30 m from the working face.

It is evident that the abutment stress distribution in the advanced coal body is directly influenced by the roof breaking of the overlying hard roof on the working face, which further impacts gas desorption. Prior to the hard roof breaking, stress concentration forms in the roof plate, creating a stress arch structure with the lower endowed rock layer. The two arch footings are situated in the coal body in front of the working face and the coal body at the back of the gob, respectively. When the hard roof breaking occurs, its large breaking span and high energy intensity lead to the synchronous breaking and slewing of the lower endowed rock body due to the destabilization of the roof structure. Consequently, the coal body in front of the working face is also affected, resulting in an elevation of the abutment stress in the coal body in front of the working face.

As can be seen in Figure 5, the degree of fracture development in the coal body significantly influences gas desorption release. When the hard rock layer is broken and destabilized, the overburden stress is transferred downward to the coal body of the advanced working face, leading to an increase in the vertical abutment stress (σ_v) of the advanced coal body. As the horizontal confining stress (σ_h) of the coal body in the advancing direction of the working face decreases, and assuming that the horizontal stress (σ_H) in the length direction of the working face remains unchanged, the stress difference in the coal body subjected to the three-directional stresses further increases. This can cause the coal body to undergo either shear or tensile damage, leading to further development of fissures in the coal body and subsequent gas analytical release. This phenomenon is also the primary reason why the overhanging coal wall is prone to flaking and toppling during the pressure in the working face, and why the desorption amount of gas increases.

3.2. Gas Enrichment Characterization Based on Roof Breaking

After the release of gas, the majority of the gas is dissipated through ventilation and extraction of the working face. However, a portion of the gas continues to disperse into the gob due to its low density and ability to diffuse upward. As a result, the distribution of gas in the gob is closely associated with the structural characteristics of the overlying rock layer [24]. This highlights the significance of the gas’ behavior in the gob and its relationship to the geological features of the surrounding rock strata. The model is applicable to the hard top mining conditions of extra-thick coal seams and the poor permeability of coal seams, and has some limitations for general coal seam geological conditions. Under the mining conditions of breaking an extra-thick coal seam with a hard roof, the overburden rock layer gradually destabilizes, with the near-field low-middle rock layer exhibiting characteristics of a cantilever and masonry beam structure. Furthermore, the lower part of the cantilever and masonry beam structure is prone to forming hollow areas. Additionally, the separated stratum space is created by the lower part of the overlying unbroken rock layer, which serves as the primary site for gas accumulation, as depicted in Figure 6.

3.3. The Effect of Roof Breaking on Gas Emission

During the coal mining process, as the mining face advances, the roof of the face experiences deformation, sinking, and ultimately breakage due to the combined effects of its own weight and the load from the overlying rock layer. This leads to induced pressure within the quarry. The coal and gangue left in the gob can be compacted by the breaking and slewing of the hard roof, simultaneously reducing the volume of space in the gob. This compaction causes the gas in the mining area to be squeezed, resulting in the high concentration of gas gushing towards the working face. This abnormal outflow of gas during the period of induced pressure can lead to gas-related issues in the working area.

To further analyze the impact of the rotary collapse of the hard top plate on gas emissions inside the gob, a model depicting the morphological characteristics of the gob after the breakage of the hard top plate was established. Subsequently, the changes in the volume of the gob space before and after the collapse of the hard top plate were calculated. This analysis will provide valuable insights into the alterations in gob space volume and their implications for gas emissions within the mining area.

As can be seen in Figure 7, the assumption made regarding the length of the working face (L), the breaking length of the hard top plate (L₁), the distance from the hard top plate to the top of the gob (L₂), and the angle of collapse (α) provides a basis for further analysis. The conclusion drawn, irrespective of rock strength and blockiness, regarding the relationship between the fragmentation coefficient and axial pressure (k_p = alnσ + b) where σ represents the axial pressure borne by the leftover coal and gangue, and a and b are the regression coefficients, is significant. The specific values of a (−0.15) and b (1.25) [25] provide important parameters for understanding the impact of axial pressure on the fragmentation coefficient. This relationship will be instrumental in evaluating the effects of axial pressure on the behavior of the gob and gas emissions.

The distance L₂, representing the distance of the hard roof from the bottom plate of the coal seam, can be calculated as the sum of the thicknesses m₁ of the mined coal, m₂ of the released coal, and m₃ of the weak rock layer below the hard roof. This comprehensive calculation will provide a clear understanding of the vertical distance between the hard roof and the bottom plate of the coal seam, taking into account the various layers and materials involved.

The area of the separated space can be calculated by equating it to a trapezoid, and the volume of the separated space can be determined as the change in volume before and after the top slab was broken. By employing these calculations, the morphological characteristics and volume changes associated with the separated space can be accurately assessed, providing valuable insights into the impact of the top slab breakage on the gob space.

In the process of integrated mining of extra-thick coal seams, the top coal recovery ratio is generally low, resulting in a large amount of coal left in the gob. When calculating the volume change in the gob during the collapse of the hard roof plate, it is important to consider the influence of the remaining coal in the gob. Assuming the top coal extraction rate is denoted as x, the maximum displacement ΔH of the hard roof breaking block after the compacted collapsed coal rock in the gob can be expressed as:

$$\begin{array}{l}\Delta H=L_2-\left(1-x\right)m_2{K}_p-m_3{K}_p=m_1+m_2+m_3-\left(1-x\right)m_2{K}_p-m_3{K}_p\\ =m_1+m_2\left[1-\left(1-x\right)K_p\right]-m_3\left(K_p-1\right)\end{array}$$

(1)

where ΔH is the maximum displacement of the hard roof breaking block after the compacted collapsed coal rock in the gob; L₂ is the distance from the hard top plate to the top of the gob; m₁ is the sum of the thicknesses of the mined coal; m₂ is the released coal; m₃ is the weak rock layer below the hard roof; K_p is the frag-mentation coefficient; and x is the top coal extraction rate.

The amount of change in the volume of the gob space ΔV due to the rotary collapse of the hard roof during the periodic pressure is given by:

$$\Delta V=\Delta H\times \left(L_1+{\displaystyle \frac{\Delta H}{\tan \alpha }}\right)=L_1\left\{m_1+m_2\left[1-\left(1-x\right)K_p\right]-m_3\left(K_p-1\right)\right\}+{\displaystyle \frac{{\left\{m_1+m_2\left[1-\left(1-x\right)K_p\right]-m_3\left(K_p-1\right)\right\}}^2}{\tan \alpha }}$$

(2)

where ΔV is the amount of change in the volume of the gob space; L₁ is the breaking length of the hard top plate; and α is the angle of collapse.

Amidst the rotational subsidence of the hard roof slab, the predominant volume of gas situated beneath the compromised block is channeled towards the coal face. Simultaneously, a lesser quantity of this gas advances into the deeper recesses of the gob. If we postulate that the mean gas concentration within the more superficial region of the gob is represented by C, the time span of the recurrent pressure influx is symbolized by t, and we incorporate an air infiltration coefficient of 15% in accordance with reference [26], the augmentation in gas discharge from the gob coinciding with the pressure influx duration, indicated by Q, can be quantified approximately as follows:

$$Q=0.85\Delta VC/t={\displaystyle \frac{17\mathrm{C}{L}_1\left\{m_1+m_2\left[1-\left(1-x\right)K_p\right]-m_3\left(K_p-1\right)\right\}}{20t}}+{\displaystyle \frac{17\mathrm{C}{\left\{m_1+m_2\left[1-\left(1-x\right)K_p\right]-m_3\left(K_p-1\right)\right\}}^2}{20\tan \alpha \cdot t}}$$

(3)

where Q is the augmentation in gas discharge from the gob coinciding with the pressure influx duration; C is the mean gas concentration within the more superficial region of the gob; and t is the time span of the recurrent pressure influx is symbolized.

To accurately quantify gas emissions from the gob, an analysis of the volumetric alterations within the space preceding and subsequent to roof collapse is imperative. This assessment furnishes a foundational framework for the implementation of gas regulation strategies throughout the cyclic surges of barometric pressure.

4. Gas Emission Control by Ground Fracturing

4.1. Rock Formations Weakening Based on Ground Fracturing

The preceding analysis elucidates that gas emissions are predominantly influenced by the presence of a hard roof in two principal ways. Firstly, the post-fracture torsional extrusion stress experienced by the hard roof amplifies the differential stress within the coal mass advancing towards the face, precipitating shear or tensile damage. This, in turn, promotes the development of fractures, spalling of the coal matrix, and the consequent liberation of gas. The manifestation of this process is an escalated volume of coal from the advancing mass during periods of increased barometric pressure at the working face. Secondly, the fracturing and rotation of the hard roof exerts a compressive force on the gas within the gob, precipitating a rapid surge in emissions within a brief time frame. This is evident in the heightened levels of gas discharge from the gob during the incoming pressure intervals at the working face. Therefore, mitigating the intensity of stress exerted on the advancing coal body by the fracturing hard roof, as well as the compression of gob gas due to its fracturing and rotation, are critical challenges in the prevention and control of anomalous gas emissions triggered by hard roof breakage. Central to addressing these challenges is the diminution of the hard roof fracture dimensions and the reduction in the magnitude of the resultant broken fragments.

Hard-roof-breaking face fracturing utilizes horizontal and vertical wells to fracture large-scale networks of coal seams in rock formations. Vertical wells are drilled from the surface to the target formation to achieve continuous fracturing of vertical multi-layer formations. The extension of fracturing in a single operation depends on the amount of fracturing fluid and the direction is determined by the three-way stress distribution in the formation. Horizontal well options include vertical, inclined, and horizontal sections. Horizontal segments fracture a single target formation. Volumetric fracturing is achieved by fracturing up to 1000 × 200 × 50 m (length × width × height), thus achieving the desired effect of volumetric fracturing [27]. The fracturing process is divided into perforating (using perforating charges to form channels) and fracturing (creating a large-scale network of seams). Figure 8 shows a schematic diagram of vertical and horizontal well fracturing.

Previous engineering practices in ground fracturing have demonstrated that the large-scale fracture created through ground fracturing technology effectively reduces the size of the hard roof breaking block. This technique eliminates the occurrence of large-area-overhanging roof breaking in the overlying rock and significantly reduces the abutment stress on the advancing coal body. These results indicate that ground fracturing can serve as an excellent pre-fracturing method, playing a crucial role in relieving pressure from over-advanced coal rock bodies and controlling lagging roof collapse.

4.2. Roof Fracturing Modification Affects Gas Emission

Figure 9 shows that the abutment stress distribution in the advancing coal body of a working face can be divided into three zones: the pressure relief zone, stress concentration zone, and original rock stress zone. The impact of ground fracturing on gas outflow from the advancing coal body is observed in two main aspects. Firstly, in the pressure relief zone, the coal body experiences tensile stress during the advancement process, leading to fissure opening, increased permeable volume, increased porosity, and elevated gas emission. This zone is the primary area for gas emission. The breaking and slewing of the hard roof intensify the stress difference in the pressure relief zone, further promoting coal body destruction and fissure development. This is a key factor contributing to significant gas outflow. Fracturing the hard roof slab weakens the stress transfer during the roof breaking and slewing process, reducing the extent of coal body destruction. This effectively alleviates the peak abutment stress on the advancing coal body, reduces coal wall spalling, and subsequently mitigates the problem of increased gas emission caused by coal wall destruction. Secondly, in the stress concentration zone, the high bearing characteristics of the hard roof result in a high static load transmitted downward, leading to elevated abutment stress in the advancing coal body. This compression of the coal seam pore and fissure reduces the permeability of the coal seam. Additionally, the high abutment pressure can cause some of the gas in the coal seam to transition from a free state to an adsorbed state. Consequently, gas emission in the stress concentration zone is relatively low and inversely proportional to the magnitude of the abutment pressure. In the case of mining extra-thick coal seams with a hard roof and wide mining movement, the influence range of the wide mining movement, coupled with the large breaking step of the hard overlying rock, results in high abutment stress on the advancing coal body. Previous monitoring studies have shown that the peak abutment stress on the advancing coal body can exceed 30 MPa, with an influence range of 100 m or more. This undoubtedly exacerbates the challenge of gas extraction during the advancement process. Ground fracturing can effectively reduce the distribution of abutment stress in the advancing coal body and enhance the permeability of the coal body in the stress concentration zone. This is achieved by artificially creating large-scale cracks in the hard rock strata, which facilitates the desorption and release of gas during advancement.

Additionally, early-stage mineral pressure monitoring following ground fracturing has revealed that the supports of the working face undergo increased pressure post-fracturing [28,29]. However, there is no distinct feature of pressure coming, and the maximum strength of the supports decreases to 30 MPa when subjected to pressure. This suggests that the roof breaking is more uniform after fracturing, effectively avoiding the challenges associated with large-scale gas emission caused by extensive roof breaking and slewing.

4.3. Characterization of Full Time-Space Pumping in Fracturing Wells

The formation of a large-area seam network after ground fracturing not only facilitates pressure relief of the advance coal body and gas desorption, but also eliminates the problem of a large-area overhanging roof of the hard roof plate and prevents abnormal gas emission caused by large-area roof breaking of the hard roof plate, and realizes the effect of full time-space and spatial extraction of gas, as shown in Figure 10.

Pre-mining extraction: Ground fracturing plays a significant role in pre-mining extraction by forming a large-scale seam network in thick hard rock seams. This process effectively reduces the abutment stress on the advancing coal body, facilitates gas desorption, and enables advance extraction through the fracture of the overburden and the created seam network. By implementing ground fracturing, the range of advanced extraction areas can be significantly increased compared to unfractured conditions, allowing for a larger area of coal to be extracted prior to mining and maximizing resource utilization. Additionally, the formation of the seam network enhances the concentration of extracted gas, ensuring efficient gas extraction during the pre-mining phase.

Post-mining extraction: The integrity of the rock formation and its breaking step are reduced by ground fracturing, and the breaking step of the thick, hard rock formation after fracturing is dramatically reduced, as Figure 10 shows. The gas extrusion effect on the gob by the uniform breakage of the rock layer and slewing has been greatly reduced, and the technical problem of uneven gas emission in the gob has been eliminated. In addition, the overlying rocks in the gob are fractured and the degree of fissure development is high after fracturing, which is also favorable to the extraction of gas in the gob.

Ground fracturing improves the stress environment of the mining field and enhances the fracture characteristics of the overlying rocks. After fracturing, the resulting fracture network can be utilized as extraction wells for full-time and space-time gas extraction. These wells enable the extraction of gas in the fissure development area of the advancing coal body and within the gob. Furthermore, the use of fractured wells effectively eliminates the problem of uneven or abnormal gas emission in the gob.

5. Engineering Practice

5.1. Extraction Effect of Vertical Fracturing Well

There are five hard rock strata overlying the 14 m extra-thick coal seam during mining. Production practice shows that the breakage and instability of the key strata in the range of 100–150 m above the coal seam has a more serious effect on the mining pressure [30]. In the case of the 8101 working face vertical well fracturing, two key strata, KS3 and KS4, were fractured at distances of 109.87 m and 144.65 m from the coal seam, respectively. The fracturing wells were selected at distances of 431 m from the cuttings of the 8101 working face and 139 m from the 2101 inlet lane. The cracks in KS3 and KS4 extended up to 250 m and 218 m, respectively, with the width of the crack extension area ranging from 30–40 m to 100–120 m. The extension of the cracks in the two layers of rock was predominantly horizontal. This extensive fracturing created a network of cracks that allowed for efficient gas extraction. The fractures provided pathways for the gas to flow towards the extraction wells, ensuring a high concentration of gas extraction. The horizontal extension of the cracks facilitated the extraction of gas from a larger area, increasing the overall gas extraction efficiency. By utilizing the fractured wells, the 8101 working face was able to effectively extract gas from the fissure development areas in KS3 and KS4, as well as within the gob. This comprehensive extraction method helped to eliminate the problem of uneven or abnormal gas emission in the gob, improving both the safety and efficiency of gas extraction.

The continuous vertical drilling holes were employed for gas extraction operations on the working face. These vertical extraction holes were spaced 40 m apart, and their arrangement is depicted in Figure 11. Notably, gas extraction holes #11, #12, and #13 were strategically positioned within the region enveloped by the ground fracturing seam network.

By considering the gas extraction concentration of extraction holes #6 and #12, we conducted an analysis of the concentration change in both the fracturing and unfracturing zones. Figure 12 presents the resulting concentration change curves for gas extraction holes #6 and #12, illustrating the variations in extraction concentration since the advancement of 150 m.

As Figure 12 depicts, it is evident that in the absence of fracturing, the gas extraction concentration of extraction hole #6 begins to increase beyond the 40 m limit and reaches its maximum value at the working face. On the other hand, extraction hole #12 is capable of extracting gas at a low concentration 150 m ahead of the working face. The concentration of gas extracted gradually starts to increase from approximately 80 m ahead of the working face, with the growth rate of extraction concentration accelerating 40 m ahead of the working face. Comparing the gas extraction holes in the unfractured area, it is evident that the extraction concentration and duration of the extraction holes located within the fracturing area have significantly increased. This indicates that the permeability of the formation has been enhanced by the ground fracturing network, leading to an improvement in the degree of gas desorption within the advancing coal body and an overall enhancement in gas extraction efficiency. Ground-based fracturing proves to be effective in altering the physical and mechanical properties of rock formations, enabling comprehensive time-space gas extraction.

5.2. Extraction Effect of Horizontal Well Fracturing

In the 8204-2 silo work face of the Tashan Coal Mine, there was a layer of typical sandy hard rock with a thickness of 10 m located at a depth of 60 m in the overlying rock. To address the potential hazard of the isolated work face and take advantage of the fracturing effect of surface horizontal wells, it was decided to employ surface horizontal wells for fracturing and weakening purposes. Taking into account the mining conditions of the 8204 work face and the ground fracturing environment, the design of the horizontal fracturing test section involved drilling a length of 700 m. The fracturing process was divided into five stages, as illustrated in Figure 13.

Following the fracturing process, the extended lengths of the fracture wings were determined to be 117 m and 162 m, respectively, after the 1-stage fracturing. The fracture height extended 14 m above the wellbore and 15 m below the wellbore, while the width of the fracture network measured 63 m. After the 2-stage fracturing, the extended lengths of the fracture wings were 178 m and 104 m. The fracture height increased to 37 m, with the upper and lower extension heights measuring 21 m and 16 m, respectively. The width of the fracture network decreased to 52 m. Similar effects were observed for the extended characteristics of the seam network after the 3- to 5-stage fracturing. After the 5-stage fracturing, a volumetric seam network was formed, with a length of nearly 300 m, a vertical direction of over 30 m, and a width ranging from 50 to 70 m. The expansion characteristics of the seam network are depicted in Figure 14.

After the fracturing process, horizontal fracturing wells were utilized to extract gas from the mined field. The gas concentration extracted from the horizontal well is depicted in Figure 15. The effectiveness of gas extraction was analyzed by examining the correlation between the mining position of the working face and the end of the horizontal section of the fracturing well. As illustrated in Figure 15, a certain concentration of gas could be extracted at a distance of 80 m in front of the working face, with a gas extraction capacity ranging from 15 to 20 m³/min. As the mining position of the working face approached the horizontal section of the fracturing well, the gas extraction concentration rapidly increased to 35–43 m³/min at a distance of 50 m from the fracturing well. When the mining position of the working face entered the range of influence of the fracturing, the gas extraction concentration gradually stabilized, indicating the stability and effectiveness of the gas extraction from the horizontal well. The average volume of gas extraction was around 30 m³/min.

The main reason for not maximizing the pure volume of extraction from horizontal wells at the surface was the implementation of integrated uphole and downhole gas extraction measures at the working face. This involved underground venting of gas in the vicinity of the working face through normal ventilation methods. The horizontal wells primarily extracted and discharged gas from the overlying fissure zones and gob through continuous extraction. The integrated extraction and drainage technical measures above and below the wells helped to eliminate excessive gas accumulation at the working face. Specifically, the use of ground-level fracturing wells for gas extraction after fracturing the hard roof effectively prevented and controlled the technical issues caused by uneven gas outflow due to roof breaking. The control effect of these measures was remarkable.

On-site monitoring has demonstrated that the combination of “roof ground fracturing + fracturing well extraction” is an effective solution for gas management in coal seams with hard roofs and low permeability. As China’s coal seams are being mined at increasing depths, the overall stress distribution in the coal seams is rising, posing a significant challenge in gas extraction control. Additionally, one-third of China’s mines are endowed with hard roofs, which further complicates the issue of gas emission. The ground-well fracturing synergistic extraction technology offers a new approach to gas management while simultaneously addressing roof decompression management. This technology holds high practical value in addressing the complexities of gas management in such challenging mining conditions.

6. Conclusions

(1)

The technology of “ground fracturing + gas extraction” is applied to improve the efficiency of gas extraction. This technology takes into account the characteristics of the ground fracturing seam network and analyzes the mechanisms of pressure relief and permeability enhancement through fracturing wells. Additionally, the full-time and spatial extraction characteristics of this technology have been thoroughly examined.

(2)

The field monitoring results have revealed a strong correlation between the concentration of gas extraction and the characteristics of roof breaking. The cycles of pressure exerted by the roof, both large and small, result in varying degrees of gas emission. During normal mining operations, the gas emission rate ranges from 35 to 45 m³/min. However, during periods of pressure exerted by the roof plate, the absolute gas emission rate can reach up to 55 to 75 m³/min, with individual peaks reaching 84 m³/min. This represents an increase of 20 to 30 m³/min compared to the gas emission rate during normal mining operations.

(3)

The characteristics of gas extraction under different conditions were analyzed in the 8101 working face using vertical well fracturing and in the 8204 working face using horizontal well fracturing in the Tashan Mine. The presence of a large-scale seam network resulting from ground fracturing weakened the stress concentration environment of the advancing coal body. This large-area seam network within the rock stratum facilitated the extraction of gas in the entire space. As a result, the application of “ground fracturing + gas extraction” by ground fracturing technology yielded positive results.

To sum up, the effectiveness of ground fracturing technology in the synergistic control of advanced pressure relief and gas extraction has been demonstrated. This technology provides a promising approach for managing gas in low-permeability coal seams with hard roofs. By creating a large-scale seam network through ground fracturing, the stress concentration of the advancing coal body can be weakened, and gas can be extracted efficiently. This integrated control technology has shown great potential in improving gas management in mining operations, and it has important value for efficient extraction of gas energy. In the future research of coal mine gas extraction, research may appear in the areas of strengthening extraction efficiency, expanding the scope of application, intelligent monitoring and control, multi-technology integration, environmental protection, and economic considerations.