Mechanism and Application of Hydraulic Fracturing in the High-Level Thick and Hard Gangue Layer to Improve Top Coal Caving in Fully Mechanized Caving Mining of an Ultra-Thick Coal Seam

Abstract

The thick and hard gangue layer has long been one of the key obstacles affecting the coal recovery rate in the fully mechanized top-coal caving mining of ultra-thick coal seams, and it is also one key factor restricting the development of the technology used for such work. In this study, to improve the poor top coal cavability and low recovery rate in fully mechanized caving mining of ultra-thick coal seams containing thick and hard gangue layers, the fully mechanized caving mining of longwall working face 42,108 of the Qinggangping Coal Mine is the engineering setting. Then, through a combination of theoretical analysis, numerical simulation, and field practice, a mechanical model of the cantilever beam with uniform load for fracture of the gangue layer is developed. Next, the mechanical action mechanism and influence of the gangue layer and thickness on the fracture of the cantilever beam are analyzed, and a method of pre-fracturing and weakening high-level thick and hard gangue layers using hydraulic fracturing technology is proposed. Finally, using RFPA2D-flow numerical simulation, the key technical parameters of hydraulic fracturing in the working face are designed and applied to field practice. The results show the following: After the high-level thick and hard gangue layer is treated by hydraulic fracturing technology, the amount of fractured gangue behind the support increases, while that of big coal blocks decreases significantly, and the overall fragmentation of top coal is at a reasonable level. In addition, after taking hydraulic fracturing technical measures during the mining period of the working face, the average recovery rate of the working face is 86.6%. This is an increase of 6.5% over the previous area without hydraulic fracturing.

1. Introduction

The complex coal-forming conditions of underground coal seams often lead to gangue layers between coal seams, which are generally harder than coal seams, and their thickness varies widely [1]. For ultra-thick coal seams (≥8 m) containing gangues, fully mechanized caving mining is an important method to extract all the coal seams at one time, but the presence of thick and hard gangue layers abates the top coal cavability and affects the recovery rate [2,3]. The occurrence conditions and mechanical characteristics of gangue layers are the main factors affecting the top coal cavability in fully mechanized caving mining. However, slice mining is unable to utilize the entire thickness of each thick coal seam at once, due to a series of problems, including increased tunneling and high labor intensity, low production, complex mining process, the tension between roadway tunneling and working face mining, frequent natural fires in the gob, and low extraction rate [4,5,6,7,8].

Due to the gangue layer in the coal seam, especially the medium- and high-thickness and hard gangue layers, a “skeleton” is easily formed in the top coal. This makes the caving block of the gangue layer larger, and the blocks are hinged to each other, in turn forming an isolation zone for top coal. Even the high-level thick and hard gangue layer is not broken as a whole; this seriously affects the cavability of top coal from the upper layer, resulting in a significant quantity of coal staying in the goaf and causing a serious waste of resources. Therefore, how to solve this problem is very important for the fully mechanized caving mining of ultra-thick coal seams containing gangue to achieve high production and high efficiency. Now, a variety of methods have been studied both in China and internationally to deal with the fully mechanized caving mining of ultra-thick coal seams with hard top coal. Intensive drilling technology, deep hole pre-split blasting technology, and hydraulic fracturing technology are among the more effective means [9,10,11,12,13,14]. However, intensive drilling technology involves problems such as a large amount of drilling construction and small effective influence range, and the deep hole pre-split blasting technology is only suitable for low-gas mines. Hydraulic fracturing technology is safe, environmentally friendly, and has a good pre-fracturing effect. As an economical and efficient coal and rock mass fracturing technology, it has been widely used in coal mine production. It is one of the effective technologies to deal with hard roofs and coal and rock mass strength weakening [15,16,17]. Many scientific research practices have shown that hydraulic fracturing technology plays a crucial function in the treatment of hard roofs in coal mines, weakening the strength of coal and rock masses to prevent rock bursts, improving coal seam permeability, and preventing coal and gas outbursts [18,19,20,21,22]. Yu Bin et al. [23] conducted a field test of hydraulic fracturing to weaken the overlying hard roof of the coal seam given the severe mine pressure in the Tashan Coal Mine. Sun Shoushan et al. [24] introduced the directional hydraulic fracturing technology of the hard roof of a Polish coal mine. Yan Shaohong et al. [25] explored the mechanism of hydraulic fracturing to treat hard roofs. Huang Bingxiang et al. [26] constructed the framework of the hydraulic fracturing theory of coal and rock mass and its technological process. Wang Yaofeng et al. [18] proposed a new technology of directional hydraulic pressure penetration and permeability enhancement of pre-fabricated guide grooves given the problem of drilling in low-permeability coal seams.

2. Engineering Background

The Qinggangping Coal Mine is approximately 30 km northeast of Xunyi County in Shaanxi Province. The mine primarily mines the #4 coal seam and is currently mining working face 42,108 in the first mining area’s east wing. The buried depth of the working face is 400–480 m, the strike length is 1360 m, and the incline length is 150 m. The #4 coal seam has a complex structure, and the thickness of the gangue and coal seam varies greatly. Bifurcation occurs in the east of the mining area (Figure 1). The layout of the working surface is shown in Figure 2. Rock layer column diagram in Figure 3. The thickness of the upper layer 4-1 is 0.8–2.0 m, and the average thickness is 1.5 m. The thickness of the coal gradually increases along the strike, with an average thickness of 12.0 m. The thickness of the high-level gangue layer between the two coal seams is 0.2–2.0 m, and the height from the immediate floor of the 4-2 coal seam is 12.0 m. The gangue layer is mainly mudstone, and its thickness gradually decreases along the strike. The 970–1360 m range of the working face is a thick gangue layer, with an average thickness of 1.5 m; the 870–970 m range of the working face is a medium-thick gangue layer, with an average thickness of 0.8 m; and the 600–870 m range of the working face is a thin gangue layer, with an average thickness of 0.3 m. The compressive strength of the #4 coal seam is 7.66–16.38 MPa, the tensile strength is 1.63–2.13 MPa, the compressive strength of the high-level gangue layer is 13.58–19.48 MPa, and the tensile strength is 2.64–4.08 MPa. Its strength is higher than that of the #4 coal seam, and this is the main reason for the top coal cavability.

In working face 42,108, fully mechanized caving combined mining is adopted. The coal cutting thickness is 3 m, the coal caving thickness is 10.5 m, and the coal caving step distance is 0.8 m. In the combined mining of fully mechanized caving, due to the presence of the high-level thick and hard gangue layer between the 4-1 and 4-2 coal, the 4-1 coal in the upper layer has a poor crushing effect and low caving rate. The gangue layer forms a “skeleton” in the top coal, and the gangue layer’s caving blocks are relatively large. Furthermore, the blocks are hinged together to form an isolation belt for the top coal, making it hard for the top coal to collapse. Even if the top coal collapses, the gangue will form large blocks that will affect the fluidity of the top coal during the caving process, leading to a huge overspend of top coal resources and a low coal recovery rate at the working face (Figure 4). Judging from the gangue layer thickness and layer situation, the 4-2 and 4-1 coal gangue layers in the working face 870–1360 m range are medium- and high-thickness gangue layers, which is negative for the top coal caving. Therefore, we must use auxiliary methods to the gangue for pre-fracture loosening and weakening.

3. Analysis of the Crushing Mechanism of Top Coal Containing Gangue

3.1. Mechanical Analysis of the Top Coal Crushing Mechanism

The abutment pressure fracturing caused by the accumulation of small cracks in the initial coal mass during the mining process is constantly densified, enlarged, and damaged, resulting in caving and the release of top coal in fully mechanized caving faces. The coal mass damage principle is as follows: If the internal fissures of the top coal are highly developed and the degree of damage is quite high, then the more uniform the block size of the top coal broken and the better the top coal cavability, from the initial micro-fracture damage to the local macro-fracture damage, then to the overall macro-fracture propagation through the damage. On the contrary, if the internal fracture extension of the top coal under the action of the abutment pressure is insufficient, the damage extent is small, and the size of the broken top coal is not uniform, and the top coal release will be poor, causing a small top coal recovery rate.

During the progress of the fully mechanized caving face, the stress condition and damage change features of the top coal at the front and back of the working face can be classified into four regions (Figure 5).

(1) Area I is the widest range, from the primary rock stress area of the coal mass to the location of the peak front abutment. The coal mass is subjected to more peak front abutment the closer it is to the working face and thus increases the degree of fine view damage that develops in the coal mass. The fine view damage is currently limited to the development of fissures, and the overall integrity of the coal mass is good.

(2) Area II is the area from the peak front abutment to the coal rib zone, and the top coal in this area is in the limit equilibrium stage. At this time, the closer to the coal rib, the smaller the stress value is. In addition, the fine view fracture developed in the coal mass has penetrated and developed into macroscopic cracks, which shows that the coal mass crushing state has been intensified, plastic deformation has been generated, and the bearing capacity has been gradually lost. Taking a small unit as the research object, it is approximated as an elastic medium, and according to its spatial stress state, we obtain the following:

$${\epsilon }_3=\frac{1}{E}\left[{\sigma }_3-\mu ({\sigma }_1+{\sigma }_2)\right]$$

(1)

Let ${\sigma }_3={\sigma }_2$, then:

$${\epsilon }_3=\frac{1}{E}\left[(1-\mu ){\sigma }_3-\mu {\sigma }_1\right]$$

(2)

Due to

$${\epsilon }_3=-\mu \frac{{\sigma }_c}{E}$$

(3)

Substituting Equation (3) into Equation (2) yields the following:

$${\sigma }_1={\sigma }_c+\frac{1-\mu }{\mu }{\sigma }_3$$

(4)

where ${\sigma }_1$ and ${\sigma }_3$ are the maximum and minimum principal stress on the coal mass, respectively; ${\sigma }_c$ is the uniaxial compressive strength of the coal mass; and $\mu$ is the Poisson’s ratio of the coal mass. As shown in Equation (4), the compressive strength and lateral restraint force of the coal mass increase the tension required to damage it.

(3) Area III includes everything from the coal rib to the roof crack located behind the hydraulic support. To maintain balance, this portion of the top coal produces a structure resembling an articulated rock beam in the top coal caving area. As with the procedure of top coal caving, its sinking speed is much higher than that of the roof. Additionally, separation is produced by the junction of coal and rock, so that under the disturbance of repeated support of hydraulic-powered support, vertical fracture of top coal occurs more often, which is favorable to the coal caving. If there is a gangue layer in the top coal, it will impede vertical fracture formation, which is detrimental to top coal caving.

The top coal goes through a transition from a three-way stress state to a uniaxial stress state as the working face moves forward. The top coal in Area III can be approximated as being in the uniaxial stress state. The Mohr–Coulomb criterion states that we obtain the following:

$$f=\left\{\begin{array}{l}{\sigma }_1-K(\xi {\sigma }_3+{\sigma }_c)\\ {\sigma }_3-K\left(\frac{1}{\xi }{\sigma }_1-{\sigma }_c\right)\end{array}\right.$$

(5)

where $K$ is the crushing coefficient of coal mass; ${\sigma }_c$ is the uniaxial compressive strength of the coal; and $\xi =(1-\sin \phi )/(1+\sin \phi )$ is a lateral coefficient. If $f=0$, then the expression of the crushing coefficient of the top coal is:

$$K=\frac{{\sigma }_1}{\xi {\sigma }_3+{\sigma }_c}$$

(6)

The crushing coefficient of the top coal is proportional to the maximum principal stress it is subjected to and inversely proportional to the minimum principal stress it is subjected to. This clearly explains the rapid fracture and crushing of the top coal in Area III after it is in a unidirectional stress state.

(4) Area IV is the caving area of the mining area behind the hydraulic-powered support, and there will be part of the top coal that has not been released during the sinking of the roof. This is tightly linked to the coal mass in front of it, limiting the amount of top coal crushing above the hydraulically powered support. For the large coal mass above, the formation of a balanced arch structure similar to a pressure arch should be avoided to increase the cavability of top coal. In addition, if necessary, corresponding measures should be applied to increase the crushing degree of the upper coal mass. Under normal conditions, the process of the four areas is continuous, and the top coal can be fully released. When the top coal has a thick hard gangue layer, it cannot fully develop vertical fissures and cannot be broken. In the meantime, even if the top coal vertical breakage is produced, its fractured block is still larger, which impedes top coal movement and does not promote coal caving.

3.2. Mechanical Analysis of the Crushing Mechanism of Top Coal Containing Gangue

The gangue layer is common in ultra-thick coal seams. The strength, thickness, and distribution position of the gangue layer differ in fully mechanized caving mining, as does its impact on top coal caving. In general, the gangue layer has a lower strength than the coal, which corresponds to the coal seam’s weak surface. At this time, the presence of gangue facilitates the crushing, caving, and release of the top coal, resulting in improved cavability. However, this has a negative impact on the cavability of top coal [27]. The coal mining machine can immediately cut through the low-level gangue layer, which has no impact on working face recovery, and while the gangue’s impact on crushing is minimal, it has a significant impact on the cavability of the top coal. Furthermore, the thick layer of gangue in the top coal acts as a cantilever to support the upper top coal. Consequently, the upper top coal cannot be caved in time, and its fall in the gob cannot be recovered.

The coal seam in the high-level thick hard gangue layer in the three-way stress state is surrounded by the coal seam of lower strength during the mining influence process. In addition, through the upper and lower relatively soft top coal acting on the gangue layer, the abutment pressure’s stress concentration degree is relatively weak, and the presence of the gangue layer hinders the lower top coal in the fissure upward propagation; thus, the upper top coal crushing effect is poor. When the gangue layer is located above the roof control area of the hydraulic-powered support, the overhanging top at the back of the support is easily formed, exhibiting the force state as shown in Figure 6a. That is to say, the lower top coal of the gangue is broken and sinking, while the upper top coal has a small amount of sinking with the deformation of the gangue, and the direct top produces separation. At this time, the gangue layer is equivalent to the cantilever beam, and the beam is subject to the upper top coal $q_m$ and its own gravity $q_p$, for the uniform load $q$; then its value is $q=q_m+q_p$. Figure 6b depicts the simplified mechanical model.

The average thickness of the gangue layer is 1.5 m, according to geological data. Based on the theory of mine pressure and material mechanics, after the initial fracture of the gangue layer, it is equivalent to a cantilever beam, which is subject to the action of the upper top coal and its own gravity. The fixed end of the suspended gangue beam has the greatest bending moment, which can be calculated as follows:

$$M_{\max }=\frac{1}{2}q{l}^2$$

(7)

$${\sigma }_{\max }=\frac{M_{\max }}{J_z}\cdot \frac{h_p}{2}$$

(8)

$${\sigma }_{\max }=\frac{3({\gamma }_m{h}_m+{\gamma }_p{h}_p)l^2}{h_p^{{}^2}}$$

(9)

where l is the gangue layer breaking interval; J_z is the section distance of the beam; ${\gamma }_m$ and ${\gamma }_g$ are the capacity of the top coal and the gangue layer, the respective values of which are 0.014 MN/m² and 0.025 MN/m²; $h_m$ and $h_p$ are the thicknesses of the top coal of the gangue and the gangue itself; R_t is the tensile strength of the gangue; finally, σ_max is the maximum tensile stress that the gangue is subjected to. Let σ_max = R_t, then we have:

$$l=\sqrt{\frac{R_t{h}_p^2}{3({\gamma }_m{h}_m+{\gamma }_p{h}_p)}}$$

(10)

From Equation (10), we know that the breaking interval of the gangue layer is proportional to its strength. In other words, the greater the strength of the gangue layer, the greater its breaking distance, and the greater its influence on the top coal cavability. At the same time, the thickness and position of the gangue layer (the thickness of the upper stratum’s top coal) are the two main factors affecting the gangue layer breaking interval. The fracturing step of the gangue layer determines the quality of the top coal cavability. In the practical production process, the gangue assignment conditions are complex and changeable, while the thickness and position of the gangue layer are the two factors of the law of change, which are also unknown and difficult to grasp in real time. As a result, it is essential to study the fracturing step of the gangue layer under the coupling effect of gangue thickness and level. The three-dimensional distribution relationship between the breaking interval and thickness of the gangue layer and the thickness of the top coal of the upper stratum is established using the Qinggangping Coal Mine 4-1 coal seam and high gangue thickness distribution range and combined with Equation (5), as shown in Figure 7. The uniaxial tensile strength of the gangue layer is taken to be a maximum of 4.08 MPa.

The average thickness of the top coal seam 4-1 in the upper part of the gangue layer is 1.5 m, as shown in Figure 7. The thickness of the high-level gangue layer is 0.2~2.0 m, and the breaking interval of the gangue layer under the uniform load is 1.4~8.7 m The top coal thickness of the gangue layer is 0 < $l$ < 1.5 m (high-level gangue layer), and the gangue $l$ breaking interval with the thickness of the gangue $h_p$ changes more sharply. When 1.5 m < $h_m$ < 10 m, the three-dimensional distribution relationship surface is relatively flat, that is, the gangue-breaking interval with the gangue thickness changes more smoothly. The effect of the change in the thickness of the gangue is not significant. It can be concluded that the high-level gangue layer is more sensitive to the change of the thickness of the gangue layer. Similarly, when the thickness of the interlayer 0 < $h_p$ < 1 m, $l$ varies significantly only in the $h_m$ < 1.5 m range. When the gangue thickness 1 < $h_p$ < 3 m, in the range of 0 < $h_m$ < 10 m, there are more moderate and uniform changes. This demonstrates that while the gangue thickness is thin, the high-level gangue layer has a more obvious influence on $l$. When the gangue layer thickness is thicker, the effect of layer level change on $l$ becomes less obvious, but $l$ still gradually becomes larger with the increase of gangue layer level.

Based on the above analysis, and combined with the occurrence conditions of the gangue layer in working face 42,108, the gangue layer between the 4-2 and 4-1 coal seams has a thickness of 0.8–1.5 m in the range of 870–1360 m of the working face, and the gangue layer has a high level of thickness, which bears a great negative impact on the top coal cavability.

4. Hydraulic Fracturing Technology and Process Flow of High-Level Thick and Hard Gangue Layer

According to the previous analysis of the crushing mechanism of top coal in fully mechanized caving mining of ultra-thick coal seams containing high-level, thick, and hard gangue layers, when there is a gangue layer in the top coal, the overall crushing effect of the top coal becomes poor. It leads to a huge overspend of greater coal resources, as well as a low coal recovery rate at the working face. Meanwhile, the gangue layer holds the overall stability and bearing capacity of the top coal when it reaches the working face, and it can transmit a large roof pressure, which leads to the appearance of severe rock pressure on the working face, which seriously affects the threats to the working face safety. Therefore, it is critical to select an appropriate strength-weakening plan for the top coal high-level gangue layer in the fully mechanized caving face in order to improve top coal caving and recovery rate while also relieving roof pressure. Since the #4 coal seam is a high-gas coal seam, the traditional method of deep hole pre-split blasting of roof coal and rock mass is limited. As a result, hydraulic fracturing technology is chosen to achieve the goal of pre-fracturing the high-level, thick, and hard gangue layer in the working face’s top coal, thereby reducing the overall strength and integrity of the coal and rock mass, weakening the gangue strength, and improving top coal cavability.

4.1. Principle of Hydraulic Fracturing Pre-Fracturing of High-Level Thick and Hard Gangue Layer

The purpose of hydraulic fracturing technology is to change the stress state of the coal and rock mass inside the borehole while using high water pressure. This aids in controlling fracture opening and expansion around the borehole, increasing fracture density in the coal and rock mass surrounding the borehole, and improving fracture network connectivity, thereby achieving the goal of reducing and weakening the overall mechanical strength of coal and rock mass. Simultaneously, the permeability of the coal and rock mass can be altered so that some of the fracturing water enters the coal and rock mass, allowing the coal and rock mass to fully absorb water to wet and soften it. During the working face mining period, the mine pressure and repeated disturbance of the top coal by the hydraulic support are fully utilized to reduce top coal lumpiness, improve top coal cavability, and meet reasonable coal caving requirements.

The hydraulic fracturing of fully mechanized caving face 42,108 primarily weakens the strength of the coal seam’s middle and high gangue layers but has no significant effect on the movement and caving law of the stratum coal and rock mass. The combined effect of mine pressure and repeated support of the hydraulic-powered support remains unchanged during the process of full crushing of the top coal.

4.2. Implementation Process of Directional Hydraulic Fracturing Technology in High-Level Thick and Hard Gangue Layers

The implementation of hydraulic fracturing technology for high-level thick and hard gangue layers generally includes the three steps of drilling, sealing, and fracturing. The main components of a hydraulic fracturing power system include a water tank, high-pressure pump station, control valve, high-pressure pipeline, and hole sealer. The auxiliary equipment is a pressure gauge and a flow meter, which can observe the pressure change and amount of water injected in real time during the water injection process. The on-site hydraulic fracturing equipment is shown in Figure 8.

The specific implementation process and process of pre-splitting high-level thick and hard gangue layer includes six steps (Figure 9): (1) Determine the hydraulic fracturing parameters. (2) Construct the hydraulic fracturing drilling between supports in the working face. (3) Install the hydraulic fracturing system (push the special hole sealer into the fracturing hole, connect the high-pressure line, perform hydraulic fracturing technique operation inspection, and seal the hole with a special hole sealer). (4) Open the water injection valve for the hydraulic fracturing field test. (5) Hydraulic fracturing ends. (6) Finally, a borescope is used to observe the weakening effect of the intercalated gangue layer and make the necessary optimization adjustments to the fracturing parameters. Among them, the determination and optimization of hydraulic fracturing parameters are to be based on theoretical analysis and numerical simulation results, combined with the actual production requirements for the pre-fracture degree of high-level thick and hard gangue layers. Then the crack propagation distance of hydraulic fracturing in top coal containing thick and hard gangue layers is analyzed. Finally, parameters such as weakening radius, fracturing hole arrangement spacing, and water injection pressure are determined.

5. Design of Key Technical Parameters of Hydraulic Fracturing High-Level Gangue Layer

5.1. Analysis of Hydraulic Fracturing Fracture Initiation Pressure

The study results show that when water is injected into the fracturing hole, the original tangential stress in the borehole’s inner wall is transformed into tensile stress, and the crack tip begins to generate tension cracks. The mechanical mechanism is as follows: The tensile stress on the tip of the hydraulic crack exceeds the tensile strength $R_t$ of the coal and rock mass when the water pressure in the hole and the in situ stress field environment cooperate. That is, when $\sigma \theta \ge R_t$, cracks begin to crack and expand. In most cases, the crack of the fracturing hole is determined by applying the following formula:

$$P{\prime }_w\ge 3{\sigma }_{\min }-{\sigma }_{\max }+R_t$$

(11)

Water infiltration will occur under the action of high-pressure water since coal and rock are porous medium materials. The pressure $P{\prime }_w$ in Equation (11) is the water pressure in the borehole that causes the hole wall to rupture, and its value is less than the pressure $P_w$ of the water in the borehole.

Due to the presence of the skeleton structure of coal and rock mass and the internal pressure of pore water, the water pressure in the borehole causes the stress environment field, which can be represented by the Terzaghi equation:

$$\sigma =\sigma \prime +P_w$$

(12)

where $\sigma$ is the stress caused by the water pressure in the borehole; $\sigma \prime$ is the effective stress borne by the coal skeleton; and $P_w$ is the pressure of the water infiltrating the coal.

The atmospheric pressure is equivalent to the initial water pressure infiltrated into the coal. The effective penetration thickness d is defined as follows to analyze the change in water pressure in coal and rock: the distance that the seepage water penetrates in the coal mass during the time $\Delta t$ when the hole wall is in contact with the water until the borehole is filled with water. It is known from this definition that:

$$d={\displaystyle {\int }_0^{\Delta t}vdt}$$

(13)

where $v=KJ$, $K$ is the permeability coefficient of the pore wall coal, and $J$ is the hydraulic gradient.

Due to the large flow rate of the water pump selected during hydraulic fracturing construction, the time $\Delta t$ from when the hole wall is in contact with water to when it is filled with water is very short. As a result, the seepage effect of the borehole wall is ignored in the calculation of the hydraulic fracturing crack initiation pressure. It is approximately considered that the pressure $P_w\approx P{\prime }_w$ of the water in the borehole can also meet the actual engineering requirements for calculation accuracy.

Therefore, the critical borehole initiation pressure of hydraulic fracturing crack initiation in hard coal and rock mass can be approximated according to Equation (14):

$$P_w\approx 3{\sigma }_{\min }-{\sigma }_{\max }+R_t$$

(14)

The burial depth of the coal seam in working face 42,108 is between 400 and 480 m, according to the current mining situation at the Qinggangping Coal Mine. The average bulk density of the overburden is 25 kN/m³. Through laboratory measurement, the uniaxial tensile strength of the gangue layer is shown to be about 3.65 MPa. According to the mine’s previous in situ stress test results, its maximum principal stress value is about 9.27 MPa, and minimum principal stress value is around 5.41 MPa. According to Equation (14), the theoretical pressure value of hydraulic fracturing fracture initiation in a high-level gangue layer is calculated to be about 10.61 MPa.

5.2. Analysis of the Law of Hydraulic Fracturing Fracture Propagation and Its Influencing Factors

For top coal with a thick, hard gangue layer, hydraulic fracturing is mainly used to pre-fracture it. Therefore, the pore size and spacing of the fracturing holes determine the propagation and development law of hydraulic fracturing cracks in the gangue layer. Reasonable fracturing parameters must cause the hydraulic fracturing cracks to propagate in the gangue layer as much as possible. The gangue can be fully broken without forming large pieces, but not excessively broken, which leads to difficulties in the stability control of the working face roof. Therefore, to obtain the key technical parameters of hydraulic fracturing in the high-level gangue layer, the size of the fracture hole aperture, hole spacing, and the fracture initiation and expansion law are studied using the Rock Failure Process Analysis (RFPA) 2D-Flow.

5.2.1. Model Building and Parameter Selection

The simulation adopts a two-dimensional rectangular planar structure model. Considering the small fracturing hole aperture, rectangular models of 8 × 8 m (25,600 units) and 60 × 30 m (18,000 units) are established to simulate the fracture initiation and propagation law under different fracturing hole aperture and hole spacing. The fracturing hole apertures are 45 mm, 60 mm, 75 mm, and 90 mm, and the spacings are 10 m, 15 m, and 20 m, respectively (Figure 10 and Figure 11). Holes are excavated in rectangles to serve as fracturing holes. Using the plane strain method, and according to the specific conditions of the high-level gangue layer in working face 42,108, the parameters of the numerical simulation scheme are obtained according to the laboratory and field test results (

Table 1. Mechanical parameters of gangue layer in working face 42,108.

Homogeneity	Compressive Strength/MPa	Tensile Strength/MPa	Poisson’s Ratio	Elastic Modulus/GPa	Friction Angle	In-Situ Stress/MPa
3	12.58	3.65	0.25	1.67	25°	9.27/5.41

). The internal seepage field is set to be stable. In the test, the technique of gradually raising the borehole’s water pressure is employed to replicate the hydraulic fracturing process. The pressure begins from 1 MPa, each step is increased by 0.3 MPa, and the cumulative loading is 100 steps.

5.2.2. Effect of Fracturing Hole Aperture on Hydraulic Fracturing Fracture Propagation

Figure 12 depicts the hydraulic fracturing propagation law at various fracturing hole apertures. The fracture morphology in the coal rock body under different sizes of fractured apertures changes as the water injection pressure changes at different stages of hydraulic fracturing. However, it could also be divided into four phases: hydraulic pressure accumulation, micro-fracture development, micro-fracture extension penetration, and fracture destabilization extension. (1) Hydraulic pressure accumulation stage. At this stage, the high-pressure water mainly enters the original pores and fissures of the coal-rock mass by seepage. (2) Micro-fracture development stage. While the injection pressure of water keeps going up, with the continuous increase of water injection pressure, many sporadically distributed micro-fractures are generated in the annular pressure increase zone at the fracturing hole edge. At this stage, the water injection pressure is the fracture initiation pressure for hydraulic fracturing crack propagation. Since the pressure around the fracturing hole wall is basically isotropic, and the material properties of the coal–rock medium follow the Weibull distribution, the resulting micro-cracks are random. (3) Micro-fracture extension penetration stage. With the gradual development of micro-cracks, some cracks gradually expand and penetrate, and stress concentration occurs at the crack tip, which bears a certain inhibitory effect on the expansion and extension of cracks. When one of the fractures begins to expand deeply, the area of pressure acting in the direction of the fracture increases. This allows the fracture to expand in a symmetrical direction, and the adjacent fractures on the left and right gradually disappear, contributing to the formation of the later symmetrical fracture expansion pattern. At this stage, the expansion of the fracture still requires the continuous driving of water pressure. Meanwhile, the high-pressure water in the fracturing hole is continuously collected into the fracture, and the pore water pressure is transmitted along the fracture. (4) Fracture destabilization extension stage. As the crack continues to expand, a bifurcation phenomenon occurs at its tip, that is, multiple irregular cracks suddenly sprout at the tip of the main crack. This indicates that the crack is about to enter an unstable expansion stage. The pressure at this time is the critical transition pressure (fracture pressure) from stable to unstable failure. There is no need to increase the pressure, and the cracks still expand gradually. Also at this time, the coal and rock mass begin to become unstable and damaged, and the critical pressure in the hole can be used as the criterion for the failure mechanism of the coal and rock mass. Comparing the propagation law of hydraulic fracturing fractures under different fracturing hole apertures, the fracture initiation pressure of hydraulic fracturing fractures does not change much, in the range of about 6.7–7.3 MPa, which is close to the theoretical calculation results. The coverage of the hydraulically fractured coal mass increases as the fracturing hole aperture increases, and the time for the fracture to penetrate the fracturing hole wall and expand outward to penetrate shortens. Meanwhile, the larger the fracturing hole aperture is, the smaller the water pressure when the fracture penetrates. When the hole aperture is 45 mm, the propagation penetration pressure is 25 MPa; when it is 60 mm, the propagation penetration pressure is 22.9 MPa; when it is 75 mm, the propagation penetration pressure is 21.4 MPa; and when it is 90 mm, the propagation penetration pressure is 16.3 MPa. Therefore, the overall fracturing effect is better if a fracturing hole with a larger aperture is selected. However, given the complexity of the field situation, in the field practice of hydraulic fracturing, it is crucial to evaluate not just the effect of fracturing but the economy of construction as well. If the fracturing aperture is too large, it will lead to high costs and large engineering volume. Otherwise, the fracturing effect may be insignificant or even ineffective. Ultimately, the desirable hydraulic fracturing hole aperture is 75 mm.

5.2.3. Analysis of the Effect of Fracturing Hole Spacing on Hydraulic Fracturing Fracture Propagation

The propagation law of hydraulic fracturing cracks is depicted in Figure 13 for various hole spacings. The propagation law of hydraulic fracturing fractures is similar to that of single-hole hydraulic fracturing when the coal–rock mass is initially fractured. It starts out by extending along a curved line, forming a diverging propagation path. However, the gradual propagation of the fractures under the movement of high-pressure water, influenced by the coupling effect between each fracturing hole, causes the tensile stress between the holes to become superimposed (Figure 13a). Fractures are rapidly expanding and developing in this area. In addition, the fractures essentially extend along the center line of the fracturing hole. The fractures are easily influenced by the fractures that are formed, and they expand continually in the same direction. Thus, the impacts of porous linear co-directional hydraulic fracturing on directional expansion and infiltration are accomplished. The coupling effect of the fracture becomes the primary component determining the propagation law of hydraulic fracturing when the distance between the fracture points of each fracturing hole constantly decreases. Subsequently, the fractures begin to penetrate each other, eventually forming a macroscopic main fracture. The closer the spacing between the fracturing holes, the stronger the coupling influence between the holes, and the easier it is for fractures to propagate along the center line of the fracturing hole. When the hole spacing is 10 m, the propagation penetration pressure is 22.9 MPa; when it is 15 m, the propagation penetration pressure is 23.1 MPa; and when it is 20 m, the propagation penetration pressure is 29.2 MPa. The coverage of hydraulic fracturing of the coal–rock mass likewise declines at the same time. Therefore, considering the fracture initiation and propagation penetration radius, coverage of high-pressure water fracturing of the coal and rock mass, and propagation penetration pressure and fracturing time, combined with on-site engineering practice, if the fracturing hole spacing is too small, this will inevitably increase the number of fracturing holes, in turn increasing the workload of hydraulic fracturing technology implementation. If the spacing is too large, there will be a fracturing blank area, which will affect the overall fracturing effect. After comprehensive numerical simulation and with the rationality of on-site construction, the designed on-site fracturing hole spacing is determined to be about 15 m.

5.3. Determination of the Key Technical Parameters of Hydraulic Fracturing

According to the results of the analysis of the initiation, propagation, and penetration of cracks in coal and rock mass under the action of hydraulic fracturing by using RFPA2D-Flow, and combined with the true issue of the site, the key technical parameters of hydraulic fracturing in the high-level gangue layer of working face 42,108 are determined, as detailed in

Table 2. Determination of parameters related to hydraulic fracturing.

Crack Initiation Pressure of Gangue Layer/MPa	Fracturing Hole Aperture/mm	Fracturing Hole Spacing/m	Gangue Rupture Pressure/MPa
6.7~7.3	75	15	25.0

.

6. Hydraulic Fracturing Effect

6.1. Design of Working Face Hydraulic Fracturing Scheme

According to the numerical simulation study on the strength weakening mechanism of hydraulic fracturing in the high-level gangue layer, the arrangement of the fracturing holes in working face 42,108 is determined. Given the complexity of the on-site situation and from the perspective of safety, the spacing of the fracturing holes can also be flexibly adjusted, so as to ensure that there is no pressure relief blind area between the fracturing holes.

Considering that there are seven groups of hydraulic-powered supports at the upper and lower ends of working face 42,108 on site, top coal caving is not performed. These include three sets of headstocks and four sets of tailstocks, and the hydraulic-powered support model is ZFG8000/21/34. Between hydraulic-powered supports #4 to #97 on the working face, an MQT-120/2.7 pneumatic bolting rig is used to construct a row of fracturing holes on the vertical roof 0.5 m away from the coal rib (Figure 14). The coal seam and gangue layer of working face 42,108 distribution dictate the specific construction depth, which must be greater than the high-level gangue layer. The effective sealing length of the hole sealer must not be less than 1 m, and the fracturing hole spacing must be 15 m. After the working face has advanced by 5 m, an additional row of fracture holes is placed along the workface with the same construction parameters.

6.2. Analysis of Hydraulic Fracturing Effect of High-Level Thick and Hard Gangue Layer

The top coal caving in the working face is seen and noted in this study to assess the pre-fracturing influence of hydraulic fracturing technology on high-level thick and hard gangue layers. Next, the development of fractures in the high-level gangue layer is observed and analyzed using the borehole peeping instrument. Finally, the recovery rate of the working face during the fracturing period is calculated and compared in the unfractured area so as to further evaluate the overall weakening effect of the high-level thick and hard gangue layer by hydraulic fracturing.

6.2.1. Observation of Pre-Fracturing Effect of Gangue Layer

The propagation of cracks in the gangue layer in between hydraulic fracturing is contrasted and examined using the ZKXG30 borescope, which is utilized to monitor some fracturing holes (Figure 15). As detailed in Figure 16, the observation results show that before fracturing the high-level gangue layer, the mudstone is uniform and dense. After fracturing, the fractures in the gangue layer of the original fracture holes have been significantly expanded, and many new fractures have been created. This demonstrates that hydraulic fracturing can produce fractures within a certain range, but the direction of the fractures is unknown due to the intricate nature of the geological structure in coal rock. At the same time, obvious deformation, rupture, and collapse occur in some areas of the fracturing hole. This demonstrates how hydraulic fracturing can efficiently degrade the gangue layer’s integrity while increasing the cavability of the top coal.

6.2.2. Statistical Analysis of Working Face Recovery Rate

To more intuitively analyze the pre-fracture effect of the gangue layer after hydraulic fracturing, the quantity of gangue and the block degree of top coal behind the hydraulic-powered support are observed and analyzed. Figure 17 shows that the amount of gangue behind the hydraulic-powered support increases, the number of large coal masses is considerably reduced, and the top coal cavability is effectively improved. At the same time, during the mining period of the working face, the recovery rate increases significantly, the overall fragmentation of the top coal is at a reasonable level, and there is no large coal mass falling.

The recovery rate of the working face refers to the ratio of the actual coal production to the resource reserves of the working face. This is an important technical and economic indicator of fully mechanized caving mining and one of the important criteria for evaluating the success of fully mechanized caving mining. In this test, the recovery rate of working face 42,108 during the joint mining of the 4-1 and 4-2 coal seams is statistically analyzed. According to GB/T31089-2014 “Calculation Method and Requirements of Coal Mine Recovery Rate”, namely the calculation method of working face recovery rate, during the field test, the recovery rate of the working face is counted in sections along the strike according to the monthly progress. The calculation method is as follows:

$$R_{gi}=\frac{W_{gi}}{S_{gi}}\times 100\%$$

(15)

where $R_{gi}$ is the recovery rate of the working face in the i-th section, %; $W_{gi}$ is the coal production volume of the working face in the i-th section, t; and $S_{gi}$ is the resource reserves of the working face in the i-th section, t.

The formula for calculating the reserves produced in each section of the working face is as follows:

$$S_{gi}=a\times b_i\times m_i\times d$$

(16)

where a is the measured length of the working face along the inclination, 150 m; b_i is the advancing length of the working face in the i-th section along the strike, m; d is the apparent density of coal, t/m³; and m_i is the average coal thickness of the working face coal, as shown in

Table 3. Working face 42,108 recovery rate statistics.

Time	Segmentation	Working Face Advance Distance/m	The Average Thickness of 4-2 Coal/m	The Average Thickness of 4-2 Coal/m	The Thickness of Gangue/m	No.4 Coal with Gangue Rate/%	Utilized Resource Reserves/t	Statistical Yield/t	Amount of Coal Mined/t	The Recovery Rate of Working face/%
2020.12	1	39.5	1.7	8.8	2.3	13.70	89,165.3	77,298.6	70,173.1	78.7
2021.01	2	57.5	1.3	9.4	1.6	14.65	132,191.1	113,538.3	105,488.5	79.8
2021.02	3	62.9	1.5	8.8	1.5	12.80	139,251.2	109,797.8	108,058.9	77.6
2021.03	4	57.1	1.1	9.8	1.7	15.25	133,691.1	115,692.9	109,359.3	81.8
2021.04	5	55.6	1.3	9.8	1.4	13.27	132,194.6	115,974.5	109,192.7	82.6
2021.05	6	55.3	1.3	9.4	1.1	9.21	127,137.5	127,795.6	109,973.9	86.5
2021.06	7	49.1	1.6	10.4	1.1	10.75	126,619.1	118,245.3	110,158.6	87.0
2021.07	8	42.5	1.1	12.7	0.8	9.59	125,944.5	108,958.3	108,564.2	86.2
2021.08	9	48.2	1.4	11.1	0.7	8.12	128,397.6	105,521.6	109,651.5	85.4
2021.09	10	45.6	1.9	11.1	0.3	10.51	127,415.5	126,253.7	109,195.1	85.7
2021.10	11	47.3	1.6	10.9	0.3	10.10	127,050.2	118,436.7	109,263.1	86.0
2021.11	12	45.1	1.6	11.3	0.2	8.80	125,010.4	117,628.3	107,884.0	86.3

Note: The thicknesses of the 4-1 and 4-2 coal seams and the gangue layer in Table 3 are obtained from the average values of the coal exploration drilling data carried out on-site in working face 42,108.

below.

The hydraulic fracturing test for the high-level thick and hard gangue layer of working face 42,108 began in May 2021. According to the analysis of the statistical results, after taking hydraulic fracturing technical measures, during the coal mining period of the 4-1 and 4-2 coal seams in the working face 42,108, as of the end of November 2021, a total of 605.7 m of coal had been mined from the working face. The calculated coal volume of the working face is 1,266,962.9 t. The monthly statistical results of the working face’s recovery rate are shown in Table 3.

The comprehensive chart analysis (Table 3, Figure 18) shows that, after the hydraulic fracturing technology was adopted in working face 42,108 to pre-fracture the high-level thick and hard gangue layer, the top coal cavability of the working face has been greatly improved. In addition, the recovery rate of the working face has been significantly increased, with an improved overall caving effect of the 4-1 coal seam. The average recovery rate of the working face before the implementation of hydraulic fracturing technical measures is 80.1%, the average recovery rate of the working face after the implementation of hydraulic fracturing technical measures from May to July is 86.6%, and the recovery rate of the working face began in May. The recovery exceeded 85%, then continued to steadily increase. In June, the recovery rate of the working face reached a maximum of 87.0%. After August, the high-level gangue layer became thinner, which had little impact on the top coal cavability. No hydraulic fracturing measures were taken, and the recovery rate of the working face decreased slightly but remained above 85%. Overall, in the process of joint mining, the implementation effect of hydraulic fracturing technology measures has improved.

7. Conclusions

(1) In this study, the cantilever beam mechanics model for the fracture and instability of the gangue layer under uniform load is established, and the formula for the maximum fracture step distance of the gangue layer is deduced. The results of the theoretical analysis show that the thickness of the gangue layer and the top coal thickness of the upper layer (i.e., position of the gangue layer) are the two main factors affecting the fracture step distance of the gangue layer. In the high-level gangue layer, the change in the thickness of the gangue layer is more sensitive to the influence of $l$. When the gangue thickness is thin, the high-level gangue layer has a more obvious influence. The influence of the layer varies as the gangue layer thickens, becomes less pronounced, but nevertheless steadily grows with the gangue layer thickness. The 4-2 and 4-1 coal gangue layers in the working face 870–1360 m range are medium- and high-thickness gangue layers, which is negative for the top coal caving. Therefore, we must use auxiliary methods to the gangue for pre-fracture loosening and weakening.

(2) RFPA2D-Flow is used to numerically simulate and analyze the crack initiation, propagation, and extension processes of the fracturing hole. The model hydraulic fracturing process is divided into the stages of hydraulic pressure accumulation, micro-fracture development, micro-fracture extension penetration, and fracture destabilization extension. As the fracture continues to expand, bifurcation occurs at its tip, and many irregular fractures sprout at the tip of the main one. The hydraulic fracturing then enters the stage of unstable expansion. The pressure at this time can be used as the criterion for the failure mechanism of coal and rock mass (fracture pressure).

(3) When fracturing holes with different pore sizes, the fracture initiation pressure does not change much, about 6.7–7.3 MPa, which is close to the theoretical calculation results, which can verify the rationality of the numerical model. Meanwhile, the larger the fracturing hole aperture is, the smaller the water pressure when the fracture penetrates. When the hole aperture is 45 mm, the propagation penetration pressure is 25 MPa; when it is 60 mm, the propagation penetration pressure is 22.9 MPa; when it is 75 mm, the propagation penetration pressure is 21.4 MPa; and when it is 90 mm, the propagation penetration pressure is 16.3 MPa. Based on the numerical simulation results and considering the actual construction on site, the optimal fracturing hole diameter is finally determined to be 75 mm.

(4) The closer the spacing between the fracturing holes, the stronger the coupling influence between the holes, and the easier it is for fractures to propagate along the center line of the fracturing hole. When the hole spacing is 10 m, the propagation penetration pressure is 22.9 MPa; when it is 15 m, the propagation penetration pressure is 23.1 MPa; and when it is 20 m, the propagation penetration pressure is 29.2 MPa. Combining the numerical simulation and the rationality of the site construction, the optimal spacing of the hydraulic fracturing holes in the site is determined to be about 15 m.

(5) After hydraulic fracturing, the amount of gangue behind the hydraulic-powered support of the working face grows, the amount of large coal masses drops dramatically, and the overall fragmentation of the top coal is at a reasonable level. The monitoring results of the borescope show that the original borehole fractures developed and expanded after fracturing, accompanied by many new fractures. Meanwhile, the average recovery rates of the working face in between hydraulic fracturing are 80.1% and 86.6%, representing a 6.5% increase. It is thus fully demonstrated that hydraulic fracturing technology bears a positive effect on weakening the strength and integrity of high-level thick and hard gangue, thus promoting the crushing and caving of the 4-1 coal seam in the upper part of the gangue layer. In turn, this enhances the working face’s recovery rate as well as the cavability of top coal.