Research on Deep Coalbed Methane Localized Spotting and Efficient Permeability Enhancement Technology

Featured Application

It solves the bottleneck problem of the low permeability of deep coal seams and the difficulty of coalbed methane mining, greatly improves the efficiency of coalbed methane mining, and reduces the risk of coal mining, which is of great significance to the project.

Abstract

To solve the bottleneck problem of low deep coal seam permeability and difficult coalbed methane (CBM) mining. Combining hydraulic splitting technology and directional drilling technology, a directional hydraulic splitting enhancement method of deep CBM mining was proposed. The selection equation for the directional hydraulic splitting of deep coalbed was constructed. The numerical simulation reveals the variation in coal fractures around different split angles. The split angle under the maximum coal damage effect was obtained. It was found that the combined effect of the double crack damage disturbance region led to reciprocal stress fluctuations during crack development and, eventually, the formation of a zigzag fracture. The larger the splitting angle, the larger the fissure development length and the larger the coal-damaged area. A double crack takes 25% less time to complete propagation than a single crack. When the splitting angle is 90°, the disturbed area occupies 2/3 of the area around the borehole, and the overall fracturing effect is the best. In the application process, the new directional hydraulic splitting technology can increase CBM mining by 5.08%, greatly improve CBM mining efficiency, and reduce the coal mining risk, which is of great significance to the project.

1. Introduction

Coalbed methane (CBM) is an important clean energy with broad application prospects [1,2,3]. However, due to the anisotropy of coal, deep coal tends to have local low permeability during CBM extraction, resulting in poor CBM extraction [4,5].

To solve the problem of low permeability in the process of deep CBM extraction, experts [6,7] usually pretreat low permeability coalbeds to improve the efficiency of CBM mining. Some experts [8,9,10,11] inject liquid nitrogen into the borehole to crack the coal. The method has achieved certain results in laboratory and field tests. To a certain extent, it improves the efficiency of CBM extraction. Other experts [12,13,14] used gas blasting to break up the coal and increase permeability. With the development of technology, some scholars have proposed the use of hydraulic technology to increase the permeability of coal. Hydraulic fracturing is applied to assist in cutting and breaking the rock to improve the coal-crushing effect.

The above methods show a good effect on the fracturing and penetration of the coalbed, but some limitations remain. Liquid nitrogen fracturing is influenced by the direction of the maximum main stress in the deep coalbed, and a single crack is easily formed by a single liquid nitrogen injection process [15,16]. Thus, finding the optimal combination of cyclic/pulsating liquid nitrogen injection parameters that are safe, efficient, and economical is necessary. At the same time, the crack scale produced by using low-temperature liquid nitrogen is small, and the penetration has some limitations [17,18]. The CBM blasting method has certain safety problems in fracturing the coalbed, which may cause mine vibration, workforce collapse, and pose a hidden risk to the safety of workers. Hydraulic fracturing may cause stress changes in the fault during the process, resulting in the “jumping” of the fault and earthquake [19,20]. In addition, although the above methods improve the permeability of the coalbed and increase the extraction efficiency of CBM, liquid tends to flow out from the crack region of the coalbed when they encounter the crack region [21]. As a result, the above methods cannot reach the deep part of the coalbed, thereby being less effective in increasing the recovery of deep CBM. Moreover, the above methods have low working efficiency, high construction costs, and insufficient crushing accuracy.

When the traditional technology is limited, hydraulic splitting technology shows obvious advantages. Hydraulic splitting technology is able to split the coal directionally and rapidly through rigid crushing, causing directional displacement of the coal and forming longer cracks. Moreover, hydraulic splitting technology requires very low geological conditions. It is well-suited to fracturing and increasing the coalbed permeability under various conditions. Meanwhile, hydraulic splitting technology can be applied to prefabricated fractures before directional water injection fracturing, allowing the fractures to propagate along the prefabricated fractures to form directional fracturing to increase permeability. The joint action of multiple cracks is required to increase the range of the disturbance area as much as possible. Through technical updating, it is now possible to install two rows of outward extending split pistons in the unit. However, the damaging effect of coal under different split angles is not yet clear. Therefore, to achieve the best crushing effect of coal, it is urgent to conduct research on the mechanism of coal breaking and permeability enhancement under the combined action of double cracks.

Based on the crack propagation criterion, the selection equation of the deep coal seam directional hydraulic splitting cleavage angle was derived to solve the above problems. Numerical models matching the actual borehole surrounding the rock were established using XFEM finite element software. The crack propagation law under the action of the splitting piston was explored and revealed the deformation characteristics of the disturbed region. The effect of the new hydraulic directional splitting technology on the increase in CBM recovery was analyzed. The new hydraulic directional splitting technology improves the efficiency of CBM mining, reduces the risk of coal mining, and effectively ensures the safe production of coal mines.

2. Materials and Methods

2.1. Selection Equation of Deep Coalbed Directional Hydraulic Splitting Angle

2.1.1. Technology of Directional Hydraulic Splitting and Permeability Enhancement for Deep Coalbed Methane Mining

As shown in Figure 1, the new hydraulic directional drilling splitting device consists of two parts: the directional drilling system and the hydraulic splitting system. The directional drilling system consists of a directional drilling rig and pipe. The directional drilling system can carry the hydraulic splitting system into deep coalbeds. When the designated position is reached, the splitting pistons in the hydraulic splitting device protrude outward to create a directional seam. The hydraulic splitting system consists of a hydraulic control valve, a hydraulic splitting device, and a splitting piston. The hydraulic splitting device was modified and installed at the end of the directional drilling device, as shown in the yellow part in Figure 1.

An analysis of Figure 1 shows that the splitting pistons protrude outward to cause fracturing of the coal, but the damage effect of the coal is different under different splitting angle selection equations.

2.1.2. An Angle Selection Equation for the Directional Hydraulic Splitting of Deep Coalbeds

As can be seen from the analysis of Figure 1, when the splitting piston extends outward, it acts on the coal wall around the borehole and causes the coal to be rigidly displaced. In this process, only the splitting piston plays a role, so the splitting process is simplified as shown in Figure 2. When the hydraulic oil is introduced, the splitting piston extends out, exerting stress on the coal wall. When the stress exceeds the bearing limit, the coal body breaks up. As the crack develops, a coal damage area is generated around the crack, and a disturbed area is formed at the periphery.

It can be seen from Figure 2 that the angle between the two cracks $\alpha$ is related to the width of the damaged area $R$ and the length of the crack $H$. Suppose the angle between the two split pistons is the ratio of the width of the damaged area to the length of the crack, then:

$$\tan \alpha =\frac{R}{H}$$

(1)

The analysis of the above equation shows that if the expressions of damaged area width and crack length can be solved, the expressions of splitting pinch angle can also be solved in order to get the selection equation of deep coalbed directional hydraulic splitting pinch angle. First, the damaged area width was explored, and the damaged area width depends on the damaged area boundary range, so the problem was transformed into solving the damaged area boundary range.

Damaged area is the pressure exceeding the ultimate bearing capacity of the material, so that the local deformation of the material is not in recoverable yield region. To judge whether the material in the region yields, the criterion often used is the Mises criterion [22], The expression is:

$${\left({\sigma }_1-{\sigma }_2\right)}^2+{\left({\sigma }_2-{\sigma }_3\right)}^2+{\left({\sigma }_3-{\sigma }_1\right)}^2=2{\sigma }_s^2$$

(2)

Among them, ${\sigma }_s$—principal stress and ${\sigma }_1$${\sigma }_2$${\sigma }_3$—component stress.

The Mises yield criterion is represented by the principal stress, so the stress at the crack front is represented by the principal stress. If the stress component is known and the principal stress is $\sigma$, the following determinant is made zero [23].

$$\left[\begin{array}{cc}({\sigma }_x-\sigma )& {\tau }_{xy}\\ {\tau }_{yx}& \left({\sigma }_y-\sigma \right)\end{array}\right]=0$$

(3)

Solving this formula, we obtain:

$$\sigma =\frac{{\sigma }_x+{\sigma }_y}{2}\pm \sqrt{{\tau }_{xy}^2+\frac{1}{2}{({\sigma }_x+{\sigma }_y)}^2}$$

(4)

The component stresses are:

$$\left\{\begin{array}{l}{\sigma }_1=\frac{{\sigma }_x+{\sigma }_y}{2}+\sqrt{{\tau }_{xy}^2+\frac{1}{2}{({\sigma }_x-{\sigma }_y)}^2}\hfill \\ {\sigma }_2=\frac{{\sigma }_x+{\sigma }_y}{2}-\sqrt{{\tau }_{xy}^2+\frac{1}{2}{({\sigma }_x-{\sigma }_y)}^2}\hfill \\ {\sigma }_3=\mu ({\sigma }_1+{\sigma }_2)\hfill \end{array}\right.$$

(5)

Substitute the stress component formula near the crack tip into Equation (5) to obtain the principal stress at any position near the crack tip as [24]:

$$\left\{\begin{array}{l}{\sigma }_1=\frac{K_{\mathrm{I}}}{\sqrt{2\pi r}}\cos \frac{\theta }{2}(1+\sin \frac{\theta }{2})\hfill \\ {\sigma }_2=\frac{K_{\mathrm{I}}}{\sqrt{2\pi r}}\cos \frac{\theta }{2}(1-\sin \frac{\theta }{2})\hfill \\ {\sigma }_3=\frac{2\mu K_{\mathrm{I}}}{\sqrt{2\pi r}}\cos \frac{\theta }{2}\hfill \end{array}\right.$$

(6)

Substituting Equation (6) into the yield criterion (3), the yield region boundary equation is obtained as:

$$R=\frac{1}{2\pi }{\left(\frac{K_{\mathrm{I}}}{{\sigma }_S}\right)}^2{\cos }^2\frac{\theta }{2}\left[{\left(1-2\mu \right)}^2+3{\sin }^2\frac{\theta }{2}\right]$$

(7)

As the width of the damaged area is perpendicular to the crack when $\theta$ = 90°, the expression of the width of the damaged area R is:

$$R=\frac{1}{4\pi }{\left(\frac{K_{\mathrm{I}}}{{\sigma }_s}\right)}^2\left[{\left(1-2\mu \right)}^2+\frac{3}{2}\right]$$

(8)

The evolution of coal damage [25] is the further crack expansion of macroscopic cracks. In the stable expansion stage of cracks, the relationship between crack expansion resistance and crack expansion increases. The relationship between crack expansion resistance and crack expansion is established as follows:

$$\frac{r^{\prime }}{r}={\left(\frac{H}{a}\right)}^n$$

(9)

In the relationship between the propagation stress G of the plane crack tip of coal and rock, the stress intensity factors $K_{\mathrm{I}}$ and $K_{\mathrm{II}}$ are:

$$G=\frac{1-v_0^2}{E_0}\left(K_{\mathrm{I}}^2+K_{\mathrm{I}\mathrm{I}}^2\right)$$

(10)

Among them, $K_{\mathrm{I}}=\sigma \sqrt{\pi a}$ and $K_{\mathrm{II}}=\tau \sqrt{\pi a}$ are substituted, and the expansion stress at the tip of the crack is:

$$G=\frac{1-v_0^2}{E_0}\pi a\left({\sigma }^2+{\tau }^2\right)$$

(11)

According to the crack propagation criterion [26], combined using Equations (9) and (11), the length of the crack after expansion can be obtained as:

$$H=a^{\frac{n}{n-1}}{\left[\frac{\pi \left(1-v_0^2\right)}{E_{0}r}\left({\sigma }^2+{\tau }^2\right)\right]}^{\frac{1}{n-1}}$$

(12)

Combining the above expressions of the width of the damaged area and the length of the crack, we obtain:

$$\tan \alpha =\frac{R}{H}=\frac{\frac{1}{4\pi }{\left(\frac{K_{\mathrm{I}}}{{\sigma }_{\mathrm{s}}}\right)}^2\left[{\left(1-2\mu \right)}^2+\frac{3}{2}\right]}{{\mathrm{a}}^{\frac{n}{n-1}}{\left[\frac{\pi \left(1-{\mu }^2\right)}{E_{0}r}\left({\sigma }^2+{\tau }^2\right)\right]}^{\frac{1}{n}}}$$

(13)

In the formula: $K_{\mathrm{I}}$—stress intensity factor, $K_{\mathrm{I}}=\sigma \sqrt{\pi a}$, ${\sigma }_s$—principal stress around the borehole, $\mu$—Poisson’s ratio, and $E_0$—disturbed modulus. The n-crack expansion resistance growth index generally takes 1/2~3/2, with $r$—crack expansion resistance.

To obtain the appropriate splitting angle, the coal sample data measured in the laboratory were brought into the formula. Due to the anisotropy of the coal, the parameters measured by each numbered coal sample were sorted and brought into the formula to obtain $\alpha$. The range is 70.5~112.26°.

2.2. Simulation Research of Splitting Disturbance Range Based on Extended Finite Element

2.2.1. Governing Equations

The propagation finite element constitutes the displacement principle based on the propagation finite element by adding the jump function and the asymptotic displacement field function of the crack tip on the basis of the conventional finite element [27]:

$$u(x)={\displaystyle {\sum }_{i\in N}{N}_i(x)u_i+}{\displaystyle {\sum }_{j\in N}{N}_j(x)H(x)a_j+}{\displaystyle {\sum }_{k\in N}{N}_k(x){\displaystyle {\sum }_{a=1}^4{\phi }_{\alpha }(x)b_k^{\alpha }}}$$

(14)

where $i$ is the set of all nodes, $j$ is the set of nodes that are completely penetrated by the crack (represented by ‘’ in Figure 3), and $k$ is the set of nodes containing crack tip elements (represented by ‘’ in Figure 3). $N_i$, $N_j$, and $N_k$ are the shape functions of the corresponding nodes. $u_i$, $a_j$, and $b_k^{\alpha }$ are the displacements of the corresponding nodes.

2.2.2. Physical Model

As shown in Figure 4, the blue device is a new hydraulic splitting device. The device has two rows of split pistons, which can rotate at a certain angle and is more adaptable to complex geological conditions. The white device is a traditional hydraulic device. The device only has a column of split pistons, and the split pistons cannot turn. The piston is out of a short trip and has high geological requirements. The specific differences are shown in

Table 1. Differences between traditional hydraulic device and new directional splitting device.

	Number of Split Pistons	Angle of Rotation	Extended Distance
Prototypes	2	70°~110°	3 cm
Developed device	1	Can’t rotate	5.5 cm

.

2.2.3. Model Design and Mesh Parameters

When the new hydraulic directional drilling splitting device acts on the coal around the borehole, the model is simplified because only the splitting pistons act on the coal around the borehole. In this paper, the quadrilateral mesh division method and advanced algorithm were used, and the boundary constraints were fixed, as shown in Figure 5.

In this paper, the actual coalbed parameters of a mine in Hebei Province were used for simulation research, as shown in

Table 2. Mine coal parameters.

$\mathbf{\rho }$	$\mathit{E}$	$\mathbf{\mu }$	$\mathbf{\sigma }$	Damage Evolution Displacement	Number of Units	Primary Fissure
1.35 × 103 kg/m3	3500 MPa	0.35	20 MPa	0.001 mm	26,460	3 mm

. The contents of the table mainly include the disturbed modulus of coal, Poisson’s ratio, maximum principal stress, etc.

2.2.4. Numerical Simulation Scheme

As coal is anisotropic, there are some differences in the parameters measured in the laboratory. The splitting angle range was obtained by bringing the actual parameters into the formula. The distribution of crack patterns greatly influences the permeability effect of the coal seam. The greater the disturbance range, the better the penetration effect of the coal. Thus, it was necessary to ensure the maximum disturbance range of the coal to improve the coalbed permeability. Therefore, we needed to explore the effect of the two crack splitting angle changes on the coalbed disturbance to further determine the optimal splitting angle by setting different splitting angles, as shown in

Table 3. Coalbed drilling and splitting angle simulation scheme.

Simulation Parameters	Numerical Value	Numerical Value	Numerical Value	Numerical Value	Numerical Value
Splitting angle	70°	80°	90°	100°	110°

. Exploring the mechanism of crack joint development damage extension, the article used a static analysis step, and the time step was set to 1 s. The model mesh was divided by the quadrilateral method, and a displacement method was used, instead of load, to simulate the coal damage effect under the action of splitting pistons.

2.2.5. Grid Independence Test

Great quality mesh is a prerequisite for accurate simulation results. To test the grid independence, four grids with different densities were selected. The corresponding grid numbers were 22,743, 23,101, 26,460, and 29,928, respectively. The measured crack lengths are shown in the Figure 6.

It can be seen from Figure 6 that the final simulation results show little difference; the maximum error of the crack length is 0.05 mm, and the error rate is negligible. In theory, a finer mesh is ideal for this simulation. However, the higher the mesh density, the longer the computation time required. Therefore, considering the computational cost and error, meshing method No. 3 was chosen in this study.

3. Results

3.1. Characteristics of Crack Disturbance Range under Different Splitting Angles

To select the splitting angle that has the best effect of increasing the permeability, the permeability increase in the field drilling was analyzed. After the technology upgrade, the included angle of the two groups of splitting pistons range from 70 to 110°. The representative splitting angles of 70, 80, 90, 100, and 110° were selected for analysis, as shown in Figure 7.

As can be seen from Figure 7, at the same stage, the disturbance range of the coal increases, and the area of damage first increases and then decreases with the increase in the angle. At the same angle, the areas of disturbance and damage increase as time increases. From the overall analysis, the expansion of the coal disturbance area can be divided into three stages. In the first stage, the coal damage area around the borehole is small. The damaged area is mainly located in the contact area between the splitting pistons and the borehole wall. In the second stage, the length of the crack, the area of disturbance, and the area of damage increase. The area of damage begins to approach or intersect with the boundary. In the third stage, as the crack length increases, the area of damage further expands. Finally, the two damaged areas are closer together or even merged because after the split piston applies stress to the coal rock, the first elastic deformation occurs, and when the local load exceeds the maximum stress sustained, the test sample will undergo local damage, and the stress is released [28,29]. As the stress increases, new supports reappear, increasing the coal rock-bearing stress. When the maximum stress is broken through again, the crack expands again. The stress is released again until the split piston is fully extended. The stress fluctuates back and forth, and the stress distribution on both sides of the crack is asymmetrical, resulting in irregular changes in the crack shape. Eventually, jagged cracks are formed. Compared with laboratory fracturing, the crack surface is not obviously jagged, although the propagation direction appears distorted [30].

A comparison of the damaged area under different splitting angles shows that the damaged area increases at an angle between 70 and 90°. When the splitting angle is 100 and 110°, the size of the damaged area is similar. A clear separation exists between the two damaged areas, and as the angle increases, the distance between the two damaged areas also increases. When the splitting angle is 90°, the damaged areas are finally connected, and a larger area of damage is formed. The main reason for this condition is that when there is crack propagation, the stress redistributes on both sides of the crack, and then the coal damage area is generated. With the short distance between the two cracks, the damaged areas affect each other, thus aggravating the damage to the coal and finally forming a large-scale damaged area.

An analysis of the coal perturbation effect under different splitting angles shows that when the splitting angle increases, the disturbed area increases, and the damaged area first increases and then decreases. To further accurately characterize the changing relationship between the parameters, a comparative study was conducted on the appearance and the morphological data of double cracks with different splitting angles, as shown in Figure 8.

As shown in Figure 8, the overall length and area of the cracks show an increasing trend over time, and the increase trend is slow at 0~0.2 s. At 0.65~1 s, the stress applied to the coal is not enough due to the maximum travel of the splitting pistons. The next stage of the crack propagation proceeds, and the crack growth stops. The main crack propagation stage is concentrated at 0.2~0.65 s, and the crack propagation length at this stage accounts for 95% of the total. In engineering applications, the crack propagation at this stage can be completed. The crack propagation is mainly concentrated in the second stage because the coal damage belongs to the disturbed deformation. The whole process needs to go through the following processes: stress accumulation–breakthrough limit–stress release [31,32]. Therefore, at 0~0.2 s, it is in a state of stress accumulation, and the crack propagation length is very short at this time. When the stress exceeds the limit, the coal is broken, the crack rapidly propagates along the principal stress direction, and the stress is released. For the next stage of the crack propagation, the stress needs to accumulate again, but as the accumulated stress is still not enough to initiate the next stage of the coal after the splitting pistons travel to the maximum, the length of the crack in the coal stops increasing.

As the splitting angle increases, the overall length of the crack increases. The main reason for this phenomenon is that with the development of cracks, the stress of coal and rock on both sides of the cracks changes. The distance between the damaged area on both sides of the cracks is close, and the damaged areas are connected; thus, the damage to the surrounding coal is greatly reduced. The cracks propagate into the damage in the coal. Deformation, which occurs when the damage to the coal is reduced, will lead to a reduction in the length of crack propagation. Therefore, when the crack-splitting angle increases, the distance between the damaged area of the two cracks increases, and the superposition effect weakens. The influence on the coal around the borehole becomes smaller. Therefore, the damage to the coal is less affected, and the crack propagation length is longer.

The data in Figure 8 indicate that when the included angle is 70°, the crack length is about 392 mm, and the crack area is about 441 mm². When the included angle is 80°, the crack length is about 400 mm, and the crack area is about 458 mm². When the included angle is 90°, the crack length is about 560 mm, and the crack area is about 480 mm². When the included angle is 100°, the crack length is about 481 mm, and the crack area is about 507 mm². When the included angle is 110°, the crack length is about 460 mm, and the crack area is about 530 mm². In comparison, when the splitting angle is 90°, the crack length and area are in the middle. Combined with the qualitative analysis, the damage effect on the coal around the borehole is the best when the included angle of splitting is 90°.

Whether the combined effect is optimal can be judged according to the range change in the crack-damaged area. The data of the width R of the damaged area of the crack was extracted, and the joint damage effect of the crack was analyzed. The extraction path and fitting results are shown in Figure 9.

An analysis of Figure 9 indicates that the overall change trend is consistent, and the two results are roughly the same, as shown by a comparison of the simulation results with the numerical substitution results. However, the width of the damaged area obtained by the simulation is slightly larger than that obtained by the calculation. This is mainly because the interaction between the damaged areas is not considered when calculating through the formula. The damaged areas interact with each other because of the close distance between the two cracks. This situation intensifies the degree of damage to the coal, and eventually, a larger area of damage is formed.

After Origin software was used for fitting, the overall change conformed to the exponential function [33] change law, the goodness of fit was greater than 0.99, and the fitting effect was good. The analysis of the curve change shows that the width of the damaged area increases, and the increasing trend is initially fast and then becomes slow with the increase in the angle of the splitting. With the empirical formula integrated into the angle selection equation of the directional hydraulic splitting in deep coalbeds, it can be better applied to the engineering practice process. The specific formula is as follows:

$$\tan \alpha =\frac{R}{H}=\frac{-400.29\ast e^{(-\mathrm{x}/11.71)}+9.55}{{\mathrm{a}}^{\frac{n}{n-1}}{\left[\frac{\pi \left(1-{\mu }^2\right)}{E_{0}r}\left({\sigma }^2+{\tau }^2\right)\right]}^{\frac{1}{n}}}$$

(15)

where $\sigma$ is the principal stress around the borehole, $\mu$ is the Poisson’s ratio, and $E_0$ is the disturbed modulus. $n$ is the crack expansion resistance growth index, which generally takes 1/2~3/2, and $r$ is the crack expansion resistance.

3.2. Comparison of Coal Damage Effect under the Action of Single and Double Cracks

A comparison of the coal damage effect under different splitting angles shows that the best damage effect is achieved when the splitting angle is 90°. However, the damage effect of a single crack on the coal is still unclear. Thus, judging which case shows a better effect on the coal damage effect and permeability enhancement is impossible. To determine the better effect of splitting and seam-making under engineering conditions, two cases of single and double cracks with 90° splitting angles are discussed.

An analysis of Figure 10 shows that the development direction of the single crack is almost the same as the direction of the stress as a whole. The disturbance range of a single crack is symmetrical around the crack as a whole, and the displacement of the disturbance area decreases in a stepwise manner from the inside to the outside. The disturbance area expands outward in a fan shape with the development of the crack. A certain angle deflection occurs between the development direction of the double crack and the principal stress direction, and the coal is damaged and displaced irregularly. The range of the coal disturbance area expands outward with the increase in the crack length. The damaged area formed by the double cracks is much larger than the damaged area formed by the single cracks.

During the whole propagation process, the crack propagation takes place in stages, undergoing multiple stress accumulations and releases in the process. The asymmetry of the range of the damaged areas on both sides of the crack leads to a reciprocal fluctuation in stress during crack development and a reciprocal change in the crack propagation direction. Eventually, a jagged crack is formed.

The above analysis indicates that the crack shape is closely related to the joint action between the two cracks. During the development of double cracks, the stress distribution on both sides of the cracks is uneven because the damaged areas of the two cracks are adjacent to each other. As a result, the cracks propagate along the direction of the maximum principal stress and shift to a certain extent. During the development of a single crack, the damaged areas on both sides of the crack increase in a symmetrical shape, and the stress distribution on both sides of the crack is also symmetrical overall. Therefore, the development direction of a single crack does not deviate considerably and only propagates in the direction of the maximum principal stress.

The morphological change characteristics of crack propagation were studied through qualitative analysis methods, and the crack propagation mechanism at different stages was analyzed above. To further quantitatively characterize the change law of crack propagation parameters, the data on the appearance of single and double cracks were compared and analyzed.

Figure 11 shows that the length and area of the single and double cracks exhibit an overall increasing trend. When the length of double cracks is 0.3 s, an obvious upward trend occurs, which lasts up to 0.65 s. The length of the crack propagation at this stage accounts for a large part of the overall length of the double crack because the coal breaks through an elastic yield phase, which occupies the whole for a long time. Due to the nature of the coal, when the elastic stage is broken, the crack expands rapidly, releasing the coal body energy [34]. This change occurs between 0.5 and 0.8 s in a single crack, which is 25% longer than a double crack. This finding indicates that the double crack will complete the splitting process first, whereas the single crack needs a longer time to accumulate stress and complete the entire splitting process.

At 0.3 s, the double crack length increases faster because the coal crushing needs to accumulate stress and break the yield limit. At 0.3 s, the stress increases continuously until the coal breaks through the yield limit at 0.3 s, and the macroscopic crack length increases rapidly. The stress builds up faster, and the time required to break the yield limit is shorter because of the superposition of the stress between the two cracks. At 0.5 s, the double crack propagation rate decreases sharply due to the extension of the splitting piston to the maximum distance. As a result, the time required for the double crack to complete propagation is shorter.

In sum, a comparison of the length of the single and double cracks, the damaged area of coal, and the completion time for crack propagation shows that when the included angle of double cracks is 90°, the best cracking effect on coal is achieved, which can effectively improve the deep cracking effect.

3.3. Engineering Application of Directional Hydraulic Splitting and Penetration Enhancement Technology for Deep Coalbed Methane Mining

3.3.1. Engineering Background

The 1203 working surface of a mine in Hebei Province is at a depth of 1200 m. The average thickness of the coalbed is 8.4 m, and the average inclination angle is 4.5°. The in situ stress of the coalbed is large, and the coal texture is hard. Extracting CBM is difficult. The traditional directional drilling technology and the technology of directional hydraulic splitting and permeability enhancement for deep CBM mining are used to induce cracks and increase the permeability of the coal. The effect of the technology is verified by comparing the change in the CBM extraction volume after fragmentation between the traditional directional drilling technology and the new hydraulic directional drilling and splitting technology.

3.3.2. Hole-Forming Effect of Working Surface

Ultra-long directional drilling at a depth of 1570 and 2353 m was completed using a directional drilling rig. The drilling layout is shown in Figure 12.

Three drilling spots were arranged in the working surface. The length of the two extra-long directional boreholes was 2353 m. The first of the two extra-long directional boreholes was drilled using conventional directional drilling technology, and the other was drilled using the new hydraulic directional drilling splitting technology. The last directional drill hole was 1570 m in length and was drilled using a new hydraulic directional drilling splitting technology.

3.3.3. Comparison of Coalbed Methane Extraction Effects of Different Additional Extraction Methods at the Same Depth

To illustrate the effect of directional hydraulic splitting and permeability enhancement technology for deep CBM mining on coal fracturing and permeability enhancement, the traditional directional drilling technology and the technology of directional hydraulic splitting and permeability enhancement for deep CBM mining were successively used on the same working surface to increase the production of a deep coalbed. The depth was determined using a number of directional drill pipes. The other operations were the same during the drilling process. When the same number of drill pipes were used, the drilling depth was considered the same.

A comparison between the traditional directional drilling technique and directional hydraulic splitting and permeability enhancement technique for deep CBM mining, as shown in Figure 13, indicates that directional hydraulic splitting and permeability enhancement technology for deep CBM mining has a better effect on the coal mass fracturing, with an improved CBM extraction effect after permeability enhancement. The traditional directional drilling technology achieved a CBM extraction effect of 5.11 m³/min, whereas the new hydraulic directional drilling and splitting technology can achieve an average extraction efficiency of 5.37 m³/min. The 5.08% increase in the drainage efficiency indicates that directional hydraulic splitting and permeability enhancement for deep CBM mining can significantly increase the CBM drainage efficiency by fracturing the coal.

3.3.4. Drainage Effect of New Hydraulic Splitting Technology at Different Depths

To illustrate the effect of the technology of directional hydraulic splitting and permeability enhancement technology for deep CBM mining on the penetration enhancement effect of coal, different depths of 1570 and 2353 m were drilled in the same coalbed. The drilling depth was determined by the number of directional drill pipes, and the other operations were the same during drilling.

An analysis of Figure 14 shows that the CBM extraction efficiency is significantly improved with the increase in drilling depth, achieving an average efficiency of 3.36 m³/min and 5.37 m³/min at drilling depths of 1570 and 2353 m, respectively. The extraction efficiency increased by 59.8%. Thus, the efficiency of CBM extraction is improved, and manpower and material resources are saved. Therefore, the method is of great significance to the project.

4. Discussion

With the increase in the coal mining depth, the coal seam stress increases, the coal permeability decreases, and coalbed methane extraction is difficult. In order to solve this problem, experts have investigated different methods to increase coal seam permeability and improve the efficiency of coalbed methane extraction. Many methods include liquid nitrogen freezing and cracking, gas blasting technology, hydraulic fracturing technology, and so on. These methods all show some effects, but they also have some defects. The freezing technology of liquid nitrogen is limited, the gas blasting technology is dangerous, and the hydraulic fracturing technology is easily affected by geological conditions.

When the traditional technology is limited, the advantages of hydraulic splitting technology are clear. In the face of the uncontrollability of liquid nitrogen freezing–cracking technology and small cracking, hydraulic cracking technology shows the advantages of long macroscopic crack lengths and the controllable expansion direction of cracks. Liquid nitrogen cannot be recycled after use, and the cost is high. The hydraulic splitting technology device can be reused many times to reduce the construction cost. In addition, although the effect of the common gas blasting is good, the range is large and easily causes working face vibration; mine environments are complex, the increase in vibration means that they may cause collapse and other accidents, where workers may be injured. Hydraulic splitting technology is a static mining technology, which can ensure the safety of underground operations. In addition, the hydraulic measures should improve the extraction efficiency and reduce the dust concentration of coal seam mining. However, when encountered in a fault, there may be running water and other phenomena, resulting in a poor effect. In the face of this situation, hydraulic splitting technology has very low requirements on geological conditions, which can be well-suited to coal seam cracking under various conditions. At the same time, the liquid pressure produced by the device can reach 40 MPa, which shows a good effect on the coal seam with higher hardness. Of course, the device can also be applied to the prefabricated cracks before the directional hydraulic pressure, which extend along the prefabricated cracks to form the directional cracks.

According to the present situation in deep CBM mining, the following was proposed. The directional hydraulic splitting of a deep coal seam was constructed. The expansion law of the cracks from a hydraulic splitting device on the coal around the drilling holes was simulated. The change characteristics of the coal rock disturbance region under different splitting angles were analyzed. Compared with the work of previous scholars, the work of this paper is more targeted. According to the actual operation situation, we simulated the range of coal rock disturbance under different splitting angles, and this research provides some guidance for the deep directional hydraulic penetration project of coal mining. In fact, the research work in this paper is based on the high hardness of the coal body, and the coal-crushing characteristics of the softer coal layer need to be further discussed.

5. Conclusions

To solve the bottleneck problem of the low permeability of deep coalbeds and the difficulty in mining CBM, this work proposes a method of directional hydraulic fracturing and permeability enhancement for deep CBM mining that combines hydraulic fracturing technology with directional drilling technology. The selection equation of the included angle for the bidirectional hydraulic splitting of deep coalbeds was constructed through theoretical derivation. The joint propagation law of hydraulic splitting double cracks was revealed using the numerical simulation results. The following conclusions are drawn:

(1)

According to the basic knowledge of theoretical mechanics, the hydraulic splitting angle selection equation was deduced. The split angle is negatively correlated with the crack length. The longer the crack, the smaller the split angle. The split angle is positively correlated with the width of the disturbed region and the wider the split angle.

(2)

The characteristics of the coal body disturbance area are different. The combined action of the double fissure damage disturbance region will lead to asymmetric stress distribution on both sides of the fissure, resulting in recurrent stress fluctuation during fissure development and forming a zigzag fracture. When the split angle is 90°, the disturbance occupies 2/3 of the area around the borehole, and the overall crack effect is the best.

(3)

The designed new hydraulic splitting device is combined with directional drilling technology. By installing the new hydraulic cracking device at the end of the directional drilling rod, the hydraulic splitting device is transported to the designated position inside the coal body, and deep directional cracking is realized, and the gas extraction efficiency of the coal seam is improved. After the new directional hydraulic splitting technology, the CBM extraction efficiency can reach 5.37 m³/min, which is improved by about 5.08%. At the same time, the deeper the drilling distance, the higher the CBM extraction efficiency.