Open AccessArticle

Comparison of Hydraulic Measures for Improving Coal Seam Permeability: A Case Study

Yuxi Huang

¹,

Xiaoyang Cheng

^2,3,4,*

and

Huan Zhang

School of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan 030024, China

China Coal Technology and Engineering Group Chongqing Research Institute, Chongqing 400037, China

State Key Laboratory of Coal Mine Disaster Prevention and Control, Chongqing 400037, China

⁴

China Coal Research Institute, Beijing 100013, China

Author to whom correspondence should be addressed.

Processes 2025, 13(3), 626; https://doi.org/10.3390/pr13030626

Submission received: 16 January 2025 / Revised: 17 February 2025 / Accepted: 21 February 2025 / Published: 22 February 2025

(This article belongs to the Topic Advances in Coal Mine Disaster Prevention Technology)

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Browse Figures

Figure 1
Mechanical model of coal damage caused by high-pressure water jet [<a href="#B25-processes-13-00626" class="html-bibr">25</a>]. "> Figure 2
Study site. "> Figure 3
Schematic diagram of hydraulic punching measure system modified from [<a href="#B27-processes-13-00626" class="html-bibr">27</a>]. "> Figure 4
Coal seam pressure relief effect by hydraulic punching [<a href="#B29-processes-13-00626" class="html-bibr">29</a>]. "> Figure 5
Borehole layouts of hydraulic reaming. "> Figure 6
Change trend of pure gas flow rate in Unit 1, 3# hydraulic reaming borehole. "> Figure 7
Change trend of pure gas flow rate in Unit 2, 2# hydraulic reaming borehole. "> Figure 8
The relationship between the amount of washed-out coal and water pressure. "> Figure 9
The relationship between the amount of washed-out coal and reaming time with a water pressure of 8 MPa. "> Figure 10
Borehole layouts of hydraulic punching. "> Figure 11
Change trend of pure gas flow rate of each inspection borehole in Unit 2. "> Figure 12
Change trend of pure gas flow rate of each inspection borehole in Unit 3. "> Figure 13
The relationship between the amount of washed-out coal and the water flow rate and punching pressure. "> Figure 14
The relationship between the amount of washed-out coal and punching time. "> Figure 15
The gas concentration of extraction boreholes with the same spacing to the measured holes by hydraulic reaming and punching. "> Figure 16
Comparison of the amount of washed-out coal and time by hydraulic reaming and punching. ">

Versions Notes

Abstract

Hydraulic measures are widely used to improve coal seam permeability, but not all hydraulic measures have a positive effect on coal permeability in soft coal seams, and the permeability-enhancing effect of hydraulic measures in soft coal seams is not clear. To further study the permeability-enhancing mechanism of hydraulic measures and compare the effect of hydraulic punching and reaming in soft coal seams, this study takes Changping Mine, China, as its case study. A comparative analysis was conducted on the influence range and gas extraction effect of hydraulic reaming and punching on coal seam permeability enhancement. The following conclusions were mainly drawn: A mathematical calculation model was established for the strength and impact velocity of high-pressure water jet damage to the coal body, and the critical theoretical pressure threshold and jet velocity were obtained. During the implementation of hydraulic measures at the Changping Mine, the effective radius of hydraulic reaming is around 4.5 m, and the influence radius of hydraulic reaming is approximately 7.5 m; the effective radius of hydraulic punching is about 6.5 m, and the influence radius of hydraulic punching is approximately 7–9 m. The gas data from field monitoring show that hydraulic measures have significantly improved the extraction gas concentration and purity, and hydraulic punching has a more significant effect on enhancing permeability in soft coal seams.

Keywords:

hydraulic measures; coal seam permeability; gas extraction

1. Introduction

Gas disasters are the biggest threat to safe production in coal mines and the “first killer” that threatens miners’ lives in China. The continuous upgrade of mining equipment has meant that modern mines are characterized by concentrated production and fast mining speed, which aggravates gas emission and results in increasingly difficult gas control during the mining process [1,2].

Gas extraction is the most effective technical measure for gas control in mines. Coal seam permeability is a decisive factor affecting gas extraction efficiency [3,4,5]. Therefore, to improve gas extraction efficiency, it is necessary to improve coal seam permeability [6,7]. The principle behind the most commonly used methods for improving coal seam permeability is to use external force to damage the coal body and release in situ stress, thereby improving coal seam permeability. With the development of science and technology, some coal seam permeability improvement technologies have been proposed by experts and scholars, such as protective layer mining [8,9], borehole presplitting basting [10], hydraulic fracturing [11,12], hydraulic slotting [13,14], and high-energy gas fracturing [15,16,17].

The hydraulic measures are widely used due to their characteristics of minimal environmental impact, no pollution to groundwater and coal seam, strong adaptability to the geological environment, etc. Thus, scholars both at home and abroad have investigated the impacts of hydraulic measures on the gas permeability of coal/rock reservoirs using a variety of methods [18]. Ma et al. introduced a new stimulation technique, which consists of two procedures: fracturing the coal seam with a fracturing borehole and flushing the coal with adjacent fracturing boreholes [19]. Zhang and Li developed the three-level flow-increasing hydraulic punching system, and it can effectively prevent borehole blockage and improve gas extraction efficiency [20]. Gu and Wu conducted a field test in a deep “three-soft” outburst coal seam, and the relationship between coal output V (the volume of coal produced during the hydraulic punching process) and hydraulic punching fracture radius R under the same water pressure was established [21]. Aiming to simulate hydraulic fracturing in rocks, Pakzad et al. [22] incorporated an elastic-brittle-damage constitutive model into ABAQUS’s coupled fluid/solid analysis. The model was applied to simulate cavity fluid pressurization under different conditions, considering the capillary effect in low-permeability media. The results showed its ability to capture fracture and fluid evolution for various permeability levels and pressurization rates, despite some limitations in ABAQUS’s fluid modeling. Llanos et al. [23] investigated hydraulic fracture propagation through orthogonal discontinuities experimentally, numerically, and analytically, finding that higher normal stress across interfaces promotes crossing, the viscous-dominated fractures cross natural fractures more easily, the fracture geometry is elliptical due to interaction with interfaces, and the coefficient of friction has less impact on crossing under certain conditions. A 2D coupled displacement discontinuity numerical model that couples fluid flow and rock deformation was established by Sesetty and Ghassemi [24], and sequential and simultaneous hydraulic fracturing in single- and multi-lateral horizontal wells, including zipper fracturing in various scenarios, were simulated. The results reveal that fracture geometries are influenced by factors such as fracture spacing and the boundary conditions of pre-created fractures, while also comparing the stimulated reservoir volumes of different fracturing methods to provide insights for optimizing fracturing design.

Although previous research has shown that hydraulic measures can improve coal seam permeability and enhance gas extraction efficiency to some extent, not all of the hydraulic measures have a positive effect on the permeability of soft coal seams, and the permeability-enhancing mechanism of hydraulic measures in soft coal seams is not clear. To further study the mechanism and the effects of hydraulic punching and reaming in soft coal seams, this study takes Changping Mine, China, as its case study. Firstly, a mechanical model of coal seam damage by water jet was proposed. Based on this, quantitative research was conducted on the coal-breaking water pressure and threshold velocity. In the field test, the permeability-enhancement effects of hydraulic reaming and hydraulic punching on coal seams were compared and analyzed. This study’s results provide theoretical and on-site experience references for gas extraction in coal seams with similar geological conditions.

2. Methodology and Materials

2.1. Coal Damage Mechanism by High-Pressure Water Jet

2.1.1. Mechanical Analysis of Coal Damage Caused by High-Pressure Water Jet

In Figure 1, a simplified mechanical model of coal damage by high-pressure water jet is established. The mechanical model can be divided into four main areas: the free jet zone, the jet deformation zone, the fractured zone, and the pore-forming zone. We make the following assumptions:

(1): The coal body is assumed to be a rigid, fractured, fluid medium. When P < R_t, the coal body is a rigid body; when P ≥ R_t, the pressure on the interfaces f-f and e-f between the pore-forming zone is the strength of the solid coal R_t.
(2): During the coal impacting process, both the free jet zone and the jet deformation zone of the high-pressure water jet move at velocity v.
(3): In any cutting plane of the mechanical model, all physical quantities are one-dimensional. On any section perpendicular to the transverse axis of the impacting zone, it is uniformly distributed along the axis and has equal physical quantities.

The length of the free jet is L. The area of a-a is S₀. In the second stage, the jet velocity is v, the total mass is M_ab, and the area of the b-b section is S_bb. The jet velocity in the third stage is u, and the cross-sectional area is S_dd. The impact pressure is P, and the mass is M_dd. In the fourth stage, the total mass within Δt is M_ff, and the f-f area is S_ff. In this zone, the impact stress on section f-f is R_t.

2.1.2. Mass and Momentum Conservation Equations for Free Jet Zone and Jet Acceleration Zone

The kinematic relationship between the free jet stage and the jet deformation stage can be expressed as follows:

\begin{matrix} d L / d t = - (v - u) \\ d H / d t = u \end{matrix}\}

(1)

where L is the jet length, m; v is the velocity of the free jet, m/s; u is the jet impact velocity, m/s; and H is the depth of the coal matrix damage and fragmentation.

The momentum equation for the free jet and jet deformation zone is as follows:

\frac{d}{d t} [ρ_{w} S_{0} L (v - u)] + \frac{d}{d t} [M_{a b} (v - u)] + ρ_{w} S_{b b} {(v - u)}^{2} + [ρ_{w} S_{0} L + M_{a b}] \frac{d u}{d t} = 0

(2)

where ρ_w is the water density, kg/m³; S₀ is the sectional area of the free jet, m²; M_ab is the mass of the water in the jet acceleration zone, kg; and S_bb is the sectional area of the b-b section, m².

By introducing Equation (2) into Equation (1), it can be concluded that

\frac{d v}{d t} = \frac{{(v - u)}^{2}}{L} (1 - γ)

(3)

where γ is the area coefficient of the jet acceleration zone, γ = S_bb/S₀.

2.1.3. Mass and Momentum Conservation Equations for the Fractured Zone

Ignoring the influence of water weight, the mass conservation equation is established as follows:

\frac{d M_{d d}}{d t} = ρ_{w} S_{b b} (v - u) - ρ_{w} S_{b c} u_{g} = 0

(4)

where M_dd is the mass of the fractured zone, kg, and S_bc is the sectional area of the b-c section, m². u_g is the motion speed of the water jet relative to the d-d plane reference coordinate system.

u_{g} = (v - u) / β

(5)

where β is the deformation coefficient of the water jet, β = S_bc/S_bb.

The momentum conservation equation for the fractured zone is as follows:

\frac{d}{d x} [M_{d d} (u_{b c} - u)] = - P (S_{b b} + S_{b c}) + ρ_{w} S_{b b} {(v - u)}^{2} - ρ_{w} S_{b c} u_{g} (- u_{g}) M_{d d} \frac{d u}{d t}

(6)

where M_dd is the mass of the water in the fractured zone, kg.

Similarly, ignoring the mass of the water jet in the area and incorporating Equation (6) into Equation (5), the following equation is obtained:

P = ρ_{w} \frac{{(v - u)}^{2}}{β}

(7)

2.1.4. Momentum and Mass Conservation Equations in the Pore-Forming Zone

Ignoring the influence of coal slag on the quality of water in the pore-forming zone, according to the mass conservation equation,

\frac{d M_{f f}}{d t} = ρ_{c} S_{f f} u - ρ_{c} \int_{s} \bar{u} \bar{n} d s = 0

(8)

where M_ff is the mass of the water in the pore forming zone, kg; ρ_c is the coal density, kg/m³; S_ff is the sectional area of the f-f section, m²;

\bar{u}

is the reverse motion velocity of coal slag on the e-f plane relative to the d-d coordinate system, m/s; and

\bar{n}

is the reverse normal vector of coal slag relative to the e-f plane, m/s.

By integrating (8), the following equation can be obtained

ρ_{c} \int_{s} \bar{u} \bar{n} d s = ρ_{c} S_{f f} u

(9)

Using the d-d cross-section as the coordinate system, the momentum conservation equation for the pore-forming zone can be established as follows:

\frac{d}{d t} [M_{f f} (u_{f f} - u)] = P S_{e e} + R_{t} (S_{f f} + S_{e f} \cos θ) - ρ_{c} S_{f f} u^{2} - ρ_{c} (u_{d e} - u) \int_{S} \bar{u} \bar{n} d s - M_{f f} \frac{d u}{d t}

(10)

where u_ff is the average velocity of the coal slag in the pore-forming zone, m/s; P is the water jet pressure, MPa; S_ee is the sectional area of the e-e section, m²; and R_t is the critical strength of the coal body, MPa.

Due to the vector sum of momentum being 0, it can be concluded that

R_{t} (S_{f f} + S_{e f} c o s θ) = R_{t} S_{e e}

(11)

By incorporating both Equations (9) and (10) into Equation (11), it can be concluded that

\begin{matrix} P = R_{t} + α ρ_{c} u_{d e} u \\ α = S_{f f} / S_{e e} \end{matrix}\}

(12)

In Equation (12), α is the damage diffusion coefficient, i.e., the expansion coefficient; u_de = 0.5u. Thus, the following can be obtained:

P = ρ_{w} \frac{{(v - u)}^{2}}{β} = R_{t} + \frac{1}{2} α ρ_{c} u^{2}

(13)

When the jet flow rate is large enough, and according to Equation (13), the following can be obtained:

\begin{matrix} μ = \frac{v - ε \sqrt{v^{2} + \frac{2 R_{t}}{α ρ_{c}} (1 - ε^{2})}}{1 - ε^{2}} \\ ε = \sqrt{\frac{α β ρ_{c}}{2 ρ_{w}}} \end{matrix}\}

(14)

where ε is the strain of the coal unit, the ratio of stress on the f-f section σ to the elastic modulus of the coal body E.

Based on the hypothesis that the coal matrix belongs to brittle materials and the analysis and calculation of the strength characteristic parameters of brittle target plate materials by Wei et al. [26], it can be summarized that

R_{t} = Y {[\frac{\frac{E}{3 Y}}{[1 - \sqrt{\frac{σ_{t}}{Y} \cdot \frac{1 - λ}{\sqrt{2}}}]}]}^{\frac{2 η}{3}}

(15)

where Y is the compressive strength of the coal body, MPa; σ_t is the tensile strength of the coal body, MPa; λ is Poisson’s ratio; η is the calculation coefficient, η = 6k/(3 + 4k); k is the compression shear coefficient; and the magnitude of the compression shear coefficient depends on the shear stress and compressive strength Y. The numerical relationship is as follows:

β^{'} = \frac{Y - σ_{θ}}{2 Y} = \frac{Y - (Y \tan θ + C)}{2 Y}, k = \frac{3 β^{'}}{3 - 4 β^{'}}

(16)

2.1.5. Mechanical Parameters Analysis of Coal Damage by High-Pressure Water Jet

Based on the above analysis, the following mechanical equation system for coal damage by high-pressure water jet can be established:

\begin{matrix} \frac{d L}{d t} = - (v - u) \\ \frac{d H}{d t} = u (v) \\ \frac{d v}{d t} = \frac{{(v - u)}^{2}}{L} (1 - γ), f (v) = (γ - 1) {(v - u)}^{2} \\ u = \frac{v - ε \sqrt{v^{2} + \frac{2 R_{t}}{α ρ_{c}} (1 - ε^{2})}}{1 - ε^{2}}, ε = \sqrt{\frac{α β ρ_{c}}{2 ρ_{w}}} \\ t = 0, v (0) = v_{0}, L (0) = L_{0}, H (0) = 0 \end{matrix})

(17)

According to Equation (7), the movement speed of the impact reference interface d-d surface u is greater than 0, the water pressure P of the water jet is greater than the critical crushing strength R_t of the coal body, and the high-pressure water jet can damage the coal body. When u = 0, the water jet stops breaking the coal, and the water pressure P ≤ R_t (critical crushing strength of the coal body). Therefore, the threshold velocity of the pressure of the high-pressure water jet at μ = 0 can be calculated as follows:

v = \sqrt{\frac{β R_{t}}{ρ_{w}}} = v_{c}

(18)

By combining Equations (17) and (18), it can be concluded that the coal damage process of the high-pressure water jet can be divided into two situations:

(1): When v > v_c, the damage and failure depth H of the coal wall under high-pressure water jet is

H = - \int_{v_{0}}^{v_{c}} \frac{u (v) L (v)}{f (v)} d v

(19)

(2): When v ≤ v_C, the remaining length of the water jet is

L = L_{0} e x p \{\int_{v_{0}}^{v} \frac{v}{f (v)} d v\}

(20)

The detailed parameters of the Changping Mine are shown in Table 1.

According to Table 1, the shear stress of the 3# coal seam τ is 1.49 MPa. Combined with Equation (15), the calculation coefficient η is 0.44. Substituting the parameters in Table 1 into Equations (16) and (18), the theoretical threshold velocity v_c of the high-pressure water jet can be obtained as 16 m/s. The threshold pressure R_t of the high-pressure water jet is 14.19 MPa.

2.2. Study Site and Equipment

The Changping coalfield is located in the southeast of Shanxi province, China (Figure 2). The annual coal production is 5 million tons. The major mining seam is 3# coal seam, with an average thickness of 5.67 m and an average dip angle of 3°, respectively. A fully mechanized top coal caving method is used for mining. The original gas content in the mining area is between 3.5 and 15.09 m³/t. Table 2 lists the coal and gas characteristics of 3# coal seam, which is obviously characterized by high gas content and low permeability.

During the field test, the equipment related to hydraulic punching and reaming mainly included high-pressure water pumps, high-pressure hoses, drilling bits for punching and expanding, drilling rigs for connecting drilling bits, devices for preventing spraying, and sedimentation tanks. The specific system connection diagram is shown in Figure 3.

The operational process of hydraulic punching includes drilling, nozzle installation, water supply pressurization, coal body impacting, slag discharging, continuous punching, and borehole sealing. The operational process of hydraulic reaming includes drilling, nozzle installation, water supply pressurization, rotary reaming, slag discharging, and borehole sealing. The primary differences between the two techniques are the type of nozzle utilized and the mechanism by which the water jet induces coal body fragmentation.

The high-pressure hoses used in hydraulic expansion systems are mainly composed of rubber pipes, nylon pipes, resin pipes, etc., and they are reinforced by wire weaving, winding, or fiber weaving. The local resistance of the pipeline system is calculated as 5% of the resistance loss along the pipeline, resulting in a total pipeline resistance loss of 0.95 MPa per 100 m. According to Section 2.1, the threshold R_t is 14.2 MPa. After considering pipeline losses (calculated based on a total pipeline length of 100 m), the theoretical value of coal-breaking water pressure is 15.2 MPa, and the speed threshold of coal-breaking water pressure is synchronously corrected to 16.5 m/s.

2.3. Hydraulic Punching and Reaming Technologies

Hydraulic punching utilizes high-pressure water jets to directly destroy the coal structure and flush out the coal. Thus, more cavities are created in the coal seam, which greatly changes the stress distribution around the borehole and significantly increases the permeability of the coal seam. Hydraulic reaming uses high-pressure water jets to destroy the coal body around the borehole through a specially designed drilling bit or nozzle. Under the action of impact and the shear force of the water jet, the coal body peels off rocks, thereby increasing the diameter of the borehole. Both of these techniques have certain effects on enhancing permeability in soft coal seams. Hydraulic reaming mainly focuses on increasing the borehole diameter by peeling off the coal body around the borehole, while hydraulic punching aims to directly break the coal structure and create cavities in the coal seam. According to previous numerical simulation [28], hydraulic punching can cause a large plastic area and pressure relief area, as shown in Figure 4. During the 3-month extraction period, the amount of coal from punching out is 0.5 t/m, and the effective radius is 5.6 m; the amount of coal from punching out is 0.7 t/m, and the effective radius is 6.4 m; the amount of coal from punching out is 1.0 t/m, and the effective radius is 7.1 m; and the amount of coal from punching out is 1.2 t/m, and the effective radius is 7.4 m.

3. Results

3.1. Analysis of Coal Seam Permeability Enhancement Effect by Hydraulic Reaming

3.1.1. The Variation of Gas Parameters and Influence Radius

The 5302 bottom extraction roadway with hydraulic reaming is located in the floor strata of the 3# seam. The roof is mainly composed of sandy mudstone with a thickness of 9.31 m, and the floor is composed of fine-grained sandstone with a thickness of 1.87 m. The distance between the 5302 bottom extraction roadway and the 3 # coal seam is not less than 6 m. The net height of the roadway is 2.9 m, the net width of the roadway is 4.4 m, and the net section is 12.76 m².

To investigate the influence radius of hydraulic reaming, field tests were conducted on the 6th group 3# boreholes of extraction Unit 1 and the 6th group 2# boreholes of extraction Unit 2. Each inspection hole is equipped with a single hole inspection measuring device, and the flow method was used to comprehensively investigate the influence radius, as shown in the green borehole in Figure 5.

The coal section of the 1-6-3 borehole is 13 m long, with a water pressure of 4–8 MPa and an average water pressure of 6 MPa. The borehole expansion time per meter is 0.5 h, and the total amount of washed-out coal from the coal section is 3.5 t. The borehole diameter is 113 mm. The variation of pure gas flow rate before and after hydraulic reaming is recorded every 2 h, as shown in Figure 6. The basic parameters of the 2-6-2 borehole are basically the same as those of the 1-6-3 borehole. The variation of pure gas flow rate before and after hydraulic reaming is shown in Figure 7.

From Figure 6, during the initial stage of hydraulic reaming measures, the pure gas flow rate of the 1-7-3 hole is at a high level. Compared with the 1-7-3 borehole, the pure gas flow rate of the 1-6-4 borehole (with a normal distance of 8.9 m) has decreased to a certain extent, with an average pure gas flow rate of 0.074 m³/min. But the average pure gas flow rate of the 1-8-3 borehole (with a normal distance of 10 m) is 0.063 m³/min. That is, the changes of pure gas flow rate of the 1-7-3 and 1-6-4 boreholes are more significant, while that of the 1-6-4 borehole has undergone a process of pure gas increase and attenuation, which are 8.9 m apart from the measure holes. Therefore, the effective radius of hydraulic reaming should be around 4–5 m, and the influence radius should be around 5–8 m. After a period of hydraulic reaming, the net pure gas flow rate began to decrease, indicating that the disturbance caused by hydraulic reaming was gradually eliminated, and the cracks closed again. Therefore, the effective radius of hydraulic reaming should be around 4.5 m, with an influence radius of around 7.5 m. The field test results of hydraulic reaming shown in Figure 7 are similar to those in Figure 6. There is a significant change in the pure gas flow rate of the 2-7-2 and 2-6-3 boreholes, but there is little change in the 2-8-2 borehole, with an interval of 10 m from the measurement holes. After 40 h of hydraulic reaming, the pure gas flow rate of the 2-6-3 borehole experienced an attenuation process. Therefore, it can be considered that the effective radius of hydraulic reaming should be around 4.5 m, with an influence radius of around 7.5 m.

3.1.2. Analysis of the Main Technical Parameters of Hydraulic Reaming

The relationship between the water pressure of the expansion hole and the amount of washed-out coal was studied, as shown in Figure 8. The results show a significant positive correlation between the amount of washed-out coal and water pressure, with the water pressure increasing from 3–6 MPa. However, due to the structural characteristics of the expanding drill bit, it is determined that the water pressure cannot continue to rise, resulting in extremely limited changes in the amount of washed-out coal.

From Figure 9, it can be seen that the amount of washed-out coal increases logarithmically with reaming time in first 200 min. Subsequently, the increasing trend gradually becomes gentle, and the amplification of the amount of washed-out coal decreases with reaming time. From the perspective of coal-breaking, hydraulic reaming relies on high-pressure water like rigid umbrella wings to directly destroy the coal body, achieving crushing of the coal body, and then inducing severe disturbance in the deep part of the coal body, generating cracks. Under the action of high-pressure water, the cracks further expand, achieving damage to the coal body. However, with the continuous progress of the hydraulic process, the range of high-pressure water disturbance around the borehole gradually decreases, resulting in a smaller amount of washed-out coal with time.

3.2. Analysis of Coal Seam Permeability Enhancement Effect by Hydraulic Punching

3.2.1. The Variation of Gas Parameters and Influence Radius

The west wing gangue discharge machine roadway implementing hydraulic punching measures is located between K7 and K6, with a direct top of K7 siltstone and a thickness of 1.25 m. The old roof is made of sandy mudstone with a thickness of 5.8 m. The west wing gangue discharge machine roadway is excavated from the north section lane 2 of the bottom extraction 5302 towards the west, with a cross-section of 5.7 m (width) × 3.3 m (height). The hydraulic punching test section was arranged in the second and third extraction units of the west wing gangue discharge machine roadway. The test section is designed as a 100 m unit. Starting from the west side of lane 2 at the 5302 bottom extraction roadway, the second extraction unit is located at a distance of 0–100 m. In sequence, one extraction unit is located every 100 m. A total of 20 hydraulic punching boreholes were conducted in each unit, with 10 boreholes for each group of two sides, with a spacing of 5 m. In order to study the influence radius of hydraulic punching, the gas extraction improvement effect of supplementary punching measure holes in Unit 2 of the 14th group in west wing gangue discharge machine roadway was analyzed. The cross-sectional diagram of the borehole layout is shown in Figure 10, and the green borehole is conducted by hydraulic punching.

The 12-4 (12 group 4# borehole, with a distance of 7.5 m), 13-2 (with a distance of 12 m), 13-5 (with a distance of 6.7 m), and 15-5 (with a distance of 6.7 m) boreholes in Units 2 and 3 were selected as the inspection boreholes to analyze the gas extraction improvement effect by hydraulic punching. The variation of the pure gas flow rate of each borehole in Unit 2 is shown in Figure 11.

From Figure 11, the pure gas flow rate is changed by hydraulic punching. Specifically, the average pure gas flow rate of the 12-4 borehole is 0.177 m³/min; the maximum pure gas flow rate of the 13-5 inspection borehole increased to 0.389 m³/min, with an average of 0.182 m³/min; and the pure gas flow rate of the 15-5 inspection borehole is 0.094 m³/min. However, the 13-5 borehole may be not evenly affected by the hydraulic punching measure, and the average pure gas flow rate is at a lower level of 0.038 m³/min. The change trend of the pure gas flow rate of the 13-5 and 12-4 boreholes proved that the effective radius of hydraulic punching measures should be around 6.5 m, while the influence radius was between 7–9 m.

The variation of pure gas flow rate of each borehole in Unit 3 is shown in Figure 12, and the same method is used to analyze the gas extraction effect by hydraulic punching in Unit 3. The average pure gas flow rate of the 12-3 borehole is 0.1162 m³/min; the maximum pure gas flow rate of the 13-5 inspection borehole is 0.289 m³/min; and the average pure gas flow rate of the 12-5 inspection borehole is 0.106 m³/min. However, the 12-2 borehole may not be evenly affected by the hydraulic punching measure, and the average pure gas flow rate is at a lower level of 0.037 m³/min. The change trend of the pure gas flow rate of the 14-5 and 12-3 boreholes can also prove that the effective radius of hydraulic punching measures should be around 6.5 m, while the influence radius was 7–9 m.

3.2.2. Analysis of the Main Technical Parameters of Hydraulic Punching

The relationship between the amount of washed-out coal and the water pressure and the pressure of the punching were studied, as shown in Figure 13. It can be seen that the amount of washed-out coal and the water pressure and the pressure of the punching exhibit a linear growth relationship.

In addition, the measured borehole in Unit 3 is selected to investigate the impact of the punching time on the amount of washed-out coal. Under a stable pressure of 18 MPa and a water flow rate of 150 L/min, the amount of washed-out coal showed a pattern of rapid increase and then slowed down with punching time, as shown in Figure 14. The results indicate that there is a logarithmic relationship between the amount of washed-out coal and punching time.

3.3. Effect of Coal Seam Permeability Enhancement by Different Hydraulic Measures

3.3.1. Comparison of Gas Extraction Concentration

The gas concentration of extraction boreholes with the same spacing to the measured holes is collected to study gas extraction efficiency by hydraulic reaming and punching, as shown in Figure 15.

It can be seen that both hydraulic measures have significant effects on improving coal seam permeability. After being treated by hydraulic reaming and punching, the corresponding average gas extraction concentrations are 18.3% and 25.3%, respectively, and the gas extraction concentration after hydraulic punching is at a higher level than that after hydraulic reaming.

3.3.2. Comparison of the Amount of Washed-Out Coal

The relationship between the amount of washed-out coal and time by the two hydraulic measures is shown in Figure 16.

According to Figure 15, the amount of washed-out coal by two hydraulic measures shows a logarithmic relationship with time. Overall, the amount of washed-out coal achieved by hydraulic punching is higher than that by hydraulic reaming, and the cumulative amount of washed-out coal in 210 min is 1.40 t and 0.71 t, respectively.

3.3.3. Comprehensive Comparison of Coal Seam Permeability Enhancement Effect

Three indicators—gas extraction concentration, gas extraction purity, and the amount of washed-out coal after the hydraulic measures—were selected to compare the coal seam permeability enhancement effect of the two hydraulic measures, as shown in Table 3. From several gas extraction indicators, hydraulic punching has a better coal seam permeability enhancement effect than hydraulic reaming. Our field test results are consistent with the numerical study results. Hydraulic punching can pose a good pressure relief effect on the coal seam in the axial direction of drilling boreholes [30]. Thus, hydraulic punching may be more suitable for enhancing permeability in soft coal seams.

4. Conclusions

(1): A simplified mechanical model of coal damage by a high-pressure water jet was established, and a mathematical calculation model for the damage strength and jet impact pressure and velocity was established. Based on the on-site engineering parameters, the critical theoretical pressure threshold and theoretical velocity for a high-pressure jet to damage the coal body were calculated, i.e., 16.5 m/s and 15.2 MPa, respectively.
(2): For hydraulic reaming, the amount of washed-out coal per meter linearly increases with the increase of water pressure and logarithmically increases with reaming time. The effective radius of hydraulic reaming is about 4.5 m, and the corresponding influence radius is approximately 7.5 m.
(3): For hydraulic punching, the amount of washed-out coal linearly increases with the water pressure and flushing flow rate and logarithmically increases with punching time. The effective radius of hydraulic punching is about 6.5 m, and the influence radius is approximately 7–9 m.
(4): Both hydraulic reaming and hydraulic punching measures can achieve permeability enhancement effects on soft coal seams. Under the same conditions, it can be found that hydraulic punching has a more significant effect on improving gas concentration and gas flow rate.

Author Contributions

Methodology, H.Z.; Writing—original draft, X.C.; Writing—review & editing, Y.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (52474276); National Natural Science Foundation of Chongqing, China (CSTB2024NSCQ-MSX0384).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank all the reviewers and editors for their hard work.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

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Figure 1. Mechanical model of coal damage caused by high-pressure water jet [25].

Figure 2. Study site.

Figure 3. Schematic diagram of hydraulic punching measure system modified from [27].

Figure 4. Coal seam pressure relief effect by hydraulic punching [29].

Figure 5. Borehole layouts of hydraulic reaming.

Figure 6. Change trend of pure gas flow rate in Unit 1, 3# hydraulic reaming borehole.

Figure 7. Change trend of pure gas flow rate in Unit 2, 2# hydraulic reaming borehole.

Figure 8. The relationship between the amount of washed-out coal and water pressure.

Figure 9. The relationship between the amount of washed-out coal and reaming time with a water pressure of 8 MPa.

Figure 10. Borehole layouts of hydraulic punching.

Figure 11. Change trend of pure gas flow rate of each inspection borehole in Unit 2.

Figure 12. Change trend of pure gas flow rate of each inspection borehole in Unit 3.

Figure 13. The relationship between the amount of washed-out coal and the water flow rate and punching pressure.

Figure 14. The relationship between the amount of washed-out coal and punching time.

Figure 15. The gas concentration of extraction boreholes with the same spacing to the measured holes by hydraulic reaming and punching.

Figure 16. Comparison of the amount of washed-out coal and time by hydraulic reaming and punching.

Table 1. Physical and mechanical parameters of 3# coal in Changping Mine.

Mechanical Parameters of 3# Coal Seam		In-Situ Stress Around 3# Coal Seam
Tensile strength/MPa	0.29	Burial depth/m	485
Internal friction angle/°	25.1	Vertical stress σ_v/MPa	18.57
Elastic modulus/MPa	2360	Maximum horizontal principal stress σ_H/MPa	14.97
Cohesion force/MPa	0.25	Minimum horizontal principal stress σ_H/MPa	7.94
Poisson’s ratio	0.24

Table 2. Coal and gas characteristics of 3# coal seam in Changping Mine.

Gas Content (m³/t)	Gas Pressure (MPa)	Coal Solidity Coefficient f	Initial Speed of Methane Emission ∆P (mL/s)	Permeability Coefficient of Coal Seam (mD)
3.5–15.09	0.38–0.55	0.44–0.56	14.3–20.6	0.0116–0.0520

Table 3. Comprehensive comparison of coal seam permeability enhancement effect by hydraulic measures.

Hydraulic Measures	Gas Extraction Concentration/%	Gas Extraction Volume/(m³/min)	The Amount of Washed-Out Coal/t	Effective Radius/m	Influence Radius/m
Hydraulic reaming	18.3	0.089	0.71	4.5	7.5
Hydraulic punching	25.3	0.181	1.4	6.5	7~9

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Huang, Y.; Cheng, X.; Zhang, H. Comparison of Hydraulic Measures for Improving Coal Seam Permeability: A Case Study. Processes 2025, 13, 626. https://doi.org/10.3390/pr13030626

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Huang Y, Cheng X, Zhang H. Comparison of Hydraulic Measures for Improving Coal Seam Permeability: A Case Study. Processes. 2025; 13(3):626. https://doi.org/10.3390/pr13030626

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Huang, Yuxi, Xiaoyang Cheng, and Huan Zhang. 2025. "Comparison of Hydraulic Measures for Improving Coal Seam Permeability: A Case Study" Processes 13, no. 3: 626. https://doi.org/10.3390/pr13030626

APA Style

Huang, Y., Cheng, X., & Zhang, H. (2025). Comparison of Hydraulic Measures for Improving Coal Seam Permeability: A Case Study. Processes, 13(3), 626. https://doi.org/10.3390/pr13030626

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