Study on Air Injection to Enhance Coalbed Gas Extraction

Abstract

Gas extraction is an important means to reduce coalbed gas and ensure safe coal production. Injecting N₂/CO₂ into a coalbed can enhance coal seam gas extraction, but problems with N₂/CO₂ sources underground have prevented the wide application of this technology in coal mines. The air contains a large amount of N₂, but only a few studies have investigated the injection of air into coalbeds to facilitate gas extraction. In this study, a thermal–hydraulic–solid coupling model for air-enhanced coalbed gas extraction (Air-ECGE) was established. Additionally, the impact of air injection on coalbed methane extraction was simulated, and field experiments were conducted on air injection to enhance gas extraction. The results showed that injecting high-pressure air into a coalbed can effectively facilitate gas desorption and gas migration within the coalbed, greatly improving the efficiency of gas extraction in the coalbed. In addition, owing to the large pressure gradient that can lead to fast coalbed gas seepage, the gas production rate of the extraction borehole is directly proportional to the gas injection pressure. Further, the spacing of the boreholes limits the influence range of the gas injection: the larger the spacing, the larger the influence range, and the higher the gas extraction rate of the extraction borehole. After injecting air into the coalbed of the Liuzhuang coal mine, the extraction flow rate and concentration of gas from the extraction boreholes both increased significantly. A certain delay effect was also observed in the gas injection effect, and the gas extraction flow rate only decreased after a period of time after the gas injection had stopped.

1. Introduction

Coal is the primary source of energy consumption in China, accounting for 64% of the total energy consumed in the country in 2021 [1,2,3]. However, coal is not a green energy source, and coal mine gas disasters are common, such as coal–gas outbursts and gas explosions, which cause human casualties and equipment damage [4]. To prevent such gas disasters in coal mines, the coalbed gas extraction method is often used. Currently, many methods for gas extraction in coalbeds have been developed, including hydraulic fracturing, liquid nitrogen fracturing, and ultrasonic pulse [5,6,7,8,9,10]. The injection of an external gas (N₂, CO₂, or N₂/CO₂ mixture) into a coalbed can improve the gas extraction efficiency by inducing gas displacement and sweeping effects [10,11]. This gas injection technique has become a research hotspot because it is highly effective.

CO₂-enhanced coalbed methane (CO₂-ECBM) recovery has been widely studied through experiments, numerical simulations, and field practice. As early as 1956, Walker and Geller [12] conducted some simple experiments on the CO₂ adsorption of anthracite. White et al. [13], Bai et al. [14], and Zheng et al. [15] conducted a series of laboratory experiments to study the effect of CO₂-ECBM, and they analyzed the gas flow characteristics. Wu et al. [11], Fan et al. [16], and Perera et al. [17] also simulated the CO₂-ECBM process and analyzed the influence of CO₂ injection parameters on methane production and coalbed permeability. Researchers in countries like the USA [18], the Netherlands [19], India [20], and China (South Qin-shui, Shanxi) [20,21,22,23] have carried out CO₂-ECBM field engineering tests and reported good results.

Injecting CO₂ into a coalbed will result in the adsorption and expansion of the coal skeleton, thus reducing the permeability of the coal and increasing the difficulty of CO₂ injection [23]. However, injecting N₂ into a coalbed increases the coalbed permeability [24]. The injection of the CO₂/N₂ mixed gas into a coalbed prevents and maintains the permeability on the one hand; on the other hand, it increases the displacement and production of the coalbed gas through the competitive adsorption of CO₂ and CH₄ [25,26]. Fan et al. [16] established a thermo-hydro-mechanical (THM) coupling numerical model for enhanced coalbed methane (CBM) recovery by injecting a gas mixture of N₂/CO₂, and the research showed that injecting a gas mixture into the coalbed was more effective from the perspective of the cumulative production of CH₄. Zhou et al. [27] analyzed how the permeability of a coalbed changes after the injection of the gas mixture, and they revealed the influence of different gas injection ratios on coalbed permeability. Fan et al. [28] used numerical simulation to investigate the optimal injection timing and ratio of a gas mixture, and they analyzed the importance of gas injection timing.

Compared with CH₄ and CO₂, N₂ has a smaller adsorption coal capacity [29,30,31]. Wang et al. [32] performed ECBM replacement experiments and found that the breakthrough time of N₂ injection is greater than that of CO₂. In the field of surface-well coal bed methane extraction, N₂ mixtures attract high gas treatment costs [33]. For instance, specialized equipment is required to separate the N₂ from the product gas stream (a mixture of N₂ and CH₄) [34]. Therefore, N₂-ECBM technology is rarely used in the field of surface well CBM extraction. Underground gas drainage is primarily performed in a coal mine to reduce the gas content in the coalbed and ensure safe coal production [35,36,37]. Lin et al. [38] used the triaxial permeability test system to conduct underground N₂-ECBM tests, revealing that nitrogen injection can effectively improve the permeability and methane drainage efficiency of a coalbed. The experimental study showed that cyclic N₂ injection can improve the gas recovery of a coalbed with a highly reduced N₂ consumption [39]. Chen et al. [40] optimized the spacing between the N₂ injection hole and the extraction hole through numerical simulations. Yang et al. [41] conducted a field test of N₂-ECGD in a coal mine in Hejin City, Shanxi Province, which showed that N₂ injection can greatly improve the CBM recovery.

The aforementioned studies predominantly focused on the ground well extraction of coalbed methane, primarily exploring methods to enhance production from coalbed methane wells. In contrast, this study is based on coal mine gas control, aiming not only to improve the efficiency of coal seam gas drainage but also to reduce the occurrence of mine accidents. Currently, most coalbeds being mined in China have a low permeability, which limits the coal mining efficiency because of the suboptimal efficiency of the coalbed gas drainage. Since 2000, gas accidents have accounted for 75.34% of major coal mine accidents in China, making gas control in mines an essential measure to ensure coal production safety.

N₂/CO₂-ECBM technology holds significant potential for low-permeability mines; however, its widespread application has been hindered by its high production costs and complex processes, and CO₂ injection may induce coal and CO₂ outbursts during mining operations. Given that N₂ accounts for approximately 78% of air, it has been speculated that air injection could significantly simplify the ECBM engineering process and reduce costs.

Based on the above analysis, while existing studies have explored the role of ECBM technology in enhancing coalbed methane extraction, they have not comprehensively examined its impact on mine safety [42,43]. Particularly, research remains scarce regarding reducing methane concentration to prevent mine accidents and ensure the safety of miners’ lives. This study aims to bridge this gap by demonstrating that Air-ECGE technology not only increases coalbed methane production but also significantly contributes to creating a safer working environment in mines.

2. Thermal-Hydraulic-Solid Coupling Model for Air-ECGE

Air injection into coalbeds to improve the coalbed gas extraction process includes multi-physical field coupling effects such as the non-isothermal competitive adsorption of multi-component gas, seepage of multi-component gas, coal deformation, temperature exchange, etc. To establish a coupling model to describe this process, the following assumptions are made:

(1)

The coal is homogeneous and isotropic, and the gas in the coalbed is evenly distributed;

(2)

The constitutive relation of stress and strain satisfies elastic mechanics; the compressive state of stress and strain is negative, while the tensile state is positive;

(3)

The gas in the coalbed is ideal gas, and the influence of temperature change on gas dynamic viscosity is ignored;

(4)

Regardless of the chemical adsorption of oxygen on coal, the adsorption of a gas on coal meets the Langmuir adsorption equilibrium equation.

2.1. Gas Migration Control Equation

The air is injected into the coalbed containing CH₄ and diffuses to each position of the coalbed under a pressure gradient. This phenomenon can be described by the following equation:

$${\displaystyle \frac{\partial m_{\mathrm{n}}}{\partial t}}+\nabla \cdot \left({\rho }_{\mathrm{g}\mathrm{n}}{\overrightarrow{q}}_{\mathrm{g}\mathrm{n}}\right)+\nabla \cdot \left(-D_{\mathrm{n}}\cdot \nabla \varphi C_{\mathrm{n}}\right)=0$$

where, the subscript n represents different components of the gas; ${\overrightarrow{q}}_{\mathrm{g}\mathrm{n}}=-kRT\cdot \nabla C_{\mathrm{n}}/{\mu }_{\mathrm{n}}$, ρ_gn = C_n·M_n; ${\overrightarrow{q}}_{\mathrm{g}\mathrm{n}}$ is the velocity vector of the gas seepage, m/s; m_n is the mass of each component gas contained in a unit volume, kg/m³; ρ_gn is the gas density, kg/m³; M_n is the molar mass of gas, kg/mol; D_n is the hydrodynamic dispersion coefficient; Φ is the coal porosity of; C_n is the gas concentration, mol/m³.

When air is injected into the coalbed, O₂, N₂, and CH₄ appear simultaneously in the coalbed. Considering the influence of temperature on gas adsorption, the adsorption capacity of a multi-component gas on coal can be described by the generalized Langmuir equation:

$$V_{\mathrm{s}\mathrm{n}}={\displaystyle \frac{V_{\mathrm{L}\mathrm{n}}{C}_{\mathrm{n}}{b}_{\mathrm{n}}RT}{1+{\displaystyle \sum _{\mathrm{n}=1}^3{C}_{\mathrm{n}}{b}_{\mathrm{n}}RT}}}$$

where, $V_{\mathrm{L}\mathrm{n}}=V_{\mathrm{L}\mathrm{n}0}\exp \left[-{\displaystyle \frac{d_2}{1+d_1{C}_{\mathrm{n}}RT}}\left(T-T_t\right)\right]$, $P_{\mathrm{L}\mathrm{n}}=P_{\mathrm{L}\mathrm{n}0}\exp \left[{\displaystyle \frac{C_{\mathrm{p}\mathrm{n}}}{R}}\left({\displaystyle \frac{1}{T}}-{\displaystyle \frac{1}{T_t}}\right)\right]$, $b_{\mathrm{n}}={\displaystyle \frac{1}{P_{\mathrm{L}\mathrm{n}0}}}$; V_Ln, and P_Ln are the modified Langmuir volume constant, m³/kg, and the pressure constant, Pa, respectively; d₁ and d₂ are the pressure correction factor, Pa⁻¹, and the temperature correction factor, K⁻¹, respectively; V_Ln0 is the initial Langmuir volume constant, m³/kg; P_Ln0 is the initial Langmuir pressure constant, Pa; T is the coalbed temperature, K; T_t is the laboratory reference temperature, K; C_pn is the specific heat capacity of the gas at a constant pressure.

The mass of the gas contained in the unit volume of coal can be defined as:

$$m_{\mathrm{n}}=\varphi C_{\mathrm{n}}{M}_{\mathrm{n}}+M_{\mathrm{n}}{\rho }_c{\rho }_{\mathrm{g}\mathrm{a}\mathrm{n}}{\displaystyle \frac{V_{\mathrm{L}\mathrm{n}}{C}_{\mathrm{n}}{b}_{\mathrm{n}}RT}{1+{\displaystyle \sum _{\mathrm{n}=1}^3{C}_{\mathrm{n}}{b}_{\mathrm{n}}RT}}}$$

where, ρ_Gan is the density of an n-component gas under standard conditions, kg/m³.

$${\displaystyle \frac{\partial }{\partial t}}\left(C_{\mathrm{n}}{M}_{\mathrm{g}\mathrm{n}}\varphi +(1-\varphi ){\rho }_{\mathrm{g}\mathrm{a}\mathrm{n}}{\rho }_{\mathrm{s}}{\displaystyle \frac{V_{\mathrm{L}\mathrm{n}}{C}_{\mathrm{n}}{b}_{\mathrm{n}}RT}{1+{\displaystyle \sum _{\mathrm{n}=1}^3{C}_{\mathrm{n}}{b}_{\mathrm{n}}RT}}}\right)+\nabla \cdot \left(-C_{\mathrm{n}}{M}_{\mathrm{g}\mathrm{n}}\cdot {\displaystyle \frac{kRT}{{\mu }_{\mathrm{n}}}}\nabla C_{\mathrm{n}}\right)+\nabla \cdot \left(-D_{\mathrm{n}}\cdot \nabla \varphi C_{\mathrm{n}}\right)=Q_{\mathrm{s}\mathrm{n}}$$

2.2. Stress Control Equation of the Coal Seam

The force balance of a gas-bearing coal body satisfies the Navier equation:

$${\sigma }_{ij,j}+F_i=0$$

where, σ_ij is the component of total force applied to a unit coal, Pa; F_i is the external force in all directions in the space, Pa.

According to the Cauchy equation, the geometric relationship between the stress and displacement components of a coal body in all directions of space is expressed as follows:

$${\epsilon }_{ij}={\displaystyle \frac{1}{2}}\left(u_{i,j}+u_{j,i}\right)$$

where, ε and u represent the strain and displacement of coal in space, respectively, m.

The total strain of coal is affected by temperature, gas pressure, and gas adsorption, and it can be expressed as:

$${\mathsf{\epsilon }}_{ij}={\displaystyle \frac{1}{2G}}{\mathsf{\sigma }}_{ij}-\left({\displaystyle \frac{1}{6G}}-{\displaystyle \frac{1}{9K}}\right){\mathsf{\sigma }}_{\mathrm{k}}{\mathsf{\delta }}_{ij}-{\displaystyle \frac{\mathsf{\alpha }\Delta {\displaystyle \sum _{\mathrm{n}=1}^3{C}_{\mathrm{n}}}RT}{3K_{\mathrm{s}}}}{\mathsf{\delta }}_{ij}+{\displaystyle \frac{{\mathsf{\alpha }}_{\mathrm{T}}}{3}}\left(T-T_0\right){\mathsf{\delta }}_{ij}+{\displaystyle \frac{1}{3}}{\displaystyle \sum _{\mathrm{n}=1}^3{\mathsf{\epsilon }}_{\mathrm{s}\mathrm{n}}}{\mathsf{\delta }}_{ij}$$

where G = E/(2 + 2v), K = E/3(1 − 2v), K_s = E_s/3(1 − 2v), α = 1 − K/K_s, ε_s1 = α_sg1V_sg1, ε_s2 = α_sg2V_sg2, ε_s3 = α_sg3V_sg3, σ_k = (σ_x + σ_y + σ_z)/3. G is the shear modulus of coal, Pa; K is the bulk modulus of coal, Pa; E is the Young’s modulus of the coal body, Pa; ν is the Poisson’s ratio of the coal body; K_s is the bulk modulus of the coal skeleton, Pa; α is the Biot effective stress coefficient of the coal body; α_T is the thermal expansion coefficient of the coal skeleton, 1/K; ΔT is the coalbed temperature change, K; Δε_s1 is the strain change caused by CH₄ adsorption on coal; α_sg1 is the adsorption strain coefficient of CH₄, kg/m³; Δε_s2 is the strain change caused by N₂ adsorption on coal; α_sg2 is the N₂ adsorption strain coefficient, kg/m³; Δε_s3 is the strain change caused by O₂ adsorption on coal; α_sg3 is the N₃ adsorption strain coefficient, kg/m³.

2.3. Control Equation of the Coalbed Temperature Field

Since there is no mutual conversion between mechanical energy and thermal energy in the coalbed, the temperature change of the coalbed for the entire gas injection displacement project is mainly caused by the heat exchange of the injected air and the exothermic or endothermic reaction caused by the adsorption/desorption of a gas on coal. The control equation of the coalbed temperature field can be obtained from the first law of thermodynamics:

$${\displaystyle \frac{\partial \left[{\left(\rho C_{\mathrm{p}}\right)}_{\mathrm{c}}T\right]}{\partial t}}+{\eta }_{\mathrm{c}}\nabla T+{\displaystyle \sum _{\mathrm{n}=1}^3{q}_{\mathrm{s}\mathrm{t}\mathrm{n}}\frac{{\rho }_{\mathrm{s}}{\rho }_{\mathrm{gan}}}{M_{\mathrm{gn}}}\frac{\partial V_{\mathrm{sgn}}}{\partial t}}+K{\alpha }_{\mathrm{T}}T{\displaystyle \frac{\partial {\displaystyle \sum _{\mathrm{n}=1}^3{\epsilon }_{\mathrm{sn}}}}{\partial t}}-\nabla \cdot \left({\lambda }_{\mathrm{c}}\nabla T\right)=0$$

In which,

$$\left\{\begin{array}{l}{\left(\mathsf{\rho }{C}_{\mathrm{p}}\right)}_{\mathrm{c}}=\left(1-\mathsf{\varphi }\right){\mathsf{\rho }}_{\mathrm{s}}{C}_{\mathrm{s}}+\mathsf{\varphi }\left(M_{\mathrm{g}1}{C}_1{C}_{\mathrm{v}1}+M_{\mathrm{g}2}{C}_2{C}_{\mathrm{v}2}+M_{\mathrm{g}3}{C}_3{C}_{\mathrm{v}3}\right)\hfill \\ {\mathsf{\eta }}_{\mathrm{c}}=-k\nabla C_{1}RT{\mathsf{\rho }}_{\mathrm{g}1}{C}_{\mathrm{v}1}/{\mathsf{\mu }}_1-k\nabla C_{2}RT{\mathsf{\rho }}_{\mathrm{g}2}{C}_{\mathrm{v}2}/{\mathsf{\mu }}_2-k\nabla C_{3}RT{\mathsf{\rho }}_{\mathrm{g}3}{C}_{\mathrm{v}3}/{\mathsf{\mu }}_3\hfill \\ {\mathsf{\lambda }}_{\mathrm{c}}=\left(1-\mathsf{\varphi }\right){\mathsf{\lambda }}_{\mathrm{s}}+\mathsf{\varphi }\left({\mathsf{\lambda }}_{\mathrm{g}1}+{\mathsf{\lambda }}_{\mathrm{g}2}+{\mathsf{\lambda }}_{\mathrm{g}3}\right)\hfill \end{array}\right.$$

where, (ρC_p)_c is the effective specific heat capacity of coal and the coalbed gas mixture, J/(m³·K); η is the convection coefficient, J/(m²·s); λ_c is the mixed thermal conductivity of coal and the coalbed gas, W/(m·K). C_s is the specific heat capacity of the coal skeleton, J/(m³·K); λ_s is the thermal conductivity of the coal skeleton, W/(m·K); q_st is the equal adsorption heat, J/mol; C_v1 is the constant specific heat capacity of CH₄, J/(m³·K); C_v2 is the constant specific heat capacity of N₂, J/(m³·K); C_v3 is the constant specific heat capacity of O₂, J/(m³·K); λ_g1 is the thermal conductivity of CH₄, W/(m·K); λ_g2 is the thermal conductivity of N₂, W/(m·K); λ_g3 is the thermal conductivity of O₂, W/(m·K).

In the Air-ECBM project, the coalbed porosity is influenced by gas pressure, gas adsorption/desorption, temperature change, and other factors. The porosity is expressed as:

$$\mathsf{\varphi }=1-{\displaystyle \frac{\left(1-{\mathsf{\varphi }}_0\right)}{1+e}}\left(1-{\displaystyle \frac{\mathsf{\alpha }}{K_{\mathrm{s}}}}\left(\Delta C_1+\Delta C_2+\Delta C_3\right)RT+\Delta {\mathsf{\epsilon }}_{\mathrm{s}1}+\Delta {\mathsf{\epsilon }}_{\mathrm{s}2}+\Delta {\mathsf{\epsilon }}_{\mathrm{s}3}+{\mathsf{\alpha }}_{\mathrm{T}}\Delta T\right)$$

where, Δε_s1, Δε_s2, and Δε_s3 represent the volume strain generated by the adsorption/desorption of CH₄, N₂, and O₂ on coal, respectively.

The relationship between permeability and porosity satisfies the cubic law. The permeability of coal can be expressed as:

$$k=k_0{\left\{{\displaystyle \frac{1}{{\mathsf{\varphi }}_0}}-{\displaystyle \frac{1-{\mathsf{\varphi }}_0}{{\mathsf{\varphi }}_0\left(1+e\right)}}\left(1-{\displaystyle \frac{\mathsf{\alpha }}{K_{\mathrm{s}}}}\left(\Delta C_1+\Delta C_2+\Delta C_3\right)RT+\Delta {\mathsf{\epsilon }}_{\mathrm{s}1}+\Delta {\mathsf{\epsilon }}_{\mathrm{s}2}+\Delta {\mathsf{\epsilon }}_{\mathrm{s}3}+{\mathsf{\alpha }}_{\mathrm{T}}\Delta T\right)\right\}}^3$$

where, k₀ is the initial permeability of coal (m²).

3. Numerical Simulation of Air-ECGE

3.1. Numerical Model and Solution Conditions

The Liuzhuang Coal Mine is located in the western part of the Huainan Coalfield, with a mining area of approximately 82.2114 km² and a production capacity of 11 million tons per year (Figure 1). The coalbed gas content in the mining area is relatively low, with an average gas content of 3.2 m³/t. Although the gas content of the mine is relatively small, due to the high mining intensity, the occurrence frequency of the gas concentration exceeding 0.6% in the upper corner of the working face during coal mining is seven times per month. This value makes it very difficult to control gas disasters in the mine. Owing to the low gas content of the mine, facilitating the coalbed gas extraction using the coalbed permeability enhancement method is not effective enough. The coalbed in Liuzhuang Mine is a spontaneous-combustion coalbed, and a short-term air injection will not cause spontaneous combustion of the coal body. Therefore, the efficiency of the coalbed gas extraction can be enhanced by injecting air into the coalbed.

The average thickness of coalbed 11-2 in Liuzhuang Mine is 4.3 m, and the spacing between the underground gas extraction boreholes is 4 m. In this study, the minimum gas extraction unit (two boreholes and their intermediate coal body) was selected for numerical simulation, with a model size of 4 × 4.3 m (Figure 2). The borehole wall was set as Dirichlet boundary conditions, with roller support conditions applied to the surrounding boundary. For the convenience of observing the simulation effect, a reference line A-B is drawn, and the coordinates of point A and point B are (0, 2.15) and (4, 2.15), respectively. The numerical simulation parameters are set according to the geological conditions of the coal and rock in Liuzhuang Mine, with initial gas pressure, temperature, and permeability of 0.45 MPa, 298.5 K, and 9 × 10⁻¹⁷ m², respectively. Other numerical simulation parameters are listed in

Table 1. Numerical simulation parameters.

Parameter	Value	Parameter	Value
Bulk modulus of coal (E)	2786 MPa	Bulk modulus of coal skeleton (Es)	8147 MPa
Poisson’s ratio of coal (v)	0.35	Coal skeleton density (ρs)	1375 kg/m3
Density of coal (ρ)	1289 kg/m3	Density of N2 at standard condition (ρga2)	1.25 kg/m3
Density of CH4 at standard condition (ρga1)	0.716 kg/m3	Initial porosity of coal (φ0)	0.032
Density of O2 at standard condition (ρga3)	1.429 kg/m3	Initial temperature of coalbed (T0)	308 K
Initial Langmuir volume constant of CH4 (VL1)	0.0223 m3/kg	Initial Langmuir pressure constant of CH4 (PL1)	1.85 MPa
Initial Langmuir volume constant of N4 (VL2)	0.0183 m3/kg	Initial Langmuir pressure constant of CH4 (PL2)	3.01 MPa
Initial Langmuir volume constant of O2 (VL3)	0.0401 m3/kg	Initial Langmuir pressure constant of O2 (PL3)	2.05 MP
Pressure correction factor (d1)	0.071 1/MPa	Temperature correction factor (d2)	0.021 1/K
Dispersion coefficient of CH4 (D1)	3.6 × 10−12 m2/s	Dispersion coefficient of N2 (D2)	2.8 × 10−12 m2/s
Dispersion coefficient of O2 (D3)	6.2 × 10−12 m2/s	Specific heat capacity of coal skeleton (Cs)	1280 J/(kg·K)
Constant specific heat capacity of CH4 (Cv1)	1624 J/(kg·K)	Constant specific heat capacity of N2 (Cv2)	741 J/(kg·K)
Constant specific heat capacity of O2 (Cv3)	918 J/(kg·K)	Thermal conductivity of coal skeleton (λs)	0.171 W/(m·K)
Thermal conductivity of CH4 (λg1)	0.031 W/(m·K)	The thermal conductivity of N2 (λg2)	0.025 W/(m·K)
Thermal conductivity of O2 (λg3)	0.024 W/(m·K)

.

3.2. The Influence of Air Injection into the Coalbed for Coalbed Gas Extraction

Figure 3 presents the variations in CH₄ production rate and cumulative CH₄ production under three different conditions: no air injection, injection starts on the first day, and injection starts on the 30th day. As can be seen in Figure 3, when no air is injected into the borehole, the maximum CH₄ production rate is only 0.71 L/min. The CH₄ production rate rapidly decreased during the extraction process, and the average CH₄ production rate after 90 days of extraction was only 0.31 L/min. After 30 days of extraction, 0.6 MPa of air was injected into the gas injection hole, and the CH₄ production rate increased rapidly from 0.34 L/min to 0.57 L/min on the day of gas injection, an increase of 67.64%. When air was injected into the coal seam on the day of extraction, the maximum CH₄ production rate was 0.72 L/min, which was 132.26% higher than that under the condition of no air injection.

As shown in Figure 4, when air was not injected, the CH₄ content in the coalbed decreased slowly. The CH₄ content was lowest near the extraction borehole. When the extraction time was 90 days, the maximum CH₄ content in the coalbed was 3.47 m³/t. When air was injected into the coalbed 30 days after extraction, a large reduction zone of CH₄ content was formed near the injection borehole, and the CH₄ content in the entire coalbed extraction area was significantly reduced. When the extraction time was 90 days, the maximum CH₄ content in the coal seam was 3.08 m³/t. When air was injected into the injection borehole at the beginning of the extraction project, when the extraction time was 90 days, the maximum CH₄ content in the coal seam was only 2.96 m³/t, which is far less than that of the above two conditions. The injection of high-pressure air into the coalbed cannot only promote CH₄ desorption but also increase the migration speed of CH₄ in the coalbed and improve the efficiency of coalbed gas extraction in the extraction borehole.

Figure 5 shows the variation of permeability under different air injection conditions. As illustrated in Figure 5, when there was no air injected into the coalbed, the permeability of the coalbed continuously increased slightly during gas drainage. This is because gas desorption shrinks the coal skeleton, increasing the coal porosity and permeability. When air was injected into the coalbed, the adsorption capacity of O₂ in the air was greater than that of CH₄, and the coal skeleton expanded after absorbing O₂, resulting in a rapid decrease in the permeability of the coal body. The decrease in coalbed permeability was closely related to the amount of air injection. When the air injection time was 90 d, the permeability of the coalbed near the gas injection port increased to the original 97.43% (Figure 6).

3.3. Effect of Injection Pressure on Air-ECGE

As shown in Figure 7, the CH₄ production rate increased with the increase in injection pressure. After 90 days of extraction, the CH₄ production rate decreased by 68.54%, 70.14%, 74.41%, and 77.73% when the air injection pressures were 0.8 MPa, 1.0 MPa, 1.2 MPa, and 1.4 MPa, respectively. Setting the injection pressure P_z = 1.0 MPa as a reference, the cumulative production of CH₄ at P_z = 1.0 MPa, 1.2 MPa, and 1.4 MPa after extraction for 90 days increased by 9.03%, 21.26%, and 31.08%, respectively.

As shown in Figure 8 and Figure 9, under the influence of air injection, the CH₄ concentration significantly decreased near the gas injection borehole in the early stages of the coalbed gas extraction, and the range of CH₄ concentration reduction was generally the same as the range of N₂ migration. The migration of N₂ promotes the migration of CH₄ and reduces the concentration of CH₄ in the area. When the gas injection pressure increased from 0.8 MPa to 1.4 MPa, it significantly increased the driving force of gas in the coalbed. However, due to the low O₂ content in the air, after 90 days of air injection, O₂ still did not break through to the extraction borehole. The injection pressure increased from 0.8 MPa to 1.4 MPa, and the average O₂ concentration in the coalbed increased by 98.31%.

Figure 10 shows the evolution of the permeability ratio on line A-B with varying injected air pressures. As illustrated in the figure, the area of coal permeability reduction near the gas injection borehole increased with time, and the permeability of the coal near the extraction borehole showed a trend of first increasing and then decreasing. The higher the air injection pressure, the greater the increase in permeability of the coal near the extraction borehole. After 90 days of extraction, the decrease in permeability of the coal near the gas injection borehole became more prominent. This is because increasing the air injection pressure facilitates the CH₄ extraction and the entry of more N₂ into the coal, which significantly increases the coal seam permeability. More O₂ is also injected into the coalbed, and when it migrates to the extraction borehole, the permeability of the coal near the area will decrease significantly.

3.4. Effect of Borehole Spacing on Air-ECGE

As shown in Figure 11, the CH₄ production rate of the extraction borehole was generally the same under different borehole spacing conditions in the early stages of the coalbed CH₄ extraction. As the extraction progressed, the closer the borehole spacing, the lower the CH₄ production rate. There are two reasons for this phenomenon: first, the larger the borehole spacing, the more CH₄ sources are controlled by a single borehole; second, the smaller the borehole spacing, the more accelerated the breakthrough of air, affecting the CH₄ production rate. After 90 days of extraction, when the borehole spacing r = 4 m, 5 m, 6 m, and 7 m, the cumulative CH₄ production was 56,614.29 L, 59,477.14 L, 61,297.14 L, and 62,485.71 L, respectively. Setting the drilling spacing r = 4 m as a reference, the cumulative gas production rate of r = 5 m, 6 m, and 7 m was increased by 5.06%, 8.27%, and 10.37%, respectively.

As shown in Figure 12, there was only a slight difference in CH₄ concentration in the coalbed during the initial stage of extraction. However, as the extraction project progressed, the smaller the spacing between boreholes, the more significant the decrease in CH₄ concentration in the coalbed. After 90 days of extraction, when the borehole spacing r = 4 m, 5 m, 6 m, and 7 m, the maximum CH₄ concentrations in the coalbed were 130.32 mol/m³, 142.04 mol/m³, 150.83 mol/m³, and 157.43 mol/m³, respectively. Setting the borehole spacing r = 4 m as a reference, the maximum CH₄ concentrations of the coalbed for r = 5 m, 6 m, and 7 m were increased by 9.00%, 15.73%, and 39.12%, respectively.

Figure 13 shows the evolution of the permeability ratio on line A-B with varying borehole spacing. As illustrated in the figure, the larger the spacing between boreholes, the higher the gas content in the coalbed after 90 days of extraction, and the smaller the increase in coal permeability near the extraction boreholes. The area of reduced permeability near the gas injection well is related to the migration of O₂. The greater the borehole spacing, the less the area near the extraction hole is affected by O₂, and the higher the permeability.

4. Field Test of Air-ECGE in Liuzhuang Mine

To verify the effectiveness of Air-ECGE, an underground Air-ECGE engineering test was conducted in the Liuzhuang Coal Mine (Figure 14). A total of six test boreholes were considered, of which 1# and 2# were air injection boreholes, and 1-1#, 1-2#, 2-1#, and 2-2# were extraction boreholes. The final spacing of the boreholes was 4 m, and the gas injection pressure during the test was 0.8 MPa.

As shown in Figure 15, after injecting air into the coalbed, the extraction flow rapidly increased. The flow rate of borehole 1-2# was 0.48 L/min before injecting air into the coalbed but increased to 12.00 L/min after injecting gas into the coalbed, which is an increase of 24.2 times. The gas extraction flow rate of the four extraction boreholes increased significantly after gas injection, with the gas extraction flow rates of the 1-1#, 2-1#, and 2-2# boreholes increasing by 4.13 times, 18.97 times, and 10.44 times, respectively, after air injection into the coalbed. Clearly, injecting high-pressure air into the coalbed accelerated the migration of gas within the coal, and the flow rate of the extraction boreholes increased significantly. The variation trend of the flow rate of the gas extraction boreholes in the same group was generally the same, which is closely related to the efficiency of injecting air into the gas injection holes.

As shown in Figure 16, the volume fraction of CH₄ in the extraction borehole decreased rapidly after injecting air into the coalbed. This is because air quickly penetrates the extraction borehole, increasing the extraction flow rate of the borehole and reducing the extraction concentration of the borehole. The air breakthrough occurred on the same day mainly because of the close location of the borehole opening. Since no gas injection was performed between 25 July and 30 July, the extraction drill hole had a large CH₄ volume fraction during this period. The CH₄ volume fraction of the 2-2# drill hole increased significantly during the initial gas injection period, indicating that air had not yet penetrated the drill hole. This was mainly due to the deviation of the drilling trajectory, which put the 2-2# borehole far away from the gas injection drill hole. After air breakthrough, the CH₄ volume fraction of the borehole decreased significantly.

Figure 17 illustrates the changes in CH₄ flow rate in the extraction borehole. As illustrated in the figure, the injection of air significantly increased the CH₄ extraction rate. Before the air injection into the coalbed, the CH₄ extraction flow rate of the 1-2# borehole was 0.331 L/min, but after the injection, it rose to 3.344 L/min, an increase of 9.10 times. After the air injection, the average CH₄ extraction rate of the 1-1#, 1-2#, 2-1#, and 2-2# boreholes increased by 3.82%, 100.19%, 60.72%, and 49.68%, respectively. Under conventional extraction conditions, the CH₄ extraction rate decreases with time, but by injecting air into the coalbed, the CH₄ extraction rate can be significantly increased. During the period from 25 July to 30 July, the CH₄ extraction rate of each borehole that had not yet been injected with air was still greater than the CH₄ extraction rate before injection. This observation indicates that the effect of injecting air on facilitating coalbed CH₄ extraction has a certain lag effect and does not disappear immediately after stopping injection.

5. Discussion

Based on the above analysis, the impact of air injection on coalbed gas extraction can be divided into two stages:

(1)

Owing to the high content of N₂ in the air and the relatively low adsorption capacity of N₂ on coal, the migration speed of N₂ inside the coal is faster than that of O₂, and the coal around the gas injection borehole is first affected by N₂. High-pressure N₂ compresses the coal skeleton, increasing the width of the coal duct and the permeability. Moreover, high-pressure N₂ increases the internal pressure difference of the coal body, promotes gas migration, and increases the extraction efficiency of methane (Figure 18a,b).

(2)

When O₂ affects the coal body, owing to the strong adsorption of O₂ on the coal body, the coal skeleton expands, the pores of the coal body become smaller, and the permeability decreases. However, O₂ can displace more CH₄, further promoting the extraction of coalbed methane (Figure 18c,d).

(3)

As shown in previous studies focusing on surface well coalbed methane drainage, the injection gas pressure typically exceeds 6 MPa. In contrast, this field test achieved effective results with only 0.8 MPa injection pressure. The key reason lies in the substantial difference in well spacing: surface operations require 100–300 m between injection and production wells, whereas coal mine underground drilling allows a spacing of merely 1–6 m between boreholes. This compact configuration enables efficient gas drainage with lower pressure. Although studies indicate higher injection pressures enhance drainage effectiveness, China’s underground air compressors are restricted to a maximum outlet pressure of 2.5 MPa per safety regulations, with pipeline pressure typically maintained below 1.0 MPa. This critical safety measure prevents gas leaks from endangering personnel in underground workings.

(4)

Not all areas within coal mines are suitable for Air-ECGE technology implementation. First, specific coal seam characteristics are a prerequisite: if geological structures are well-developed, it may cause inadvertent connectivity between injection boreholes, extraction boreholes, or underground roadways. This scenario would allow injected gas to flow directly into extraction points or tunnels, significantly compromising its effectiveness in enhancing coal seam gas drainage. Second, regarding underground infrastructure: large-scale implementation requires high-capacity air compressors, which typically exceed 3.5 m in length and 3 tons in weight. Mines must therefore possess adequate transport capacity for such equipment along with sufficient clearance for installation in confined underground spaces.

(5)

Since the technology involves injecting high-pressure air into boreholes, its core equipment requirement is specifically air compressors. Currently, certified air compressors suitable for underground coal mine operations carry an approximate price tag of $75,000. Notably, engineering materials employed are standard consumables routinely used in mine production, incurring no significant additional material costs. Mine implementation would require only one additional air compressor operator. Critical safety protocols must be established: Mine operators should enhance underground safety training programs with emphasis on strictly prohibiting unauthorized disconnection of injection pipelines to prevent high-pressure hazards. Furthermore, the efficacy of Air-ECGE technology is highly dependent on optimal borehole design and precise gas injection parameter selection. Consequently, systematic technical training for relevant personnel is imperative, with projected training expenditures estimated at $100,000. Although this technology entails additional costs, it can significantly enhance the efficiency of coalbed methane extraction, ensure production safety during coal mining, reduce accident rates, and improve coal production efficiency in mines. From this perspective, its application ultimately lowers operational costs for mines and enhances corporate profits.

6. Conclusions

In this study, a thermal–hydraulic–solid coupling model for air-enhanced coalbed gas extraction (Air-ECGE) was established. Additionally, the impact of air injection on coalbed methane extraction was simulated, and field experiments were conducted on air injection to enhance gas extraction. The following are the major findings of the study:

(1)

The results show that injecting high-pressure air into a coalbed can effectively promote gas desorption and gas migration within the coalbed, greatly improving the efficiency of coalbed gas extraction.

(2)

Owing to the large pressure gradient that can lead to fast coalbed gas seepage, the gas production rate of the extraction borehole is directly proportional to the gas injection pressure. The spacing of the boreholes limits the influence range of the gas injection: the larger the spacing, the larger the influence range, and the higher the gas extraction rate of the extraction borehole.

(3)

After injecting air into the Liuzhuang coal mine coalbed, the extraction flow rate and concentration of gas from the extraction boreholes both increased significantly. A certain delay effect was observed in the gas injection effect, and the gas extraction flow rate only decreased after a period of time post gas injection stoppage.

(4)

This study only conducted field trials of Air-ECGE technology under one borehole layout configuration. However, multiple types of boreholes exist in underground coal mines, such as cross-measure boreholes and long boreholes exceeding 300 m. The effectiveness of Air-ECGE technology under these scenarios requires further investigation. Furthermore, injecting air into coal seams may reduce the methane extraction concentration from underground boreholes. Currently, most mines utilize the extracted methane gas, and a decrease in concentration could impose limitations on its utilization. Therefore, the Air-ECGE process requires further optimization to ensure increased methane extraction while maintaining concentration. Achieving a balance between increasing the absolute methane yield and maintaining extraction concentration constitutes a critical future research direction for this technology.

(5)

Due to the O₂-rich nature of air, prolonged injection into spontaneous combustion-prone coal seams carries the risk of triggering spontaneous combustion, potentially leading to mine fires. Therefore, ensuring the safety of such coal seams during Air-ECGE application represents a critical research topic for subsequent studies.