Research on the Effectiveness of Gas Control in Low-Permeability Coal Seams Based on Microbial Gas Dissolution

Abstract

To shorten the gas control period in low-permeability coal seams, the effectiveness of microbial gas dissolution technology for gas control was investigated through a combination of field tests and numerical simulations. First, three groups of injection boreholes were constructed, and the microbial gas-dissolving solution was injected into the coal seam, followed by an evaluation of the treatment effect after three days. Furthermore, based on a multiphysics-coupled modeling framework, the conventional borehole drainage process was simulated using the Partial Differential Equation (PDE) module of COMSOL Multiphysics to compare the evolution of coal seam gas pressure under conventional drainage with the field performance of the gas dissolution treatment. The results showed that the injected bioactive solution promoted methane oxidation and rapidly reduced the gas content and pressure in the treated region. After 3 days of gas dissolution treatment, the average gas content and gas pressure within the overlapping influence zones between the injection borehole groups decreased by 38.0% and 80.7%, respectively, relative to their initial values. However, the treatment effect gradually weakened with increasing distance from the injection zone, indicating spatial attenuation of the effective influence of the bioactive solution in the low-permeability coal seam. Comparatively, conventional drainage required approximately 68 days to reduce the gas pressure to the same level achieved by the gas dissolution treatment within 3 days, demonstrating that gas dissolution technology can drastically accelerate the gas control cycle. Nevertheless, further large-scale application requires consideration of economic feasibility and continued optimization of injection parameters and field implementation procedures.

1. Introduction

Coal and gas outbursts are among the most hazardous and catastrophic dynamic disasters encountered in coal mining in China, posing a major constraint on the efficient and intensive exploitation of deep coal resources. For gas hazard prevention and control, protective seam mining and pre-drainage of coal seam gas are widely adopted as the primary engineering strategies [1]. Conventional borehole drainage mainly relies on the pressure gradient generated by negative drainage pressure to drive free and desorbed methane through the pre-existing pore–fracture network toward the drainage boreholes. Its effectiveness is therefore strongly constrained by coal seam permeability, fracture connectivity, and the effective influence radius of the boreholes [2,3]. In low-permeability coal seams, methane desorption, diffusion, and seepage are relatively slow, often resulting in low single-borehole drainage rates, small effective influence radii, rapid declines in gas concentration and flow rate, and prolonged periods required to meet drainage targets [2,3,4]. Moreover, as drainage proceeds, gas pressure decreases progressively in the vicinity of the boreholes, whereas methane replenishment from regions farther away remains limited. This results in pronounced spatial heterogeneity in gas depletion, making it difficult to effectively remove methane from deeper coal regions and zones with poor pore–fracture connectivity [5]. Consequently, conventional borehole drainage alone often fails to achieve rapid, efficient, and spatially uniform gas reduction in low-permeability coal seams.

During conventional borehole drainage, various physical permeability enhancement techniques have been widely employed to overcome the inherently low permeability of coal seams, including hydraulic fracturing [6], waterless fracturing (e.g., CO₂ fracturing) [7], and blasting-induced permeability enhancement [8]. However, statistical data indicate that more than 95% of outburst-prone coal seams in China are typical low-permeability reservoirs characterized by complex pore structures and poor connectivity [9]. In such coal seams, conventional physical stimulation methods often encounter limitations such as long drainage periods, rapid declines in single-borehole gas flow rates, and restricted site-specific applicability [10]. These limitations may disrupt the coordination between coal extraction and roadway development and can even adversely affect mine production scheduling. Therefore, the development of an in situ gas control technology that goes beyond conventional physical permeability enhancement approaches and offers the dual potential for “chemical degradation” and “physical permeability enhancement” has become an important scientific challenge in the field of coal mine gas control.

In recent years, microbial gas dissolution technology has attracted increasing attention as a promising, environmentally friendly, and efficient approach to gas control [11,12]. The fundamental principle of this technology is to inject a bioactive solution enriched with methane-oxidizing bacteria (MOB) into deep coal seams through dedicated boreholes. Driven by injection pressure, the microbial solution migrates through the coal pore–fracture network and promotes the in situ biochemical oxidation of methane, converting it into stable products such as H₂O and CO₂ [13], thereby reducing coal seam gas content and reservoir pressure. Depending on their metabolic pathways, methane-oxidizing microorganisms can be broadly classified as aerobic or anaerobic and are widely distributed in natural environments such as wetlands, marshes, and rice paddies. They can also be obtained through isolation and enrichment cultivation [14,15,16]. In laboratory studies, Yu [17] demonstrated through a series of experiments that common mine gases, including CO, H₂S, and SO₂, exert no significant toxic or inhibitory effects on these microorganisms within their typical concentration ranges. Chen et al. [18] introduced bacterial solutions into coal samples containing adsorbed methane and found that MOB achieved an average methane degradation efficiency of approximately 44%. Zhang et al. [19] employed a triaxial stress–seepage testing system to investigate the degradation of adsorbed methane by anaerobic microorganisms and found that elevated stress facilitated methane degradation. Yu et al. [20] experimentally demonstrated that a specific type of methane-oxidizing bacteria achieved degradation efficiencies exceeding 43.3% for methane adsorbed in raw coal samples with different Protodyakonov coefficients. Pan [21] investigated the effects of temperature, injection pressure, and injection volume on anaerobic methane-oxidizing microorganisms and found that degradation efficiency increased with increasing pressure and injection volume, with 30 °C identified as the optimum temperature. Zhao et al. [22] examined degradation efficiency under different pH conditions and found that pH 6.75 provided the most favorable acid–base environment for microbial activity. Xue et al. [23] further demonstrated that the synergistic action of microorganisms and a static magnetic field could enhance methane desorption and improve pore connectivity in coal samples.

Recent studies have also shown that microbial treatment can improve the average pore size, pore volume, and pore connectivity of coal to varying degrees. Some initially closed or poorly connected pores may be transformed into effectively connected pores, thereby providing more favorable pathways for methane desorption, diffusion, and seepage [24,25,26]. Studies focusing specifically on MOB have further demonstrated that the wettability and adsorption behavior of bacterial solutions on coal surfaces can affect their migration into the coal matrix. The introduction of appropriate wetting agents or chelating agents can reduce the coal–water contact angle, enhance coal hydrophilicity, and promote pore enlargement and interconnection, thereby increasing the effective contact between MOB and coal seam methane [27,28]. In addition, the synergistic interaction between MOB and surfactants can further improve pore connectivity and gas–liquid–solid interfacial mass transfer, facilitating methane migration through coal pores and its transfer into the liquid phase [29]. Overall, existing studies indicate that MOB and their associated bioactive solutions not only oxidize and consume methane but may also improve effective gas flow and mass transfer conditions in coal seams by enhancing coal wettability, modifying pore structure, and increasing pore connectivity. Nevertheless, direct quantitative measurements of intrinsic coal permeability before and after MOB treatment remain relatively limited, and the magnitude and long-term evolution of permeability enhancement require further systematic investigation.

Regarding preliminary field applications, Jiang [30] conducted microbial solution injection tests in three coal mines in Henan Province and demonstrated, based on changes in borehole gas concentration, that methane-oxidizing bacteria could be used for coal-mine gas control. Mao [31] verified the feasibility of microbial gas control through comparative surface and underground injection tests, showing varying degrees of reduction in gas content, gas pressure, and drill-cuttings gas desorption indices. Guo et al. [32] conducted a gas dissolution field test at the 15205 working face of Sijiazhuang Coal Mine, where the coal seam gas content decreased by 32.3% within 24 h. In another field test at the 11703 working face of Qingcaitang Coal Mine, Guo et al. [33] reported reductions of 64.3% in coal seam gas pressure and 63.8% in gas content after application of the gas dissolving agent, further demonstrating the effectiveness of this technology for coal mine gas control.

Although the biochemical mechanisms underlying microbial methane degradation have been increasingly elucidated, systematic investigations of the spatiotemporal evolution and engineering applicability of this technology under the specific conditions of low-permeability coal seams remain insufficient. Therefore, the No. 6 low-permeability coal seam of a mine in Guizhou Province was selected as the representative study site. By integrating in situ field tests with numerical simulations, this study systematically investigates the gas mitigation performance and temporal–spatial evolution characteristics of microbial gas dissolution technology under low-permeability conditions. The results are expected to provide a sound scientific basis and engineering guidance for the targeted application and large-scale implementation of microbial gas control technology under complex geological conditions.

2. Technical Principles

The core mechanism of gas dissolution technology is to use a specifically formulated biochemical solution as a carrier to deliver aerobic methane-oxidizing bacteria (MOB) and their associated enzymatic systems into coal seam microfractures and the interior of the coal matrix, thereby promoting the in situ biochemical oxidation of methane. These MOB utilize methane as their primary carbon and energy source, and their metabolic activity requires the participation of molecular oxygen. As shown in Figure 1, at the molecular level, CH₄ is first oxidized to methanol (CH₃OH) by methane monooxygenase (MMO) in the presence of molecular oxygen. Subsequently, CH₃OH is dehydrogenated to formaldehyde (HCHO) by methanol dehydrogenase (MDH). HCHO is then further oxidized to formic acid (HCOOH) through the catalytic action of formaldehyde dehydrogenase (FADH). Finally, HCOOH is further oxidized to carbon dioxide (CO₂) by formate dehydrogenase (FDH). Although deep coal seams are generally characterized by oxygen-deficient conditions, the bioactive solution carries a certain amount of dissolved oxygen during its preparation and injection. In addition, the injection process can introduce a limited amount of oxygen into the pores and fractures of the coal matrix. These oxygen sources can provide the conditions necessary to sustain the aerobic metabolism of MOB within a limited spatial range and for a certain period of time. Through the above microbially mediated enzymatic oxidation process, methane in the coal seam is progressively consumed, thereby contributing to a reduction in coal seam gas content and mitigating the potential risk of coal and gas outbursts.

Representative aerobic methanotrophic bacteria capable of promoting methane oxidation and conversion into stable end products include Methylococcus capsulatus, Methylosinus trichosporium, Methylomonas methanica, and Methylocystis parvus. These bacteria can utilize methane as a carbon source and oxidize it through MMO-mediated metabolic pathways.

3. Experimental Site and Program

3.1. Experimental Site

3.1.1. Overview of the Mine

The coal seams of Coal Mine A in the Guizhou Province are characterized by significantly high gas content, high gas pressure, and low permeability. There are over 60 faults within the mining area, and the mine field is topographically structured as an synclinal unit with a northeastward dip. The coal-bearing strata in the mine field are primarily composed of the Permian Longtan Formation, which is divided into upper, middle, and lower members. The area contains 24 to 38 coal seams, of which 7 are mineable. Coal Mine A employs a rising-and-dipping mining method for development, with longwall retreating mining along the strike adopted at the working faces, and a district-exhausting ventilation system utilized throughout the mine.

3.1.2. Coal Seam Characteristics

The test coal seam is Seam No. 6 of Coal Mine A, which is the uppermost mineable seam within the mine field. The seam is generally stable in occurrence, has a relatively simple geological structure, and is mineable throughout the study area. It is predominantly a medium-thick coal seam, with an average coal thickness of 2.60 m. The number of partings ranges from 0 to 3 layers, and the average dip angle is 17°. The roof consists mainly of marl and sandstone, whereas the floor is predominantly mudstone. The coal is classified as Type III according to the degree of structural damage and is not prone to coal-dust explosion. The basic gas parameters of Seam No. 6 are summarized in

Table 1. Fundamental gas parameters of Seam No. 6.

Coal Seam	Measurement Location	Borehole No.	Burial Depth (m)	Gas Pressure (MPa)	Gas Content (m3·t−1)	Permeability Coefficient (m2·MPa−2·d−1)	Borehole Flow Decay Coefficient (d−1)
Seam No. 6	Pedestrian walkway, district 11	A6-1	132.2	0.96	13.66	0.4144	0.2534
	Return air drift, district 11	A6-3	185.3	1.15	14.45

.

3.1.3. Field Implementation Site

The experimental site was established in the gas drainage roadway associated with the 110609 haulage roadway, beneath Seam No. 6. The test section was located approximately 135 m from the roadway entrance. No other boreholes had been constructed in the surrounding area prior to the test, as shown in Figure 2.

3.2. Experimental Program

In this study, gas content and gas pressure were selected as the primary indicators for evaluating the treatment effectiveness. Initially, injection boreholes were constructed to collect coal samples for the determination of baseline gas parameters prior to the application of the gas dissolvent. Three days after the injection was completed, evaluation boreholes were drilled to remeasure the gas content and gas pressure and assess the treatment effect.

3.2.1. Borehole Layout Scheme

Three groups of injection boreholes, comprising 12 boreholes in total with four boreholes in each group, were constructed in the sidewall of the gas drainage roadway associated with the 110609 haulage roadway. The spacing between the borehole groups was 3.0 m. Within each group, the horizontal and vertical spacings between boreholes were 0.4 m and 0.2 m, respectively. After 3 days of treatment, six effectiveness evaluation boreholes were constructed in the same area, with boreholes arranged between the second and third injection groups and on both sides of the injection zone. The designed positions and spacings of the injection boreholes (indicated in black) and the evaluation boreholes (indicated in red) are illustrated in Figure 3. The design and construction parameters of the injection and evaluation boreholes are summarized in

Table 2. Design parameters of the injection boreholes.

Borehole No.	Azimuth (°)	Inclination (°)	Depth (m)
1, 2, 3-1#	315	30	37
1, 2, 3-2#	315	33	34
1, 2, 3-3#	315	36	31
1, 2, 3-4#	315	39	29

and

Table 3. Design parameters of the evaluation boreholes.

Borehole No.	Azimuth (°)	Inclination (°)	Depth (m)
1#	315	30	37
2#	315	33	34
3#	315	24	43
4#	315	36	31
5#	315	39	29
6#	315	30	37

, respectively.

3.2.2. Equipment and Procedures

(1) Gas dissolving solution and its injection system

The microbial gas dissolving solution is primarily composed of a base microbial culture, bio-enzymes, and an aqueous phase. Considering the geological characteristics of the target coal seam and the requirements of field operation, 2 t of the base microbial culture was mixed uniformly with 5 t of water to prepare 7 t of working solution. Subsequently, the solution was then injected into the coal seam through dedicated injection boreholes using a pressurized injection system at a constant pressure of 7 MPa, as shown in Figure 4.

The ancillary engineering equipment and auxiliary materials required for field operations also included an emulsion pump station and associated water reservoirs, high-pressure grouting pumps, high-pressure delivery pipelines, globe valves, galvanized steel pipes, ball valves, industrial mixing tanks, sealing packers, and expansive cement.

(2) Experimental procedures

•

Borehole construction: Injection boreholes were constructed, and coal samples were collected to determine the baseline gas content and gas pressure. Subsequently, the boreholes were sealed with a sealing length of 20 m.

•

Pump station installation: The pump station was positioned at the entrance of the 110609 haulage roadway gas drainage gallery, approximately 100 m from the injection site.

•

Pipeline connection: The equipment was connected in the following sequence: emulsion pump → high-pressure steel pipe → high-pressure hose → control box → high-pressure hose → four-way joint → high-pressure hose → stop valve → injection pipe.

•

Preparation of gas dissolvent: The gas dissolvent and additives were first added to a mixing barrel according to the specified ratio and stirred until uniform. The mixture was then transferred to the water tank of the emulsion pump.

•

Emulsion pump operation: Following an equipment inspection, the emulsion pump was activated, and the pressure was adjusted to 7 MPa. The valves were then opened to facilitate sequential injection into the boreholes.

•

Injection operation: Dynamic pressure injection was employed. Once the pump pressure reached the design specifications and remained stable, the pump was stopped upon the observation of “sweating” or seepage on the roadway walls, or when the water level in the tank ceased to decrease significantly under continuous pressure. Finally, the pressure relief valve was closed, concluding the injection procedure.

4. Numerical Simulation of Conventional Gas Extraction

Numerical simulation techniques based on multiphysics coupling frameworks have been widely used to characterize the dynamic evolution of underground gas drainage and have shown good agreement with field measurements in previous studies [34,35,36]. In this study, the Partial Differential Equation (PDE) module of COMSOL Multiphysics 6.2 was used to simulate the spatiotemporal evolution of gas pressure during conventional borehole drainage. The resulting conventional drainage scenario provides a reference for comparison with the field performance of the gas dissolution technology.

4.1. Governing Equations

Coal is a representative dual-porosity (pore–fracture) medium. It is assumed that the coal skeleton behaves as a linearly elastic solid, with gas transport in the coal matrix governed by Fick’s law of diffusion and gas flow through the fracture system governed by Darcy’s law. Gas drainage is a complex multiphysics-coupled process involving interactions among multiple physical fields. The governing processes include gas diffusion in the coal matrix, gas seepage through the fracture system, deformation of the coal skeleton, and the associated evolution of coal permeability. The governing equations for these coupled processes are presented below [37].

(1) Gas diffusion field

$${\displaystyle \frac{\partial p_m}{\partial t}}=-{\displaystyle \frac{{\sigma }_{c}D{V}_M\left(p_m-p_f\right)(p_m+P_L{)}^2}{V_{L}RT{P}_L{\rho }_c+{\varphi }_m{V}_M(p_m+P_L{)}^2}}$$

(1)

(2) Gas seepage field

$${\varphi }_f{\displaystyle \frac{\partial p_f}{\partial t}}+p_f{\displaystyle \frac{\partial {\varphi }_f}{\partial t}}=\nabla \left({\displaystyle \frac{k_e}{\mu }}{p}_f\nabla p_f\right)+{\sigma }_{c}D\left(1-{\varphi }_f\right)\left(p_m-p_f\right)$$

(2)

(3) Coal deformation field

$$G{u}_{i,jj}+{\displaystyle \frac{G}{1-2\nu }}{u}_{j,ji}-{\beta }_f{p}_{f,i}-{\beta }_m{p}_{m,i}+F_i=0$$

(3)

(4) Permeability equation

$$k_e=k_0{\left\{1+{\displaystyle \frac{{\beta }_f}{M{\varphi }_{f0}}}\left(p_f-p_0\right)+{\displaystyle \frac{{\beta }_m}{M{\varphi }_{f0}}}\left(p_m-p_0\right)+{\displaystyle \frac{{\epsilon }_L}{{\varphi }_{f0}}}\left({\displaystyle \frac{K}{M}}-1\right)\left[{\displaystyle \frac{p_m}{p_m+P_L}}-{\displaystyle \frac{p_0}{p_0+P_L}}\right]\right\}}^3$$

(4)

where $p_m$ is the gas pressure in the coal pores (MPa); $V_M$ is the molar volume of methane under standard conditions (m³/mol); $p_f$ is the gas pressure in the fractures (MPa); $P_L$ and $V_L$ represent the Langmuir pressure (MPa) and Langmuir volume (m³/kg), respectively; $R$ is the ideal gas constant (J/(mol·K)); $T$ is the coal seam temperature (K); ${\rho }_c$ is the apparent density of the coal (kg/m³); ${\varphi }_m$ is the matrix porosity of the coal (%); $M$ is the molar mass of methane of the coal (kg/mol); ${\sigma }_c$ is the matrix shape factor (m⁻²); $D$ is the diffusion coefficient; ${\varphi }_f$ is the fracture porosity of the coal (%); $G$ denotes the shear modulus of the coal (MPa); $\nu$ is the Poisson’s ratio of the coal, $F_i$ is the body force (N); $k_e$ and $k_0$ signify the permeability and initial permeability (mD); $\mu$ is the dynamic viscosity of the gas (Pa·s); $K$ is the bulk modulus of the coal (MPa), ${\varphi }_{f0}$ is the initial fracture porosity (%); ${\epsilon }_L$ is the Langmuir volumetric strain; ${\beta }_f$ and ${\beta }_m$ are the effective stress coefficients for the fractures and pores, respectively, defined as ${\beta }_f=1-\left({\displaystyle \frac{K}{K_m}}\right)$ and ${\beta }_m=\left({\displaystyle \frac{K}{K_m}}\right)-\left({\displaystyle \frac{K}{K_s}}\right)$, where $K$, $K_m$ and $K_s$ are the bulk modulus of the coal mass, the coal matrix, and the coal skeleton (MPa), respectively; the relationships are given as $K={\displaystyle \frac{E}{3\left(1-2\nu \right)}}$, $K_m={\displaystyle \frac{E_m}{3\left(1-2\nu \right)}}$ and $K_s={\displaystyle \frac{K_m}{1-\frac{3{\varphi }_m\left(1-\nu \right)}{2\left(1-2\nu \right)}}}$, where $E$ is the elastic modulus of the coal (MPa) and $E_m$ is the elastic modulus of the coal matrix (MPa).

It should be noted that the gas transport model adopted in this study was based on a previously validated mathematical and numerical modeling framework [37]. The purpose of the simulation was not to reproduce the field gas dissolution process, but rather to establish a conventional borehole drainage reference scenario under the same geological and initial conditions as the field test. Because both gas dissolution treatment and conventional drainage alter the original gas pressure field of the coal seam, it is difficult to conduct the two field tests at the same location under identical initial conditions. Therefore, numerical simulation was employed to obtain the evolution of gas pressure under conventional drainage conditions.

4.2. Model Establishment and Parameter Setting

Based on the basic parameters of Seam No. 6 in Coal Mine A and the design specifications of the injection boreholes, a geometric model measuring 22.5 m in length and 12.3 m in height was established. Twelve boreholes with a radius of 0.06 m were incorporated into the model; the resulting geometry and mesh generation are illustrated in Figure 5. Regarding boundary conditions, the bottom of the model was designated as a fixed boundary, while roller supports were applied to the lateral boundaries. The top boundary was subjected to a vertical in situ stress load. The initial gas pressures for both the matrix and the fractures were set at 4.30 MPa, with a drainage negative pressure of 85 kPa applied within the boreholes. To analyze the spatial and temporal evolution of gas pressure, a horizontal monitoring line, AB, was established at the mid-height of the model, with monitoring point C located at the position corresponding to field Evaluation Borehole No. 2. The specific physical parameters used for the numerical simulation are summarized in

Table 4. Basic physical parameters for numerical simulation.

Parameter	Value	Parameter	Value
${\varphi }_{f0}$	0.012	$R$	8.4135 J/(mol·K)
${\sigma }_c$	60 cm−2	$T$	293 K
${\varphi }_{m0}$	0.06	$\mu$	1.08 × 10−5 Pa·s
$k_{e0}$	0.01 mD	${\epsilon }_L$	0.004
$E_m$	8139 MPa	$P_L$	1 MPa
$E$	2713 MPa	$V_L$	0.02 m3/kg
$\nu$	0.339	$V_M$	0.0224 m3/mol
$M$	0.016 kg/mol	${\rho }_c$	1250 kg/m3

.

5. Results and Discussion

5.1. Effectiveness of Gas Dissolution Treatment

As presented in

Table 5. Initial gas content and pressure of the coal seam.

Injection Borehole No.	Gas Content (m3·t−1)	Gas Pressure (MPa)
1-1#	11.69	1.74
1-3#	16.32	7.29
1-4#	16.99	8.74
2-1#	13.33	2.85
2-4#	14.69	4.42
3-1#	13.38	2.90
3-2#	14.32	3.92
3-3#	12.98	2.56

, the initial gas content within the experimental area of Seam No. 6 ranged from 11.69 to 16.99 m³/t, with a mean of 14.31 m³/t; these results are consistent with the baseline gas parameters detailed in Table 1. Additionally, the initial gas pressure was found to range from 1.74 to 8.74 MPa, with an average value of 4.30 MPa.

Figure 6 presents the field observations during the initial and later stages of depressurization and gas discharge from the injection borehole. During the initial discharge stage (within 1 min), a small amount of green mist was instantaneously discharged from the borehole. Its color and appearance were generally consistent with those of the original gas-dissolving solution shown in Figure 4, indicating that this liquid mainly consisted of residual solution retained within the borehole cavity and the near-borehole region. Subsequently, the borehole entered a sustained and stable gas discharge stage, accompanied by a distinct white mist. An intrinsically safe portable CO₂ gas analyzer was used on-site to analyze the gas discharged from the borehole outlet, and the measurements confirmed the presence of a substantial amount of CO₂ in the discharged gas. It should be noted that CO₂ itself is colorless; therefore, the observed white mist does not represent the color of CO₂. According to previous high-pressure CO₂ release experiments [38], rapid depressurization and expansion of high-pressure gas can produce a pronounced cooling effect and promote the condensation of water vapor into fine droplets, thereby forming a visible white cloud. Therefore, the white mist observed in the present test was mainly associated with liquid atomization and water vapor condensation during depressurization, whereas the presence of CO₂ was confirmed by the on-site instrumental measurements. Considering the biochemical methane-oxidation mechanism of MOB together with the field measurement results, it can be inferred that the bioactive solution injected into the coal interacted with coal seam methane through oxidative reactions.

The gas content and gas pressure measured within the experimental area after microbial solution injection are summarized in

Table 6. Gas content and pressure of the coal seam after dissolution.

Evaluation Borehole No.	Gas Content (m3·t−1)	Gas Pressure (MPa)
1#	9.00	0.85
2#	8.61	0.77
3#	12.79	2.41
4#	9.41	0.95
5#	11.35	1.58
6#	8.44	0.74

. Evaluation boreholes Nos. 1, 2, 4, and 6 were located within the overlapping influence zones of the three groups of injection boreholes. In these regions, both gas content and gas pressure decreased markedly, with average values of 8.87 m³/t and 0.83 MPa, respectively, indicating a pronounced superimposed treatment effect resulting from multiple injection groups.

In contrast, evaluation borehole No. 3 was located farthest from the concentrated injection zone and was therefore less directly affected by the microbial solution. Its gas content and gas pressure remained the highest among all evaluation boreholes, at 12.79 m³/t and 2.41 MPa, respectively. It should be noted that the reduction in gas pressure at borehole No. 3 was substantially greater than the reduction in gas content. This difference is mainly associated with the occurrence state and mass transfer behavior of methane in coal. Coal seam gas content consists of both free and adsorbed methane. Gas pressure primarily reflects the pressure state of free gas within the pore–fracture system and therefore responds relatively rapidly to pressure disturbances, whereas a large proportion of methane is stored in the coal matrix in an adsorbed state. Its release requires successive desorption and diffusion processes, resulting in a delayed response to external disturbances [39,40]. For borehole No. 3, which is located relatively far from the injection zone, pressure disturbances can propagate rapidly through the fracture network, whereas the adsorbed methane has not yet been fully desorbed or involved in subsequent migration and oxidation processes. Consequently, the gas pressure exhibits a relatively pronounced decrease, while the reduction in total gas content remains comparatively limited.

Evaluation borehole No. 5 was located to the left of the third injection borehole group and was mainly influenced by a single injection group. Accordingly, the reductions in gas parameters were smaller than those observed in the overlapping treatment zones represented by boreholes Nos. 1, 2, 4, and 6.

Based on the comparative analysis of the above data, the average gas content and gas pressure in the coal seam after gas dissolution treatment decreased by 38.0% and 80.7%, respectively, relative to their initial values. These results indicate that, under high-pressure injection, the bioactive dissolving solution can migrate into the coal through the pre-existing pore and fracture network and promote gas reduction through MOB-mediated methane oxidation, thereby substantially reducing the gas parameters within the treated zone over a 3-day period. Meanwhile, the treatment effects at different evaluation boreholes exhibited clear spatial variations: boreholes located within the overlapping influence zones of multiple injection groups showed greater reductions in gas content and gas pressure, whereas boreholes farther from the concentrated injection zone exhibited relatively smaller reductions. This indicates that the effective influence of the bioactive solution gradually decreases with increasing distance from the injection zone. Such spatial attenuation may be associated with poor pore–fracture connectivity, high resistance to liquid migration, and the limited mass transfer range of the bioactive solution in low-permeability coal seams. It should be noted that the spatial distribution of MOB activity and the residual concentration of the dissolving solution were not directly measured in this study. Therefore, the spatial influence range was inferred indirectly from the variations in gas parameters measured at evaluation boreholes located at different positions. In future studies, the effective treatment radius can be further quantified through direct measurements of microbial activity.

5.2. Effectiveness of Conventional Borehole Gas Drainage

According to the field results of the gas dissolution tests, the gas pressure between the borehole groups decreased to the critical threshold of 0.83 MPa within only 3 days. Using 0.83 MPa as the target pressure, Figure 7 illustrates the spatial evolution of the gas-pressure field under conventional drainage at four representative drainage durations: 30 days, 70 days, 130 days, and 200 days. The simulated pressure contours reveal pronounced spatial heterogeneity in gas pressure attenuation, with the pressure reduction rate increasing as the distance to the drainage boreholes decreases. After approximately 70 days of drainage (Figure 7b), the low-pressure zones surrounding adjacent boreholes began to overlap in regions with relatively dense borehole spacing due to the superposition of their drainage influence. However, the interconnected regions between the borehole groups did not fully reach the target pressure of <0.83 MPa until approximately 130 days of drainage (Figure 7c). The results therefore indicate a pronounced time lag under conventional drainage. In comparison, the gas dissolution treatment reduced the gas pressure between the borehole groups to the same threshold within only 3 days, demonstrating its substantial advantage in shortening the gas control period.

Figure 8 illustrates the spatiotemporal evolution of gas pressure in the coal matrix and fracture system along monitoring line AB, further elucidating the gas drainage response mechanism within a dual-porosity medium. The results reveal a pronounced asynchronicity in the pressure attenuation rates between the matrix and fracture systems. Within the initial 1 day of drainage, the fracture gas pressure in the superposition zone plummeted from the baseline of 4.30 MPa to approximately 1.48 MPa (about 34.4% of its initial value), forming a distinct “funnel-shaped” pressure sink. In contrast, the matrix gas pressure showed only a slight change during the same period. This difference is attributable to the relatively high permeability of the fracture system, which provides preferential pathways for gas flow. Driven by the large pressure gradient between the coal seam and the drainage boreholes, free gas in the fractures flows rapidly toward the boreholes. As drainage continues, the sustained decrease in fracture gas pressure increases the pressure difference between the coal matrix and fracture system, thereby promoting gas diffusion from the matrix into the fractures. As this interporosity mass transfer continues, the matrix gas pressure gradually decreases and eventually exhibits a pressure evolution trend similar to that of the fracture system, with the two systems progressively approaching dynamic equilibrium.

5.3. Comparative Analysis

Monitoring point C corresponds to the No. 2 effectiveness evaluation borehole used in the field application of the gas dissolution technology. Figure 9 shows the temporal evolution of gas pressure at monitoring point C and compares the pressure reduction performance of conventional gas drainage with that of the gas dissolution technology. The results show that after 3 days of treatment, the gas pressure under conventional drainage decreased from 4.30 MPa to 1.30 MPa, corresponding to a reduction of 69.8%. In contrast, under the gas dissolution treatment, the gas pressure decreased to 0.77 MPa over the same period, corresponding to a reduction of 82.1%. These results indicate that the gas dissolution technology achieved a faster pressure decline and a more pronounced pressure reduction effect within the same treatment period.

The observed difference is mainly attributable to the distinct methane transport and removal mechanisms of the two technologies. Conventional gas drainage primarily relies on the pressure gradient generated by negative pressure in the borehole to drive methane migration through the pre-existing pores and fractures of the coal toward the drainage borehole, and its effectiveness is therefore strongly constrained by the intrinsic permeability of the coal seam. By contrast, the gas dissolution technology uses high-pressure injection to deliver a bioactive solution enriched with MOB into the pores and fractures of the coal matrix. Previous studies have shown that microbial treatment can improve the coal pore structure by converting some closed or poorly connected pores into effectively connected pores, while also increasing the average pore size and porosity and enhancing pore connectivity to a certain extent. These changes provide more favorable pathways for coal seam gas desorption, diffusion, and migration [24,25,26]. Meanwhile, MOB can further biologically oxidize methane that enters the liquid phase or comes into contact with the bacterial solution. Therefore, the more rapid gas pressure decline observed in Figure 9 should be attributed to the combined effects of improved pore structure and mass transfer conditions and microbial oxidation and consumption of methane.

Further analysis of Figure 9 reveals a distinct two-stage evolution during conventional gas drainage: an initial rapid attenuation phase (0–3 days) followed by an asymptotic slow-decline phase (after 3 days), indicating that the rate of pressure reduction progressively decreases with drainage time. Notably, it requires 68 days of conventional drainage to achieve the same pressure reduction (0.77 MPa) attained by dissolution technology in only 3 days. To reach the regulatory threshold of 0.74 MPa, conventional drainage requires approximately 77 days, whereas the gas dissolution treatment can reach this threshold within a much shorter period with additional solution injection. Therefore, compared with conventional borehole drainage, the gas dissolution technology can shorten the gas control period by approximately two months, substantially shortening the gas control cycle and enhancing the operational efficiency in low-permeability coal seams.

6. Conclusions

Through field experiments and numerical simulations, this study evaluated the gas control performance and engineering applicability of microbial gas dissolution technology in a low-permeability coal seam. The main conclusions are as follows:

(1) Field tests demonstrated that microbial gas dissolution technology can achieve rapid gas mitigation. After 3 days of treatment, the average gas content and gas pressure within the overlapping treatment zone decreased by 38.0% and 80.7%, respectively, indicating that this technology can rapidly reduce both gas pressure and gas content in low-permeability coal seams.

(2) Unlike conventional borehole drainage, which mainly relies on pressure-gradient-driven methane migration through the pre-existing pore–fracture network, microbial gas dissolution technology actively delivers a MOB-enriched bioactive solution into the coal matrix under injection pressure and promotes the in situ oxidative reduction of methane through microbial activity. Numerical simulation results showed that conventional drainage required approximately 130 days for the interconnected region between the borehole groups to reach the target pressure of <0.83 MPa, whereas the gas dissolution treatment achieved the same pressure threshold within only 3 days. This active in situ methane reduction mechanism represents the principal innovation of the technology compared with conventional drainage and can substantially shorten the gas control period.

(3) The results indicate that microbial gas dissolution technology can serve as a promising supplementary approach for rapid gas control in low-permeability coal seams, particularly where conventional drainage is constrained by low permeability and slow methane transport. For large-scale application, further work is needed to optimize injection parameters, the effective treatment radius, and the spatial distribution of the bioactive solution, as well as to systematically evaluate its long-term effectiveness and economic feasibility.