Drainage Boreholes Research Articles

Study on Roof High-Level Borehole Drainage Technology Based on Temporal and Spatial Evolution Law of Gas Migration

Study on Roof High-Level Borehole Drainage Technology Based on Temporal and Spatial Evolution Law of Gas Migration

Study on the Spontaneous Combustion Law of Coal Body around a Borehole Induced by Pre-extraction of Coalbed Methane.

To address the challenges associated with the high gas content, high pressure, and low permeability coefficient in deep coal seams, strategies such as infilling boreholes and increasing the negative pressure of extraction are commonly implemented to alleviate issues related to coalbed methane extraction. However, long-term mining pressure can lead to the development of cracks in the coal seam near the borehole, thereby creating air leakage channels, which could potentially impact the oxygen supply during the extraction process. This leads to secondary disasters such as the spontaneous combustion of coal and gas explosions, considerably impacting the life and health of underground workers. To solve this issue, a thermal-fluid-solid coupling model for the working surface was constructed based on numerical simulation software, taking into account the multimechanism coupling effect of coal seam gas. The laws of coal oxidation and spontaneous combustion induced by coalbed methane extraction around boreholes were studied. The variation laws of the oxygen concentration, coal temperature, and oxidation heating zone around the borehole under different extraction conditions were simulated and analyzed. The findings demonstrate that the negative extraction pressure enables the gas to penetrate the fracture zone of the borehole, leading to an increase in the oxygen consumption rate and coal temperature around the borehole with an increase in negative extraction pressure. The coal gas leakage surrounding the borehole reduces as the sealing depth increases, and both the heating rate of coal and oxygen volume fraction show a downward trend. The fitting relationship between the negative pressure of drainage, depth of sealing, and temperature change in the coal body surrounding the boreholes was identified. It was determined that the negative pressure of 13 kPa for borehole drainage and a sealing depth >18 m are the optimal extraction parameters. The range of the oxidation zone and the position of the boundary line under this parameter were predicted, and the position function of the dangerous area of oxidation heating was defined. The research results have remarkable implications for the coordinated prevention and control of gas and coal spontaneous combustion in coalbed methane predrainage boreholes, as well as for efficient prevention and control of CO in on-site gas extraction boreholes, thus ensuring efficient and safe gas extraction.

Optimization of Advance Drainage Borehole Layout Based on Visual Modflow

It is an effective measure to prevent water damage in coal mines in order to construct drainage boreholes in water-filled aquifers of a working face roof. The hydrogeological parameters of the roof water-filled aquifer and the parameters of the drainage borehole are closely related to the underground drainage effect. Taking the 3085 working face of the Donghuantuo Mine in Kailuan as an example, the influence degree of hydrogeological parameters and hydrophobic borehole parameters on the amount of drainage water was analyzed using the generalized gray correlation degree. Based on Visual Modflow, the 3D groundwater visualization model was established and the dredging borehole was generalized into the pumping borehole. By changing the main influencing factors, the design optimization of the advanced hydrophobic borehole was discussed. The results showed that the aquifer thickness had a great influence on the amount of water discharged, and the influence degree of the sharp angle between the formation and the direction of drilling, the depth of the final hole, the azimuth angle of drilling, the dip angle of drilling, the elevation of the final hole and the elevation of the borehole on the amount of water discharged decreased successively. Based on the simulation calculation, it could be observed that the hydrophobic borehole should be placed in a position with a larger accumulated thickness of the aquifer to have a better effect of hydrophobic depressurization.

Impact of closed pores on gas transport and its implication for optimizing drainage borehole design

Mining disturbances can rupture the closed pores, releasing the gas and potentially triggering gas accidents. The pre-drainage of gas via boreholes is the primary measure for preventing coal and gas outbursts. Nevertheless, the influence of closed pores on gas migration remains unclear, leading to suboptimal borehole spacing and radius. Therefore, a gas–solid coupled model incorporating closed pores was developed to investigate the influence of closed pores on gas migration during gas drainage (GD). Subsequently, response surface methodology was employed to investigate the input parameters and their interactions on residual gas content (RGC) and pre-drainage time (PDT). Finally, an optimization methodology for borehole spacing/radius was presented. The results show that both RGC and PDT exhibit a positive correlation with the ratio of closed porosity to total porosity (λ) and the ratio of closed pore diffusion coefficient to that of the open pore (Do/Dc). Initially, the total gas production is primarily extracted from fractures and open pores, followed by closed pores in the later stages. Single-factor analysis demonstrates that λ, permeability, and Do/Dc have a more significant impact on RGC and PDT compared to borehole spacing and borehole radius. Borehole spacing interacts more strongly with λ, permeability and Do/Dc than borehole radius. An optimization method for borehole spacing and borehole radius, constrained by PDT, RGC, and the number of boreholes, is proposed using response surface optimization maps. This method provides guidance for borehole construction to optimize GD efficiency and minimize RGC.

A case study for coupling mechanism of anti-rockburst and pressure relief in gas drainage borehole along seam

After high gas mines coal mining reaches deep, the issue of rockburst has become increasingly prominent. Aiming at solving the problem of rock burst in high gas mines, the coupling mechanism of anti-rockburst and pressure relief in gas drainage boreholes along seam has been explored. The research results indicate that gas drainage boreholes can reduce the stress environment of coal, making the peak stress transfer to deep coal. The pressure relief effect of gas drainage boreholes is slightly weaker than that of large diameter pressure relief boreholes. Moreover, the presence of gas pressure can weaken the mechanical properties of coal. The compressive strength, elastic modulus, cohesion, and internal friction angle of coal all show varying degrees of attenuation with the increase of gas pressure. Besides, a stress analysis was conducted on borehole surrounding rock, and mathematical derivation was also carried out. The expressions for radial stress and circumferential stress of the coal at the boundary of the plastic zone of the borehole surrounding rock, as well as the analytical solution for the boundary of the plastic zone, are obtained. In addition, the gas in pressure relief borehole act on the coal in the pressure relief zone, causing deterioration of the mechanical properties of the coal around the pressure relief boreholes. The expansion range of the radius of the borehole pressure relief zone will increases, and the borehole pressure relief effect will enhance, promoting the formation of coal weakening zones. The above is the coupling mechanism of anti-rockburst and pressure relief in gas drainage boreholes, which has been confirmed by numerical simulation for reliability.

EXPERIMENTAL STUDY ON CREEP LOADING OF POROUS COAL UNDER DIFFERENT INFLUENCING FACTORS

After the gas drainage borehole is formed in the coal seam, its stress deformation process is a creep process without the influence of mining. By arranging boreholes on coal samples and carrying out creep loading test of porous coal through rock mechanics testing machine, the creep process of boreholes in actual coal seams is simulated. The results show that the creep loading test results of drilled coal are quite different from those of conventional loading. Creep loading produces large deformation due to rheological and aging characteristics. Compared with non-porous coal, the yield of drilled coal occurs earlier and the yield platform is wider during creep loading. Also, the larger the borehole diameter, the deeper the borehole depth, and the lower the peak stress. The up dip angle is easier to destroy than the down dip angle. Compared with hard coal, soft coal produces greater deformation under smaller peak stress. The research results have guiding significance for drilling gas drainage in the soft coal seam.

Failure Behavior and Fracture Evolution Mechanism of Coal with Fissures around Holes.

The fracture of coal is the main channel of gas flow and an important factor affecting the stability and efficiency of gas drainage boreholes. The coal structure, the development of hole cracks, and the degree of deformation are different. It affects the strength and mechanical deformation characteristics of coal to a great extent. In order to investigate the law of fracture evolution around the borehole of fractured coal, uniaxial and triaxial compression tests of raw coal samples have been carried out. The stress field evolution characteristics of fractured coal under compression were analyzed by Particle Flow Code (PFC2D). The strength, deformation, and fracture evolution behavior of fractured coal around boreholes under different confining pressures were studied. The results show that the compressive strength and fracture morphology evolution characteristics of coal around the hole are obviously related to the confining pressure and fracture occurrence of raw coal. The borehole structure itself has an important influence on the distribution location of the shear failure zone of the fracture around the hole, and its influence degree increases with the decrease of borehole confining pressure. During the deformation of coal with cracks around the hole, the initiation, propagation, and union behavior of cracks are related to the crack angle β. The cracks with β 0 and 180° are most easily closed during compression and the cracks with β 90° have little effect on the crack propagation zone. When the crack angle β is 45°, it is most easy to sprout and expand at the end; when the coal is compressed to the ultimate strength, the increase rate of the tensile crack increases, and the polymerization and combination behavior of the crack is more obvious. The evolution cloud map of the stress field can better reflect the evolution characteristics of fracture development, expansion, and fracture in the process of coal loading. Studying the failure behavior and fracture evolution mechanism of the coal around the hole can better predict and control the gas migration and extraction effect, which is of great significance to prevent the occurrence of gas accidents.

Study on key grouting blocking parameters of gas drainage boreholes in soft coal seams

The construction of gas extraction boreholes in soft coal seams is prone to collapse and deformation, and grouting reinforcement is one of the main methods to solve the problem of efficient sealing. However, the reasonable selection of key grouting parameters still needs further research. In response to the problem of selecting grouting sealing parameters for gas drainage drilling in soft coal seams, based on the “concentric ring” reinforcement sealing technology obtained in previous research, the key parameters and sealing technology of the “concentric ring” reinforcement were studied through theoretical calculation and numerical simulation experiments. The slurry diffusion morphology and range under different grouting pressures and grouting time slurry viscosity were obtained. Finally, in order to explore the application effect of key grouting parameters, on-site industrial tests were conducted in a soft and high gas coal seam. The research results indicate that the optimal grouting pressure for the “retaining wall rock hole ring” should not be less than 3 MPa, the reasonable grouting time should be 10–15 min, and the water material ratio of the grouting material should be greater than 1:1; The use of new reinforcement sealing and grouting technology can ensure long-term good extraction effect. Compared to the testing of drilling sealing effect using ordinary cloth bags with two plugs and one injection, The adoption of new reinforcement sealing technology can effectively prevent the deformation and collapse of the borehole before sealing, and due to two rounds of grouting and pre reinforcement of fractured coal, the sealing effect of the borehole is also relatively good. The research results have important theoretical value for guiding the drilling and sealing grouting engineering of gas extraction in soft coal seams.

Research on the construction and verification of evaluation model for borehole collapse in gas extraction

To improve the precision prevention of gas extraction borehole deformation and collapse, it is an important prerequisite to achieve precise monitoring of boreholes. This work is based on distributed optical fiber monitoring and Brillouin Optical Time Domain Analysis Technique (BOTDA). We carried out simulation and validation experiments of borehole plugging with different particle size ratios, constructed a mathematical model for calculating the plugging rate (denoted by ω) of boreholes, and put forward a new methodology for accurate monitoring and evaluation of gas extraction boreholes. The experimental results show that there is a linear relationship between the strain of optical fiber (denoted by ε), the accumulation mass of the simulated coal samples (denoted by m), and the deformation collapse of the borehole. And a mathematical model for calculating the plugging rate of boreholes with more universality has been built by introducing the ratio, volumetric, and strain correction coefficients (denoted by φ, β, and γ). Based on the verification experiment, it was found that the change in the actual and theoretical plugging rates is consistent. With the increase in the ratio of medium coal (2.0–5.0 cm), the plugging rate error decreases first and then increases. The maximum errors are −18.36 % and 19.04 %, but the plugging rate results are still within the middle boundaries of the collapsed boreholes (29.31 % to 75.07 %) and have a neglected effect on the stage classification. Inevitable errors in model construction and actual accumulation errors are two important factors contributing to errors in mathematical models. Taking ω as 0 %, 27.71+40.05·rord.-2%, 74.50+14.13·rord.-2%, and ε limit value 143.77·φ·γ με as the critical value to distinguish the primary, middle, and late periods of borehole collapse. In this work, a new technology of borehole monitoring and evaluation with plugging rate and strain critical value as evaluation indexes is proposed, which can provide a reference for accurate monitoring of drainage boreholes and gas prevention.

A 3D Numerical Model for Improving Methane Drainage Efficiency in Underground Coal Mines Using the Cross‐Measure Borehole Method

The release of methane into the working area of coal mines can affect safety and production. Therefore, methane drainage reduces the risk of explosion and can be used as a fuel and energy source. One methane drainage method is cross‐measure borehole drilling, which involves drilling boreholes from tailgate roadways to the roof or floor layers of a mined seam. In this method, determining the appropriate distance between methane drainage stations has a significant impact on increasing the efficiency of operation and reducing drilling costs and the time required for drainage operations from each station. In this paper, a new 3D numerical model is developed for methane drainage from an underground coal mine to determine the suitable distance between methane drainage stations in the cross‐measure borehole method using COMSOL Multiphysics software. For this purpose, by considering the porous media flow module and the laws related to the movement of gas in the goaf, different numerical models for distances of 10, 15, 20, 25, and 30 m were simulated. The simulation results showed that by reducing the distance between methane drainage stations, the speed of gas flow in the borehole and the amount of gas removed from the boreholes in a similar period of time increase to an acceptable level. In addition, the implementation of this model in a case study showed that a distance of 10 m between drainage stations increases the efficiency of boreholes to approximately 20%. Numerical simulation in this study can provide basic support for the design of drainage boreholes and reduce the risk of gas in underground coal mines.

Research on composite municipal solid waste incineration bottom ash and granulated blast furnace slag based geopolymers with sodium alginate for sealing coal seam gas drainage boreholes

This study has created geopolymer sealing materials from sodium alginate (SA), granulated blast furnace slag (GBFS), and municipal solid waste incineration bottom ash (MSWI-BA) to see how well they can seal coal seam gas drainage boreholes. In comparison to ordinary cement sealing material (CSM) and neat MSWI-BA based geopolymer, composite geopolymer sealing material (SIG) with 0.1 wt% SA has the advantages of appropriate setting time, improved fluidity, and increased compressive strength. Additionally, SIG demonstrated a slight expansion that aids in maintaining the durability of the seal. However, when compared to neat geopolymer, the addition of sodium alginate increased porosity and permeability slightly, but not to levels higher than CSM. Based on the microstructural characterization, a mechanism for the formation of SIG with coordination crosslinking structure in the network of C-A-S-H and N-A-S-H gels was proposed. The created composite geopolymer material, which is based on MSWI-BA and GBFS, therefore, has a lot of potential for use as sealing materials in coal seam gas drainage boreholes.

COMPREHENSIVE CONTROL MEASURES IN THE VERY LARGE TAKISAKA LANDSLIDE AREA IN JAPAN

The purpose of this paper is to highlight the comprehensive control measures in the very large Takisaka landslide area in Japan. The Takisaka landslide area is one of the most representative large landslide areas in Japan. The location of the Takisaka landslide is extremely peculiar and this landslide could pose a great threat to the surrounding area. The landslide area is directly adjacent to the main channel of the Agano River, which has a length of 210 km and a catchment area of 7,710 km2 and flows from the eastern part of Fukushima Prefecture through Niigata Prefecture to the Sea of Japan. In the worst case, if the Takisaka landslide were to occur on a large scale, the sliding mass of soil could dam up the mainstream of the Agano River. In this case, a large reservoir would be created upstream and many residential areas would be flooded. In addition, if the reservoir were to burst, there would be flooding downstream. In order to avoid such a situation, the Takisaka landslide area must be managed extremely carefully, and comprehensive and drastic countermeasures must be taken. Originally, it is impossible to completely prevent such a large landslide by hard countermeasures using engineering structures alone. Therefore, for such a large landslide, it is necessary to monitor the amount of landslide movement in real-time continuously and to immediately issue an evacuation order to nearby residents when a critical amount of movement is measured. At the same time, however, it is also necessary to install hard measures using engineering structures to prevent the continuous movement of the landslide. This report first describes the characteristics of the Takisaka landslide. Then, comprehensive control measures, both soft measures and hard measures, are reported. The main part of the soft measures is the early warning system based on the comprehensive real-time monitoring system of this landslide. The main components of the hard measures are drainage works to collect and channel dangerous groundwater, such as drainage tunnels, drainage wells, and drainage boreholes. Finally, the effect of the drainage works already installed is illustrated three-dimensionally.

Study on the Coupling Effect of Stress Field and Gas Field in Surrounding Rock of Stope and Gas Migration Law

In the process of working face mining, the permeability of the coal seam and the crack evolution characteristics of overlying strata are very important for efficient gas drainage. In this study, the distribution characteristics of the stress field and crack field in the working face and their relations are analyzed mainly by 3DEC numerical simulation. Furthermore, combined with the on-site measurement of coal seam stress, gas pressure, and gas seepage in front of the working face and the gas seepage in overlying strata before and after mining, the coupling effect of stress field and gas field and the law of gas migration and distribution in the working face are deeply explored. The results show that the changing trend of gas seepage and gas pressure is controlled by the stress change of the working face, and with the increase of stress, gas pressure and gas seepage also increase. The peak position of gas pressure is the farthest from the coal wall, about 22.5~25 m, followed by the peak of stress and gas seepage. When the permeability of coal and rock mass increases, the gas seepage increases and the gas pressure decreases. The coal seam stress and gas seepage in the working face and gas seepage in the overlying strata fracture zone along the tailgate side are generally greater than those on the headgate side, but the gas pressure is the opposite. Mining cracks and strata separation provide a good channel and space for gas migration and accumulation. Along the strike and tendency of the working face, gas is mainly concentrated in the overlying strata crack space above the separation zone and the roof and overlying strata crack space on the side of the tailgate, respectively. Based on this, the directional borehole gas drainage technology and borehole layout scheme in the fractured zone are put forward, which effectively reduce the gas concentration in the working face by 30~36%.

Mechanism of Coal Permeability Enhancement by a Series of Mechanically Over-excavated Cavities Along Drainage Boreholes

Mechanism of Coal Permeability Enhancement by a Series of Mechanically Over-excavated Cavities Along Drainage Boreholes

The Preparation of Active Support-Based Sealing Material and Sealing Effect Analysis

To improve the effect of gas drainage, with the aim of overcoming the problem of air leakage of conventional sealing materials, this study proposed the concept of gas drainage borehole sealing with active support and developed a new type of sealing material with the properties of permeability, fluidity, expansibility, early strength and compressive resistance through orthogonal tests. Using a specially designed active support testing device, this study analyzed the development law of borehole fracture under the action of active support force, which was then verified via field tests at a mine in Henan Province, China. The results show that the higher the expansion rate, the smaller the internal friction angle and cohesion, which is reflected in the sharp decline in the peak strength of the sealing material. The active support force of the sealing material has a significant time effect. It increases rapidly at first, before its growth rate slows down and tends toward a certain value. The greater the active support force provided by the sealing material, the smaller the radius of the plastic zone and breakage zone; this observation is also true of the displacement around the borehole. The active support force of sealed boreholes can effectively inhibit the development of cracks around the hole and reduce the generation of gas leakage channels. Under the action of the same burial depth and active support force, the larger the mechanical parameters of rock, the smaller the radius of the plastic zone and the breakage zone, as well as the smaller the displacement around the borehole. As has been proven by engineering practice, under the same conditions, the gas drainage concentration of the new sealing material is 47.5%, which is 84.8% higher than that of the original sealing material.

Mechanical properties and energy evolution characteristics of borehole-containing specimens under disturbances of cyclic incremental stress with different lower limits

Drilling gas drainage boreholes in coal mines is a major measure to prevent gas disasters in coal mines. However, coal mining disruptions can affect the stability and shape of gas drainage boreholes. Therefore, multi-level cyclic loading and unloading tests with a constant lower limit and constant-amplitude multi-level cyclic loading and unloading tests were conducted on hole-containing specimens. The results show that cyclic loading and unloading enlarges the deformation modulus of specimens with holes. The stress change during cyclic loading affects the deformation modulus of the specimens, rendering them more brittle and vulnerable to damage under high stress-induced disturbances. Additionally, the dissipated energy per unit volume of the specimens containing holes increases exponentially during multi-level cyclic loading and unloading. The cumulative dissipated energy per unit volume of the hole-containing specimens exhibits logarithmic growth during constant-amplitude multi-level cyclic loading and unloading. Furthermore, a discontinuity in the curves is observed as the stress level rises, which promotes brittle failure around the hole. The research results expound the deformation and energy evolution characteristics of hole-containing specimens during cyclic loading and unloading, and provide a reference for clarifying the failure modes of boreholes in the gas extraction process.

Substantiation of Drilling Parameters for Undermined Drainage Boreholes for Increasing Methane Production from Unconventional Coal-Gas Collectors

Decarbonization of the mining industry on the basis of closing the energy generation, on the basis of cogeneration of coal mine methane, and on the internal consumption of the mine is a promising direction in ensuring sustainable development. Known problems of deep underground mining do not allow for realizing the potential of man-made gas reservoirs due to the deterioration of the conditions of development of reserves of georesources. The aim of the work was to improve recommendations for the substantiation of drilling parameters for undermined drainage boreholes for increasing methane production from unconventional coal-gas collectors. The authors’ approach innovation lies in the possibility of using the established patterns of better natural stability of undermined boreholes to optimize them as spatial orientation parameters in an existing drilling passport for the improvement of methane extraction productivity. For this purpose, smoothing (LOESS) of the experimental data of two similar types of wells was used; then deterministic interpolation methods in combination with a three-dimensional representation of the response function in “gnuplot” were used. As a result, it was found that the increase in the inclination angle from 40° to 60° leads to a significant transformation of the model of the studied process, accompanied by a decline in the dynamics of methane emission and a decrease in the distance of the productive work zone of this type of well from 13 to 5 m before the roof landing, which then is replaced by a sharp increase in the productive work zone up to 35 m ahead of the longwall face. This allows under specific conditions for recommending increasing the productivity of methane capex from technogenic disturbed coal-gas reservoir replacement of wells with a smaller angle of rise to the transition to a more frequent grid of clusters from wells #4.

Construction Method of Honeycomb Gas Flow Network Channel: Structure-Based and Engineering Response on Plastic Zone Distribution Form

The plastic zone of the surrounding rock of the borehole is a high-quality channel for gas flow. Studying the distribution form of the plastic zone in the surrounding rock of the boreholes and the characteristics of the extended connection network is essential for the optimal layout of the gas drainage boreholes. Considering gas pressure parameters based on the modified Terzaghi effective stress principle and Mohr–Coulomb strength criterion. The implicit equation of the plastic zone boundary of the surrounding rock of the borehole is derived. The influence of two-way confining pressure ratio, coal body strength, gas pressure and hole diameter on the distribution of the surrounding plastic rock zone and the permeability-increasing circle is analyzed, and a honeycomb gas flow network channel structure based on the distribution of plastic zones is constructed. The gas flow and enrichment zone of the coal seam are connected, and the gas drainage rate is improved.

Crack closure effect during the impact coal seam with high-pressure air blasting and the influence of gas drainage efficiency

The crack closure in impact coal seams induced by high-pressure air blasting greatly affects gas drainage efficiency. The length of the crack closure was calculated and analyzed based on energy and elastic theories. The closure region was then determined to be 3.8 m from the blasting hole. The results of a high-pressure air blasting experiment in the underground of one coal mine in China showed that the effect of crack closure on gas drainage efficiency manifested as a decreased amplitude of gas emission in the crack closure region. At 1.0–4.0 m from the blasting hole, the amplitude of gas emission in the observation holes first increased and then decreased with increasing distance from the blasting hole. At 1.8–2.5 m from the blasting hole, the amplitude of the gas emission was maximal. At 4.0 m from the blasting hole, the crack was nearly closed, and the gas emission in the observation holes was minimal. The theoretical calculation had good consistency with the field test results; thus, it can provide an important reference for an appropriate arrangement of gas drainage boreholes.

Parameter Optimization of Constant Pressure Grouting Technology for Borehole Sealing with Inorganic Noncondensable Material in Tectonic Coalbed of South China

To solve the problem of a rapid attenuation of gas concentration along with drainage borehole collapse in the tectonic coalbeds of South China, a constant pressure grouting technology with inorganic, noncondensable material was proposed. Firstly, the slurry fluidity, water separation rate, and sealing performance of the inorganic sealing materials were tested under different water-cement ratios. The seepage model of slurry in a layer-through borehole was built with COMSOL Multiphysics simulation software, to explore the scopes of loose circles around drainage roadway and borehole, and to analyze the seepage capacity of the slurry under different grouting pressures. Eventually, the sealing performance of the slurry was investigated in the field. The results showed that the inorganic, noncondensable material with the water-cement ratio of 5 : 1 has a strong fluidity, low water retention, high permeability, and good sealing performance. After the excavations of No.2164 drainage roadway and layer-through borehole, there are obvious stress concentrations both at the shoulder corners of the roadway and at the borehole bottom, and the scope of the loose circle around the roadway is about 6.2 m. The effective seepage radius of the inorganic slurry gradually increases with a rising grouting pressure, and the slurry seepage range in the sandstone section is broader than that in the mudstone section. Adopting the constant pressure grouting technology with the slurry, the average drainage concentration of boreholes in Puxi coal mine is 51.5%, and the average gas flow rate is 0.005 m3/min, which are 1.35 times and 1.67 times than those with the cement grouting method.