Coal Seam Permeability Research Articles

A new technology to enhance gas drainage in the composite coal seam with tectonic coal sublayer

Composite coal seam is a kind of special coal seam that is formed after complex geological structure action, including a tectonic coal sublayer. Gas can't be drained easily from this coal seam, which cause coal and gas outburst accidents (CGOA) occur frequently. A new hydraulic flushing technology was proposed in this paper to improve gas drainage. Accordingly, a new set of mechanical equipment, technology process, roadway layout system and gas extraction monitoring system are provided by drawing lessons from the Guhanshan coal mine. The stress variation and damage failure were investigated by numerical simulation on the basis of flushed-out coal and borehole geometry. The gas drainage effect, coal seam permeability and outburst risk-sensitive indexes of the special coal seam were obtained by field measurements. According to the results, hydraulic flushing can enlarge the stress unloading and plastic damaged zone around boreholes dramatically, and the coal seam permeability can be increased largely.

Nano-CT measurement of pore-fracture evolution and diffusion transport induced by fracturing in medium-high rank coal

Fracturing, as a common fracture-making technique, can improve the permeability of coal seams to enhance fluid transport efficiency. To quantitatively evaluate the microscopic characteristics of medium-high rank coal, the loaded pore-fracture system was characterized by computerized tomography (CT) scanning under triaxial loading, followed by the analysis of stress-strain evolution, stress sensitivity and three-dimensional (3D) fractal dimension. Combined with snow algorithm and incompressible steady laminar flow simulation, the heterogeneous distribution of fluid pressure is investigated, focusing on the diffusion effect of gas transport. The results show that the strain of the high-rank coal Chengzhuang (CZ) in the linear elastic stage increases from 0.25% to 1.25%, greater than that of the medium-rank coal Qiyi (QY) from 0.75% to 1.63%, demonstrating a slight lag of the high-rank coal from the linear elastic stage into the yielding stage. The porosity of CZ changes from 1.66% to 13.58% and that of QY varies from 1.74% to 22.28% after fracturing, reflecting that the primary and secondary pores of the medium- and high-rank coals form a complex network structure for fluid transport through continuous connection-expansion. When the strain is between 0.75% and 1.25%, the stress sensitivity coefficient of CZ decreases from 0.13 to 0.02. Moreover, there are many mutation points in the 3D fractal dimension of coal samples after fracturing, mainly due to the generation of new pore-fractures at different locations of the computational domain. For fluid transport, the pressure of QY after fracturing spreads in a wider range than CZ, accompanied by more distribution of high fluid pressure. The diffusion coefficient of the fractured CZ is 350 times higher than that of the original coal under the gas pressure condition of 0.5 MPa, which provides the possibility for more gas to be converted from Knudsen diffusion to transition diffusion or Fick diffusion in the channel.

Permeability-Enhancing Technology through Liquid CO2 Fracturing and Its Application

Liquid carbon dioxide (CO2) phase change fracturing (LCPCF) is an innovative technique to improve the efficiency of gas drainage from low-permeability coal seams of high gas content. However, fracture sprouting, extension and displacement changes of coal under LCPCF need further study, and corresponding field tests are also lacking. Therefore, a mechanical model based on the thermodynamic theory of CO2 phase change is developed in this paper. Then, the pressure change characteristics, crack propagation and displacement change of coal subjected to LCPCF were analyzed through numerical simulation. In addition, the permeability-enhancing effect of the field LCPCF test was analyzed. The results obtained from the numerical simulation show that during the LCPCF process, the crack-generation process changes with pressure as follows: microfracture–numerous microfractures–major macrofracture–macrofractures. During the development of fractures, the stress is incompletely symmetrically distributed in coal centered on the fracturing borehole. The failure occurs stochastically in the coal in the vicinity of the fracturing borehole at first, and then it gradually propagates to the inner seam of coal as the gas pressure increases. The following result can be obtained from field experiments: the permeability coefficient of coal seams after increasing the permeability through LCPCF is 2.60~3.97 times that of coal seams without presplitting. The average concentration of gas extracted in coal seams within the zone having undergone an increase in permeability through liquid CO2 fracturing is 2.14 times greater than that within the zone without presplitting. The average pure amount of gas extracted within the zone having undergone an increase in permeability through LCPCF is 3.78 times greater than that within the zone without presplitting. By comparing coal seams before and after fracturing in the field test, it can be seen that the LCPCF presents a favorable effect in increasing the permeability of low-permeability coal seams. This provides an effective approach for increasing the permeability of coal seams in coal mines with similar geological conditions.

Study on the combustion characteristics of bituminous coal modified by typical inorganic acids

Acid fracturing technology is applied to increase the permeability of coal seams to promote gas extraction, but the effects of acid solutions on coal combustion cannot be ignored. This paper focuses on HF, HCl, and HNO3. The Bagchi method determines the mechanism function of each nonisothermal TG stage of acidified coal. Combining FTIR, TG-DTA, and KTA, this paper studies the combustion performance of acidified coal from the perspectives of the coal molecular structure evolution, combustion phenomenon, and kinetics. The results show that inorganic acids can destroy C=O in coal, resulting in the carbonyl groups in the HF, HCl, and HNO3 samples decreasing to 21%, 35%, and 0% of that in the RAW samples, respectively. Inorganic acid significantly enhances the reaction rate and thermal effect of combustion stage and improves the comprehensive combustion performance of coal, and the effect is in the order of HF > HNO3>HCl. Inorganic acids change the most likely mechanism function of initial weight loss stage. The oxidation increase of the HNO3 sample is greater than the corrosion weight loss, resulting in a corrosion ratio of −1.74%. HF and HCl reduce the activation energy of oxidation weight gain stage and increase the spontaneous combustion tendency of coal samples.

Influence of hygroscopic expansion of clay minerals on the dynamic evolution of coal permeability

Exploring the evolution characteristics of hygroscopic expansion of clay minerals in coal on its permeability has a positive effect on improving coal seam permeability. Firstly, the dynamic evolution model of coal permeability affected by hygroscopic expansion was established. Secondly, numerical simulation is carried out after an adaptive multi-field multiphase coupling tester verifies the model. Finally, the evolution law of coal permeability under different confining pressures and different moisture absorption expansion coefficients is analyzed. Research shows that the permeability of coal samples decreases first and then increases with the stress in the shape of a deep “V”. Due to the hydration and softening of clay minerals caused by hygroscopic expansion, the peak stress and permeability decreased by 3.8% and 1.69%, respectively, and the plastic strain increased by 6%. In this process, the change of permeability of axially compressed coal samples plays a leading role. Before reaching the yield stress, the influence of hygroscopic expansion on the change of permeability of the coal sample is more obvious. The research results enrich the theory of permeability dynamic evolution and have great significance in guiding coalbed methane exploitation.

Coupling Mechanism of Coal Body Stress–Seepage around a Water Injection Borehole

In China, coal seam water injection technology is widely used in the prevention and control of mine gas and dust disasters. The evolution law of stress–seepage field in the range of hydraulic influence in the process of coal seam water injection is the most basic theoretical problem in coal seam water injection technology. Therefore, this paper first uses the radial seepage experimental system to test the permeability of raw coal samples, and then numerically inverts the stress–seepage evolution law of the coal body around the water injection hole. The results show that the coal seam permeability decreases exponentially with the increase in the experimental overburden pressure. The dynamic water intrusion causes the pressure relief area and the stress concentration area to transfer to the deep part of the coal body, and abnormal stress occurs near the water injection hole wall. The composite stress area formed by the superposition of stress and pore water pressure reduces the permeability of the hole wall. During the radial development of water injection, the pore water pressure dropped sharply and eventually tended to zero, while Darcy’s velocity decreased significantly (within the pressure relief zone, it rapidly decreased from 7.26 × 10−3 m s−1 to 2.5 × 10−3 m s−1 (by 65.56%)). Near the initiation point, the maximum step-down height of Darcy’s velocity decreased from 6.73 × 10−3 m s−1 to 5.27 × 10−3 m s−1 (by 22.73%). This can make it clear that the seepage velocity presents a non-Darcy seepage phenomenon under the influence of pore pressure.

Research on the influence of liquid nitrogen freeze-thaw cycles on gas emission characteristics of coal

ABSTRACT To research the effect of freeze-thaw cycles of liquid nitrogen (LN2) on gas emission characteristics of different coal types, using low-field nuclear magnetic resonance device (NMR) and the emission of gas (Eg) experimental platform built independently, the pore structure characterization and dispersion characteristics of saturated anthracite and coking coal samples after 3, 6, and 9 exposures were studied. The experimental results show that the LN2 freeze-thaw cycles can significantly increase the Eg of coal under experimental conditions and have different effects on the gas emission characteristics of anthracite and coking coal. After exposure, the T2 spectra area (AT2) of anthracite and coking coal increases by 51.58% and 67.06%, the Eg max increases by 37.26% and 41.44%, and the initial gas emission velocity (Vg0) increases by 91.18% and 80.77%. In the freeze-thaw cycles, the changes of Eg and Vg0 are correlated with the changes of AT2 to a certain extent. The freeze-thaw cycles of LN2 can effectively transform the pore structure of coal, improve the permeability of coal seam, and promote the migration of coal gas. The research results of this paper provide theoretical support for the cryogenic cracking of coal seam by LN2, anti-permeability, and suction promotion in coal mine site.

Study on Mechanism of Pressure Relief and Permeability Enhancement in Soft-hard Composite Coal Seam by Directional Hydraulic Flushing Technology

The soft-hard composite coal seam composed of tectonic and primary structural coal has the problems, such as high gas content, high pressure, poor permeability and difficult extraction, which seriously affect the production safety in the mining. To study the evolution characteristics of pressure relief in soft-hard composite coal seam based on directional hydraulic flushing technology, taking Hudi coal mine in China as the engineering background, and the pressure relief effect of directional hydraulic flushing on 'two soft and one hard' composite coal seam was studied by theoretical analysis and numerical simulation. The stress evolution of the whole coal seam under a single hole and porous interaction of different flushing radii were analyzed. The field test of directional hydraulic flushing in tectonic soft coal seam similar to protective layer mining was conducted. Results show that when the flushing diameter is 3-4 m, the influence range of stress disturbance covers the whole coal seam, and the vertical stress relief of the primary coal seam is the largest, which provides a theoretical basis for coal seam pressure relief and permeability enhancement technology. The average daily gas extraction (ADGE) concentration has doubled, and the ADGE purity has increased from 0.03 m3/d to 1.07m3/d. The target period of gas extraction is shortened from the expected 6 months to 1 month, and the average daily driving speed of coal roadway increases from 2.32 m/d to 5.37 m/d. The gas extraction efficiency is significantly improved, and the risk of gas outburst is effectively reduced, which has important theoretical and practical significance for improving the pressure relief and permeability enhancement efficiency of the soft and low permeability coal seam.

Water-gas combined methane control in low permeability coal seam near fault: a case study

ABSTRACT Coal and gas outburst (CGO) is one of the most devastating disasters in underground coal mines, and it always triggers substantial casualties and property losses. Therein, the coal seam permeability and gas extraction rate are usually low in tectonic coal reservoirs, threatening safe coal production activities. In this work, water-gas combined methane control technology was proposed, based on the merits of hydraulic fracturing (HF) and liquid CO2 phase change fracturing (LCPCF), to improve the permeability of tectonic coal reservoirs. A field experiment was then conducted in a coal seam near a fault to verify its technical practicality, and the tectonic coal seam gas governance effect was accordingly evaluated. The research results suggested that the maximal concentration and average concentration of a single drill hole after HF increased by 13 times and 3 times, respectively, while the maximal flow and average flow increased by 23.4 times and 24.7 times, respectively. On the other hand, the peak concentration and mean concentration of a single hole after LCPCF increase by 2.7 times and 3 times, respectively, and the peak flow and mean flow increase by 6 times and 6.6 times, respectively. Additionally, the destructive degree triggered by LCPCF can be intuitively quantified through grayscale analysis. Furthermore, through the application of water-gas combined methane control technology in engineering practice, the gross quantity of extracted gas is 11 times larger than when LCPCF or HF is individually conducted, and the mean concentration and flow are 1.9 times greater and 2.1 times greater, respectively, than when LCPCF or HF is separately performed, indicating that this technology is practical and has a superior tectonic coal seam permeability improvement effect. This study has profound implications for enhancing coalbed methane (CBM) recovery and for eliminating CGO disasters in tectonic coal reservoirs or complex geologic structure areas.

Engineering practice of underground heat injection-enhanced gas extraction in low-permeability coal seams

ABSTRACT The heat injection mechanism for improving gas desorption, coal seam gas pressure, and coal seam permeability in the Yangquan mining area was investigated via theoretical analysis. This analysis was conducted to address the problems of poor permeability, difficult gas extraction, and low extraction efficiency of 15# coal seam. An underground heat injection-enhanced gas extraction technology was proposed; accordingly, a heat injection-enhanced gas extraction system was developed and appropriate extraction methods were devised. The findings of an industrial test on 15# coal seam in the coal mine show that the proposed technology not only increased the amount of gas extracted but also halved the extraction time and realized efficient gas extraction. Two mature underground heat injection-enhanced gas extraction methods were established. These methods have similar gas production principles in that the gas production speed and gas concentration in the heat injection stage are relatively low, while the average degree of improvement in the extraction stage after heat injection can reach more than 50 times. The average degree of improvement in gas production speed and gas concentration after the second heat injection is greater than that after the first.

Numerical simulation of coalbed methane under the mining conditions based on dynamic virtual well and variable permeability field

After coal seam mining,stress change and rock movement in adjacent rock strata, make permeability and other fields change in adjacent coal seam. At the same time, due to the exploitation of coal seams, material exchange may occur in the adjacent coal seams. It also caused desorption of near coal seam near the mining working face. Recent years, with the development of coalbed methane mining technology under pressure relief in mining areas, it is necessary to carry out numerical simulation under coalbed methane mining conditions to realize the optimization of coalbed methane extraction simulation under mining conditions. The coupling effect of dynamic virtual well and variable permeability field advancing with the mining face is used to simulate the output of coalbed methane under mining conditions: the dynamic virtual well is used to advance with the mining face to characterize the influence of the working face on the material exchange of adjacent coal seams in the mining process; the use of variable permeability field to realize the influence of the permeability of the adjacent coal seam during the advancement of the working face. Finally, through the dynamic coupling of variable permeability field and dynamic virtual well with the mining face, the rapid pressure relief effect of high speed and short time on the adjacent layer under the influence of mining is characterized. The simulation results of the computer simulation software written by the algorithm show that when the mining effect is not considered, that is, when the algorithm does not consider the dynamic virtual well production and the variable permeability field, it is basically consistent with the commercial software simulation results, but for the consideration of mining condition is a situation that commercial software cannot simulate, and the simulation results of this algorithm are more consistent with the actual monitoring data of the mine. This method achieves numerical simulation of CBM under mining conditions by improving the mature algorithm, which can provide effective guidance for future CBM capacity prediction under mining conditions.

Ultra-High-Pressure Hydraulic Slitting Pressure Relief and Penetration Enhancement Technology and Equipment for Soft Coal Seams

There is a “bottleneck effect” in the extraction process of ordinary boreholes. It is crucial to enhance the permeability of the boreholes. In this paper, a set of ultra-high-pressure water jet-based drilling and slotting integrated equipment was designed and applied, and the ultra-high-pressure safeguarding technology of several key components has been studied. Finally, the gas drainage effect of slotted boreholes and ordinary boreholes was investigated through field tests. The test results showed that after using ultra-high-pressure hydraulic slotting, the average drainage concentration of slotted boreholes is 1.49 times that of the ordinary boreholes. The average extraction scalar volume of slotted drilling is 3.02 times that of ordinary boreholes. Under the condition of original content of 12 m3/t, the extraction radii of slotted boreholes in 3 months and 6 months are 3.76 m and 4.76 m, respectively. The effective radius of the slotted boreholes is 1.76 times that of the ordinary boreholes, which indicate that the ultra-high-pressure hydraulic slotting technology can effectively relieve the pressure and increase the permeability of the soft coal seam.

Dynamic Effect of Shaped Charge Blasting and Its Application in Coal Seam Permeability Enhancement.

The dynamic effect of shaped charge blasting and its application in coal seam permeability enhancement have been investigated. Comparative experiments of shaped charge blasting and conventional blasting to fracture the concrete are carried out. Then, the propagation characteristics of explosion stress waves under shaped charge blasting and conventional blasting are analyzed by ANSYS/LS-DYNA. Finally, the fracture mechanical model of shaped charge blasting is established. The experimental results show that the width of the four main cracks formed after conventional blasting is 0.3 cm, while the width of the cracks in the energy accumulation direction after shaped charge blasting is 1.1 cm and the width of that in the vertical energy accumulation direction is 0.4 cm. The numerical simulation results show that the crushing area after shaped charge blasting is “dumbbell type”, and the area is smaller than that of conventional blasting. However, the cracking area is “spindle type”, and the development of the fracture degree is better than that of conventional blasting. In addition, shaped charge blasting is used to improve the permeability of coal seams. The results show that shaped charge blasting effectively improves the permeability and gas extraction rate of coal seams.

Experimental Study on Hydraulic Fracturing of Laminated Sandstone Combined with Industrial CT and L-NMR

Hydraulic fracturing in the immediate roof is a suitable method to increase the permeability of soft outburst coal seam. In this work, the propagation law of hydraulic fractures in laminated virgin sandstone and the change of microstructure after hydraulic fracturing were revealed. The results show that the hydraulic fracturing pressure of sandstone with different bedding angles is 15.69~22.17 MPa. In the experiments, the hydraulic fractures extend through the bedding planes when the bedding angle is 45°. When the bedding angle is 0°, 15°, 30°, or 60°, the hydraulic fracture is prone to extend along a bedding plane. The hydraulic fracture in sandstone is jointly determined by the internal fractures and the cementation strengths of the bedding planes. The internal fractures may not necessarily guide the extension of the hydraulic fractures. The hydraulic fracturing pressures of the samples affected by the bedding plane and internal fractures are significantly lower than that of samples with uniform strength. Besides, hydraulic fracturing cannot change the connectivity between pores. The hydraulic fractures are the main channel for gas migration. The results can provide some guidance for the design and application of hydraulic fracturing in the immediate roof of the soft outburst coal seam.

Anisotropy and fracture analysis for coalbed methane reservoir development in Bokaro coalfield, India

ABSTRACTThe low permeability of coal seams is a constraint in the efficient production of coalbed methane. However, the presence of natural fractures in coal enhances the permeability, and prior knowledge of sub‐surface fractures in coal seams is vital to identify the prospective seam. This paper investigates the anisotropy and identifies fractures by processing the advanced sonic and resistivity image logs to mitigate challenges in the reservoir. Anisotropy is estimated from the difference in the travel time between fast and slow shear waves. The application of Alford's rotation technique determines the fast shear wave polarization angle which is consistent with the fracture orientation along the NE–SW or NW–SE direction in coal seams. Moreover, the crossover of fast and slow shear waves in the slowness versus frequency plot indicates stress‐induced anisotropy that originates from fractures. Besides, drilling‐induced fractures observed along NE–SW in the resistivity image log indicate the maximum horizontal stress direction. Results from this study compare coal seams based on fractures in adopting better operational activities for optimized production and future geomechanical studies in the Bokaro coalfield situated in India.

Seepage Law of Nearly Flat Coal Seam Based on Three-Dimensional Structure of Borehole and the Deep Soft Rock Roadway Intersection

Exploring the evolution characteristics of gas seepage between boreholes during the drainage process is critical for the borehole’s layout and high-efficiency gas drainage. Based on the dual-porous medium assumption and considering the effect of stress redistribution on coal seam gas seepage characteristics, a coal seam gas seepage model with a three-dimensional roadway and borehole crossing structure has been established and numerically calculated, concluding that the coal seam is between the drainage boreholes. The temporal and spatial evolution characteristics of gas pressure and permeability help elucidate the gas seepage law of the nearly flat coal seam associated with the deep soft rock roadway and borehole intersection model. The results indicate that: (1) The roadway excavation results in localized stress in some areas of the surrounding rock, reducing the strength of the coal body, increasing the expansion stress, and increasing the adsorption of gas by the coal body. (2) Along the direction of the coal seam, the permeability decreases initially and then increases. The gas pressure in the coal seam area in the middle of the borehole is higher than the pressure in the coal seam around the borehole, and the expansion stress and deformation increase, reducing the permeability of the coal body; when near the next borehole, the greater the negative pressure, the faster the desorption of the gas attracts the matrix shrinkage effect and causes the coal seam permeability rate to keep increasing. (3) The improvement of gas drainage with the overlapping arrangement of two boreholes firstly increases and then decreases as time goes on. (4) When the field test results and numerical simulation of the effective area of gas extraction are compared, the effectiveness of the model is verified. Taking the change of the porosity and the permeability into the model, it is able to calculate the radius of gas drainage more accurately.

Experimental Study on the Dynamic Response and Pore Structure Evolution of Coal under High-Pressure Air Blasting.

High-pressure air blasting (HPAB) is one of the main feasible technologies to improve the extraction efficiency of unconventional gases. At present, there are few visual studies on the evolution characteristics of pore structure in coal under HPAB, resulting in an unclear understanding of the mesoscopic damage evolution mechanism of coal under HPAB. To study the dynamic response and mesoporous structure evolution characteristics of coal under HPAB, simulated coal specimens were used in HPAB experiments. The pore structure characteristics of coal at different locations away from the blasthole after HPAB were analyzed by using computed tomography scanning and 3D reconstruction technology. The maximum sphere algorithm was used to study the law of pore connectivity and reveal the mesoscopic damage evolution mechanism of coal under HPAB. The results indicate that the stress wave and attenuation and the crack propagation direction are greatly affected by the confining pressure. Compared without confining stress, the radial strain attenuation index decreases by 11.97% and the lateral strain attenuation index increases by 15.36% under confining pressure. Without confining pressure, the crack direction is disordered. On the contrary, the crack expands along the σ1 and σ2 directions with confining pressure, while the expansion along other directions is inhibited. The stress wave has a great influence on the pore structure in the nearby zone. Compared with before HPAB, at 25 mm distance from the blasthole, the number of pores increased by 24.80%, the number of throats increased by 12.96 times, the maximum equivalent radius of throats increased by 52.15%, and the maximum channel length of the throat increased by 56.06%. With the increase of the distance, the stress wave has little influence on the pore structure in the middle and far zones. The porosity of representative elementary volume and the distance from the blasthole decay in a power function trend. The maximum disturbance distance under HPAB can reach nearly 110 times of the blasthole radius. The study results provide a theoretical basis for enhancing the coal seam permeability and gas drainage of low-permeability coal seam by HPAB.

Study on Crack Penetration Induced by Fatigue Damage of Low Permeability Coal Seam under Cyclic Loading

For low permeability coal seam permeability is weak, low degree of gas migration, prone to gas accidents and other issues. In this paper, a numerical model is established to simulate the process of hydraulic fracturing under monotonic loading and cyclic loading, and a method of increasing permeability of coal seam by cyclic loading hydraulic fracturing technology is proposed. Combined with similar experiments, the influence of cyclic load and cyclic load applied parameters on the fracturing effect of coal and rock mass was analyzed by applying a cyclic load with a pulse pump. The effect of cyclic load pressure technology on coal seam drainage was analyzed by application in 20915 gas control roadways of a coal mine in Guizhou. The results show that after fracturing, the fracture extends along the weak plane of the prefabricated fracture, the pore pressure in the fracture is high pressure, and the pore pressure around the fracture decreases step by step. Due to the compression of the crack, the energy is transferred to the two ends of the crack. The pore pressure has an irregular oval distribution, and there is stress concentration. The pressure value reaches 41.48 MPa. After the cyclic load was applied to the model, the pressure reached the maximum value of 27.64 MPa at 3.37 s. Compared with the monotonic load, the pressure value was reduced by 46.27%. Through pressure and ringing analysis, the fatigue damage of specimens can be realized under cyclic loading. In the experiment, the unconstrained initiation pressure was 2.48 MPa, but after the constraint was applied, the initiation pressure increased to 4.58 MPa, and the pressure increase reached about 55%. After multiple loading and unloading, the peak pressure of the specimen can be reduced and the number of cracks can be increased. In the experiment, the gas extraction rate of ordinary drilling was maintained at about 0.019 m3/min, and the gas extraction rate of ordinary fracturing drilling fluctuated at 0.025 m3/min after 21 days of gas extraction. The pumping capacity of 15 Hz and 20 Hz cyclic loading fracturing boreholes tended to be stable after 15 days, which were about 0.041 m3/min and 0.062 m3/min, respectively. Cyclic loading hydraulic fracturing is better than monotonic loading hydraulic fracturing, and the lower the cyclic loading frequency, the better the fracturing effect.

Variable Weight-Projection Gray Target Evaluation Model of Degree of Protection of Protective Layer Mining

In order to quantitatively evaluate the degree of protection in protective layer mining and provide guidance for the design of a secondary outburst elimination scheme, a variable weight-projection gray target dynamic evaluation model for the effectiveness of protective layer mining is established. The improved order relation analysis method was used to determine the subjective weight of each index toward the decision-making goal based on numerical diversity characteristics, and the initial fixed-weight calculation for mixed multi-attribute metrics was processed through the degree of index action. The variable weight function was used to dynamically adjust the fixed weight through the penalty and incentive index methods. Four indexes (gas content, gas pressure, coal seam permeability coefficient, and expansion deformation) were selected, the outburst elimination and anti-reflection were taken as the guide, and the critical value of each index for eliminating burst and the critical value of pressure relief were taken as the positive and negative bullseyes. Based on the variable weight-projection gray target decision model, the distance between the two target centers of each scheme was calculated; at the same time, the variable weight vector changed dynamically with the evaluation scheme to achieve the dynamic quantitative evaluation of the degree of protection. Additionally, compared with the calculation results of fixed weights, it was found that the variable weight-projection bullseye distance can more accurately reflect the dynamic control effect of differences in numerical combinations of multi-attribute indexes in different decision schemes based on the degree of protection of protective layer mining. Taking a mine in PingMei as the engineering background, Ding protected the Wu area, and the degree of protection in the Wu group coal seam reached 116.29%, eliminating the outburst risk of the coal seam. The Wu protected the Ji group coal seam, with the degree of outburst risk in the Ji group being reduced by 14.27, and the Ding + Wu group protected the Ji group coal seam, with the degree of outburst risk of the Ji group being reduced by 20.71%, but not eliminated. The evaluation model quantifies the degree of protection of protective layer mining, and provides a theoretical basis for further assessing whether the working face should strengthen the enhancing permeability or whether it needs to be used in tandem with high-strength outburst elimination methods.

Modern Technologies for Coal Bed Methane Production in China: Trends and Development Prospects

Coalbed methane (CBM) is a new clean and unconventional energy that has broad space for development. There are many coal deposits in China where methane is present, especially in the landfills of medium and low grades of coal. In the country, significant attention will be paid to the development of coalbed methane resources in light of the carbon doubling policy. The gas from these reservoirs has the advantages of high calorific value when burned, cleanliness and no pollution to the environment. In the presence of rich reserves of these sources of raw materials, due to late development of their industrial extraction, the complexity of operation and low profitability of this business, exploration and exploitation have lagged far behind. There are also a number of technical problems (issues of methane distribution in coal seams, determination of their recoverable part, assessment of the permeability of coal seams) that need to be addressed in the processes of applying technologies and organizing production processes and using promising, effective methods of methane extraction in China. Accordingly, the research is focused on the development of CBM operation technologies, discussion and analysis of problems, and trends in their use. The existing technologies of methane extraction from coal seams have been identified and evaluated, including from the point of view of economic feasibility, the problems of their application and further development have been identified, as well as solutions to the identified problems and directions for improving coal development technologies have been proposed.