An Experimental Investigation of the Gas Permeability of Tectonic Coal Mineral under Triaxial Loading Conditions

Underground coal mining of CH4 gas-rich tectonic coal seams often induces methane outburst disasters. Investigating gas permeability evolution in pores of the tectonic coal is vital to understanding the mechanism of gas outburst disasters. In this study, the triaxial loading–unloading stresses induced gas permeability evolutions in the briquette tectonic coal samples, which were studied by employing the triaxial-loading–gas-seepage test system. Specifically, effects of loading paths and initial gas pressures on the gas permeability of coal samples were analyzed. The results showed the following: (1) The gas permeability evolution of coal samples was correlated with the volumetric strain change during triaxial compression scenarios. In the initial compaction and elastic deformation stages, pores and cracks in the coal were compacted, resulting in a reduction in gas permeability in the coal body. However, after the yield stage, the gas permeability could be enhanced due to sample failure. (2) The gas permeability of the tectonic coal decreased as a negative exponential function with the increase in initial gas pressure, in which the permeability was decreased by 67.32% as the initial gas pressure increased from 0.3 MPa to 1.5 MPa. (3) Coal samples underwent a period of strain development before they began to fail during confining pressure releasing. After the stress releasing-induced yield stage, the coal sample was deformed and cracked, resulting in a quickly increase in gas permeability. With a further releasing process, failure of the sample occurred, and thus induced rapidly increasing gas permeability. These obtained results could provide foundations for gas outburst prevention in mining gas-rich tectonic coal seams.

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Coupled Effects of Stress, Moisture Content and Gas Pressure on the Permeability Evolution of Coal Samples: A Case Study of the Coking Coal Resourced from Tunlan Coalmine

Water ◽

10.3390/w13121653 ◽

2021 ◽

Vol 13 (12) ◽

pp. 1653

Author(s):

Guofu Li ◽

Yi Wang ◽

Junhui Wang ◽

Hongwei Zhang ◽

Wenbin Shen ◽

...

Keyword(s):

Moisture Content ◽

Coalbed Methane ◽

Gas Permeability ◽

Axial Stress ◽

Gas Pressure ◽

Pressure Curve ◽

Permeability Evolution ◽

Confining Stress ◽

Qinshui Basin ◽

Gas Seepage

Deep coalbed methane (CBM) is widely distributed in China and is mainly commercially exploited in the Qinshui basin. The in situ stress and moisture content are key factors affecting the permeability of CH4-containing coal samples. Therefore, considering the coupled effects of compressing and infiltrating on the gas permeability of coal could be more accurate to reveal the CH4 gas seepage characteristics in CBM reservoirs. In this study, coal samples sourced from Tunlan coalmine were employed to conduct the triaxial loading and gas seepage tests. Several findings were concluded: (1) In this triaxial test, the effect of confining stress on the permeability of gas-containing coal samples is greater than that of axial stress. (2) The permeability versus gas pressure curve of coal presents a ‘V’ shape evolution trend, in which the minimum gas permeability was obtained at a gas pressure of 1.1MPa. (3) The gas permeability of coal samples decreased exponentially with increasing moisture content. Specifically, as the moisture content increasing from 0.18% to 3.15%, the gas permeability decreased by about 70%. These results are expected to provide a foundation for the efficient exploitation of CBM in Qinshui basin.

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Guiding Effect of Soft Tectonic Coal on Coal and Gas Outburst and its Experimental Study

10.21203/rs.3.rs-605221/v1 ◽

2021 ◽

Author(s):

Qingyi Tu ◽

Sheng Xue ◽

Yuanping Cheng ◽

Wei Zhang ◽

Gaofeng Shi ◽

...

Keyword(s):

Physical Simulation ◽

Local Existence ◽

Coal Pillar ◽

Gas Pressure ◽

Coal And Gas Outburst ◽

Control Group ◽

Long Distance ◽

Gas Outburst ◽

Soft Coal ◽

Tectonic Coal

Abstract Soft tectonic coal commonly exists in coal and gas outburst zones. The physical simulation experiment was carried out to reproduce the influences of soft coal area on the outburst, and the guiding action mechanism of soft tectonic coal on the outburst was investigated. This study concludes that the amount of outburst coal in the experiments of group with local existence of soft coal area are relatively lower. The outburst coal amount (3.8035 kg) and relative outburst intensity (21.02%) in the GR5# experiment were both lower than that in the GN6# experiment of control group. However, the outburst coal in the experiments of group with local existence of soft coal area could be commonly migrated to a long distance, the maximum throwing distances in the three experiments were all over 16.73 m, reaching as high as 20.10 m. Under the gas pressure of 0.30 MPa in the group with local existence of soft coal area, the outburst coal amount (2.7355 kg) was smaller than the amount (2.803 kg) of pulverized coal filled, and the 2.0 cm coal pillar experiences failure only nearby the outburst mouth. As the gas pressure increases, the failure degree of the coal pillar becomes higher and higher until complete failure. The outburst development sequence is changed due to the existence of the soft tectonic soft area. Once the sealing conditions are destructed, the outburst firstly develops in the soft tectonic coal area. Nevertheless, sufficient energy is supplied to transport the coal mass in the soft tectonic coal area to a farther distance, while the residual outburst energy can just result in the outburst of a small quantity of coal masses in the normal area. This research will be of great scientific significance for explaining the soft tectonic coal-induced change of outburst starting and development sequence.

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Model development and analysis of coal permeability based on the equivalent characteristics of dual-porosity structure

Journal of Geophysics and Engineering ◽

10.1093/jge/gxz108 ◽

2019 ◽

Vol 17 (2) ◽

pp. 313-327

Author(s):

Haijun Guo ◽

Kai Wang ◽

Yuanping Cheng ◽

Liang Yuan ◽

Chao Xu

Keyword(s):

Coalbed Methane ◽

Model Development ◽

Gas Pressure ◽

Dual Porosity ◽

Permeability Evolution ◽

Evolution Law ◽

Fracture Width ◽

Coal Permeability ◽

Porosity Structure ◽

Gas Seepage

Abstract Mining is a dynamic fracture process of coal and/or rock. The structural failure of coal bodies will change the coal matrix-fracture characteristics and then affect the distribution characteristics of the coalbed methane (CBM). Because of the structural complexity of coal, the coal matrices and fractures will be assumed to the geometries with rule shapes when the gas seepage characteristics in coals are analyzed. The size of the simplified geometries is the equivalent scale of dual-porosity coal structures (i.e. the equivalent fracture width and equivalent matrix scale). In this paper, according to the reasonable assumptions with regarding to dual-porosity coal structures, a new coal permeability evolution model based on the equivalent characteristics of dual-porosity structure (ECDP model) was built and the effect of the equivalent characteristics of dual-porosity structure on the coal permeability evolution law was analyzed. It is observed that if the initial fracture porosity is constant and the equivalent matrix scale increases, the range in which the permeability of coal rises with rising gas pressure increases; if the equivalent fracture width decreases and the equivalent matrix scale is constant, the range in which the permeability of coal rises with rising gas pressure decreases. The ECDP model is more suitable for revealing the evolution law of the coal permeability when large deformations occur in the coal bodies and/or the coal structure is damaged irreversibly, especially during enhancing CBM recovery.

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Features of Jianshanchong Klippe and its Control to Gas Geology at Qingshan Coal Mine, Jiangxi Province

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.164.501 ◽

2012 ◽

Vol 164 ◽

pp. 501-505

Author(s):

Zhi Gen Zhao ◽

Jia Chen ◽

Jia Ping Yan

Keyword(s):

Coal Seam ◽

Coal Mine ◽

Emission Rate ◽

Coal And Gas Outburst ◽

Jiangxi Province ◽

Coal Seams ◽

Gas Emission ◽

Gas Outburst ◽

Transverse Profile ◽

Tectonic Coal

The coal and gas outburst is serious at Qingshan Coal Mine of Jiangxi Province, so it is of significance to research the features of Jianshanchong klippe and its control to gas geology. The research reveals that: Jianshanchong klippe is distributed from the east boundary of Qingshan Coal Mine to No. 45 Exploration Line, its transverse profile is like a funnel while its longitudinal profile is like a wedge, northwest side of the klippe is thicker and deeper while southeast side is thinner and more shallow. Because of the cover and insert of Jianshanchong klippe, the structure of coal-bearing strata is more complex, some secondary folds are formed, and also, the coal seam is changed greatly, the tectonic coal is well developed and the coal seam is suddenly thickening or thinning. Due to the effect of Jianshanchong klippe, the coal and gas outbursts occur in the area of secondary folds, thicker coal seams or tectonic coals. Concerning the prediction of gas geology in deep area, in view of the facts including simpler structure, stable coal seam and decreased thickness, the gas emission rate and the coal and gas outburst will decrease in Fifth and Sixth Mining Level than that in Second and Third Mining Level

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Experimental Study on Adsorption Pore Structure and Gas Migration of Coal Reservoir Using Low-Field Nuclear Magnetic Resonance

Advances in Civil Engineering ◽

10.1155/2020/8839819 ◽

2020 ◽

Vol 2020 ◽

pp. 1-9

Author(s):

Jiazhuo Li ◽

Penghui Guo ◽

Wenhao Xie ◽

Jiaqi Chu ◽

Zhiqiang Yin ◽

...

Keyword(s):

Nuclear Magnetic Resonance ◽

Magnetic Resonance ◽

Pore Structure ◽

Methane Adsorption ◽

Gas Pressure ◽

Low Permeability ◽

Coal Seams ◽

Low Field ◽

Tectonic Coal

For the quantitative recognition and characterization of the flow characteristics of polymorphism coalbed gas in tectonic coal, experiments on pore morphology, pore diameter distribution, and methane adsorption law in outburst tectonic coal were carried out by field emission scanning electron microscopy and low-field nuclear magnetic resonance. The results revealed abundant round and dense “pyrolysis pores” in outburst tectonic coals, most of which were adsorption and seepage pores, with micropores accounting for 78.2%. Most pores were independent and formed the network pore space for gas enrichment and migration in outburst tectonic coal. The transverse relaxation time (T2) of methane adsorption in tectonic coal and crushed outburst tectonic coals presented three peaks, namely, adsorption, drifting, and free peaks. The isolation of nanopores and micropores revealed lower adsorption capacity of outburst tectonic coal than that of crushed outburst tectonic coal. The gas staged adsorption of raw coal with outburst tectonic low-permeability was observed. Under low gas pressure, the T2 spectral peak area of methane adsorption increased remarkably, whereas that of desorbed methane increased slightly. As gas pressure was increased to a certain numerical value, the increment of methane adsorption decreased and tended to reach equilibrium. This finding reflected that methane adsorption tended to be saturated after gas pressure reached a certain value, but desorbed methane in isolated micropores increased quickly. The quantitative recognition and characterization of pore structure and gas adsorption in tectonic low-permeability outburst coal seams based on low-field magnetic resonance imaging provide an experimental method for gas exploitation in coal seams and the study and control of coal and gas outburst mechanism.

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Numerical Investigation of Complex Thermal Coal-Gas Interactions in Coal-Gas Migration

Advances in Civil Engineering ◽

10.1155/2018/9020872 ◽

2018 ◽

Vol 2018 ◽

pp. 1-9

Author(s):

Xiaoyan Ni ◽

Peng Gong ◽

Yi Xue

Keyword(s):

Coal Seam ◽

Variable Temperature ◽

Gas Pressure ◽

Gas Migration ◽

Coal Seams ◽

Coal Seam Gas ◽

Coal Gas ◽

Thermal Coal ◽

Gas Seepage ◽

Gas Interactions

Understanding the influence of temperature on the gas seepage of coal seams is helpful to achieve the efficient extraction of underground coal seam gas. Thermal coal-gas interactions involve a series of complex interactions between gas and solid coal. Although the interactions between coal and gas have been studied thoroughly, few studies have considered the temperature evolution characteristics of coal seam gas extraction under the condition of variable temperature because of the complexity of the temperature effect on gas drainage. In this study, the fully coupled transient model combines the relationship of gas flow, heat transfer, coal mass deformation, and gas migration under variable temperature conditions and represents an important nonlinear response to gas migration caused by the change of effective stress. Then, this complex model is implemented into a finite element (FE) model and solved through the numerical method. Its reliability was verified by comparing with historical data. Finally, the effect of temperature on coal permeability and gas pressure is studied. The results reveal that the gas pressure in coal fracture is generally higher than that in the matrix blocks. The higher temperature of the coal seam induces the faster increase of the gas pressure. Temperature has a great effect on the gas seepage behavior in the coal seams.

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Numerical Simulation Research on Coal-Gas Outburst in the Process of Development

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.204-208.3377 ◽

2012 ◽

Vol 204-208 ◽

pp. 3377-3383 ◽

Cited By ~ 1

Author(s):

Feng Cai

Keyword(s):

Coal Seams ◽

Simulation Research ◽

Coal Gas ◽

Rock Materials ◽

Gas Outburst ◽

Whole Process ◽

Deformation And Fracture ◽

Gas Seepage ◽

Sph Algorithm ◽

The Impact

Coal-gas outburst is a kind of complex disaster induced by engineer operations in the process of extracting or developing in coal seams. According to the theories of gas seepage and deformation of coal or rock seams, taking consideration of the heterogeneity property of coal or rock materials in mechanics of materials as well as non-linear permeability characteristic of coal or rock materials in the process of deformation and fracture, SPH algorithm of LS-DYNA software is used to numerically simulate the coal-gas outburst induced by development in coal seams. Simulation results represent the whole process of coal-gas outburst from the growing, extending and connecting of cracks to final outburst under the impact of gas pressure, ground stress and mechanical properties of coal-rock. The nonlinear nature of the mutation of outburst induced by gradual destruction of the coal or rock materials under the influence of developing operations is revealed, including the evolution of the stress field in the rupture process of the coal or rock material. This provides a theoretical foundation for further understanding the mechanism of coal-gas outburst as well as preventing outburst technology.

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Research and Optimization of Gas Extraction by Crossing-Seam Boreholes from Floor Roadway

Geofluids ◽

10.1155/2021/7499012 ◽

2021 ◽

Vol 2021 ◽

pp. 1-10

Author(s):

Gang Li ◽

Jiafei Teng

Keyword(s):

Coal Seam ◽

Gas Permeability ◽

Gas Pressure ◽

Gas Content ◽

Gas Extraction ◽

Gas Migration ◽

Governing Equations ◽

Coal Seam Gas ◽

Gas Seepage ◽

Gas Disaster

Deep coal seams are characterized by large stress, high gas pressure, and low permeability. The gas disaster threatens the safe production of coal mine seriously. Gas extraction by crossing-seam boreholes from floor roadway (GECMBFR) can reduce the pressure and content of coal seam gas, which is the main measure to prevent gas disaster. Considering the Klinkenberg effect, governing equations of gas adsorption/desorption-diffusion, gas seepage, and stress fields within the coal seam are established to form the seepage-stress coupling model. The governing equations are embodied into a finite element driven software to numerically simulate gas migration and fluid-solid coupling law in coal seam. On this basis, the process of gas extraction under different borehole spacings and diameters is simulated. The effects of these two key parameters on coal seam gas pressure, gas content, and gas permeability were analyzed. The borehole spacing and diameter were determined to be 5 m and 0.09 m, respectively. Combined with the actual situation of a mine, the process of gas extraction from floor roadway with different cross-sectional schemes, ordinary drilling boreholes and punching combined drilling boreholes, is comparatively analyzed. The results show that the gas extraction effect by ordinary drilling boreholes is lower than that of the punching combined drilling boreholes, and the extraction is uneven and makes it difficult to meet the standard. Hydraulic punching was carried out, and coal was washed out of the borehole, which expanded the contact area between the borehole wall and coal seam. The coal seam around the punching borehole is unloaded, which improves coal permeability and accelerates gas migration towards the borehole, thus promoting the efficiency of gas extraction. It is more reasonable to use punching combined drilling borehole scheme when implementing the GECMBFR technology.

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Numerical Study on Shocking Coal Seam of Multi-Points Explosion with High-Pressure Gas

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.524-527.450 ◽

2012 ◽

Vol 524-527 ◽

pp. 450-454

Author(s):

Kun Gao ◽

Ji Ren Wang ◽

Bao Shan Jia

Keyword(s):

High Pressure ◽

Coal Seam ◽

Percolation Theory ◽

Gas Permeability ◽

Numerical Study ◽

Pressure Change ◽

Gas Pressure ◽

Low Permeability ◽

Gas Extraction ◽

Coal Seams

The permeability of coal seam is an important parameter for the gas extraction and gas outburst control. However, most of the coal seams are low-permeability with outstanding characteristic in China. Therefore，it is a good technology to provide the theoretical basis for increasing the permeability of low-gas- permeability coal seam by shocking with high-pressure air. Based on the percolation theory of porous media and combining the gas pressure change after shocking the coal seam with high-pressure air, the solid-gas coupled mathematical model is presented for the flow in the coal seam. By applying the software, the numerical simulation is computed and analyzed for the gas pressure evolution owing to the multi-spot continuous shocking the coal seam by the high-pressure air under the different pressure values and strata pressure.

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Mechanical criterion for coal and gas outburst: a perspective from multiphysics coupling

International Journal of Coal Science & Technology ◽

10.1007/s40789-021-00447-z ◽

2021 ◽

Author(s):

Ting Liu ◽

Baiquan Lin ◽

Xuehai Fu ◽

Ang Liu

Keyword(s):

Control Method ◽

Risk Identification ◽

Controlling Factors ◽

Gas Pressure ◽

Critical Values ◽

Coal And Gas Outburst ◽

Strong Interactions ◽

Gas Outburst ◽

Multiphysics Coupling ◽

Coal Strength

AbstractAlthough a series of hypotheses have been proposed, the mechanism underlying coal and gas outburst remains unclear. Given the low-index outbursts encountered in mining practice, we attempt to explore this mechanism using a multiphysics coupling model considering the effects of coal strength and gas mass transfer on failure. Based on force analysis of coal ahead of the heading face, a risk identification index Cm and a critical criterion (Cm ≥ 1) of coal instability are proposed. According to this criterion, the driving force of an outburst consists of stress and gas pressure gradients along the heading direction of the roadway, whereas resistance depends on the shear and tensile strengths of the coal. The results show that outburst risk decreases slightly, followed by a rapid increase, with increasing vertical stress, whereas it decreases with increasing coal strength and increases with gas pressure monotonically. Using the response surface method, a coupled multi-factor model for the risk identification index is developed. The results indicate strong interactions among the controlling factors. Moreover, the critical values of the factors corresponding to outburst change depending on the environment of the coal seams, rather than being constants. As the buried depth of a coal seam increases, the critical values of gas pressure and coal strength decrease slightly, followed by a rapid increase. According to its controlling factors, outburst can be divided into stress-dominated, coal-strength-dominated, gas-pressure-dominated, and multi-factor compound types. Based on this classification, a classified control method is proposed to enable more targeted outburst prevention.

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