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The stress of a coal seam increases with an increase in the mining depth, which makes the failure mechanism of a coal mass more complex. To reveal the deformation and failure law of deep coal, a series of triaxial experiments was carried out via laboratory experiments and numerical simulation experiments to analyze the influence of the confining stress on the nonlinear failure characteristics of coal. Based on the crack-propagation model, the values for the inelastic flexibility S 1 and the damage variable D decreased with an increase in the confining stress, which indicated that the increase in the confining pressure could inhibit the crack propagation and that the inhibitory effect was more obvious when the confining pressure increased in a small range of 4 to 12 MPa. The damage variable decreased with an increase in the confining pressure at the yield point; moreover, with an increase in the initial confining pressure, the damage rate gradually decreased. The coal body changed from the compression state to the expansion state when moving from the yield point to the peak point, and the compression value of the yield point and the dilation value of the peak point increased with the increase in the confining pressure. After the coal body entered the yield stage, the change in the confining pressure had a more significant effect on the damage to the coal body.S 1 decreased with an increase in the confining stress, which indicated that the increase in the confining pressure could inhibit the crack propagation and that the inhibitory effect was more obvious when the confining pressure increased in a small range of 4 to 12 MPa. The damage variable decreased with an increase in the confining pressure at the yield point; moreover, with an increase in the initial confining pressure, the damage rate gradually decreased. The coal body changed from the compression state to the expansion state when moving from the yield point to the peak point, and the compression value of the yield point and the dilation value of the peak point increased with the increase in the confining pressure. After the coal body entered the yield stage, the change in the confining pressure had a more significant effect on the damage to the coal body.

Coal mining leads to stress loading-unloading variation in front of the working face, which influences the occurrence of disasters. In order to study the influence mechanism of stress loading-unloading to the coal failure, a series of experiments of gas-bearing coal deformation and failure under triaxial stress were conducted and acoustic emission (AE) was monitored. In this study, the effect of gas pressure on the mechanical behavior of gas-bearing coal in conventional triaxial stress (CTS) experiments and fixed axial stress and unloading confining stress (FASUCS) experiments was analyzed, and the damage evolution rules of gas-bearing coal in the CTS experiments and FASUCS experiments were determined using AE. The results show that with the increasing of gas pressure, the peak strength and peak strain of gas-bearing coal in the CTS experiments and FASUCS experiments gradually decrease, and the peak of AE ring-down counts lags behind the peak strength. Compared with the CTS experiments, the strength of gas-bearing coal in the FASUCS experiments is lower and the precursor information appears later. The trends in calculated stress and damage coefficient D are consistent with the stress path during unloading, and both begin to rise sharply after the sample enters the plastic stage. Therefore, AE ring-down counts, damage coefficient D, and calculated stress can be used as precursor information for failure of coal and rock, which has great significance for the further study of coal-rock material and for early hazard warning.

Coal bed methane (CBM), the high-quality and efficient fuel, has caught the interest of many nations as they strive for environmentally friendly development. Therefore, the efficient exploitation and utilization of CBM has become one of the international focal research problems. A significant factor affecting the mining of CBM is coal permeability. To better capture the changes that occur during the extraction of CBM, the internal swelling coefficient of matrix (ISCM) has been gradually in permeability introduced into the permeability models, and such models have become an important type of the development of permeability models. The goal is to find out more precisely the evolution mechanism of the ISCM and its influence on the permeability models. In this paper, the selection of coal structure, determination of boundary conditions and influencing factors of permeability for were first analyzed. Then, according to the research process of ISCM, the permeability models including the ISCM were reviewed and divided into four phases: proposal phase, development phase, evaluation phase and display of internal structure phase. On the basis of the ISCM values in the current coal permeability models, the primary influencing factors and evolutionary laws of the ISCM are explored. The results obtained provide guidance for future theoretical refinement of permeability models with the ISCM. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a type of coal mine dynamic disaster with complex causes and serious consequences. In China, the number and scale of outburst disasters are among the highest in the world. It is an inevitable stage for outburst that the mechanical instability of the combination of intact coal and tectonic coal (CICTC) in strata. The dip angle of the interface has an important influence on the instability of CICTC-containing gas. To further understand the instability process of CICTC, the uncoordinated deformation of CICTC was defined. According to this, three strain conditions were delimited. The variations of stress under three strain conditions were analyzed, the expressions of additional interfacial stress were established. Then, based on the Mohr–Coulomb criterion, the strength characteristics and failure forms were analyzed. It was found that the dip angle deflects the principal stress in the interface, and the influences of additional stresses on the value of principal stress are opposite. These two reasons make the principal stress state and instability process of the combination complicated. Finally, combined with actual parameters, the instability processes of CICTC under different stress paths and buried depths were studied. It can be found that the failures of CICTC under different stress paths were concentrated appearing in the tectonic coal body, intact coal interface, and tectonic coal interface. With the increase of buried depth, the initial maximum principal stress increases and the reduction from additional stresses to the strength of intact coal interface enhances, which makes the combination damage easily and the failure form change. The research results in this paper can provide a theoretical basis to prevent and control the outburst of the combination containing gas considering the dip angle of the interface. Highlights: The uncoordinated deformation of combination considering the dip angle was first defined. The expressions of additional interfacial stress of combination considering the dip angle were established The influences of different stress paths and buried depths for the instability of combination were discussed quantificationally. [ABSTRACT FROM AUTHOR]

Coal mining has to develop to deeper strata with the increasing demand for coal, which leads to frequent coal and gas outburst disasters. Especially, when there is a deformed coal in the coal seam, the outburst disasters are easier and more intense. Coal and gas outburst disasters can be promoted by the fragmentation. In order to clearly explain the reasons for the impact of fragmentation on deformed coal and gas outburst, mercury intrusion porosimetry, low-temperature liquid N2 adsorption test, and scanning electron microscope were carried out in this paper. The differences in pore characteristics of intact coal and deformed coal under different particle size conditions were analyzed from pore shapes, pore size distributions, and fractal dimensions. The damage paths of fragmentation on various kinds of pores in intact coal are as follows: macropores, mesopores, minipores, and micropores, and those on various kinds of pores in deformed coal are as follows: macropores, micropores, mesopores, and minipores. According to the fractal dimension theory and experimental results, the calculative results of fractal dimensions show that the fragmentation and tectonism reduce the surface roughness and structural complexity of macropores and mesopores and increase those of minipores and micropores in deformed coal. Finally, combined with the coupling effect of fragmentation and tectonism on pores, the reason of deformed coal with high crushing degree in fields of outburst was explained. The research provides the basis for the prevention of coal and gas outburst. [ABSTRACT FROM AUTHOR]

Coal mining and production activities lead to static loading and unloading changes of coal stress in front of the working face, and the stress change process has a significant influence on the occurrence and development of coal-rock gas dynamic disasters. In this paper, the macroscopic failure characteristics, acoustic emission timing characteristics, and acoustic emission nonlinear characteristics of coal with different gas pressures under true triaxial loading and unloading conditions were experimentally studied. The results showed that the macroscopic failure form of coal with different gas pressures under unloading conditions was tensile-shear composite failure, and the crack structure was formed near the unloading surface. The fractal dimension D and multifractal parameter Δ α of acoustic emission time series both could reflect the complexity of coal fracture process. With the increase of gas pressure, the fractal dimension D and multifractal parameter Δ α decreased, which indicated that the greater the gas pressure, the lower the complexity of coal fracture process. Under different gas pressures, the dynamic change trends of multifractal parameter Δ α were similar, taking the beginning of unloading as the dividing point, the change was roughly in the form of "W." When the stress state and failure form of coal body changed, the multifractal parameter Δ α changed synchronously, indicating that the change of Δ α could reflect the transformation process of failure mechanism of coal body under load to a certain extent, which was of great significance for clarifying the occurrence and development mechanism of coal-rock gas dynamic disasters and ensuring the safe production in underground coal mines. [ABSTRACT FROM AUTHOR]

• The distribution of plastic failure and gas pressure of the coal in front of roadway during mining were analyzed. • The calculation model of gas outburst energy and the energy criterion of outburst were established. • The energy instability of gas outburst of coal combinations was assessed. The complexity of the coal and gas outburst process makes the gas outburst mechanism difficult to understand, leading to serious disasters of coal and gas outbursts in China. However, the presence of deformed coal may promote the occurrence of coal and gas outbursts, and several normal coal layers and deformed coal layers often occur at the same location. To better assess the energy instability of gas outbursts of coal combinations, the coal and rock were assumed to be homogeneous, isotropic mediums, and the deformation of coal and rock was assumed to be infinitesimal. Then, ignoring the influence of the change of gas pressure on the stress field during the process of gas emissions, the distribution of the horizontal stress, plastic failure and gas pressure in the coal in front of the roadway were analyzed in three different cases. The results show that there will be a large horizontal tensile stress between the normal coal and deformed coal, which may make plastic failure extend from the deformed coal to the normal coal through the interface. Due to plastic failure, the permeability of deformed coal in coal combinations will increase. Moreover, the deformed coal itself not only enables gas to migrate rapidly but also increases the gas emissions of the normal coal adjacent to the deformed coal. Then, a new model of gas outburst energy and an energy criterion of outburst were established. It was found that the total outburst energy of the normal coal in combinations is approximately 149 times that of a normal coal monomer, so the deformed coal increases the total outburst energy of the normal coal in coal combinations. In regard to the outburst energy of coal combinations, the desorbed gas expansion energy is the highest, which is 1–6 times the elastic energy and 5–17 times the gas expansion energy. In addition, measures should be taken to first reduce the expansion energy caused by desorbed gas, and second measures should be taken to reduce the elastic energy caused by ground stress. Finally, a series of control measures of gas outbursts for different coal seams were proposed. These research results, which were obtained under certain assumptions, can provide a theoretical basis for the selection of measures for the control of coal and gas outbursts of coal combinations. [ABSTRACT FROM AUTHOR]

• A general form of simplified diffusion model considering steady-state and unsteady-state diffusion was established. • The n in the model, as a gas diffusion state discriminant index, was determined. • The gas diffusion state was divided into three zones. As the prediction indexes of gas outbursts, the gas content and gas desorption index K 1 of drilling cuttings are affected by the gas diffusion in coal particles. According to the current research results, the steady-state and unsteady-state gas diffusion cannot be excellently defined, which leads to the measurement distortion of prediction indexes of gas outbursts in coal particles. Therefore, a general form of simplified diffusion model (GFSDM) considering steady-state and unsteady-state diffusion was established. And the experimental validation and reliability analysis were carried out. The results show that the fitting accuracy of GFSDM for gas diffusion data and the calculation accuracy of the lost gas amount are both greater than those of the simplified unipore diffusion model and simplified mathematical unsteady-state diffusion model. Then, the discriminant index of gas diffusion state and the boundaries of pure diffusion and pure steady diffusion were determined. Based on the boundaries of pure unsteady diffusion and pure steady diffusion, the gas diffusion state was divided into three zones, namely the ultra-negative unsteady diffusion zone, negative unsteady diffusion zone and positive unsteady diffusion zone. The influence mechanism of gas pressure, particle size and tectonism on the gas diffusion state was discussed. It is found that the main reasons for controlling the change of effective diffusion coefficients with time include the influence of diffusion length, influence of free gas pressure and influence of absorbed gas thickness. The research results can improve the gas diffusion theory and provide a theoretical basis for predicting coal and gas outbursts. [ABSTRACT FROM AUTHOR]

Coal seam gas drainage is not only the effective measure for gas outbursts control, but also has the great significance for the use of gas energy and reduction of greenhouse-gas emissions. Coal permeability is the key parameter of gas drainage, and the permeability model of mining-damaged coal has the important theoretical guidance value for gas drainage in low permeability coal seams. In order to better understand the permeability model of damaged coal, the key points during the establishment of permeability model, which are the selection of coal structure, selection of boundary conditions and influencing factors, were first analyzed. Then, the expression and application of permeability models of mining-damaged coal in recent 20 years were reviewed. According to the contribution of new fractures, the permeability models of mining-damaged coal were divided into the empirical permeability models (EPM), volumetric strain permeability models (VSPM), plastic strain permeability models (PSPM) and damage variable permeability models (DVPM). After that, the quantitative characterization of key parameters, scientific issues of permeability models of mining-damaged coal were reviewed and discussed. Finally, the prioritization of 18 indexes affecting the permeability were determined, and the potential research directions of permeability models of mining-damaged coal were pointed out. The results of this paper can provide the theoretical guidance for the improvement of permeability model of mining-damage coal in the future. • The expression and application of permeability models of mining-damaged coal in recent 20 years were reviewed. • The quantitative characterization of key parameters, scientific issues of permeability models of mining-damaged coal were reviewed and discussed. • The prioritization of 18 indexes of factors affecting the permeability were determined. [ABSTRACT FROM AUTHOR]

• A new mathematical time-dependent diffusivity model and its simplified model of deformed coal were established and validated. • The change law of gas diffusion coefficient of deformed coal with time was obtained. • The engineering application of SNMTDM was analyzed. Deformed coal provides favorable conditions for the occurrence of gas outburst disasters. The existing gas diffusion model cannot accurately calculate the lost gas amount during the sampling period of deformed coal. In order to solve the above problems, a new mathematical time-dependent diffusivity model (NMTDM) and its simplified model (SNMTDM) of deformed coal have been established and validated. At the same time, the performance and engineering application of models were also discussed. The results show that the fitting accuracy of NMTDM and SNMTDM to gas diffusion data is greater than that of unipore diffusion model (UDM) and simplified unipore diffusion model (SUDM), respectively, which indicates that it is reasonable to regard the diffusion coefficient of deformed coal as a time-dependent variable. The diffusion coefficient of adsorbed gas, diffusion coefficient of free gas and total diffusion coefficient of gas decrease with the increase of time, and SNMTDM can realize the accurate calculation of lost gas amount. However, when the application of SNMTDM extends beyond its applicable range, the calculating results of the model may deviate seriously from the actual values. The gas diffusion models established in this paper can improve the calculation accuracy of lost gas amount and gas content of deformed coal. [ABSTRACT FROM AUTHOR]

The mechanical properties of coal are important parameters for coalbed methane (CBM) extraction and gas outburst control. However, the effect of adsorbed gas on strength cannot be evaluated quantitatively yet. To better understand the weakening mechanisms of free and adsorbed gas on the strength of coal, normal coal, and deformed coal are chosen to test their mechanical properties of CH4-saturated and non-gas-saturated specimens under different conditions. Under the same effective stresses, the peaks of strength of CH4-saturated specimens with high-pressure gas are lower than those with low-pressure gas, implying that the adsorbed gas can also weaken the peak strength of coal. Then, a new model for the weakening values of strength induced by free and adsorbed gas was developed, and the effects of free gas and adsorbed gas on the strength of coal were assessed by our model. The results show that the free gas and adsorbed gas can weaken the strength of coal for different weakening mechanisms. The ratio of weakening value of strength due to free gas to that of adsorbed gas of the normal coal is 1.3–3.4, and that of the deformed coal is 8.4–19.8. These results can help us to better understand why the weakening effect of adsorbed CH4 in the laboratory is ignored in earlier studies. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mixed solution of HF and HCl can effectively expand the pores and cracks inside the coal. • The effect of acidification of coal will change with temperature. • The best acidification temperature for this experiment is 50 °C. • The acid-heat coupling affects the element composition on the coal surface and the internal pore structure of the coal. To study the effect of acid-heat coupled fracturing on the damage of coal pores and fractures, scanning electron microscope-energy dispersive spectrometry (SEM-EDS), mercury intrusion porosimetry (MIP) were used to analyze. The results of the study show that the acid-heat coupling effect not only changes the microscopic morphology and element composition of the coal sample, but also promotes the formation of a pore-fracture network within the coal, and the connectivity is significantly increased, which is conducive to the diffusion and migration of coalbed methane. The optimum acidification temperature for the experiment is 50 °C. SEM observed that the acid-heat coupling changed the local stress sensitivity of the coal body and weakens the strength of the coal body to form a fracture network. EDS analysis shows that the element distribution on the coal surface changes regularly with the increase of acidification temperature, and the reduction rate of mineral content on the coal surface is highest at 50 °C. MIP and fitting analysis show that the pore parameters of coal samples will increase with the increase of acid-heat coupling temperature, reaching a peak at 50 °C, and the acidification effect will be relatively slow after increasing the temperature. This research is of great significance for exploring the optimal acidification conditions of coal mines, reducing acidizing fracturing costs, and ensuring coal mine safety production. [ABSTRACT FROM AUTHOR]