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Coal permeability is a key parameter that affects the drainage of coal bed methane (CBM). Owing to the lower original permeability of coal seams in China, more attention was paid to the study of damage‐induced permeability laws. However, the theory of damage‐induced permeability still needs to be improved. In this paper, the permeability evolution experiments during the complete stress–strain process of coal were carried out firstly. The results are shown that with the increase in axial strain, the coal permeability decreases slowly at first, then increases rapidly, and finally increases slowly or even remains unchanged, and the peak stress point can be regarded as the extreme point of permeability. Then, the equivalent plastic strain was used to describe the damage of coal. Based on the above results, a new damage‐induced permeability model was developed, and the new model can match the experimental data of permeability very well. In addition, solid–gas coupling models of coal were developed based on the new permeability model, and the numerical results show that there is a region of sudden increase in permeability around the borehole where plastic failure occurs, which indicates that the damage‐induced permeability model is reasonable. Finally, the results of numerical simulation of gas predrainage in combination of soft coal and hard coal under different cases were used to optimize the borehole layout of gas predrainage in the field. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a potentially fatal risk during the mining of gassy coal seams, which seriously threatens the safe mining of collieries. To understand the outburst mechanism and evolution rules, a new apparatus (LSTT) was developed to conduct simulated experiment. In the context of an outburst accident in Dingji coal mine, the authors launched an authentic outburst experiment to replay the outburst accident. Experimental apparatus, similar criterion, coal‐like materials and gas sources, and experimental design were discussed systematically in this paper. Experimentally, the study analyzed the geo‐stress has significant influence on the outburst evolution. At the driving face, the stress concentration possibly caused gas outburst, under the influence of mining‐induced stress. After the outburst occurred, the stress balance of the coal changed, resulting in the instability of the coal. Furthermore, the elastic energy, gas enthalpy, and gravitational potential energy were released rapidly. The experimental result stated that outburst coal has the sorting characteristics, in line with the field outburst law. The intensity prediction model has been built based on the energy model. Moreover, the factors that impact outburst intensity were analyzed. In the process of coal and gas outburst, the gas enthalpy of gas and the elastic potential of coal are the main energy sources. This study provides guidance for the development of the outburst mechanism and outburst mine management. [ABSTRACT FROM AUTHOR]

China is vigorously promoting the development of coalbed methane (CBM) resources because CBM is cleaner than coal and a hazardous gas in coal mining. However, the CBM production in China is significantly lower than that in other coal‐rich countries, such as Australia and the United States. In this paper, the main problems hindering CBM development in China are discussed to provide management suggestions for the government. These key problems include exploration and development techniques, the overlap of mining rights, management, financial supporting policy, investment environment, regulations, and market conditions. The results of this study indicate that to improve the development of the CBM industry in China, national and local governments must enact specific and exercisable regulations to increase financial support, improve technology, increase investment in CBM exploration, strengthen public project establishment, and improve the reform of CBM exploration and development policies. [ABSTRACT FROM AUTHOR]

With the increase of mining depth, the occurrence mechanism of coal and gas outburst becomes more complex, and stress‐dominated outburst disaster has become one of the major safety problems in deep coal mines. Therefore, stress relief of coal seam has become an urgent technical demand for outburst prevention method in deep mines. In this study, based on zonal disintegration characteristics of deep surrounding rock, a regional strip outburst prevention method, called strip stress relief combined with gas extraction through stress‐relief floor roadway, was proposed, and relevant in‐site investigations were carried out in the Qujiang coal mine in China. Our results show that the zonal disintegration phenomenon occurs in the high‐stress surrounding rock at great depths, and induces the stress‐relief effect on the upper coal seam, in which the permeability and extraction radius of boreholes are increased greatly. The results of deformation and failure of surrounding rock of floor roadway show that from the surface to deep regions, fractured and nonfractured zones were alternatively distributed in the surrounding rock of deep floor roadways, the damage degree of surrounding rock was gradually weakened, and the width of fractured area was gradually reduced. The displacement of surrounding rock also presented the characteristics of alternative distribution of wave peak and trough. Moreover, the deformation and failure of surrounding rock presented a time‐dependent rheological behavior. In the stress‐relief zone, the coal seam permeability was enhanced significantly, with the maximum value being 43 times larger than that of the original coal seam. The effective gas extraction radius of the borehole was improved by 15%‐45%. During the driving period of coal roadways, the outburst risk prediction indexes K1 and S of the working face were lower than the critical value, and the driving speed of the coal roadway improved from 40 m per month to 100 m per month. This indicates that the proposed strip regional outburst prevention method can simultaneously reduce the ground stress and the gas content, and can effectively eliminate the outburst risk of coal roadway in deep mines. [ABSTRACT FROM AUTHOR]

To explore practical approaches for the evaluation of favorable shale gas areas, a structural decomposition analysis with respect to the index system, evaluation criteria, weight determination, and evaluation methods was carried out. Five types of indexes were determined from 25 subindexes that formed the index system, and improved whitenization weight functions are proposed to establish evaluation criteria. The index weights calculated using entropy weight and Euclidean distance methods are consistent, whereas the weights determined by gray relational analysis are significantly affected by the relative number of indexes and evaluation objects, selected mother factor, and resolution coefficient. The significant linear positive correlation between the relative approach degree values calculated using TOPSIS and the scores obtained using fuzzy comprehensive evaluation (FCE) is indicative of a consistent evaluation result, but it should be noted that extreme indicator values can have strong effects on the evaluation using the fuzzy similarity method. Finally, an exploration strategy for China's shale gas is suggested: Areas with weak reformation and relatively low maturity are expected to be favorable for marine shale production in South China, and continental and transitional shale‐bearing basins are promising for the prospecting multiple‐origin shale gas and coal measure gas, respectively. [ABSTRACT FROM AUTHOR]

In China, one or several ultrathin soft coal sublayers are widely developed in the coal seam. Therefore, a method of using hydraulic flushing in soft sublayer to enhance the gas extraction in these particular coal seams is developed in this work. We first established a new fully coupled gas extraction model by combing gas diffusion, gas flow, and a permeability model that considers the effect of stress change and plastic failure. By adopting this model, the gas extraction enhancement mechanism and its main influence factors were studied using the numerical simulation method based on the engineering and geological background in the Yangquan No.5 coalmine. Thereafter, a hydraulic flushing equipment, which could move freely in the underground coalmine, was developed to apply the hydraulic flushing method in soft coal sublayers used in the 8402 working face. Its gas extraction effect was systematically investigated. Our simulation results match well with the field results, suggesting that our model is feasible. Meanwhile, after adopting this method, the gas extraction condition in this coalmine improves significantly. The borehole number decreases by 66.7%, while the gas extraction rate and gas extraction concentration increase by 1.33 times and 3 times, respectively. Moreover, during the coal mining process, the gas adsorption index of drilling cuttings, the quantity of drilling cuttings, and the CH4 concentration also decrease dramatically. [ABSTRACT FROM AUTHOR]

Coal seams in China are mainly characterizsed by low permeability and high risk of coal and gas outburst. Therefore, the elimination of coal and gas outburst risk of low‐permeability coal seam remains a hard challenge. Taking Shihuatian Coal Mine in Guizhou Province, China as an example, boreholes were slotted using high‐pressure water jet to relieve the gas pressure inside the coal seam and increase the permeability of coal mass, which thus improves the gas drainage efficiency and eliminate the outburst risk. The results show that the gas drainage efficiency of the slotted boreholes significantly increases. Compared with the conventional boreholes, the average gas concentration and flow velocity improved by 1.6 and 7.5 times, respectively. Moreover, the prediction index of outburst risk is less than the critical value after high‐pressure water jet slotting, which indicate that the coal and gas outburst risk has been effectively eliminated. The measured values of gas desorption index of drilling cuttings are all lower than the critical value after the application of the high‐pressure water jet slotting technology. The research achievements could provide a practical reference for the effective gas disaster prevention and control in low‐permeability coal seams. [ABSTRACT FROM AUTHOR]