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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]

A systematic analysis on gas occurrence in geological structure zone is crucial to coalbed gas exploitation, gas disaster prevention and outburst risk prediction. To study the geological control of fold structure on gas occurrence and its contribution on coalbed gas outburst, a typical case taken from Qinan coal mine was investigated by laboratory tests, theoretical analysis and on-site exploration. Thereby, a comprehensive understanding was made from the influence of fold structure on tectonic stress, coal structure and gas occurrence based on integrated factors outburst hypothesis. The results indicate that burial depth from 470 to 675 m has a slight influence on the fundamental properties of 72 coal seam, whereas the existence of Wanglou anticline is more relevant to affect vitrinite reflectance, gas adsorption and emission capacity with a positive correlation. By comparing the field data of gas content, pressure and emission quantity, it can be discovered that gas occurrence in 81 mining area appears to be primarily controlled by fold structure. Similar trends can be found in drilling cuttings index, indicating that geological stress increases with the distance approaching Wanglou anticline. In this case, a detailed description of tectonic stress in studying area is conceptually presented. Based on physical, pore structure and surface analyses, further knowledge on structural transformation by fold structure demonstrates adsorption and seepage pores are well developed with complex and disordered surface morphology, providing an evidence on the geological formation mechanism of tectonic coal in Wanglou anticline zone. Moreover, a conceptual description is proposed to explain the generation, migration and preservation mechanism of gas occurrence in the fold zone, which is consistent with roof and floor lithological properties as well as gas content data. Additionally, the locations of six gas outburst accidents were investigated to verify the relationship with fold structure. Based on the above results, theoretical analysis for coal mine outburst is discussed, which highlights the effect of tectonic stress on gas occurrence and coal structure as well as their contributions to gas outburst. [ABSTRACT FROM AUTHOR]

In order to obtain the weakening influence of coal seam water injection on coal and gas outburst, taking the 1100 m outburst coal seam in Sunjiawan Coal Mine in Fuxin of China as the research object, coal and gas outburst energy conversion model was established. The influence of water content on coal and gas outburst test was studied by using a self-developed simulation experimental system. The relationships between water content and critical gas pressure, outburst strength and energy conversion rate were analyzed. The results show that gas pressure is the main power source of coal and gas outburst, and the relationship between water content and critical gas pressure increases exponentially. When the water content increases from 0% to 3% and 6%, the relative strength of outburst decreases by 9.6% and 1.2%, respectively. With the increase in water content, the relative outburst strength and absolute outburst strength will gradually decrease. Gas internal energy is the key factor affecting coal and gas outburst energy which is closely related to the outburst-forming and the outburst-triggering process. As the water content increases, the outburst-forming energy and the outburst-triggering energy increase linearly. However, the energy conversion ratio decreases linearly and the less prone to coal and gas outburst. [ABSTRACT FROM AUTHOR]

The Pingdingshan mining region in China has witnessed severe coal and gas outbursts. A total of 153 coal and gas outburst accidents have occurred in this mining region. As the mining depth progressively increase, mining conditions in the region have become more complex, and gas outburst disasters have become more severe. This research statistically analyzed the 153 outburst accidents in the Pingdingshan mining region. Additionally, based on historical data and typical outburst cases, the characteristics and primary factors affecting coal and gas outbursts within the mining region were obtained. The results indicate that the outburst accidents in this mining region were primary influenced by the following factors: geological structure, mining depth, seam thickness, the lithology of roof and floor, and the mode of operation. As mining depth increased, the geotechnical environment and the mechanical properties of coal varied. Moreover, under the combined action of high stresses and considerable amounts of high-pressure gas accumulated within the coal and rock mass, the structures of both coal seam and rock strata in the mining faces or roadways were destroyed instantly. Therefore, coal and gas outbursts occurred. The outburst intensity increased significantly when mining depth is greater than 500 m. Coal and gas outburst accidents happened most frequently and severely in regions with geological structures containing such features as faults and folds due to the increasing gas pressure and sharply rising ground stress. Moreover, variation of seam thickness and the lithology of the roof and floor of the seam have significant control effect on coal and gas outburst. In addition, engineering disturbance was an external factor inducing coal and gas outbursts. For example, blasting and coal cutting caused changes in the stress state in the coal at the end of excavation roadways and mining faces. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is one of the main gas hazards in coal mines, and pulse hydraulic fracturing is a new technique for preventing and controlling gas hazards in coal mines and is presented based on conventional hydraulic fracturing. S2107 is a coal seam with high gas content and coal and gas outburst hazard but with low gas concentrations when extracted. To safely drive air tunnels into S2107, the pulse hydraulic fracturing technique is used on cross-measure boreholes placed from a high-level roadway to the driving roadway to improve the effects of gas drainage and ensure safety during production. The mechanism of coal fracturing via pulsed pressure is analyzed. A variable frequency method is used to strengthen the fatigue damage effect, and the initial pressure is estimated based on previous studies. The layouts of fracturing borehole and guide borehole are designed according to the layer relation, and a new hole sealing method is developed. The results indicate the pressure changes during pulse hydraulic fracturing reflect crack extension, and it is conducive to extending and connecting cracks under pulse pressures. The initial pressure during pulse hydraulic fracturing is negatively related to the water volume and fracturing time but cannot directly influence the fracturing radius. Moreover, this pressure is 30-43 % less than the calculated value. After this application, the gas desorption index, K, at the driving workface decreases to below the critical value. The gas drainage volume of fracturing boreholes and their guide boreholes increases relative to normal boreholes by 3.32-fold and 3.07-fold, respectively. The new technique is promising for preventing and controlling gas hazards in the future. [ABSTRACT FROM AUTHOR]

Coal and gas outbursts took place about thirty times at one coal mine in China, which not only caused safety problems but also hindered the regular production in different degree. In-situ geo-stresses of two points are successfully measured near coal and gas outburst locations, and the measured maximum principal stress is approximately at the horizontal direction, while the minimum principal stress is at the vertical direction. A three-dimensional model is built with FEM software of ANSYS, the measured geo-stress are taken as boundary pressures to be applied on the FEM model, and the geo-stress fields in the coal seam are calculated. In the end, the qualitative influence of the maximum principal geo-stress on coal and gas outburst is analyzed based on the relationship between geo-stress and number of coal and gas outburst. [ABSTRACT FROM AUTHOR]

Intrusive igneous rock is usually found in the overlying strata above mining stopes, and its occurrence, lithology, and distribution play important roles in coal mining safety. Of the numerous coal mine disasters in China, a large number have been caused by magma intrusion. Magmatic activity is intense and widely distributed in the Haizi Coal Mine which has suffered eleven coal and gas outburst accidents and one water inrush accident under a thick-hard igneous rock with 120-m-thick. Based on theoretical analysis, laboratory testing and field observations, we found that under the effect of thermal evolution and entrapment of the igneous rock, the coal pore structure developed, the gas adsorption capacity was enhanced, and the risk of gas outburst increased. The igneous rock, as the main key stratum, will not subside or break for a long time after mining. The closing time of fractures and separations is also prolonged and provides good conditions for gas drainage. The distant penetration borehole for draining pressure relief gas is proposed which can ensure effective gas drainage and reduce the number of rock laneways. However, with the continuous mining of a large area, the igneous rock could suddenly break, instantly releasing a tremendous amount of elastic strain energy, which will easily induce the occurrence of complex dynamic disasters, such as rock bursts, water inrush, gas outbursts, and surface subsidence. Based on the cause analysis of dynamic disasters, a reasonable goaf filling height is proposed for fully eliminating mine disasters under the special geological condition. [ABSTRACT FROM AUTHOR]