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The coal mine in deep mining can easily form a compound dynamic disaster with the characteristics of rock burst and gas outburst. In this paper, the analytic hierarchy process (AHP) and the entropy weight method (EWM) are combined, and the fuzzy comprehensive evaluation (FCE) secondary evaluation model of compound dynamic disaster is proposed to evaluate the risk of compound dynamic disaster, which avoids the problems of the imperfect evaluation index system and strong subjectivity of index weight. Based on the statistical analysis of typical compound dynamic disaster cases in China, three first-level indicators were established, and sixteen second-level indicators were developed. The verification results show that the accuracy and weight are better than the traditional evaluation methods. Combined with geological and mining data, the compound dynamic disaster risk assessment was carried out on the second mining area of mine B, in the Pingdingshan mining area, and the result was grade II (weak risk). Corresponding prevention measures and parameters were implemented, and no compound dynamic disaster occurred during the working face excavation. [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]

Igneous intrusions are distributed extensively in the Huaibei coalfield, China. In the Haizi coal mine, coal and gas outbursts have occurred 11 times under an extremely thick sill (average thickness 120 m). This paper presents the results of a study on the influences of the igneous rock on coal pore structure, methane desorption and diffusion properties, and coal seam gas occurrence. The results show that the thermal evolution effect of the igneous sill prominently increases the specific surface area and pore volume of the affected coal. Samples HZ1 and HZ2 (No. 7 and No. 9 seams, respectively) closer to the sill possess improved pore connectivity, while samples HZ3 and HZ4 (away from the sill) and sample HZ5 (without sill covering) of the No. 10 coal seam have poor pore connectivity. Moreover, the effective diffusivity and desorption indexes of the coal increase progressively closer to the sill. The thermal effect of the igneous sill promotes the development of coal pores, thus leading to better pore connectivity, more desorbed gas, and much higher gas desorption and diffusion rates. Consequently, the thermal evolution effect of the igneous sill can change the occurrence and characteristics of the entrapment effect in the underlying coal seams, thus resulting in a high probability of gas hazards or even coal and gas outbursts in the coal seam close to the igneous sill. Engineering practices show that the affected coal seams have high gas content, gas pressure, and gas emission amounts as well as a high propensity for coal and gas outburst. [ABSTRACT FROM AUTHOR]

For the characteristics of high frequency and strong suddenness of dynamic disasters in deep coal mines, the traditional detection and evaluation techniques applied to shallow coal mine failed to accurately judge the risk degree of disasters. Therefore, it is of great significance to use advanced detection technologies and appropriate evaluation methods to improve the accuracy and efficiency of risk assessment in the process of coal mining. The present paper applies the rapid detection and multi-source dynamic detection technologies used in the field of mining with the purpose of improving the reliability of detection technologies for typical dynamic disaster. In this study, the data fusion technology was used to analyze data obtained from laboratory experiments, engineering survey, detection and historical data, so as to form the final dynamic and static indicators. Then, the new combined evaluation models with time series of coal and gas outburst as well as rock burst were established respectively to carry out the comprehensive risk evaluation using the least-squares method and the time-varying weight method. After the comprehensive analysis on the results of the above two evaluation models, the risk areas of the typical dynamic disasters were judged and classified. Finally, the evaluation models were coded to build an early-warning software platform that could achieve automatic evaluation and the actual 3D visualization of coal mining areas. The early-warning software platform was applied to risk assessment of dynamic disasters in Gengcun Coal Mine in Yima City, Henan Province, China. The results of the 6-month experiment showed that the risk assessment accuracy and reliability of the proposed evaluation models was 100% and 90% respectively, which indicates that the newly developed approach is reliable and can be recommended for applying in more coal mines to improve the process safety risk control. [ABSTRACT FROM AUTHOR]

The deep mining of coal mines leads to some significant changes in the physical properties of coal and rock, stress environment and gas occurrence law. The perturbed range of overlying rock and level of dynamic-static load have also increased significantly. In addition, the disaster effect caused by the linkage of mine groups and the control effect of large geological bodies appear in deep mining. The same disaster effect with the coexistence and conversion of rock burst and coal & gas outburst occurs. Therefore, above characteristics make the prevention and control of dynamic disaster in deep coal mines become a problem to be solved urgently. Considering the situation that the conditions of disaster breeding and mutual transformation mechanism are complex and still unclear, the quick detection method, prevention- control technology and equipment are still lacking, in this paper, the key scientific and technical problems of prevention- control for dynamic disaster in deep mining are condensed, the thought of prevention and control with different sources in multi-scales for coal and rock dynamic disasters in deep mining is proposed, the research direction of prevention and control technology for dynamic disaster in deep mining is determined, and the theory and technical framework of prevention and control with different sources in multi-scales for coal and rock dynamic disasters in deep mining are established. Based on the above achievements, the theory and technology system of prevention and control with different sources in multi-scales for coal and rock dynamic disasters in deep mining are formed successfully, which can provide a theoretical and technical support for the prevention and control of rock burst, the outburst of coal and gas or compound dynamic disaster in deep mining of coal mines in China. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing can increase coal seam fractures and reduce gas outburst risks, but the main challenge of its implementation in coal mines is to achieve uniform fracturing and gas reduction. In this paper, we studied the mechanism of water-driven gas release through two aspects: the H 2 O molecule is more easily adsorbed by coal than CH 4 , and water can decrease the gas adsorption capacity of coal. Comparative experiments of conventional hydraulic fracturing and blasting-fracturing technologies were carried out in #8 coal mine in Pingdingshan, China. We found that fractures and water contents around the blasting-based fracturing hole were more uniform than the conventional fracturing hole. After hydraulic fracturing, low water content coal usually has high gas content, which confirmed that gas in wet areas was displaced by water and driven to dry areas. To improve the uniformity of hydraulic fracturing, we proposed the uniform fracturing technology by combining blasting-fracturing holes and control holes. Blasting before hydraulic fracturing will increase the uniformity of fracturing and water distribution, and the control holes are used to monitor and release gas driven by water injection. The effective wetting range of conventional hydraulic fracturing was from −6 to 6 m along the strike direction and from −3 to 9 m along the dip direction with 94.71 m3 water injected. While the effective wetting range using uniform fracturing was increased to from −21 to 18 m along the strike direction and from −12 to 18 m along the dip direction with78.13 m3 water injected. Furthermore, number of boreholes to be drilled was largely reduced from 1300 to 132 when using the uniform fracturing technology to replace the dense hole technology in a 300 m long roadway. This research has suggested that uniform fracturing can effectively reduce gas outburst risk in gassy mines in China. • Analyzed water-driven gas release mechanism for coal and gas outburst control. • Developed uniform hydraulic fracturing by blasting and water-driven gas release. • Demonstrated uniform fracturing in field trails with less borehole required and less outburst risk. [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 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]

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]

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]

In Western China, there are some mining areas with complex geological structure and significant hazards of gas, coal and gas outburst. The outburst hazard not only threatens safety of mine gravely, but also makes it difficult to explore coal resources in some regions. Aiming at the characteristics of the complex structure, poor coal seam permeability and multi-seam mining in the mining area, with the purpose of eliminating the danger of outburst and decreasing the gas content of coal seam, this paper made a systematic research on the mining of the lower distant protective layer, the technology of pressure-relieved gas drainage using surface boreholes and eliminating the potential dangers of coal and gas outburst. By constructed the model of similar materials experiments of stope and numerical simulation, the author studied the depressurization effect of protected coal seam under the mining condition of lower distant protective layer: (1) During extraction of the lower distant layer, the stress of protected layer could be reduced sufficiently, and gas permeability coefficient could be increased substantially, providing favorable condition for gas drainage; (2) As the double protective seam mining,40-50 m area of intake entry and return entry of mining face in protected coal seams is dilated area, which is the proper location of surface boreholes; (3) Repeat depressurization mode is found that the gob of the extracted seam below the gassy one provides a "Buffering Effect" for the overlying coal seam which makes the depressurization of the overlying seam weaken. The results from mechanical model for analysis of the stability of surface borehole are: (1) The greater the difference between the strength of adjacent strata is, the bigger the squeezing and shearing stress in the casing become; (2) Under the condition of leaving the space between the casing and borehole wall, the max shearing stress of casing decreased by 50.8% evenly, and the probability of failure of casing was reduced significantly; (3) The longer the production casing is, the bigger the deflection value is and the greater anti-bending ability the casing gets. Based on the above results, the author designed a structure of surface borehole against shearing and rupturing. There leaves the "buffer spacing" between the cementing casing or borehole wall and the production casing which is whole-tube and screen section was handled by "casing enhancement technology", effectively improving the stability of surface borehole. The law of pressure relief methane drainage using surface borehole is demonstrated: (1) Illustrated the distance from working face to borehole (showing the methane permeability coefficients of overlying coal seam and fractured zone), is the key influencing factor of gas production rate of borehole; (2) Revealed the changing mechanism of "fast increase-slowly decrease" of the gas production rate of borehole during the protective coal seam mining, and determined the best layout parameters of surface boreholes; (3) Built a mathematical model for determining the gas flow rate of overlying coal seam and gob, which can provide a basis of assessment for gas drainage of overlying coal seam; (4) Proposed the method of improving gas drainage form overlying pressure released seams with increasing the lower borehole resistance. The research results mentioned above were carried out in Wulan Coal Mine of Shenhua Group, which show that the effect of eliminating outburst by extracting pressure relieved gas with surface boreholes is remarkable. The pure gas drainage quantity of boreholes in the testing area has totally reached 1 512.96 x 104 m³. The max spacing of borehole near the intake entry and the return entry are 150 m and 169 m respectively. The remaining gas quantities of the protected coal seams are eliminated to 3.63 m³/t and 3.14 m³/t, and the gas drainage rates are 65.8% and 68.0%, respectively, completely eliminating the outburst danger of the coal. [ABSTRACT FROM AUTHOR]