Simulation of Ground Stress Field and Advanced Prediction of Gas Outburst Risks in the Non-Mining Area of Xinjing Mine, China

Jilin Wang, Ming Li, Shaochun Xu, Zhenghui Qu, Bo Jiang
2018 Energies  
In order to predict in advance the coal and gas outburst risks in the No. 3 coal seam in the non-mining area of the Xinjing mine, the strata were divided into seven rock assemblage types based on the lithologic characteristics of strata in the research area, and eight geological profile models were constructed. The finite element methods were used to simulate the ground stress field of the No. 3 seam floor. Based on the log curve, the coal structural type of the No. 3 coal seam was identified,
more » ... am was identified, with the thicknesses of the coal body under different types of damage being marked off in each borehole. A damage index of the coal structure (DV) was also proposed, and the DV indexes for all boreholes were calculated. The coal and gas outburst risks in the research area were comprehensively evaluated and predicted through a superposition analysis of the spatial distribution states of three indexes, namely: ground stress, coal structure damage degree, and coal seam gas content. The results show that the equivalent stress of the No. 3 coal seam floor is usually within the range of 9-26 MPa. The high-stress zone presents a strip distribution along the northeast-southwest (NE-SW) direction. The distribution of ground stress is mainly subject to the folds and buried depth of the coal seam. The distribution range where the damage degree of the coal structure falls under Types II and III is DV ≥ 22. The gas outburst risk in the mid-southern and northeastern parts of the research area is high, whereas that in the mid-western part is low. The zones with a high and low degrees of gas outburst risks are all mainly present as strips in the NE-SW direction. The gas outburst risk in the northwest and southeast is moderate. The research results can provide guidance for gas control in non-mining areas. Energies 2018, 11, 1285 2 of 16 an important impact on gas (or coal bed methane) accumulation. In a mining area for example, the development of mylonitized coal (one kind of tectonic deformed coal that exhibits strong deformation) is not conducive to drainage [4] . Of course, the preservation conditions for gas after accumulation are also crucial. For instance, when coal bearing strata in some mining areas were raised and eroded, this type of tectonic evolution background was not conducive to gas preservation [5] . Gas emission during the mining process is affected by various factors. During exploiting, the annular-shaped overlying zone formed around the longwall panel could cause rock stress change, which then accelerates the development of cracks, and affects the gas flow pattern [6] . A test conducted in the field has shown that there is a good correlation between rock burst and high methane gas emission events [7] . The development of minor fault groups in a coal mine and its various combinations significantly affect gas emissions. For example, small, closed faults could create a sealed space so that gas could easily accumulate [8] . In addition, the amount of gas emission is also related to the content of the vitrinite group of coal rock: the amount of coal gas emission with a higher content of the vitrinite group is higher [9] . The mechanism of gas migration has been studied through experimentation, and its behavior has been investigated via numerical simulation. Experimental studies have demonstrated that gas seepage in coal seams is mainly influenced by the effective stress effect, shrinkage effect of the coal matrix, gas slippage effect, and the effect of the superposition of these variables [10] . It has been reported that the highest permeability is in the direction parallel to the bedding surface. Therefore, gas seepage can easily occur along the direction of the bedding surface [11] . An experiment comparing primary structural coal and molded coal showed that different coal structures may react differently during the adsorption-desorption process and with a change of residual strains after desorption [12] . In order to study the gas migration pattern, FLAC 3D and other numerical simulation methods are often employed to study stress distribution in the mining area. The gas-solid constitutive coupling model that takes gas pressure and the adsorption effect into consideration can be derived with COMSOL software, and it can then be used to simulate gas migration in front of the working face, and to study the pattern of gas migration in the coal seam [13] . Coal and gas outburst is a strong gas dynamic phenomenon, and a lot of research has been done in this field. This gas dynamic phenomenon in coal mines is closely related to ground stress and the thickness of the coal seam [14] . In addition to the closure effect of the roof and the floor, the area around the high density-gas zone in the coal seam is often a circular, low-permeability zone that causes an abnormal enrichment of the gas distribution in the coal seam, thus providing the conditions necessary for coal and gas outbursts [15] . Experiments on a briquette body have shown that the smaller the coal particle size, the larger the fractal dimension of the coal pore structure. Therefore, the adsorption capacity for gas is strong, and the strength of the coal and gas outburst is high [16] . In summary, it has been well understood that accidents resulting from coal and gas outbursts in mining areas are mainly affected by the geological structure, the mining depth, the thickness of the coal seam, the lithology of the roof and floor, the physical properties of coal, the gas content, and the mining methods [17, 18] . Different field test data must be used to evaluate the coal and gas outburst risks with the aid of an approach based on a mathematical method and artificial intelligence. In the early stage, the critical value of gas outbursts was defined by the amount of drillings and the rate of flow of gas [19] . Some scholars also discussed the relationship between the anomalous electromagnetic radiation of rocks, the EPR spectra of coals and the gas outbursts [20, 21] . Another model was dichotomic functions based on a random field to represent relations between the causes and effects of outbursts [22] . In the last 10 years, researchers installed meters and gauges in a pit to regularly measure gas pressure and its change pattern, and then calculated the coal seam permeability in order to control the gas outburst risks [23] . Using Monte Carlo techniques, several scholars collected data during field testing and laboratory tests and then applied a model grid to simulate and analyze the gas outburst risks [24] . Other scholars have tried to obtain the precise content of gas in coal seams by using a rapid field sampling technique aided by instruments and in combination with numerical simulations, in order to
doi:10.3390/en11051285 fatcat:45afzib5evafpiyri7sbcwhukq