Determination of Effective Radius under the Condition of Nonuniform Gas Occurrence and Borehole Plastic Failure.

In order to solve the problem of the blindness of borehole layout when the nonuniform occurrence of gas and the neglect of plastic failure around the borehole will underestimate the gas production, a gas-solid coupling model is established for considering plastic failure. The finite element software COMSOL Multiphysics is used to investigate the evolution of the effective radius during the nonuniform occurrence of gas, and the relationship between the effective radius and the borehole spacing is determined. The results show that the permeability around the borehole increases significantly when plastic failure is considered and the rising area is butterfly shaped, with a maximum increase of 94 times. Not only does the peak value of the gas drainage flow increase but the time of its occurrence is also delayed. The maximum gas drainage flow rate increases by 40.7%, and the cumulative drainage volume increases by 37%. The relationship between the effective radius and reasonable borehole spacing in nonuniform gas occurrence is obtained. This research can provide a reference for the selection of borehole spacing under the condition of nonuniform distribution of coal seam gas.

Research on Hydraulic Fracturing Pressure Relief and Improvement Permeability Technology of the Soft Coal Seam Roof.

Aiming at the characteristics of low permeability and large gas content of deep soft coal seam, hydraulic fracturing pressure relief and improvement permeability technology of the roof are proposed. FLAC3D was used to invert the roof hydraulic fracturing of the soft coal seams. On this basis, the dynamic evolution characteristics of permeability before and after fracturing were given by the COMSOL multiphysics software. Finally, roof hydraulic fracturing technology was applied on the site. The results show that after the hydraulic fracture of the roof, the peak stress on both sides of the roadway is reduced by 1.7 MPa, the peak stress concentration in front of the roadway is reduced by 2.2 MPa, the stress concentration range is reduced, and the stress is effectively transferred to the deep part of the roadway. After fracturing, the permeability of the stress concentration area increased by 2.18 times, the permeability of the original rock stress area increased by 1.84 times, and the gas fluidity improved. After roof hydraulic fracturing, the occurrence of a coal cannon decreases from 2 to 4 times per 1 m driving to 0 to 1 times, and the phenomenon of a coal cannon at the working face decreases significantly. The concentration of the return air flow is reduced from 0.5-0.9% to 0.2-0.4%. The results of this study have a guiding role in taking the corresponding measures for gas disaster prevention and control.

Online evaluation method of coal mine comprehensive level based on FCE.

An online evaluation method of coal mine comprehensive level based on Fuzzy Comprehensive Evaluation method (FCE) is proposed. Firstly, following the principles of fairness, systematicness and hierarchy, taking research and development, production, sales, finance, safety and management as the first level indicators, a set of multi-level evaluation indicator system of coal mine comprehensive level combining objective and subjective evaluation indicators is established. Secondly, according to the characteristics of the indicator system, the specific process of FCE of coal mine comprehensive level is given. Then, taking SQL Server as the database management system and C#.NET as the development language, a set of B/S structure online evaluation system of coal mine comprehensive level based on FCE is designed and developed. Finally, the proposed method is applied to Coal group PM for test. The application shows that the method proposed can provide an efficient and convenient online evaluation platform to evaluate the comprehensive level of coal mines for the Coal group, and the horizontal and longitudinal comparison of the evaluation results can urge the coal mines to maintain their advantages and avoid their disadvantages, which is of some significance for improving the overall competitiveness of the Coal group.

Synergistic inhibition effect on the self-acceleration characteristics in the initial stage of methane/air explosion by CO2 and ultrafine water mist.

Cellular instability is responsible for the self-acceleration of a flame, and such acceleration might cause considerable damage. This paper presents an experimental study on the inhibition effect of CO2 and an ultrafine water mist on the self-acceleration characteristics of a spherical flame in the initial stage of a 9.5% methane/air explosion in a constant volume combustion bomb. Results showed that insufficient water mist enhanced the self-acceleration of the spherical flame and the intensity of the explosion; nevertheless, the synergistic inhibition effect of CO2 and ultrafine water mist prevented enhancement of the explosion and significantly mitigated the self-acceleration of spherical flames, which observably delayed the appearance time of a cellular flame, and reduced the flame propagation speed, overpressure and the mean rate of pressure rise, indicating that suppression of flame self-acceleration could effectively mitigate the damage from a methane/air explosion. The reason for the synergistic effect was a result of a combination of physical suppression and chemical suppression: due to the preferential diffusion dilution effect of CO2, the initial flame speed was reduced, and the flame became thicker, which increased the evaporation time and quantity of droplets around the flame front, accordingly enhancing the cooling effect on the flame front. The increased flame thickness could withstand greater disturbance and inhibit the formation and development of a cellular flame. Meanwhile, CO2 and H2O can also reduce the concentration of active radicals (O, H and OH) and reduce the reaction rate and combustion rate of a methane/air explosion.