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On the basis of research into solid similar materials, a similarity criterion applying to “solid–gas” coupling of gas in coal rock is presented according to the “solid–fluid” coupling similarity theory. With sand as the aggregate and paraffin and oil as the cementing agents, a kind of materials suitable for “solid–gas” coupling simulation experiment is developed. Through lots of experiments, physical mechanics of similar materials are tested, such as compressive strength, brittle parameter, and permeation rate. By resorting to the self-developed device, tests are also made on the seepage of specimens of similar materials containing cementing agents of different contents. The findings indicate that with the continuous rise of paraffin content, the compressive strength of materials is increasing accordingly, and the seepage is decreasing gradually; and the change of oil content helps eliminate the shortcoming that compressive strength is changing significantly when only paraffin is used. Rocks of different strengths and different seepages can be simulated through the reasonable adjustment of the proportions of paraffin and oil by computer. Under the precondition that mechanical parameters meet requirements of simulated rocks, the paper conducts a comparison between and experimental analysis of the seepage velocities of the developed new material and the original solid similar material, and draws the conclusion that the developed material can remarkably reduce the seepage velocity of gas in it. Meanwhile, the material is also applied to the model experiment of coal mining, and effectively reveals the relationship between gas seepage rate and overlying rock movement.

Methane safety extraction is an effective method for reducing methane concentration in the gob and enhanced coal-bed methane recovery that is widely used in China. Although there is already extensive research on this technique, dip angle coal of seams has unpredictable effects on methane extraction locations, which is an influential factor that has rarely been studied, especially where the seam dip angles are larger. The benefits of this paper include improving methane extraction techniques, exploring the fractal characteristic of channel, and developing the laws governing how channels emerge, especially where coal seam dip angles vary. The results show that with increased coal seam dip angle, the height and width of channels show an increasing trend, along with crack angle. Key parameters of the methane migration channel boundaries in mutant regions of the bed separated under inclined coal mines were also defined. In the laboratory, an effective model was put forward to calculate the evolution characteristics of channels under inclined coal mining using fractal and elliptic paraboloid zone theory (EPZ); the finding was that the height of the A region at 30° was 1.34 and 2.00 times that of 15°, 0°, respectively. Moreover, borehole methane extraction concentration in area B increased to 6.04% in field application. The research results are expected to benefit industry and mine safety staff when arranging the position of high-level boreholes. The findings can enhance safety levels and improve efficiency in inclined coal mining.

To investigate the mechanical evolution progress of mining-induced pressure-relief gas migration channel in overlying strata fissure at fully mechanized mining working face, physical similarity simulation tests were conducted to experimentally and numerically evaluate the mining process of a coal mine in Heshun, Shanxi Province. The profiles of the gas migration channels were mathematically modeled based on the theory of elliptical zone of mining-induced fissure, using the dip angle of coal seams of working face as the key parameter. The impacts of dip angle of coal seams on the evolution of pressure-relief gas migration channel in overlying strata fissure at the main mining working face were analyzed. Results suggested that the increase in the dip angles of coal seams could reduce the height of the three zones and could expand the average weighting interval; the gas migration channels were formed at lower positions in the overlying strata, resulting in the lower height, width, caving angle, and range; in addition, with the increase of the dip angle, the segregation and propagation rates of the gas migration channel gradually increased, and the fractal dimensions of coal seams are measured in between 1.68, 1.69, and 1.71, displaying an increasing trend. On-site high-level boreholes were arranged to validate the laboratory results. Based on the monitoring, the gas extraction through the high-level borehole field reached 49.94~89.88% of the total gas emission. In general, the average concentration of gas in the upper corner is maintained below 0.27% and the average concentration of gas in the return airway is maintained below 0.32% so as to ensure a safe and an efficient mining environment. The study contributes to the theoretical basis for identification of pressure-relief gas enrichment areas in overlying strata under the effect of dip angle of coal seams.

This paper evaluates the cracking mechanism and material characteristics of rocklike specimens under five loading rates (0.02, 0.06, 0.10, 0.14, and 0.18 mm/s). The rocklike cylinder speci...

Fracture randomness and irregularities have a decisive effect on the gas reservoir and migration in overlying strata at different coal mining heights. To determine the boundary of a gas migration channel, its effect on gas storage, and migration in the overlying strata at different coal mining heights, this study integrated the two-dimensional (2D) physical similarity simulation of the calculated fractal dimensions to examine the results obtained using analytical techniques. With the increase of the mining height, the range of gas migration channels increased significantly. Moreover, we identified the primary parameters of the gas migration channel boundary in the mutant regions of bed separation and the broken fracture density. Apart from the increasing working distance, the fractal dimension increased when the fractal dimension of a working face side was larger than an open-off cut side. The fractal dimensions of a mine with a height of 6.0 m were 1.07 and 1.23 times of those for heights of 4.0 and 2.0 m, respectively. According to the variation principle of fractal characteristics and Reynolds number, the gas migration channel was divided into three regions, namely a gas active, gas circulation, and gas enrichment areas. On the basis of the fractal theory, an effective model was proposed to calculate the evolution characteristics of gas migration channels at different mining heights. Compared with the previous gas migration channel model, the proposed model could be appropriately estimated the position of high-level boreholes; the improved estimations can improve gas drainage efficiency and ensure safe and efficient mining.

Combined with the coal mining process of a high gas coal mine in Jinzhong, Shanxi Province, this paper discussed the evolution mechanism of the gas safety extraction channels induced by the mining of overburden fissures in the fully mechanized mining working face and carried out the physical similar simulation test. According to the theory of mining fracture elliptic paraboloid zone, the working face advanced speed as the key parameter, mathematical modelling of the gas safety extraction channel profile was performed. The effect of advancing speeds on the evolution of gas safety extraction channels in overburden fissures was comprehensively analysed. The results suggested that increasing advancing speed reduced the height of the caving, fractured and bending zone, enlarged average weighting intervals, and the range of development was small. What's more, with increasing advancing speed, gas safety extraction channels presented gradually reduced bed-separation volume and continuity, and the fractal dimensions (7.0