Research and Application of Hydraulic Punching Pressure Relief Antireflection Mechanism in Deep “Three-Soft” Outburst Coal Seam

To solve the problems of gas predrainage in deep seams with “three softs” and low-air permeability, hydraulic punching pressure relief antireflection technology is proposed on the basis of the research background of gas predrainage technology in Lugou Mine to alleviate technical problems, such as low gas drainage efficiency, in this mine. Through the analysis of the mechanism of hydraulic punching and coal breaking, combined with FLAC3D software, a hydraulic punching pressure relief antireflection model is established. Then, the fracture radii of coal rock are simulated and calculated. The results show that, under hydraulic punching with a water pressure of 10 MPa and coal outputs of 3 m3, 6 m3, 9 m3, and 12 m3, the fracture radii of coal and rock are 3.4 m, 4.8 m, 5.5 m, and 5.9 m, respectively. Using the software to fit the relationship between coal output V and hydraulic punching fracture radius R under the same water pressure, R = 2.32479 V0.3839 is obtained. The field test is carried out in the bottom drainage roadway of 32141 in Lugou Mine. The application effect is as follows: the gas concentration of hydraulic punching with a coal output of 3 m3 is twice that of ordinary drilling, and the coal output of hydraulic punching with a coal output of 6 m3 is four times that of ordinary drilling. The extraction concentration is four times that of ordinary drilling, and the extraction concentration of hydraulic punching with a coal output of 9 m3 is 6.4 times that of ordinary drilling. Combining the results of the numerical simulation and taking into account the actual construction situation on site, the coal output of water jetting from the borehole is 9 m3, and the fracture radius is 5.5 m. This outcome means that the effective half radius is 5.5 m, and the borehole spacing is 7.7 m. These values are the construction parameters for large-scale applications. This proposal provides effective technology and equipment for gas drainage in the deep three-soft coal seam. Consequently, it has promotion and reference significance for gas drainage in coal seam of the same geological type.

Stability Assessment of Deep Three-Soft Outburst Coal Seam Roof Based on Fuzzy Analytic Hierarchy Process

The stability of deep “three-soft” coal seam roof has always been a key issue in coal mining. There are a lot of factors affecting the stability of deep three-soft coal seam outburst roof. However, there is currently no definite method able to draw an accurate assessment conclusion on roof stability. In order to accurately determine the main influencing factors of the stability of deep three-soft coal seam outburst roof and reduce the loss of coal production, this paper performed three-soft coal seam risk identification on Lugou Mine based on the introduction of the fuzzy analytic hierarchy process theory. 23 main risk factors were identified. Then, it established a hierarchical structure model of coal seam roof stability in accordance with experts’ opinions. The analytic hierarchy process was used to calculate the weights of indicators at all levels. Next, the paper used the fuzzy comprehensive evaluation method and expert scoring to evaluate various risk factors in the indicator system, as well as the overall safety level. The results showed that the deep three-soft coal seam stability of Lugou Mine ranks the third hazard level. The main risk and harmful factors include safety awareness, safety monitoring system, roof weakness, ventilation system, fire-fighting system, and rock bolt quality. In response to the evaluation results, this paper formulated corresponding control measure in terms of ventilation risk, safety monitoring risks, construction personnel risks, and fire protection risk to reduce losses in the mining process, providing a new evaluation method for the stability assessment of deep outburst coal seam roof.

Theoretical Study and Numerical Simulation Research on the Effect of Coal-Rock Interface on Multistaged Fracturing in the Roof of Outburst Coal Seam

Multistaged fracturing in the roof of outburst coal seam is an efficient and creative technology for coalbed methane (CBM) drainage, which can effectively improve the permeability of coal seam. To reveal its mechanism of permeability enhancement, the effect of coal-rock interface on multistaged fracturing in the roof of outburst coal seam was simulated and discussed in this paper. Firstly, the lithological difference between outburst coal seam and roof was compared, and the concept and significance of multistaged fracturing in the roof of outburst coal seam were explained. Then, the mechanical conditions of multiple fractures in the roof traversing coal-rock interface were analyzed. The effects of mechanical parameters on multiple fractures were numerically simulated. The results indicated that fracturing borehole in adjacent rocks of outburst coal seam is much easier to drill and maintain gas drainage. Considering gas drainage efficiency and avoiding being blocked by coal fines, multistaged fracturing borehole is generally drilled in the stable rock stratum of roof. Whether the multiple fractures in the roof can traverse coal-rock interface is related to mechanical parameters of coal and rock, friction factor of coal-rock interface, angle between horizontal profile and coal-rock interface, cementing strength of coal-rock interface, minimum horizontal stress, and other factors. Higher fracturing fluid pressure contributes to propagating from the reservoir with low elastic modulus to the one with high elastic modulus for hydraulic fracture. Hydraulic fracture is more likely to propagate in the rock stratum with high brittleness index. The research results can improve multistaged fracturing theory and provide technological support for field test.

Analysis of pressure relief and outburst prevention effect and mining depth effect of floor roadway in single serious outburst coal seam

In order to solve the serious problem of single serious outburst coal seam gas disaster, based on the gas geological conditions of Jiangxi Fengcheng Qujiang company, theoretical analysis, numerical simulation, field test and other comprehensive research methods were used to analyze the partition fracture law of roadway surrounding rock under different mining depth conditions. The results show that: under the conditions of high ground stress, high temperature and other environmental conditions, the radius of pressure relief loose circle of surrounding rock of roadway is significantly increased, and it is significantly larger than that of shallow part. In the deep area of - 800m elevation, the distance between floor roadway and coal seam is optimized to be 12 ~ 15m. The field test results show that the original permeability coefficient of overlying coal seam of floor roadway increases by 55.74 times, and the pressure relief effect is good.

Numerical Simulation Analysis Based on the Study of the Slag discharge Mechanism of the Triangular-shaped Drill Pipe

According to the characteristics of high efficiency and high hole forming rate of the triangular-shaped drill pipe in the process of drilling in soft outburst coal seam, the slag discharge model of annular gap between the triangular-shaped drill pipe and hole wall is established by using the basic theory of computational fluid dynamics.Using the finite volume element method, the flow field characteristics of the annular flow area between drill pipe and hole wall are analyzed.Through the numerical simulation analysis, the velocity fluctuation effect of the annular flow field of the slag removal model is put forward, that is, the velocity fluctuation zone is formed between the sides of the arc edge of the triangular-shaped drill pipe, which causes the precipitated coal cinder to rise under the vortex action of the flow field, and is quickly discharged out of the hole under the coordination of the axial velocity, which effectively improves the slag removal efficiency and pore forming rate of the drill pipe.Finally, the comparative test of the triangular-shaped drill pipe and wide-blade spiral drill pipe was carried out in the soft outburst coal seam of 31021 lower auxiliary roadway in a mine. The drilling depth was increased by 19.4%, the hole forming rate was increased by 17.4%, and the drilling efficiency was increased by 26.4%. There was no drill drop and broken phenomenon.