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Gas drainage is an important measure to ensure energy safety in coal mines. However, for mines with high gas content and low permeability, the critical question is how to enhance the drainage effect of drilling. By using the self‐unloading pressure effect on the coal body in front of the mine, the permeability of the coal body can be improved, which facilitates gas drainage. Therefore, establishing the fluid–solid model of gas‐bearing coal mass under the influence of mining, this paper simulates the advanced self‐pressure‐relief range in front of the coal working face and the permeability of coal seam at different heights. Through simulations and field tests, the reasonableness of the advanced self‐pressure‐relief gas drainage was verified, and the spatial and temporal characteristics of the gas drainage from the borehole were quantified and analyzed. The results show that the width of the advanced self‐pressure‐relief zone after simulated mining is 25 m, the vertical permeability of the coal seam increases first and then decreases with the increase of the height from the floor, and the maximum permeability of the layer is 2.6 m from the floor, which is 8.62 times higher than the initial permeability; the gas drainage effect of test boreholes has a periodic law with the coal seam recovery, which, divided into the initial drainage stage, the scalar volume of gas drainage increase stage, and the scalar volume of gas drainage decay stage. At a distance from the working face 20–40 m, the boreholes' scalar volume of gas drainage increased by 3.89 times compared with the initial drainage stage, and the scalar drainage volume of the boreholes through the seam increased by 8.01 times compared with the initial drainage stage; in the section of increasing scalar volume of drainage, the gas drainage volume of the boreholes at different heights of the coal seam had obvious "stratification" phenomenon. The test boreholes were optimally placed in the middle of the seam at the lower end. The study provides a reference for self‐pressure‐relief gas drainage technology and optimization of borehole placement. [ABSTRACT FROM AUTHOR]

The revelation of the distribution characteristics of stress and fractures in the floor strata is of great significance to improve the regional gas drainage efficiency and to eliminate or reduce the risk of coal and gas outburst in adjacent coal seams during multi‐seam mining. In this paper, the stress distribution in the floor under mining condition was derived, and the characteristics of stress zones in the floor were elaborated while mining the 22201 working face in Shaqu coal mine in Shanxi province, China. Besides, the propagation laws of fractures in different stress zones were theoretically analyzed, and the distribution law of fracture angles in the floor strata under different mining‐induced stresses was obtained. The research results demonstrate that the compression and transition zones show high vertical stress and less developed fractures and are dominated by fractures with a large angle (>60°), which is unfavorable for gas drainage. The expansion zone in the floor within 2‐40 m in the rear of the working face is characterized by high unloading degree of vertical stress and developed fractures. Moreover, fractures with a small angle (<60°) dominate the spatial distribution of the fractures, which is favorable for gas drainage. The research results can not only be a beneficial supplement to the theory of coal and gas co‐exploitation in multi‐seam mining but also provide theoretical support for optimizing the design parameters of cross‐measure boreholes under similar conditions. [ABSTRACT FROM AUTHOR]

Gas outbursts pose a significant threat in the mining of deep low‐permeability coal seams. As mining depth increases, there is a rise in coal and gas dynamic disasters, rendering the gas outburst prevention technology from shallow mining areas unsuitable for deep coal seam exploitation. Addressing the challenges posed by deep, highly gassy coal seams, this study introduces a gas outburst prevention technology integrating destressing in the close distance underlying roadway with gas drainage. The article investigates the relevant technical parameters and assesses the feasibility of this approach. First, it establishes destressing model for the underlying roadway to derive an analytical solution for stress distribution in the coal seam. Second, appropriate technical parameters are designed considering the seepage characteristics of gas in the coal seam. Finally, the gas drainage technical scheme is implemented at Qujiang coal mine to verify its on‐site effectiveness. Field test results indicate an increase in gas permeability within the coal seam following destressing in the underlying roadway, accompanied by significant improvements in the efficiency of gas drainage and the advancement speed of the coal roadway. Specifically, the driving speed of the coal roadway has escalated from 40 m/month to 150 m/month. These outcomes demonstrate the potential of this technology as a promising approach for preventing gas outbursts in the challenging context of deep, low‐permeability coal seams, facilitating swift coal roadway excavation. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing experiments were conducted to study the change of initial fracture pressure and roof stress under repeated hydraulic fracturing in coal mines and to investigate the actual penetration enhancement effect on coal seams. The results show that as the fracturing is repeated, a lower initiation pressure is required. The roof stress changes consistently with water injection pressure. First, the stress in the area around the water injection hole gradually increases, and then stress transfers to the surroundings. With the re‐implementation of fracturing, the area of increased stress expands, and the scope of influence of hydraulic fracturing also becomes larger. When fracturing in a coal mine, for the same total fracturing time, the total water injection volume of repeated fracturing is 35% higher than that of single fracturing, and the affected area is increased by 31%. The concentration of repeated hydraulic fracturing is about 36% higher than that of a single hydraulic fracturing, and the pure flow of gas drainage is increased by about 1.3 times. The results of this study can guide the efficient implementation of hydraulic fracturing underground in coal mines, so as to achieve a larger impact range of coal seam hydraulic fracturing and better gas drainage effects. [ABSTRACT FROM AUTHOR]

In underground mining operations, the overlying strata move, deform, and crack as the underground working face advances due to strata breakage, failure, and subsidence, forming a "vertical three zone" over the mined out area. The permeability of strata under mining changes differently in different zones. Therefore, studying the distribution of strata permeability is very important to obtain knowledge of gas migration pattern in the overlying strata in order to increase the gas extraction efficiency. In this study, a series of numerical models are developed to describe the overlying strata movement for three different zones. In detail, models for calculating the strata subsidence for fracture and bending zones and analyzing strata collapsing development for the caving zone are proposed based on the influence function method and mechanical analyses. In addition, on the basis of such models, a new comprehensive model to describe the strain‐porosity‐permeability of strata is developed. In the case study of applying such models in an underground mine, the reliability of the strata movement model is verified and the permeability change is also calculated, which gives a good guidance to design the location of gas drainage roadway practices in the future. The field engineering practice shows its good application. [ABSTRACT FROM AUTHOR]

Coal seams in China are mainly characterizsed by low permeability and high risk of coal and gas outburst. Therefore, the elimination of coal and gas outburst risk of low‐permeability coal seam remains a hard challenge. Taking Shihuatian Coal Mine in Guizhou Province, China as an example, boreholes were slotted using high‐pressure water jet to relieve the gas pressure inside the coal seam and increase the permeability of coal mass, which thus improves the gas drainage efficiency and eliminate the outburst risk. The results show that the gas drainage efficiency of the slotted boreholes significantly increases. Compared with the conventional boreholes, the average gas concentration and flow velocity improved by 1.6 and 7.5 times, respectively. Moreover, the prediction index of outburst risk is less than the critical value after high‐pressure water jet slotting, which indicate that the coal and gas outburst risk has been effectively eliminated. The measured values of gas desorption index of drilling cuttings are all lower than the critical value after the application of the high‐pressure water jet slotting technology. The research achievements could provide a practical reference for the effective gas disaster prevention and control in low‐permeability coal seams. [ABSTRACT FROM AUTHOR]

With the increase of mining depth, the high gassy coal mine confronts with a variety of gas problems, such as gas concentration exceeding the limit in the working face, gas outburst and gas explosion. According to the situation of gas drainage in Qinshui Basin (South of Shanxi Province, China) and enlightened by horizontal directional drillings (HDD) and multi‐branched horizontal wells, a new gas drainage pattern that inverted π‐shape km borehole layout was proposed. Verified in Yuwu Coal Mine, the gas drainage data were standardized via the unified theory of standardization. It could dropped the coal seam gas content to 7.71 m3/t, and increased the gas drainage rate to 34.2%, which conformed to the Chinese regulation (<8 m3/t, >30%, respectively). This pattern has a significant effect on coal mines with a single seam, where protective seam mining technology was unsuitable to prevent gas outburst and gas explosion. A double pressure relief circle model of gateway and borehole was proposed. Combined with traditional gas drainage technology, a new gas drainage scheme came up with – inverted π shape km boreholes group model. The novel gas drainage scheme of inverted π‐shape km boreholes arrangement has the advantage of reducing gas drainage time and expanding coverage area. It is a new tri‐dimensional space‐time gas drainage system for underground coal mines. [ABSTRACT FROM AUTHOR]

Degassing a coal seam with in‐seam boreholes is an important method for mitigating the gas hazards in the underground coal mine. However, the low strength of the outburst‐proven coal limits the borehole sealing performance and borehole space maintaining, and thus influences the drainage performance of in‐seam boreholes. This study was conducted to seek a method to improve the sealing performance and borehole space maintaining for high‐efficiency CMM drainage. A visco‐elastic plastic model involving the plastic softening and dilatancy features for soft coal was proposed, and the deformation, shrinkage, and fracture characteristics of the coal surrounded a borehole and a roadway were analyzed. An enormous amount of connective fractures generated in the failure zone and the plastic zone, and the plastic zone develops timely for the creep behavior of coal, which aggravates the difficulties of borehole sealing. A comprehensive approach including the theoretical method and the technical method was developed to determine the proper sealing depth which can significantly influence the sealing performance. The sieve tubes made of high‐density polyethylene has been used to maintain the borehole space during the scheduled pre‐drainage period. A series of field tests were conducted in an outburst‐proven coal seam to verify the feasibility of the comprehensive approach and the borehole space maintaining method. Field tests showed that the comprehensive approach to determine the proper sealing depth and the installing of sieve tubes to protect the borehole space can provide favorable conditions for maximum CMM pre‐drainage from outburst‐prone coal and maximum utilization of the in‐seam boreholes. Degassing coal seam with boreholes is an important method for mitigating the gas hazards in the underground coal mine. Two improvements, including presetting sieve tubes and proper sealing depth, are proposed to enhance the methane quality and quantity in pre‐drainage. [ABSTRACT FROM AUTHOR]