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Air leakage caused by mining-induced fractures around the borehole and roadway greatly affects the effect of underground gas drainage. Therefore, the study of the air leakage model considering three-dimensional stress and coal elastic-plastic deformation is of great significance to prevent air leakage. In this work, a model of air leakage outside borehole in mining-disturbed coal seam including a fully coupled gas-air flow and coal mechanics model and a mining-induced damage permeability model was developed and verified. The model was used to study the gas-air migration law and air leakage mechanism during gas drainage, and the influence of key parameters including initial permeability and sealing depth on gas drainage effect was analyzed. The results show: (1) The model can be used to characterize the air leakage of pre-drainage boreholes in mining-disturbed coal seams. (2) The gas and air pressure in the severe mining disturbance area rapidly decreased and increased in a short time. (3) Increasing the initial permeability and sealing depth will promote the gas flow in the borehole in the early stage, but the gas concentration will be reduced. The research results provide a scientific theoretical basis for improving the gas drainage effect and ensuring mining safety.

The 100-year global warming potential of methane is 28 times higher than the value of carbon dioxide. Accurate gas content measurement is a precondition of coal mine reconstruction and extension designs for reducing methane emission due to that in these scenarios the stress and gas pressure distributions were unpredictably disturbed by previous mining activities. Longer roadway exposure time induce lower gas content and high measurement error. In this study, a coupling model considering the influence of gas transport, solid mechanics, and permeability evolution was established to predict the gas pressure distribution in coal seams. Based on this model, the influence of the roadway exposure time on the gas pressure distribution in an extended mine, Hulonggou Mine, was numerically studied with the software of COMSOL. The results show that the stress concentration area near the roadways can reduce the rate of gas emission. In addition, the gas pressure relief zone becomes increasingly larger as the emission time increases. For the studied coal seam, the boundary of the gas pressure relief zone is about 25–30 m to the center line after 25 years’ gas emission. At last, actual gas content measurements validated the simulated results. The results can help scholars properly predict the gas content and design the gas drainage plans for coal seams that have been excavated for several years.

Adopting in-seam boreholes for gas pre-drainage is an important method for ensuring safety coal mine operation. However, borehole collapse often occurs during the process of construction in soft coal seams, especially in downward boreholes. The main purpose of this study is to develop an efficient drilling fluid for coal seam methane drainage boreholes. First, a drilling fluid with a formula of 1 wt.% xanthan gum, and 1.2 wt.% Na-CMC, 0.6 wt.% cellulase and 4 wt.% bentonite, which is simply referred to as the drilling fluid, was developed based on the actual requirements of the construction site. Then, five property tests (viscosity, fluid loss resistance, cuttings carrying capacity, anti-collapse performance and degradability) of the drilling fluid were conducted in the laboratory. Finally, engineering application performances were carried out in Tangshan Mine, China. The results indicate the following: (1) When the xanthan gum concentration is fixed, the viscosity of the drilling fluid improves with increasing Na-CMC concentration. The API filtrate loss of the drilling fluid decreases with increasing bentonite concentration, while the filter cake thickness increases and then stabilizes. (2) The drilling fluid has an efficient performance in the carrying capacity of drill cuttings and the anti-collapse performance of a coal mass. Thorough degradation is completed 300 min after adding cellulase. (3) Compared with traditional clear water, the effective drilling distance of downward core drilling using the drilling fluid increases by two times, and the anti-collapse performance for coal seam methane drainage boreholes is remarkable.

High concentration and large flow flux of gas drainage from underground coal seams is the precondition of reducing emission and large-scale use of gas. However, the layered occurrence of coal seams with tectonically deformed sub-layers and intact sub-layers makes it difficult to effectively drain gas through commonly designed boreholes. In this study, the gas drainage performance in coal seams with different combinations of tectonically deformed sub-layers and intact sub-layers was numerically analyzed. The analysis results show that the gas drainage curve changes from a single-stage line to a dual-stage curve as the permeability ratios of Zone II (kII) and Zone I (kI) increase, raising the difficulty in gas drainage. Furthermore, a dual-system pressure decay model based on the first-order kinetic model was developed to describe the dual-stage characteristics of pressure decay curves with different permeability ratios. In the end, the simulation results were verified with reference to in-situ drainage data from literature. The research results are helpful for mines, especially those with layered coal seams comprising tectonically deformed sub-layers and intact sub-layers, to choose appropriate gas drainage methods and develop the original drainage designs for achieving better gas drainage performance.

Predrainage of gas by cross‐boreholes is an important method to prevent coal and gas outbursts in the process of a coal roadway excavation. Limitations of technologies and poor effects of preventing coal and gas outbursts occur in the coal seam with a soft layer. In this study, a new hydraulic caving technology for cross‐boreholes has been proposed to solve this problem. A CT equipment was used to observe the development of coal fissures around cross‐boreholes in the coal seam with a soft layer. The plastic zone range and permeability distribution of ordinary hydraulic slotting technologies and the new hydraulic caving technology also were compared by numerical simulation. The field test of hydraulic caving was taken in the Xuehu Mine (China). The results show that the fracture development degree of soft coal is greater than that of hard coal at the same distance from cross‐boreholes. The fracture development degree in the plastic zone is greater than that in the elastic zone. The hydraulic caving technique can achieve the same effect as the ordinary hydraulic slotting with less workload. The mass of coal discharged from a cross‐borehole was 1.15 t/m and the average methane concentration of single drainage borehole increased by 1.09‐4.1 times, and the average methane extraction amount of single drainage borehole increased by 6.06 times. Elimination of coal and gas outburst had been completed after 110 days of extraction in the area of hydraulic caving, and Q and S values are lower than the critical value during roadway excavation. It is conclusion that hydraulic caving technique can effectively improve the permeability and rapidly eliminate coal and gas outburst in the coal seam with a soft layer.