Underground radial drilling technique is a novel hydraulic stimulation technology, which is one of the important ways to realize precise, intelligent, and efficient extraction of the gas resources. There are many factors influencing the hole‐forming effect by water jet, such as nozzle arrangement and so on. Recently, the self‐rotatory multinozzle drilling bit is frequently used to improve the rock‐breaking effect of water jets. In this paper, to study the effects of inclination angle of forward nozzles and standoff distance, the ANSYS FLUENT combined with RNG turbulence model was adopted to analyze the multinozzle jets flow field. The results indicate that the components of axial velocity and tangential velocity of forward nozzle jets are different with inclination angles of forward nozzles. Moreover, there exists an optimal standoff distance of multinozzle drilling bit for rock breaking. Besides, the drilling test was conducted using the self‐developed drilling simulation system to investigate the effect of inclination angle, radial angle, standoff distance, and jet pressure on the rock‐breaking effect of water jet. Based on the rotating characteristic of rotary jet, the asymmetric inclined linear nozzle arrangement was proposed for the multinozzle drilling bit. And the results also show that the drilling bit with three forward nozzles has larger rock‐breaking zone in terms of jetted hole diameter. Besides, the depth of jetted hole decreased, while the hole diameter increased with the inclination angle of forward nozzle.

Coal contains a large number of fractures, whose characteristics are difficult to describe in detail, while their spatial distribution patterns may follow some macroscopic statistical laws. In this paper, several fracture geometric parameters (FGPs) were used to describe a fracture, and the coal seam was represented by a two-dimensional stochastic fracture network (SFN) which was generated and processed through a series of methods in MATLAB. Then, the processed SFN image was able to be imported into COMSOL Multiphysics and converted to a computational domain through the image function. In this way, the influences of different FGPs and their distribution patterns on the permeability of the coal seam were studied, and a finite element model to investigate gas flow properties in the coal seam was carried out. The results show that the permeability of the coal seam increased with the rising of fracture density, length, aperture, and with the decrease of the angle between the fracture orientation and the gas pressure gradient. It has also been found that large-sized fractures have a more significant contribution to coal reservoir permeability. Additionally, a numerical simulation of CBM extraction was carried out to show the potential of the proposed approach in the application of tackling practical engineering problems. According to the results, not only the connectivity of fractures but also variations of gas pressure and velocity can be displayed explicitly, which is consistent well with the actual situation.

When faced with multi‐seam mining, some coal pillars are left to support the overlying strata when mining the lower seam, where stress concentration usually occurs. When the working face in the lower seam advances to the area influenced by these coal pillars, high ground pressure behavior occurs. Furthermore, the instability of coal pillars under the participation of the roof may induce rock burst, which threaten the safe production of coal mine. To study the roof strata movement and coal pillar instability in multi‐seam mining, a coal mine in Datong was taken as an example. Firstly, the bursting liability of coal and rock stratum was tested. The test results show that the 7#, 8#, and 11# coal seams in the mining area all have strong bursting liability. And the roof was dominated by siltstone and firestone, which also have strong bursting liability. Then, based on the geological conditions of the coal mine, we investigated roof movement and instability of the coal pillar during multi‐seam mining through physical simulation. The simulation results indicate that multi‐seam mining aggravates the roof damage, which leads to the increase in the fracture range of overlying strata, and that the failure height increases from 46 to 121 m. In addition, the detailed distribution of stress on residual coal pillars and its influence on the lower working face were studied through numerical simulation. The results show that, when the working face 8707 of the 8# coal seam advances to a horizontal distance of 20 m from the coal pillar, it enters the influence zone. As the working face advances beneath the coal pillar, the stress reached to 24 MPa. Moreover, when the working face passes by the coal pillar, the coal pillar is cutoff, thus affecting the working face.

During the multiseam mining process, the overlying strata were damaged due to different mining disturbances, where a large number of fractures were produced which were the main seepage paths of gas and water, easy to cause gas or water inrush accidents. Thus, it is necessary to explore the development characteristics of the fractured zone in the overlying strata from multiseam mining. In this paper, laboratory triaxial compression tests were firstly conducted on the rock specimens subjected to different mining disturbances. In these tests, the specimen was loaded to different stress before a regular test to simulate the different damage effects on the rock strata caused by different mining disturbances. And the corresponding values of m and s could be obtained through a calculation based on the test results. Subsequently, the numerical simulation was conducted to simulate the field situation, in which the m and s values were used in the numerical model combined with the Hoek-Brown-modified criterion, and the results show that the overburden failure height was increased with the mining disturbances from 48 to 120 m, and then, the effects of mining height and interval thickness on the development height of fracture zone were studied by numerical simulation. Finally, the leakage of roof drilling at different stages was tested and calculated in the field. The results showed that the loss of leakage drilling can directly reflect the development degree of roof cracks, and the detection results were close to the development height of the numerical simulation fracture zone. It is verified that the simulation method used in this paper is feasible and can be used as a reference for similar research.

In this study, an indicator model and system for coal and gas outburst prediction are identified and established based on gas geological anomalies, mining stress, and gas emission characteristics. In this regard, the coal and gas outburst prevention techniques and mine rules in China are taken into account. In addition, using this model, an online integrated system and database for coal and gas outburst prediction were built for Xinjing Mine in Shanxi Province. The application results of the prediction system indicate that the system software and hardware can guarantee early warning of coal and gas outburst. The system is continuous and stable, and the early warning results are issued in a timely and accurate manner. The accuracy of the early warning results is high and consistent with the actual risk of the mine.