In recent years, liquid nitrogen (LN₂) fracturing technology has been applied in coalbed methane (CBM) development. However, the impact of the liquid nitrogen freeze‐thaw process on the pore structure of anthracite coal has not yet been systematically investigated. In this study, nuclear magnetic resonance (NMR) analysis and scanning electron microscopy (SEM) imaging of coal samples after treatment with liquid nitrogen for different freezing times and freeze‐thaw cycles were performed to study the pore structure damage of anthracite subjected to the liquid nitrogen freeze‐thaw process as well as the variation in the porosity, fracture evolution, and permeability. The results show that the LN₂ freeze‐thaw process can enlarge the pore size, enhance the pore connectivity, and form a fracture network on the surface of coal samples, which increases the total porosity, residual porosity, effective porosity, and permeability. This study provides a theoretical value for LN₂ fracturing in the development of CBM.

To further more deeply understand the influence of cyclic loading‐unloading on coal seams during mining and reduce the rock burst and coal and gas outburst disasters and ensure safety during coal mining, triaxial cyclic loading‐unloading tests were performed on coal samples. The permeability recovery rate, residual deformation, and dissipated energy ratio were defined to analyze the evolution in the characteristics of permeability, deformation, and energy observed during the triaxial cyclic loading‐unloading test. The results show as following: (a) The absolute permeability recovery rate first gradually decreases and then increases before failure, while the relative permeability recovery rate decreases sharply during the compaction stage, remains stable during the elastic phase, and gradually increases before failure; (b) The cumulative residual deformation increases with increase number of cyclic loading‐unloading, while relative residual deformation first decreases gradually, then stabilities, and then rises sharply before failure; (c) With increases in deviatoric stress, the total energy of coal samples increased as an exponential function. The dissipated energy ratio gradually decreased in the initial stages of cyclic loading‐unloading, then tended to stabilize in the elastic stage and then increase gradually before failure. This study provides precursor conditions for the early warning of rock burst and coal and gas outburst disasters, which has great importance for reducing coal mine dynamics disasters.

The permeability of coal reservoirs prominently determines methane flow in reservoirs. A large amount of coal reservoirs containing methane, however, have low permeability because pores in reservoirs are sealed. To solve this issue, we investigated the pore structure and permeability law of coal through contrastive analysis of the microstructure and macrostructure before and after acidizing, using ASAP 2020 and triaxial servo-controlled seepage equipment. An analysis of the experimental data revealed the mechanism and the key parameters of acidizing, such as acid mass fraction and acidizing time. In addition, we hold that the optimal mass fraction for the test specimens is about 12~15%. Furthermore, the acid reaction is divided into three stages according to characteristics of the reaction progress. An analysis of the reaction kinetics of “HCl-HF” states that the significant factors will impact the process of reaction, like the composition of coal, surface area of the acid-coal reaction, mass fraction of acids, and environment of the reaction system. The results provide a new idea and research method for further enhancing permeability and gas extraction.

Coal and gas outburst is one of the major serious natural disasters during underground coal, and the shock air flow produced by outburst has a huge threat on the mine safety. In order to study the two-phase flow of a mixture of pulverized coal and gas of a mixture of pulverized coal and gas migration properties and its shock effect during the process of coal and gas outburst, the coal samples of the outburst coal seam in Yuyang Coal Mine, Chongqing, China were selected as the experimental subjects. By using the self-developed coal and gas outburst simulation test device, we simulated the law of two-phase flow of a mixture of pulverized coal and gas in the roadway network where outburst happened. The results showed that the air in the roadway around the outburst port is disturbed by the shock wave, where the pressure and temperature are abruptly changed. For the initial gas pressure of 0.35 MPa, the air pressure in different locations of the roadway fluctuated and eventually remain stable, and the overpressure of the outburst shock wave was about 20~35 kPa. The overpressure in the main roadway and the distance from the outburst port showed a decreasing trend. The highest value of temperature in the roadway increased by 0.25 °C and the highest value of gas concentration reached 38.12% during the experiment. With the action of shock air flow, the pulverized coal transportation in the roadway could be roughly divided into three stages, which are the accelerated movement stage, decelerated movement stage and the particle settling stage respectively. Total of 180.7 kg pulverized coal of outburst in this experiment were erupted, and most of them were accumulated in the main roadway. Through the analysis of the law of outburst shock wave propagation, a shock wave propagation model considering gas desorption efficiency was established. The relationships of shock wave overpressure and outburst intensity, gas desorption rate, initial gas pressure, cross section and distance of the roadway were obtained, which can provide a reference for the protection of coal and gas outburst and control of catastrophic ventilation.

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.