In view of the differences in the applicability and prediction ability of different creep rupture life prediction models, we propose a creep rupture life prediction method in this paper. Various time–temperature parametric models, machine learning models, and a new method combining time–temperature parametric models with machine learning models are used to predict the creep rupture life of a small-sample material. The prediction accuracy of each model is quantitatively compared using model evaluation indicators (RMSE, MAPE, R²), and the output values of the most accurate model are used as the output values of the prediction method. The prediction method not only improves the applicability and accuracy of creep rupture life predictions but also quantifies the influence of each input variable on creep rupture life through the machine learning model. A new method is proposed in order to effectively take advantage of both advanced machine learning models and classical time–temperature parametric models. Parametric equations of creep rupture life, stress, and temperature are obtained using different time–temperature parametric models; then, creep rupture life data, obtained via equations under other temperature and stress conditions, are used to expand the training set data of different machine learning models. By expanding the data of different intervals, the problem of the low accuracy of the machine learning model for the small-sample material is solved.

To ensure the effectiveness of the gas extraction borehole, it is necessary to investigate the stress-displacement-fracture evolution of the coal around the borehole. In this study, by constructing a numerical model of a gas extraction borehole, the burial depth and side pressure coefficient are used to characterize the overall stress level of the borehole and the difference in stress distribution caused by complex stress conditions. First, the stress time-varying pattern and force chain distribution of coal around the borehole were revealed. Then, the displacement time-varying pattern and displacement distribution of coal around the borehole were elucidated. Then, the microfracture distribution of coal around the borehole, which characterizes the microfractures, was analyzed. Finally, the validity of the numerical results was verified. The results showed that, after the stress field of the coal around the borehole was adjusted, the force chain of the borehole was unevenly distributed and the stress concentration phenomenon appeared. With the increase in burial depth, the stress around the borehole gradually increased, while the range of stress concentration zone in the borehole kept increasing, and the borehole changed from unilateral instability to bilateral instability. Moreover, the displacement field around the borehole was distributed in the shape of a ”disk leaf”. With the increase in burial depth, the deformation of coal around the borehole increased. With the increase in the side pressure coefficient, the vertical and horizontal displacement also increased gradually. Furthermore, there was a certain correspondence between the development of fracture and the deformation around the coal. With the increase in burial depth, the development of fractures was gradually obvious, and the distribution characteristics were concentrated in the middle and dispersed around. This study provides a theoretical reference for the stability of gas extraction boreholes, aiming to improve the gas extraction effect.

Liquid carbon dioxide (CO₂) phase change fracturing (LCPCF) is an innovative technique to improve the efficiency of gas drainage from low-permeability coal seams of high gas content. However, fracture sprouting, extension and displacement changes of coal under LCPCF need further study, and corresponding field tests are also lacking. Therefore, a mechanical model based on the thermodynamic theory of CO₂ phase change is developed in this paper. Then, the pressure change characteristics, crack propagation and displacement change of coal subjected to LCPCF were analyzed through numerical simulation. In addition, the permeability-enhancing effect of the field LCPCF test was analyzed. The results obtained from the numerical simulation show that during the LCPCF process, the crack-generation process changes with pressure as follows: microfracture–numerous microfractures–major macrofracture–macrofractures. During the development of fractures, the stress is incompletely symmetrically distributed in coal centered on the fracturing borehole. The failure occurs stochastically in the coal in the vicinity of the fracturing borehole at first, and then it gradually propagates to the inner seam of coal as the gas pressure increases. The following result can be obtained from field experiments: the permeability coefficient of coal seams after increasing the permeability through LCPCF is 2.60~3.97 times that of coal seams without presplitting. The average concentration of gas extracted in coal seams within the zone having undergone an increase in permeability through liquid CO₂ fracturing is 2.14 times greater than that within the zone without presplitting. The average pure amount of gas extracted within the zone having undergone an increase in permeability through LCPCF is 3.78 times greater than that within the zone without presplitting. By comparing coal seams before and after fracturing in the field test, it can be seen that the LCPCF presents a favorable effect in increasing the permeability of low-permeability coal seams. This provides an effective approach for increasing the permeability of coal seams in coal mines with similar geological conditions.

The No. 2₁ coal seam in the Zhengzhou mining area is a soft, three-layer, low-permeability coal seam prone to outbursts. The three-layer structure includes the coal seam and the roof and floor layers, which exhibit high gas contents and poor permeability. The dynamic hazards caused by coal and gas outbursts are very serious. A new permeability-increasing fracturing technique that combines hydraulic perforation and hydraulic fracturing was developed specifically for the geologic conditions of the gas-bearing No. 2₁ coal seam. Numerical simulations were developed to study the influence of the technique on the stress distribution and permeability of the coal around the borehole. In addition, the extracted borehole gas concentrations and extraction amounts at multiple sites were investigated before and after using the technique. The study shows that the permeability-increasing fracturing technique destroys the concentrated stress coal pillars via the development of fractures between boreholes in exposed hydraulically perforated coal. The coal stress within the zone with an effective increase in permeability decreased by 30. Furthermore, the permeability in this zone increased by three times, and the average extracted gas concentration increased by over six times. The gas pressure in the No. 2₁ coal seam decreased from 1.1MPa to 0.4MPa, and the gas content decreased from 15.96m³/t to 5.6m³/t. All outburst prediction indexes measured on site did not exceed their respective limits. The technique not only effectively eliminated the dynamic hazards caused by coal and gas outbursts but also achieved efficient gas extraction in the Zhengzhou mining area.

To investigate the crack propagation mechanisms in low-permeability coal seams induced by liquid CO₂ phase change blasting under different blasting pressures, this research presents an experimental study conducted on a small liquid CO₂ phase change blasting test system. The failure mode, crack morphology, and distribution characteristics of the coal rock model specimens under different liquid CO₂ phase change blasting pressure were revealed, analyzing the crack shapes and expansion process. The results show that with increasing blasting pressure, both the number and complexity of cracks significantly increase under liquid CO₂ phase change blasting, evolving from simple linear cracks to more complex multi-directional networks. Furthermore, the process of crack generation and expansion during liquid CO₂ phase change blasting in coal and rock is controlled by the interaction of shock waves and quasi-static stress resulting from high-pressure CO₂ phase transition in the borehole. Cracks form in distinct zones: the broken zone, where shock waves cause severe crushing near the borehole; the crack zone, where quasi-static tensile stress drives crack propagation. Higher confining and CO₂ blasting pressures increase crack propagation. The research results offer valuable insights for optimizing blasting design in liquid CO₂ phase change fracturing.

The evolution characteristics of pores and fractures in coal after liquid carbon dioxide (CO₂) phase change are important factors that determine the permeability increase effect. Therefore, it is critical to correctly understand the influences of liquid CO₂ phase change on pores and fractures in coal. The changes of adsorption and desorption isotherm, pore size, pore volume, and specific surface area of fractured coal and fractured coal were compared by low temperature liquid nitrogen adsorption experiment. In addition, a scanning electron microscope was adopted to observe fracture characteristics of fractured and unfractured coal samples and analyze changes in the connectivity and fracture development. Experimental results show that the fractured coal samples exhibit better hysteresis loops and a larger proportion of gas desorption than the unfractured ones. Fractured coal samples contain more developed pores and fractures compared with unfractured ones, and their fragmentation degree, pore diameter, fracture width, and connectivity of pores and fractures are also better. Besides, the closer the samples from the fracturing boreholes are, the better the fracturing effect. This indicates that liquid CO₂ phase change can effectively enhance the gas transport capacity in pores and fractures in coal. The research results provide a solid basis for the better application of liquid CO₂ phase‐change fracturing to the prevention of coal and gas outburst disasters and the realization of efficient gas extraction in deep coal seams.