The permeability of more than 70% of coal seams in China is less than 1 mD, creating difficulties in recovering underground coal methane. Therefore, a new technology of high-temperature nitrogen (HTN₂) injection into the coal seam was proposed to improve the coal permeability and gas extraction rate. In this paper, the effects of the N₂ temperature, injection pressure and cycle number on the permeability of naturally fractured coking coal has been investigated. When HTN₂ was injected into coal samples, the results indicated that the permeability decreased over time in the beginning, suddenly increased to a large value, and was subsequently maintained in a relatively stable range. The maximum permeability ratio increased with the rise of the N₂ temperature and injection pressure. An analysis indicated that the increase of coal permeability was the result of the increase of the global coal strain caused by thermal expansion and the adsorption-induced expansion. The maximum permeability ratios in various cycles of multicycle N₂ injection into the coal sample were all greater than 1.0 while progressively declining. Obviously, the alternating stress was conducive to the further expansion of the coal fractures to increase the coal permeability. However, on the basis of the first period of expansion, re-expansion was difficult and required more energy. The effects of multicycle N₂ injection on coal permeability have been considerably improved when compared with N₂ injection with only one cycle. The research results are helpful for rapidly extracting methane and guaranteeing mine safety.

Deep grouting rock engineering is faced with the dual influence of high temperature and dynamic load, which has become a hot issue in geotechnical engineering. This study analyzes the mechanical responses and failure properties of deep-grouted fractured rock under real-time coupling of temperature and dynamic loads through the high-temperature-split Hopkinson pressure bar (HT-SHPB), high-speed imaging, and scanning electron microscopy (SEM) tests. Key findings reveal that (1) the dynamic compressive strength of grouted fractured rock exhibits significant temperature dependency, where the strength increases with the increase of temperature, which has been verified by relevant references. From indoor temperature to 100 °C, the dynamic strength increases moderately, while a pronounced increase is observed between 100 °C and 300 °C. (2) In contrast, the dynamic peak strain demonstrates a two-stage evolution, which sharply rises from indoor temperature to 100 °C, followed by a slowly rise from 100 °C to 300 °C. (3) Macroscopically, impact fractures preferentially initiate as parallel lines at the extremities of pre-existing grouted fractures, consistent with stress concentration patterns under dynamic loading. Microscopic analysis reveals that grouting materials effectively suppress micro-crack generation and propagation at 300 °C, attributed to thermally enhanced cementation and pore-filling effects, explaining the variation of the macroscopic dynamic strength with temperature from the microscopic point of view.

Conventional seismic advanced detection methods suffer from relatively low lateral resolution and symmetry artifacts in imaging results due to their narrow-azimuth observation systems. To fundamentally solve this problem, starting from a wide-azimuth perspective, constructing a reasonable wide-azimuth observation system is crucial for breaking through the bottleneck of traditional methods. Accordingly, this research investigates wide-azimuth detection utilizing a CO₂-concentrated source through theoretical analysis, three-dimensional numerical simulation, and physical analysis. The research results indicate that: (1) Studies establish that the dominant excitation direction for channel wave advanced detection using concentrated force sources is horizontal and perpendicular to the tunnel axis (typically designated the Y direction). The CO₂ source controls the excitation direction by adjusting the outlet of the pressure relief head, achieving directional excitation of the concentrated force source. To further identify the seismic wavefield characteristics of this Y-direction concentrated force source, mechanical mechanisms and three-dimensional numerical simulations were adopted. These analyses examined amplitude differences of multi-component P-waves, S-waves, and channel waves across various directions under Y-direction excitation in both homogeneous models and coal-rock-coal models, thereby uncovering the developmental mechanism of channel waves at coal seam interfaces. (2) Based on the Y-direction concentrated force source, a study was conducted on the characteristics of a wide-azimuth three-dimensional three-component seismic wavefield. The reflected trough waves had a large time interval and clear wavefronts compared to other wavefronts in the time domain. The signal-to-noise ratio of seismic records in the Y component is higher than that in the X and Z components, and the reflected trough wave is a characteristic wave for fault detection. (3) The migration processing is carried out using fault characteristic waves. Compared with the narrow-azimuth observation systems, the wide-azimuth observation systems can effectively eliminate symmetry artifacts, further verifying the effectiveness of the method. Comparative experiments were conducted between CO₂ seismic sources and explosive seismic sources to further assess the effectiveness of the CO₂ source. The frequency spectrum of the CO₂ source closely resembles that of the explosive source, with the CO₂ source richly exciting high-frequency seismic signals.