The law of gas initial desorption from coals is greatly important for understanding the occurrence mechanism and predicting coal and gas outburst (hereinafter referred to as ‘outburst’). However, dynamic characterization of gas initial desorption remains to be investigated. In this study, by monitoring the gas pressure and temperature of tectonically deformed (TD) coal and primary-undeformed (PU) coal, we established the evolution laws of gas key parameters during the initial desorption. The results indicate that the gas pressure drop rate, mass flow rate, initial desorption rate, and gas velocity increase with increasing gas pressure, with stronger gas dynamic effect, generating a high pressure gradient on the coal surface. Under the same gas pressure, the pressure gradient formed on the TD coal surface is greater than that formed on the surface of the PU coal, resulting in easily initiating an outburst in the TD coal. Moreover, the increased gas pressure increases temperature change rates (falling rate and rising rate) of coal mass. The minimum and final stable temperatures in the TD coal are generally lower compared to the PU coal. The releasing process of gas expansion energy can be divided into two stages exhibiting two peaks which increase as gas pressure increases. The two peak values for the TD coal both are about 2–3 times of those of the PU coal. In addition, the total gas expansion energy released by TD coal is far greater than that released by PU coal. The two peaks and the total values of gas expansion energy also prove that the damage of gas pressure to coal mass increases with the increased pressure, more likely producing pulverized coals and more prone to initiate an outburst.

The performance of coal mine gas extraction borehole sealing material plays an essential role in the efficiency of gas extraction. Microcapsule technology was proposed to delay the expansion time of sealing materials to address the ineffective expansion of cement-based sealing materials in hydration. Based on conventional cement based sealing materials, delayed-expansion microcapsules were prepared by phase separation with ethyl cellulose (EC) as the capsule wall material and montmorillonite (MMT) as the core material. A single-factor experiment showed that the delayed expansion microcapsule had the best comprehensive effect when the EC content was 3%, the stirring rate was 400 rpm, the MMT content was 3.5%, and the core-wall ratio (CWR) was 10:3. Second, the slow release effect of the cement samples under the action of microcapsules is remarkable through physical tests. In addition, the triaxial compression test results show better mechanical properties of the delayed-expansion sealing materials. Then, nuclear magnetic resonance scanning was applied to the coal samples injected with different sealing materials. It was found that the proportions of macropores and mesopores in the total pore volume in the coal sample at 50 and 100 mm from the borehole wall decreased by 12.28 and 11.82%, respectively, indicating that the delayed-expansion sealing material has a better sealing effect.

Borehole-sealing solidified material plays a significant role in improving sealing quality and enhancing gas drainage performance. In this study, the MTS815 electro-hydraulic triaxial servo test system and MR-60 NMR test system were adopted to conduct triaxial compression control experiment on the coal sample material, concrete material, and new solidified sealing material, respectively. This paper aims to analyze the difference of support effects, porosity, and stress sensitivity between those materials. Experimental results show that under the same stress condition, the stiffness of traditional concrete solidified material is the largest, while the new solidified material is the second, and the coal sample material is the smallest. Compared with the traditional concrete solidified material, the new solidified sealing material has better strain-bearing capacity and volumetric expansion capacity under each confining pressure in the experiment. The axial strain and volume increment of new solidified material is higher than those of the traditional concrete solidified material at the peak stress. Meanwhile, the confining pressure has a certain hysteresis effect on the postpeak stress attenuation. Fracture has the strongest stress sensitivity in three pore types, and its T₂ map relaxation area has a larger compression than adsorption pore and seepage pore under the same pressure. The relative content of seepage pore and fracture in the new solidified material is less than that of coal and concrete samples, and the stress sensitivity of the new solidified materials is weaker than that of coal and concrete materials, thence, new solidified material will have better performance in borehole sealing. Outcomes of this study could provide guidance on the selection of the most effective sealing materials for sealing-quality improvement.

To solve the problem of unstable sealing in the sealing section of a gassy, soft coal seam, a seal reinforcement material for gas extraction boreholes was developed, which was mainly made of ordinary Portland cement and blended with additives such as aluminium powder, quicklime, and gypsum. Firstly, in order to obtain the necessary expansion and compressive strength of reinforcement material, key factors affecting the material properties were determined. Key factors affecting the expansion properties and compressive strength of reinforcing materials were investigated by a single‐factor test. Moreover, according to the central combination (Box–Behnken) experimental principle and response surface analysis (RSA), the interactions of various factors on the expansion and compressive strength were determined, and the optimal experimental conditions were acquired. The experimental results indicated that the optimum ratio of the material was 2% for gypsum and 0.52% for aluminium powder and quicklime at the experimental temperature of 20°C, and the ratio of water to material was 0.6. Finally, in the N1103 working face of No. 3 coal seam of Yuwu coal mine, Luan Group, China, the sealing property of the reinforcement material was validated, and the problem of hole collapse at the borehole orifice was solved (resulting in a gas concentration 2.48 times that measured before borehole reinforcement), and the gas drainage effect was enhanced.

It is difficult to monitor the collapse position of a gas extraction borehole in a coal seam. In order to solve this problem, a fiber-optic grating collapse monitoring technology was proposed. Five kinds of grating arrangements (0°, 45°, 90°, 135°, 180°) were examined to simulate the hole collapse. The relationship model between the center wavelength offset and the amount of collapsed coal and extraction flow was constructed to obtain the distribution curve of the hole collapse position and quality along the length of the hole, and flow decay rates of 80% and 50% were used as the critical values to classify the three levels of hole collapse. The results show that the hole collapse monitoring accuracy is the highest with the grating measurement points located below the substrate material arrangement. Finally, the effectiveness of fiber-optic grating monitoring technology was verified in the 2202 working face of the Changcun coal mine of the Lu’an Group, and the pure amount of gas extraction from the repaired borehole after monitoring was increased by 62.6% compared with that before the repair.