No abstract

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.

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.