True triaxial experimental study on the influence of axial pressure on coal permeability

The permeability of coal is a key parameter affecting coal and gas outbursts and coal seam gas drainage. The permeability is clearly affected by geo-stress. In this study, the influence of the axial pressure on the permeability of the coal seam was studied using a self-developed true triaxial stress permeability experimental device to set fixed gas and confining pressures, and to change the magnitude of the axial pressure. The experimental results show a polynomial relationship between the axial pressure and the permeability of the coal seam. With an increase in axial pressure, the permeability initially decreased slightly and then increased gradually. When the axial pressure exceeded 30MPa, the permeability of the coal seam sharply increased. This may be due to plastic deformation of the coal seam under a large axial pressure, resulting in new fractures and significantly improving the permeability of the coal seam. Using the COMSOL numerical simulation software, the effect of in situ stress on the coal seam gas drainage efficiency was calculated by comprehensively considering the adsorption/desorption, diffusion, and seepage of gas. The calculation results show that with an increase in the axial pressure, the gas drainage efficiency of the coal seam increases continuously. As the axial pressure increased from 5 to 30MPa, the gas drainage efficiency increased to 2–3 times that of the original value.

煤的渗透率是影响煤与瓦斯突出以及煤层瓦斯抽采的关键参数。渗透率明显受地应力影响。在本研究中，利用自主研发的真三轴应力渗透率实验装置，设定固定的瓦斯压力和围压，并改变轴压大小，研究了轴压对煤层渗透率的影响。实验结果表明，轴压与煤层渗透率之间呈多项式关系。随着轴压的增加，渗透率起初略有下降，随后逐渐上升。当轴压超过30MPa时，煤层渗透率急剧上升。这可能是由于煤层在较大轴压下发生塑性变形，产生了新的裂隙，从而显著提高了煤层的渗透率。利用COMSOL数值模拟软件，综合考虑瓦斯的吸附/解吸、扩散和渗流，计算了地应力对煤层瓦斯抽采效率的影响。计算结果表明，随着轴压的增加，煤层的瓦斯抽采效率不断提高。当轴压从5MPa增加到30MPa时，瓦斯抽采效率提高到原来的2 - 3倍。