Significance of gas flow in anisotropic coal seams to underground gas drainage

Abstract

At present, gas drainage technology from boreholes is an effective means of preventing gas disasters in deep mines and realise better utilisation of gas. To study the spatio-temporal evolution of gas flow fields around boreholes for gas drainage, it was postulated that the permeability of a coal mass is a non-free vector relating to spatial position based on anisotropic characteristics of the coal mass. The permeabilities of anisotropic coal mass in different directions were represented based on the principle of equivalent displacement and a model for coupling multi-physical fields of coal seams containing gas with structural anisotropy was established. Moreover, the spatio-temporal evolution of gas pressures and permeabilities of fractures in coal masses around boreholes were analysed. The results demonstrate that, differing from the conclusions of traditional mathematical models, gas pressures and permeabilities around boreholes for gas drainage in coal seams are anisotropic. The distributions of gas pressures and permeabilities present the vertical and horizontal figure of eight (8) shapes, separately. Over time, the vertical 8-shaped distribution of gas pressures gradually shrinks inwards, while the horizontal 8-shaped distribution of permeabilities expands outwards. With increasing distance from drainage boreholes, the trends therein were mutually contrary. The residual gas concentration in the vertical bedding direction of the borehole is about 1.4 times that of the parallel bedding direction under a same time period after gas drainage. The decrease of gas pressure can shrink the coal matrix and increase the effective stress on the coal mass. In the decrease of gas pressure caused by drainage over time, matrix shrinkage affects the permeability, while increasing effective stress on the coal mass plays a leading role in affecting the permeability in decreasing gas pressure resulting from spatial distance. Finally, the arrangement of boreholes in a coal mass and drainage affected zones under isotropic and anisotropic conditions were discussed. In the gas drainage project of underground coal seam, the layout of the borehole should consider the coal seam bedding direction.