The adsorption gas
of coal matrix will cause its swelling, change
the fracture structure, and further affect the permeability of the
coal seam. In this study, CT image processing technology was used
to numerically simulate the swelling of different extents (0.05%,
0.10%, 0.15%, 0.20%, and 0.25%). At the same time, the evolutions
of geometric parameters, connectivity, and fractal dimension of fractures
under different swelling extents were quantified. The results show
that with the increase of the swelling extents, the average length,
width, and aperture of fractures decreased by 18.69%, 24.14%, and
52.24%, respectively. The connected porosity of fractures decreased
by 44.83%, indicating fractures are closed and the connectivity becomes
worse. It is noteworthy that when the swelling extent is less than
0.1%, the average aperture and connected porosity of fractures are
significantly reduced by 32.84% and 31.03%, respectively. When it
is greater than 0.15%, and the average aperture and width of fractures
are clearly reduced by 49.25% and 23.28%, respectively. The fractal
dimension increased from 2.132 to 2.211, indicating that the fracture
structure became complex. Additionally, using Darcy’s law to
determine the change in absolute permeability during swelling, the
results show that there was a significant negative exponential correlation
between permeability and swelling extents. The decrease of aperture
and connected porosity are the main factors of poor permeability.
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