Permeability of a coal seam with respect to fractal features of pore space of fossil coals

•Permeability by Kozeny-Carman representation is possible to explain the change in the total porosity of fossil coal with increasing coal bed occurrence to greater depths.•Calculation of permeability of coal beds with respect to their depth is based on the measurement of coal porosity taking into account their hierarchical structure.•The specific surface area of closed and open pores, measured by the small-angle neutron scattering, provides a reliable determination of the surface fractal dimension.•The reduction of the fractal dimension results in enlarged volume of macropores that provide enhanced permeability and gas content of deep seated coal beds.