Carbon dioxide gas permeability of coal core samples and estimation of fracture aperture width

The structure of cleats, or natural fractures in coal seams, is an important factor affecting gas permeability. Coal seam permeabilities have been classified into fracture and matrix flows. However, fractures in coal seams are generally not regular and usually are discontinuous. In addition, it is difficult to definitively distinguish the fracture and matrix permeabilities, and to measure the fracture widths or other necessary properties of fractures for permeability estimation. Typically gas permeability has been determined by using the Darcy equation for incompressible fluid flow and homogeneous porous media, even if the gas flow through fractures has different characteristics with respect to effective stress, which is given by the confined stress minus the gas pressure.In this study, permeability measurements were carried out with a new procedure and methodology to estimate the fracture width in coal core samples. This measurement method is an improvement on conventional permeability measuring methods. In this research, it was identified that fracture permeability can be estimated from measurements of gas flow rates in a limited small area using a pipe attachment on the end of the surface of the core. Finally, a linear equation composed of fracture permeability and matrix permeability is presented.