Investigating the Effects of Seepage-Pores and Fractures on Coal Permeability by Fractal Analysis

Permeability is one of the key petrophysical properties for the coalbed methane (CBM) reservoirs, which directly impact the CBM production rate and the amount of CBM that can be ultimately recovered. Due to the complex and heterogeneous nature of coals, an accurate determination for permeability of coals is required. Coal permeability is often determined by fractures (or cleats), which correlates with the aperture of cleats and cleats frequency. However, the gases flow path in coals covers fractures (or cleats), and pores with pore width > 100 nm; thus, the effect of pores on permeability should not be neglected, especially in the process of outgassing for methane release from pores over 100 nm (defined as seepage-pores) in the coal seams. Understanding of this issue is limited. Therefore, the determination of the pore size distribution in coal cores was conducted by using mercury intrusion porosimetry. With a specific interest in larger pores ( > 100 nm), the larger pores are linked to their contribution to permeability in coal cores with core permeability by the transient pulse method. Based on 33 coal samples with vitrinite reflectance in the range of 0.54–2.99