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

Abstract

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 %, this work examines the seepage-pores contribution on core permeability by using classic geometry and thermodynamics fractal model. Moreover, a pore fractal permeability model was established to acquire the seepage-pores permeability, which can be used to interpret the permeability evolution of coals during coalification. Coal pore surfaces generally have very high heterogeneity with fractal dimension of 2.75–2.96 from thermodynamics model $$(D_{\mathrm{ts}})$$ , which presents a cubic polynomial relation with coal rank. The fractal dimensions from thermodynamics model show a consistent result, which indicates that the pore size/volume distribution was one of the key parameters affecting seepage-pores permeability. Finally, an ideal coal permeability (including seepage-pores and fractures contributed) evolution model during coalification was proposed and the seepage-pores contributed permeability generally covers $$\sim $$ 10 to $$\sim $$ 30 % of the entire coal permeability.