Investigation of coal pore and fracture distributions and their contributions to coal reservoir permeability in the Changzhi block, middle-southern Qinshui Basin, North China

Abstract

Pores and fractures in coal have an important influence on gas desorption, diffusion, and seepage during coalbed methane (CBM) recovery. The occurrence and spatial distribution of pores and fractures in the Changzhi CBM block coals of the middle-southern Qinshui Basin, North China, were investigated by means of scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and low-temperature liquid nitrogen isotherm adsorption/desorption (LT-N2AD). In addition, pore and fracture contributions to the permeability of the coal reservoir were evaluated. The results show that there are many pores and fractures with different widths and lengths in the coals, most of which are isolated. The microfractures serve as a bridge for gas migration between pores, cleats, and macrofractures. Based on the relationship between cross-cutting microfractures and cleats, the multiple populations of microfractures in the coals can be classified as 1st class and 2nd class microfractures. Statistical analyses reveal that the aperture distributions of these fractures are heterogeneous and that the expected fracture aperture distributions can be described by a log-normal Gaussian function. Based on the statistically expected fracture aperture and its actions on gas flow, three levels of potential flow paths and three permeability domains are concluded to occur in coal reservoirs, including cleats, 1st class microfractures, and 2nd class microfractures. For this study area, due to the occurrence of mineralized cleats, the CBM reservoir permeability is significantly reduced. The current coal reservoir permeability contribution is mostly from the 1st class and 2nd class microfractures. Lastly, based on the pore and fracture spatial distributions of the coal and the three statistically expected permeability domains, the coupled pore-fracture network configuration models of coal reservoirs were constructed, which are useful for evaluating coal reservoir permeability.