Micrometer-scale pores and fractures in coals and the effects of tectonic deformation on permeability based on fractal theory

Micrometer-scale pore and fracture structures of coal seams are the crucial parameters in the case of enhanced coalbed methane (CBM) recovery as they determine permeability and productivity. A significant study has been made in fracture characteristics; however, the detailed structural and fractal characteristics of micro-fractures and micro-pores of tectonically deformed coals are poorly understood. To get deep insight into the variability and heterogeneity of micro-pores and micro-fractures in different tectonically deformed coals collected from the Guojiahe coal mine, the multifractal analysis using the Menger sponge and box-counting model was employed to study deformed coals based on mercury injection porosimetry and scanning electron microscopy. The results show that tectonic deformation changed the structure of the micro-pores by increasing their diameter and that of the micro-fractures by expanding, intersecting, and creating additional microfracture networks at any scale. For the coals investigated in this study, permeability was nonlinearly correlated with their structural fractal. For undeformed coals, with the increase in the fractal dimension of micro-pores, the distribution of coal permeability exhibits a U-shape. However, it exhibited an inverted U-shape as the fractal dimension of micro-fractures increased. The distribution of the permeability of deformed coal samples is characterized by a U-shape as the fractal dimension of micro-fractures increased, while the fractal dimension of micro-pores decreased. Thus, the structural proportional odds of micro-pores and micro-fractures are vital in defining the maximum value of permeability. As a whole, deformed coals have a relatively high permeability as local micro-pores and micro-fractures are well connected. We suggest that coal seams with brittle deformation at syncline, anticline, and folded areas are favorable for CBM exploitation.