Pore-fracture and permeability heterogeneity of different marcolithotypes of medium-rank coals in Jixi Basin, China

The pore-fracture system of different marcolithotypes (bright, semi-bright, semi-dull and dull coals) is quite different, which has great influences on coalbed methane (CBM) exploitation. Here the optical microscope (OM), scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), low-temperature nitrogen adsorption (LTND) and X-ray computed tomography (X-CT) are performed on different marcolithotype coals from Jixi Basin. From bright coals to dull coals, "ink bottle" shaped pores account for a gradually increasing proportion of the transition pores, but contribute fewer mesopores. The results of LTND and MIP indicate that bright coals have the poorest mercury removal efficiency and lowest BET specific surface area, while the opposite appears to be the case for dull coals. According to the pore size distribution based on LTND and MIP, from bright coal, semi-bright coal, semi-dull coal to dull coal, the total pore volume gradually decreases, with the proportion of macropore volume occupying 76.2%, 5.1%, 3.7% and 1.7%, respectively. On the basis of the 3D reconstruction of the porefracture system obtained from X-CT, the bright coal is characterized by high connected porosity and total porosity, presenting the best pore-fracture system. The pore network of bright coal is three-way interconnected with the largest number of pores and the shortest and densest throats, which facilitates the communication of fluids between the pores. In contrast, dull coals with a unidirectionally connected pore network model, fewer pores and sparse throats, have a poor seepage capacity. In addition, the X-CT seepage simulation results and the gas permeability results confirm the superiority of the pore-fracture system in bright coals.