Porosity type analysis and permeability model for micro-trans-pores, meso-macro-pores and cleats of coal samples

Porosity and permeability interact strongly, and the former describes the pore space for fluid storage, whereas the latter is used to describe the ability of fluid to migrate through the rock. Coal pore systems can be commonly classified as micro-trans-pores, meso-macro-pores and cleats, which indicates that the interactions between permeability and porosity in each pore system should be diverse. To investigate this diversity, measurements of nuclear magnetic resonance and routine core analysis are conducted for fifteen coal samples collected from underground mines in China. Then, the total porosity, movable porosity and unmovable porosity of each pore system are calculated, and the scatter diagrams of the three types of porosity versus air permeability are plotted. The results indicate that air permeability is influenced by both movable meso-macro-pore porosity and movable cleat porosity, and is weakly affected by the unmovable porosity of each pore system. According to the relative values of movable meso-macro-pore porosity and movable cleat porosity, coal samples are classified as cleat dominated permeability samples, meso-macro-pore dominated permeability samples, and combined effect samples. The former two types of samples can be used to establish models of cleat permeability and matrix permeability, respectively, based on the modified matchstick and Coates models.