Poroperm characteristics of high-rank coals from Southern Qinshui Basin by mercury intrusion, SEM-EDS, nuclear magnetic resonance and relative permeability analysis

Abstract Poroperm characteristics (porosity and permeability) of a coal seam play crucial roles in exploration and recovery of coalbed methane (CBM). In order to comprehensively understand the poroperm characteristics of high-rank coals, a series of laboratory tests including water-saturated porosity measurement, high-pressure mercury intrusion porosimetry (MIP), low-field nuclear magnetic resonance (NMR), scanning electron microscope and energy dispersive spectrometry (SEM-EDS) analysis and relative permeability tests were performed on the selected high-rank coal samples from underground coal mines in Southern Qinshui Basin, China. The cleat size distribution index ( λ ) and cleat tortuosity ( η ) were derived from the relative permeability experimental data and the correlations between these two parameters and coal rank parameters were analyzed. The results shows that: adsorption pores account for a dominant percentage in the total pore volume, while seepage-pores and fractures are poorly developed; clay minerals fill most of the micro-fractures and have a strongly negative impact on the coal permeability; the relative permeability curves for the coal samples is characterized by a higher residual water saturation, a higher water saturation at cross point, a narrow span of two-phase flow region, and a lower gas relative permeability. The cleat size distribution index ( λ ) is positively related to the water and gas relative permeability, while the cleat tortuosity ( η ) has a negative effect on fluid flow in coals. A mathematical model was proposed to relate η and λ to coal rank parameters respectively, which can be used to evaluate the relative permeability of high-rank coals. The producible porosity is very low, ranging from 0.17% to 0.37% for the high-rank coal samples, but it plays a dominant role in determining the permeability of coal. A modified producible porosity (PP) model was proposed to evaluate the absolute permeability and the effective gas permeability under the residual water saturation of high-rank coals.