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.