Effects of the coalification jump on the petrophysical properties of lignite, subbituminous and high-volatile bituminous coals

Abstract

The evolution of the petrophysical properties (porosity/permeability) of lignite, subbituminous and high-volatile bituminous coals (LSBC) and the effects of the first coalification jump on the pore-cleat physical properties are still poorly understood. We investigated the pore-cleat heterogeneity, volume, aperture, connectivity and pore-contributed porosity/permeability with increasing coal ranks based on measurements obtained by photometer microscopy, mercury intrusion porosimetry and nuclear magnetic resonance. The results show that the values of adsorption-pore fractals (DA)<seepage-pore fractals (DS)<cleat fractals (DC), which indicates that the heterogeneity is in the order adsorption-pores<seepage-pores<cleats. The first coalification jump makes abrupt changes in the pore volume, efficiency of mercury withdrawal and coal cleat/total porosity, and results from the generation of hydrocarbons, asphalt and cleats in vitrinite. Additionally, coal porosity displays a “U” shaped trend with increasing coal rank. Cleat-contributed porosity also exhibits a “U” shaped trend, with ∼95% for 0.3 Ro, m%, ∼80% for 0.6 Ro, m% and ∼95% for 0.9 Ro, m%. The pore-contributed porosity decreases with increasing coal rank, which may result from pore space evolution by mechanical and chemical compaction. It is found that coal permeability has no obvious trend with increasing coal ranks, possibly related to the degree of cleat development. The cleat characteristics, including the aperture, length, frequency and connectivity, have a significant influence on LSBC porosity and permeability. However, certain subbituminous coals have pore-contributed permeabilities of 35–79%, which demonstrates that the influence on coal permeability of seepage-pores (providing methane flow pathway) should not be ignored.