On the fundamental difference of adsorption-pores systems between vitrinite- and inertinite-rich anthracite derived from the southern Sichuan basin, China

Abstract

To further understand the fundamental difference of adsorption-pores systems between vitrinite- and inertinite-rich anthracite, six coal core samples with >85% vitrinite and three samples with >80% inertinite were collected from coalbed methane wells in the southern Sichuan basin, China, by macerals analysis method. The differences of pore genetic types, pores shapes, pore surfaces roughness, pore-size distribution, specific surface area, total pore volume, physical properties, and CH4 adsorption capacity between vitrinite- and inertinite-rich samples, were studied via ESEM observation, low-temperature N2 adsorption experiments, NMR tests, and CH4 isothermal adsorption experiments. Results show that plant tissue holes are easier to observe in inertinite than vitrinite, and blowholes, breccia pores and broken pores are all common in vitrinite and rare in inertinite of the study coals. Both vitrinite- and inertinite-rich coal samples exhibit complex nano-pore structures, and pore shapes in inertinite-rich coals are more special than those in vitrinite-rich samples. Fractal dimensions analysis from the N2 adsorption isotherms indicates that inertinite-rich coals have the higher surfaces roughness of irregular pores than vitrinite-rich coals in the P/Po intervals of 0.5–1. In addition, it can be predicted that pores of D2 and D3 type hysteresis loops with diameters of <2.76 nm each are almost semi-closed pores, and the narrow neck in “ink bottle” pores generally measure 2.76 nm. Nano-pores measuring <4 nm comprise the largest proportion among all adsorption-pores. The proportion of pores with diameters of <0.64 nm each in inertinite-rich coals is greater than that in vitrinite-rich coals. NMR porosities, permeabilities, and irreducible water saturations between vitrinite- and inertinite-rich coals are all similar, and the porosities display a good exponential positive correlation with the permeabilities. Vitrinite- and inertinite-rich samples are both characterized by a large VL, and there are positive relationships of VL with vitrinite and inertinite, indicating the CH4 adsorption capacity of organic macerals is much stronger than that of inorganic minerals in anthracite.