Comparison of nanopore evolution in vitrinite and inertinite in coalbed methane reservoirs during coalification

Abstract

The different responses of vitrinite and inertinite to thermal metamorphism strongly affect the evolution of nanopores in different macerals of coal reservoirs. In order to reveal these differences during coalification, vitrinite and inertinite samples were handpicked from 10 block coal samples. A series of laboratory experiments were performed to explore the macromolecular structure and nanopores of the samples. The results indicate that the nanopore structure of the vitrinite showed obvious regularities with the reflectance of vitrinite (in oil, Ro), but that of the inertinite did not. The macromolecular structure of the inertinite did not correlate with Ro, whereas the vitrinite showed obvious correlations with Ro. Therefore, the nanopore structure of the inertinite cannot be affected by changes in Ro. The nanopore structures of both the vitrinite and inertinite were affected by the changes of macromolecular structures. The detachment of the aliphatic side chains and the promotion of the aromatic degree and the aromatic ring polycondensation degree decreased the micropore surface area (micro-SA) because the longer aliphatic side chains and lower degree of aromatic ring polycondensation make the surface of the coal particle more heterogeneous. The increases in the aromatic degree and aromatic ring polycondensation degree increased the mesopore volume (Vmes), total pore volume excluding micropores (VT), mesopore surface area (meso-SA), and Brunauer–Emmett–Teller (BET) specific surface area (BET-SA) because more isolated pores were connected with each other; thus, more interspace emerged among the aromatic ring layers with an increase in Ro. However, this does not mean that these regularities of inertinite are related to the coal rank, because macromolecular structures of the inertinite had been fixed prior to the coal formation and is not sensitive to coalification. These results are expected to enhance the understanding of the differences in vitrinite and inertinite nanopore structure evolution that occurs during coalification.