Characterization of the micropore systems in the high-rank coal reservoirs of the southern Sichuan Basin, China

Abstract

To gain a better understanding of the characteristics of micropore systems in high-rank coal reservoirs, 51 coal core samples collected from 16 wells were examined by maceral analysis, proximate analysis, scanning electron microscopy observation, low-temperature nitrogen gas adsorption, and nuclear magnetic resonance (NMR). The results show that pores in coals can be divided into plant tissue hole, blowhole, dissolved pore, and intercrystalline pore, and they have three structure shapes: open pore, semiclosed pore, and “ink bottle” pore. The total specific surface area (Brunauer-Emmet-Teller [BET]) ranges from 0.611 to (7 to ), averaged at . The total pore volume (Barrett-Joyner-Halenda model [BJH]) ranges from 0.0018 to (0.0001 to ) with an average of , and it shows a good positive relationship with . The adsorption amount shows a good positive relationship with the total and . The average pore size ranges from 5.775 to 17.842 nm. Pores that are smaller than 5 nm make up the main contribution to the pore surface area, and those larger than 10 nm contribute greatly to the pore volume. Inertinite-rich coals have higher total specific surface area, pore volume, pore size, and adsorption capacity than those in vitrinite-rich coals. Lopingian coal reservoirs are characterized by low porosity and extremely low permeability obtained from NMR tests, and the permeability has a positive correlation with the porosity. The average permeability of inertinite is almost twice that of vitrinite .