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

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 (N-2) 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 (S-BET) (Brunauer-Emmet-Teller [BET]) ranges from 0.611 to 12.854 m(2)/g (7 to 138 ft(2)/oz), averaged at 2.638 m(2)/g (28 ft(2)/oz). The total pore volume (V-BJH) (Barrett-Joyner-Halenda model [BJH]) ranges from 0.0018 to 0.0112 cm(3)/g (0.0001 to 0.0007 in.(3)/oz) with an average of 0.0037 cm(3)/g (0.0002 in.(3)/oz), and it shows a good positive relationship with S-BET. The N-2 adsorption amount shows a good positive relationship with the total S-BET and V-BJH. 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 peimeability of inertinite (8.9464 x 10(-3) md) is almost twice that of vitrinite (4.5056 x 10(-3) md).