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 (N2) 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 (SBET) (Brunauer-Emmet-Teller [BET]) ranges from 0.611 to 12.854  m2/g (7 to 138  ft2/oz), averaged at 2.638  m2/g(28  ft2/oz). The total pore volume (VBJH) (Barrett-Joyner-Halenda model [BJH]) ranges from 0.0018 to 0.0112  cm3/g (0.0001 to 0.0007  in.3/oz) with an average of 0.0037  cm3/g(0.0002  in.3/oz), and it shows a good positive relationship with SBET. The N2 adsorption amount shows a good positive relationship with the total SBET and VBJH. 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 (8.9464×103  md) is almost twice that of vitrinite (4.5056×103  md).

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