Coal
mainly consists of complex organic macromolecular polymers.
Their spatial arrangement and macromolecular structural parameters
control the coal reservoir property. This study evaluated the effects
of coal’s macromolecular structural evolution on the micropores
and established the relation between the coal heterogeneity and the
coalbed methane content in the vertical direction. Five groups of
samples with a reflectivity range of 0.60–1.01% and a burial
depth range of 400–1200 m were collected from the Fukang mining
area, Xinjiang, China. Variations of coal’s macromolecular
structural parameters, namely, the preferential development direction,
fringe length, fringe separation, and fringe tortuosity, with the
burial depth were experimentally determined via high-resolution transmission
electron microscopy. As the burial depth increased, coal’s
mean fringe length increased and the fringe separation dropped linearly,
while the fringe tortuosity gradually decreased at a burial depth
of approximately 800 m. In terms of the aromatic ring species, with
an increase in the burial depth, the share of aromatic fringes below
2 × 2 dropped while that of aromatic fringes above 3 × 3
increased steadily. The increase in aromatic fringes above 3 ×
3 was not conducive to the development of micropores. In addition,
the mean fringe separation in the research area decreased linearly
with the buried depth, and it decreased by 0.00295 nm per 100 m at
the buried depth of 400–1200 m. The reduction in aromatic fringe
spacing further reduces the intermolecular pores volume, which is
illustrated in low-pressure CO2 measurements. At a burial
depth exceeding 1000 m, the content of adsorbed-state methane in the
reservoir dropped. Both the fringe separation and the tortuosity degree
influenced the Langmuir volume and pressure parameters. The Langmuir
pressure dropped with increasing fringe separation. However, it decreased
rapidly at first and then slowly with increasing tortuosity degree
of the aromatic fringes.
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