Effect of the Coal Molecular Structure on the Micropore Volume and the Coalbed Methane Content

Abstract

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.