Coal microstructure and secondary mineralization: their effect on methane recovery

Abstract

Methane production from coal seams rather than porous sandstone reservoirs is now recognized as a valuable and recoverable energy source in Australia. The Bowen Basin of Australia possesses well defined coal seams that contain major methane resources. However, commercial gas production to date has been hampered by the low permeabilities of the coal seams. Recovery of this valuable resource will be assisted by a fundamental understanding of coal microstructures and presence of mineralization, and their influence on the gas flow behaviour through coal. This paper examines the relationships between coal type, microstructure, secondary mineralization and gas flow behaviour. The study demonstrates that Bowen Basin coals should not be viewed as simply a dual porosity system of micropores which are surrounded by cleats. Instead, studies using scanning electron microscopy show the Bowen Basin coals have a third porosity system comprising a hierarchy of micron-sized fractures and micronsized cavities at a level between the micropores and the cleat/macropore system, which vary according to coal type. To determine the influence such microstructures have on the flow of gas through the coal matrix, sorption experiments were carried out on small solid blocks of coal, using a new gravimetric technique. The results demonstrate that a clear distinction exists between diffusivity of dull and bright coals in response to coal microstructure. The gas sorption data suggests that both dull and bright coals can be divided into two categories: coal which have a rapid sorption behaviour and coals which have a slow sorption behaviour. The results of the sorption experiments indicate that size, continuity, connectivity of the microstructures, and the extent of minerals infilling the fractures and cavities, play a significant contribution to overall permeability, and are likely to play a major rate-limiting factor in the flow of methane through coal at a level between diffusion at the micropore level and laminar flow at the cleat level. The studies indicate that the flow behaviour of gas through coal seams in the Bowen Basin is unlikely to be solely dependent on the cleat system but rather a combination of the cleat, microstructure and secondary mineralization in coal.

The effect of rank, lithotype and roughness on contact angle measurements in coal cleats

“…Figure 6 illustrates the variation in the surface morphology with dull inertinite bands having a rougher texture compared to the smooth surface displayed on vitrinite rich bright bands. The image closely corresponds to the lithotype features reported by Gamson et al (1996) who also examined different lithotype morphology using SEM. They reported that the bright bands displayed a smoother surface texture compared to the rougher dull band coal.…”

“…They reported that the bright bands displayed a smoother surface texture compared to the rougher dull band coal. cavities permeated throughout, as seen in Figure 6C (Gamson et al, 1996). The cylindrical cavities between the sheets that are ~2 µm wide and ~10 µm in length are due to the original cell lumen (Gamson et al, 1996).…”

Coupling Effect of Intruding Water and Inherent Gas on Coal Strength Based on the Improved (Mohr-Coulomb) Failure Criterion

“…When using the mixture theory for derivation, the pore sizes of the porous medium are assumed to be uniform. In fact, the pore sizes of coal span a wide range, and the pores also have different forms [34]. When the differences of pore sizes and forms in a coal specimen are notable, the original assumption may cause a significant deviation.…”

The Effect of Water Occlusion on Gas Production in Coal

“…This difference is due to the dull banded coal possess phyterals that create a more porous and interconnected structure, compared to the bright band. Gamson et al (1996) also reported that these microstructures in the dull band are heavily mineralised and both the dull and bright bands have different diffusivity rates, as well as the potential to exhibit significant differences in gas and water flow behaviour. …”

“…Work by Gamson et al (1996) shows that the Bowen Basin dull and bright band lithotypes typically exhibit very different topographies at a micron scale, illustrated in Figure 2.28. This difference is due to the dull banded coal possess phyterals that create a more porous and interconnected structure, compared to the bright band.…”