Microstructure of Coal and Its Influence on Flow of Gas

This article discusses the results of an investigation carried out to study the microstructure of coal enhancing the understanding of the phenomena involved in release of methane and its movement in coalbeds. Scanning electron microscopy and transmission electron microscopy were used to obtain information about the structure of bituminous coal and its surface properties tike roughness, size and distribution of pores, and presence of microfractures and cleats. Micrographs obtained clearly showed the highly porous nature of coal. Most of the pores, however, appeared to have a very small volume and large surface area, which explains the large quantities of methane that are retained in adsorbed form in coalbeds. On the surface of coal, a network of microfractures and cracks was seen, indicating that it is these fractures that are chiefly responsible for flow of gas. The results illustrate the use of physical models for simulation of gas flow in coalbeds, involving dual flow behavior. Once the gas desorbs, there is diffusion flow in the coal matrix toward the fractures, followed by flow of gas in these fractures toward the gas producing wells. Furthermore, the microstructure of coal explains some of the unusual gas flow characteristics of coal.