Publications

The pore character in coal determines all facets of its gas holding properties and thus must be understood if the reservoir behaviour is to be understood. In general, coal is thought to have a dual porosity system: large fractures and smaller pores. The pores themselves are classed into three size classes: macro-, meso- and micropores representing diameters >50 nm, 2-50 nm and <2 nm, respectively. Methane will move through the coal, from pore to pore and finally to the fracture system, by the process of diffusion. This is the same process by which a balloon slowly deflates from the migration of its gases from a higher-pressure environment (i.e. inside the balloon), through the plastic surface to the lower-pressure environment of the surrounding atmosphere. Most of the methane holding potential is thought to occur within the micropores where gas is adsorbed onto the surface of the pores. In some cases the methane molecule is thought to stretch the pore when it is absorbed onto the coal surface with the result that upon degassing, the matrix of the coal will shrink. This is important as fracture porosity may close when dewatering a coal reservoir resulting in loss of permeability, but matrix shrinkage may increase permeability in some reservoirs by reopening those fractures. Unlike conventional reservoirs where pore volume is the key indicator of gas holding potential, in coal it is the pore surface area. Thus, pore size distribution and shape become important when estimating gas holding potential in coal. The pore character is linked to the organic composition of the coal, which of course is a reflection of the original peat-forming environment.