HOW MANY HOLES DOES IT TAKE TO FILL THE ALBERT HALL? THE IMPORTANCE OF POROSITY IN A COALBED METHANE RESERVOIR

摘要

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.