Gas Sorption and Transport in Poroelastic Coals

摘要

In this paper, natural gas sorption and transient diffusion processes are described within coals exhibiting bimodal (macro- and micro-) pore structure on space-time continuum. Single-component gas is distributed in the microporous solid at adsorbed and dissolved states and in the macropores as free gas. The coal matrix is poroelastic, namely, its solid material manifests swelling and shrinkage effects due to the sorption phenomena under effective overburden stress. Gas transport is Fickian in nature and described by molecular and surface diffusion processes simultaneously taking place in the macroand micropores, respectively. A free gas concentrationdependent apparent diffusion coefficient is explicitly derived. Initial/boundary value problems are constructed considering the cases of gas uptake by and release from the coal. Consequently, influences of sorption phenomena on solid/macropore volumes and on the overall gas transport are numerically investigated using a finite difference approach. It is found that transport is primarily hindered by equilibrium sorption and takes place at a rate significantly less (typically 1-10 per cent) than that in the macropores only. Macroporosity variations are non-uniform in space and time. These mainly relate to availability of the sorbed gas in microporous solid; thus, the swelling and shrinkage effects are closely associated with the affinity of solid material to the gas component. Only in gas-coal systems with large sorption capacity the solid material is observed to shrink during the gas release and swell as the gas is sorbed by the coal. The estimated macropore volume changes could be as high as ±10 percent, although their effect on the overall gas transport is negligible.