Evolution of coal permeability: Contribution of heterogeneous swelling processes

This study hypothesizes that coal swelling is a heterogeneous process depending on the distribution of coal voids such as fractures, and that coal matrixes swell due to CO₂ sorption while fractures are compressed in response. This explains why coal permeability reduces even when the effective stress on coal samples is kept constant. A dual porosity-dual permeability model, which separately accommodates gas flow and transport in the coal matrix (swelling component) and fracture systems (non-swelling component) and rigorously accommodates the role of mechanical deformations for a dual porosity continuum, was developed and applied to prove this hypothesis. We use observations of a CO₂ flow-through experiment on coal constrained by X-ray CT to define the heterogeneous distribution of fracture porosity within the coal sample as a basis of mapping material properties for modeling. Matches between experimentally-measured and model-predicted ensemble permeabilities are excellent. More importantly, the model results illustrate the crucial role of heterogeneous swelling in generating swelling-induced reductions in permeability even when the fractured sample is mechanically unconstrained. These results prove that coal swelling is a heterogeneous process depending on the distribution of coal voids: matrix (swelling component) swells while fractures (non-swelling component) are compacted in response.