Diffusional Effects in the Recovery of Methane From Coalbeds

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Abstract
This paper presents evidence that the unipore model is inadequate for describing diffusional fluxes from coal over the entire timescale of desorption. Field desorption of methane from coal pieces shows pronounced curvature, which is attributable to the bidisperse pore structure generally found for coal. A model is presented that accounts for the bidisperse pore structure of coal and predicts diffusion rates for both field and laboratory desorptions. Transient diffusivities are determined for Pittsburgh bituminous and Madrid (NM) anthracite coals by using a pulse tracer technique coupled with Fourier analysis of the elution curves. This technique allows the rapid determination of the diffusion parameters with minimal experimental effort. In addition, steady-state diffusivities are determined to identify mechanisms of diffusion. Diffusion was found to be a combination of bulk, Knudsen, and surface diffusions, depending on the coal pore structure and gas pressure.

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