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Investigations of the coal permeability in the flow of nitrogen, methane and carbon dioxide |
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Investigations of the coal permeability in the flow of nitrogen, methane and carbon dioxide Authors: J. Żółcińska, L. Dyrga
The permeability of gases depends on the type of coal, and its pore structure (evaluated by the porosity and the nitrogen reference permeability kNl), on the type of flowing gas (sorption affinity), and on the moment of the flow. Eighteen different coal samples cut of coal blocks received from the Miechowice, Zofiówka and Thorez mines have been investigated. For the research the samples of different structure, and of nitrogen permeabilities varying in the interval 0.01-600 x 10"15 m2 (0.01-600 mD) have been applied. For each sample the flow of various gases: nitrogen, methane and carbon dioxide were investigated. The magnitude of permeability determined for the steady stage of the first nitrogen flow through coal has been accepted as the reference value for a given sample and denoted by kNr The variations occurring during the methane and carbon dioxide flow are better represented by the normed permeabilities kCHt/kfi2 and kcoJkN2 than by feCH4 and feCO2 permeabilities because the normed permeabilities are respected to the steady nitrogen flow. For the methane flow the normed kCHJkNl permeability increases until the maximum value the location of which depends on the coal type. For small pore dimension samples, i.e. for samples of small reference permeability the sorption process is the most significant phenomenon. The normed permeability kCHJkNl decreases with the increase of pore dimensions, i.e. with the increase of the reference permeability owing to the increasing role of hydrodynamic effects. For carbon dioxide the normed kcoJkN2 permeability varies in the monotonie mode. For small reference permeability samples the highly developed sorption of carbon dioxide implies the following situation: the normed kcoJkN2 permeability is several times less than the normed kCHJkNl permeability for the same coal sample. Appointing samples with increasing feN2 permeability the hydrodynamic effects imply the approximate equality of the permeability magnitudes for methane and for carbon dioxide. Nevertheless the considered kCHa and kCOl permeabilities are about 20-25% less than feN for the nitrogen flow.
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2011