The effect of drying and re-saturation on the gas permeability of coal

Predictions of potential gas production from coal seam reservoirs rely on accurate measurements of porosity and permeability of coal samples. We have performed gas flow measurements on two coal samples from the San Juan Basin to examine the effect of drying and resaturation on flow rates. Gas flow rates were measured as a function of hydrostatic pressure, gas inlet pressure, and gas type (methane, helium, nitrogen) for samples under three different conditions: (1) ‘un-disturbed’, (2) dried, and (3) resaturated after drying. Drying the samples increased the flow rate by as much as two-orders of magnitude. The magnitude of the increase in flow caused by drying depended on the initial permeability of the sample and on the hydrostatic confining pressure. Resaturating the dried samples decreased the flow rate, but did not restore flow rates to the values obtained on the ‘undisturbed’ samples. Drying of coal samples results in permanent damage to the coal and thus greatly influences gas permeability.