Analyzing slippage coefficients and intrinsic permeabilities of dissimilar Indian coals considering their pore-size distributions: implications to CO2 storage in coal seams

Abstract:

Gas transport in dominantly microporous rocks such as coal, shale is significantly controlled by Klinkenberg or slippage effect at low reservoir pressures. Pore size fractions of the concerned reservoir rock control the degree of Klingenberg effect. This study tries to assess the differences in Klinkenberg effect in dissimilar coals owing to their difference in pore-size distributions. We report apparent permeabilities of three Gondowana coal samples from the Bokaro, Jharia, and Bansgara coalfields at room temperature and constant isostatic stress (900 psi). The linear plots of apparent permeability versus inverse pore pressure were modelled using the Klinkenberg equation, and non-linear plots by the Ashrafi equation to determine the slippage coefficient and intrinsic permeabilities. Ashrafi model reports lower values of slippage corrected permeability compared to the Klinkenberg model. The slippage coefficient and intrinsic permeability obtained from N₂ and CO₂ is different for the Bansgara sample. The trends between transport parameters and pore-size parameters were examined. The slippage coefficient increased with interconnected porosity and decreased with the percentage of micropores. Intrinsic permeabilities showed an opposite trend to that of the slippage coefficient. In dissimilar samples, larger volumetric proportion of micropores in the pore volume gamut resulted in higher slippage coefficients and lower intrinsic permeabilities. The average pore diameter of the samples determined from the BJH method was different from Klinkenberg or Randolph’s method, which utilized the value of the slippage coefficient to determine the pore diameters. Reporting the slippage coefficients and intrinsic permeabilities with corresponding pore size distributions of coal samples from different global basins will lead to statistically significant empirical relationships between pore size parameters and slippage coefficients.