Abstract
Knowing the difference between gas and water permeabilities is significant not only for solving gas-water two-phase flow problems, but also for quick measurements of permeability using gas as pore fluid. We have thus measured intrinsic permeability of porous sedimentary rocks from Western Foothills of Taiwan, using nitrogen gas and water as pore fluids, during several effective-pressure cycling tests at room temperature. The observed difference in gas and water permeabilities have been analyzed in view of the Klinkenberg effect. This effect is primarily due to slip flow of gas at pore walls which enhances gas flow when pore sizes are very small. Experimental results show (1) that gas permeability is larger than water permeability by several times to one order of magnitude, (2) that gas permeability increases with increasing pore pressure, and (3) that water permeability slightly increases with increasing pore-pressure gradient across the specimen. The results (1) and (2) can be explained by the Klinkenberg effect quantitatively with an empirical power law for the Klinkenberg constant. Thus water permeability can be estimated from gas permeability. The Klinkenberg effect is important when permeability is lower than 10-18 mm2 and at low differential pore pressures, and its correction is essential for estimating water permeability from the measurement of gas permeability. A simple Bingham-flow model of pore water with a fixed pore size can explain the overall trend of the result (3) above. More sophisticated models with a pore-size distribution and with realistic rheology of water film is needed to account for the observed deviation from Darcy's law, i. e., the dependence of water permeability on the pore-pressure gradient.
