Page 276 - gas transport in porous media

CHAPTER 15

AIR PERMEABILITY MEASUREMENTS IN
POROUS MEDIA

VINCENT C. TIDWELL
Sandia National Laboratories, P.O. Box 5800, MS 0735, Albuquerque, NM 87185, USA

15.1 INTRODUCTION

Fundamental to continuum-based modeling of flow and transport is some knowl-
edge of the permeability of the medium. Permeability quantifies the resistance of
the medium to flow and depends only on the characteristics of the porous medium.
Permeability is an integrated measure representing the complex relationship between
the geometry of the pore system and hydraulics of the flow through that system.
Permeability is not measured directly, rather is calculated by inverting an assumed
model populated with measured state data (i.e., flux, pressure). As such, permeabil-
ity has meaning only in the context in which it is measured. That is, permeabilities
measured over different scales or using different flow geometries are likely to exhibit
very different values.
Belowweexploresomeofthemethodsavailableforcharacterizingthepermeability
of a porous medium. Our survey focuses on techniques that employ gas as the test
fluid. Measurements at both the lab and field scale are reviewed.

15.2 PERMEAMETERS

Simply stated, permeability is determined by applying a pressure gradient across a
porous medium while measuring the resulting flux. Both steady and non-steady state
methods are routinely employed in these measurements. The non-steady or pressure-
decay methods are generally reserved for materials with low permeabilities, where
achieving steady-state flow in a reasonable time is impractical.
The constant head permeameter is a common laboratory technique for determining
permeability. Its popularity stems from its relative simplicity, both in measurement
and permeability calculation. Testing is performed on rock cores or unconsolidated
samples loaded in laboratory columns. A constant head difference is applied across
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C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 273–278.
© 2006 Springer.