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International Journal of Geophysics
Volume 2012, Article ID 897807, 12 pages
http://dx.doi.org/10.1155/2012/897807
Research Article

On the Validity of the “Thin” and “Thick” Double-Layer Assumptions When Calculating Streaming Currents in Porous Media

Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK

Received 10 August 2011; Revised 1 December 2011; Accepted 19 January 2012

Academic Editor: Laurence Jouniaux

Copyright © 2012 Matthew D. Jackson and Eli Leinov. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

We find that the thin double layer assumption, in which the thickness of the electrical diffuse layer is assumed small compared to the radius of curvature of a pore or throat, is valid in a capillary tubes model so long as the capillary radius is >200 times the double layer thickness, while the thick double layer assumption, in which the diffuse layer is assumed to extend across the entire pore or throat, is valid so long as the capillary radius is >6 times smaller than the double layer thickness. At low surface charge density (<10  m C m 2 ) or high electrolyte concentration (>0.5 M) the validity criteria are less stringent. Our results suggest that the thin double layer assumption is valid in sandstones at low specific surface charge (<10  m C m 2 ), but may not be valid in sandstones of moderate- to small pore-throat size at higher surface charge if the brine concentration is low (<0.001 M). The thick double layer assumption is likely to be valid in mudstones at low brine concentration (<0.1 M) and surface charge (<10  m C m 2 ), but at higher surface charge, it is likely to be valid only at low brine concentration (<0.003 M). Consequently, neither assumption may be valid in mudstones saturated with natural brines.