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| Titel |
Water saturation dependence of streaming potential coupling coefficient and excess charge density in sandstones during drainage and imbibition |
| VerfasserIn |
Matthew Jackson, Eli Leinov, Murtaza Gulamali, Jan Vinogradov |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250050802
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| Zusammenfassung |
| The streaming potential resulting from the flow of water through porous media can be
characterised in terms of either the streaming potential coupling coefficient (C), or the
density of excess charge transported by the flow of water (Q). These properties are related
by
C = Qkw-
Ïăμw
where kw is the permeability to water, Ïă is the rock conductivity and μm is the
water viscosity. When an immiscible, non-polar phase such as oil or gas is also
present in the pore-space, both C and Q are water saturation dependent, but their
relationship with saturation is poorly understood. Streaming potential measurements
during unsteady-state drainage and imbibition in sandstones saturated with water and
undecane have recently been reported (Vinogradov and Jackson, in press). However, the
saturation dependence of C and Q cannot be directly obtained from their experimental
results. In this paper, we use numerical methods to invert the measured data and
report the saturation dependence of C and Q during drainage and, for the first time,
imbibition.
We find that C generally decreases with decreasing water saturation while Q generally
increases. However, the saturation dependence of both C and Q is different during drainage
and imbibition. During drainage, C initially decreases slowly with water saturation, before
rapidly decreasing to its minimum value. Even very small rates of water flow, at water
saturations close to the assumed ‘irreducible’ saturation, result in non-zero values of C,
because water flow occurs through wetting layers which contain a very high excess charge
density. During imbibition, C increases with increasing saturation and, close to the residual
non-wetting phase saturation (Sw = 1-Snwr), C exceeds the value observed at saturation (Sw
= 1). A number of previous publications have assumed that Q is inversely proportional
to water saturation, but we find that this is not the case during either drainage or
imbibition.
These results are relevant to the interpretation of streaming potential measurements in oil
reservoirs, contaminated aquifers and the vadose zone. They suggest that (i) the behaviour of
the multiphase streaming potential coupling coefficient depends upon the phases present and
the direction of saturation change, (ii) the excess charge density transported by the flow of
water does not scale inversely with water saturation, and (iii) the results of drainage
experiments cannot be applied to imbibition.
Vinogradov, J. and Jackson, M.D., Multiphase streaming potential in sandstones saturated
with gas/brine and oil/brine during drainage and imbibition, Geophys. Res. Lett., in press. |
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