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| Titel |
A Circuit Model for the Measurement of the Streaming Potential in a Rock Sample |
| VerfasserIn |
C. Yin, J. Wang, A. Qiu, X. Liu, H. Hu |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064618
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| Zusammenfassung |
| Streaming potential is usually defined under the assumption that the rock sample under
consideration is not connected electrically to any external circuit. In this study we investigate
experimentally the effect of the external circuit on the measurement of the streaming
potential.
Cations usually dominate anions in the diffuse layer in the pore canals in a fluid-saturated
porous sandstone sample. When a pressure difference is applied to the sample, fluid flows in
the pores and causing a convective current due to the cation-dominate motion. With the
separation of opposite ions at the two ends, a streaming potential occurs, and results in a
conductive current. Those two current will be opposite and equal in value so that the
streaming potential does not change.
But in any experimental measurement of the streaming potential, the rock sample is not
isolated in the circuit. An external circuit is necessary for the measurement of the potential
difference at the ends of the sample. This external circuit will divert the flow of charges. This
study investigates the effect of the external circuit on the convective current and conductive
current in the pores by experiments, and gives an equivalent circuit model for the two
currents.
We connect an external resistance Rext to the ends of the fluid-saturated rock sample, and
measure the potential difference at the ends of the sample . The impedance of the
fluid-saturated rock sample Zrock is definite under a given salinity and can be separately
measured. The circuit is governed by the following equations,
Urock = ZrockIcond,
(1)
Urock = RextIext,
(2)
Iconv + Icond +Iext = 0,
(3)
where Iconv is the convective current, Icond is the conductive current, Iext is the external
current and Urock is the potential difference at the ends of the rock sample.
From the above three equations, we get
- Urock(Zrock + Rext)
Iconv = ––-Z–-R––––- .
rock ext
(4)
We repeated the measurement under different external resistance Rext. The computed
convective current changes slightly. Thus we conclude that the measurement circuit does not
change the convective current, although it changes the conductive current. So the
convective current behaves as a constant current source. This conclusion is in consistence
with the electrokinetic model, in which the convection current is caused by the
cation-dominated fluid flow under pressure difference. We notice that the measured
streaming potential is smaller than that without measuring circuit connected to
it, unless the external resistance Rext is orders higher than the resistance of the
rock.
This study is supported by project No.41174110 of the National Natural Science
Foundation of China. |
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