Отправлено: 04.10.19 12:18. Заголовок: Characterising the self-potential (SP) response to concentration gradients in heterogeneous

Characterising the self-potential (SP) response to concentration gradients in heterogeneous sub-surface environments
DJ. MacAllister, Graham, M , J. Vinogradov , A.P. Butler, M.D. Jackson

© 2019 American Geophysical Union

Abstract
Self-potential (SP) measurements can be used to characterise and monitor, in real-time, fluid
movement and behaviour in the sub-surface. The electrochemical exclusion-diffusion (EED)
potential, one component of SP, arises when concentration gradients exist in porous media. Such
concentration gradients are of concern in coastal and contaminated aquifers, and oil and gas
reservoirs.
It is essential that estimates of EED potential are made prior to conducting SP investigations
in complex environments with heterogeneous geology and salinity contrasts, such as
the UK Chalk coastal aquifer. Here, we report repeatable laboratory estimates of the EED potential of
chalk and marls using natural groundwater (GW), seawater (SW), deionised (DI) water and 5 M NaCl.

In all cases the EED potential of chalk was positive (using a GW/SW concentration gradient the
EED potential was c.14 to 22 mV), with an increased deviation from the diffusion limit at the higher
salinity contrast.
Despite the relatively small pore size of chalk (c.1 m), it is dominated by the
diffusion potential and has a low exclusion-efficiency, even at large salinity contrasts. Marl samples
have a higher exclusion-efficiency which is of sufficient magnitude to reverse the polarity of the EED
potential (using a GW/SW concentration gradient the EED potential was c.-7 to -12 mV) with respect
to the chalk samples.
Despite the complexity of the natural samples used, the method produced repeatable results.
We also show that first order estimates of the exclusion-efficiency can be made
using SP logs, supporting the parameterisation of the model reported in Graham et al. (2018), and that
derived values for marls are consistent with the laboratory experiments, while values derived for
hardgrounds based on field data indicate a similarly high exclusion-efficiency.
While this method shows promise in the absence of laboratory measurements, more rigorous estimates should be made
where possible and can be conducted following the experimental methodology reported here.