Characterising the self-potential response to concentration gradients in heterogeneous sub-surface environments

D. J. MacAllister (Corresponding Author), M. T. Graham, J. Vinogradov, A.P. Butler, M.D. Jackson

Research output: Contribution to journalArticle

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.
Original languageEnglish
JournalJournal of Geophysical Research: Solid Earth
Early online date10 Jul 2019
DOIs
Publication statusE-pub ahead of print - 10 Jul 2019

Fingerprint

self potential
exclusion
Calcium Carbonate
gradients
chalk
Seawater
Groundwater
ground water
salinity
seawater
Aquifers
groundwater
aquifers
estimates
coastal aquifer
Deionized water
marl
Geology
Parameterization
Pore size

Keywords

  • self-potential
  • exclusion-diffusion potential
  • membrane potential
  • electrochemical potential
  • geoelectric methods
  • groundwater
  • seawater intrusion
  • water resources
  • reservoir monitoring

Cite this

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

In: Journal of Geophysical Research: Solid Earth, 10.07.2019.

Research output: Contribution to journalArticle

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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.",
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author = "MacAllister, {D. J.} and Graham, {M. T.} and J. Vinogradov and A.P. Butler and M.D. Jackson",
note = "DJM was supported by NERC CASE studentship NE/I018417/1. Additional support was provided by NERC to MG under the Science and Solutions for a Changing Planet Doctoral Training Partnership, run by the Grantham Institute for Climate Change at Imperial College London. Two anonymous reviewers are thanked for their comments, which greatly helped to improve the manuscript. The authors would also like to thank Southern Water for access to the borehole at Saltdean. Atkins Global and Southern Water are thanked for some additional funding under the NERC CASE studentship. The laboratory components of this work were carried out with support from TOTAL who we gratefully acknowledged. All data supporting the conclusions of this work are available in the supporting information.",
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