Exploring geological resources and reservoirs Integrity by geophysical prospecTING of clays properties from nano to field scale – ExCiTING

In this project, we have pursued two main lines of study: the realization of measurements under controlled conditions and numerical modeling at different scales, from the nanometer to the decameter. At the heart of the project, we find the multi-scale aspect and the unification of pre-existing descriptions, but valid on particular ranges and conditions. We have also worked on upscaling relationships that allow us to move from laboratory sample models to the scale of surface geophysical measurements.
Spectral Induced Polarization (SIP) measurements have been performed in the laboratory to validate these relationships on pure clays, then on mixtures and finally including structural heterogeneities such as fractures.
In parallel, we have developed measurement procedures through significant improvements in instrumental, signal processing and imaging techniques. They have been carried out on several test sites and datasets, including the hydrogeological experimental site of Poitiers to test them and disseminate the results to the geosciences community.

Thanks to the mechanistic models, a better understanding of the physico-chemical phenomena and their relative importance in the behavior of the complex resistivity has been achieved. A better use of empirical laws has been demonstrated for the extrapolation of properties when scaling up. The method of moments was used to estimate complex resistivity in the presence of fractures.
Software for 3D imaging of complex resistivity from electromagnetic data is shared with the community upon request to the authors.
A new generation of instruments has been designed, developed and tested, and prefigures the future commercial range of the industrial partner.

The project has contributed on several important theoretical aspects for the characterization and use of the subsurface, in particular clayey rocks (e.g. integrity of a cap-rock for storage, exploration for geothermal energy).
Prospecting methods, from the measuring device to the imaging software, have been developed and tested at different scales, from the decimeter to the kilometer (for aquifers or deeper reservoirs). The contribution of an additional parameter to describe the subsurface opens additional possibilities for the description and discrimination of geological facies.

Within the framework of the project, 15 scientific papers have been published in international peer-reviewed journals, and 21 presentations have been made in international conferences.
The project partners organized and animated a workshop «Electrical properties of clays« in the framework of the international conference of geosciences EAGE 2021, and participated in many other events in the framework of geophysical applications.

Up to recent time, underground has been investigated mainly for its non-renewable natural resources neglecting its huge potential for storage and geothermal capability. A full energetic transition from traditional hydrocarbon resources to carbon free energy needs smart and safe underground use. In addition to being a source of geothermal energy, the subsurface is a vast 3D space that can be used in a carefully planned way for the management of carbon-free energies through the geological storage of CO2 and various other forms of energy vectors (e.g., H2, heat, compressed air).
For a safe and efficient exploitation of all natural resources (e.g., geothermal energy, hydrocarbon, minerals) or underground storage, one critical effort is to identify, characterize, and monitor natural clayey cap rock overlying a target (resource reservoir or storage volume), which plays an essential role in risk reduction (e.g., water table contamination, substances upward leakage) due to their low permeability. Characterization of clayey rocks is thus a key issue in this context. Focusing on this geological formation allows reducing a great part of geotechnologies issues.
The identification, characterization, and monitoring of the mineralogy and permeability of the clayey rocks is classically done using boreholes geological, geochemical and geophysical measurements. Despite having a high accuracy, boreholes measurements are invasive and can only bring punctual information at high cost. Surface-based geophysical tools, and especially electrical and electromagnetic (EM) methods (i.e. electric and/or magnetic field measurements), can provide additional information between boreholes at a significantly lower cost and repeatable in time. Interpretation of EM measurements is usually performed using only the direct current (DC) electrical resistivity, which yields to equivalency, sensitivity, and spatial resolution problems. These problems limit the method ability to identify different compartments and therefore generate interpretation difficulties. Using EM measurements and complex resistivity will improve the reliability and accuracy of the interpretation. But, this improvement requires high level of instrumental, theoretical and modeling developments at different scales, in particular for clayey rocks, as these rocks have a typical complex electrical signature associated with their strong surface electrical properties which is function of the clay mineralogy.
The main objective of the project is to improve the characterization of the complex and frequency dependence of electrical properties of different clays minerals and mixtures. For that purpose, we intend to closely combine measurements, modeling and inversion tools at different scales (from nano to pluri-m) in parallel to instrumental development. This work will require the development of upscaling procedures, from the mineral/water interface (nano/micrometric) to the field scale (decametric to kilometric). Laboratory experiments using Spectral Induced Polarization (SIP) and multi-scale simulations will be conducted in order to validate the upscaling relationships developed theoretically. These models will be included in an existing inversion code in order to characterize the complex electrical conductivity (chargeability) more precisely after inversion.
In parallel, we aim at improving the reliability of EM imaging at depth based on Controlled Source EM (CSEM) by resolving the surface heterogeneities “static” effects which often deteriorates the imaging capabilities deeper. We will develop a new prototype of EM Induction device (EMI) in order to image densely over large zone the shallow earth (from deca to hectometers). This project will help to push further the use of geophysical methods for the characterization of clayey cap rocks.