05 Sep 2022
05 Sep 2022
Status: this preprint is open for discussion.

Insights into the interaction of a shale with CO2

Eleni Stavropoulou and Lyesse Laloui Eleni Stavropoulou and Lyesse Laloui
  • Ecole Polytechnique Fédérale de Lausanne (EPFL), Laboratory for Soil Mechanics (LMS), EPFL-ENAC-LMS, Station 18, CH-1015 Lausanne, Switzerland

Abstract. Caprock formations such as shales, play a key role to safe underground CO2 storage since they serve as a hydromechanical barrier that prevents migration of the injected CO2 to the surface. While their hydromechanical response is important to ensure their sealing capacity, interaction with the injected CO2 involves additional thermo-chemo-mechanical (THMC) phenomena that may threaten the long-term caprock’s integrity. The low transport properties of shales make them a suitable caprock material, but at the same time challenging to study due to the very long time scales that are required for the various thermo-hydro-chemo-mechanical processes to manifest. In this work, the multiphysical interaction of the Opalinus Clay shale with CO2 is studied with live x-ray tomography. Long-term exposure to liquid and supercritical CO2 targets the investigation of different occurring THMC processes locally and globally in 3D that are often indistinguishable with conventional lab testing protocols. To improve spatial and temporal resolution while applying realistic pressure and temperature conditions, small size samples are studied. Long-term injection of liquid CO2 resulted to a siginificant fissuring of calcite-rich zones that were for the first time visualised and quantified from the x-ray images, while a re-arrangement of the pre-existing micro-fissures in the clay matrix were observed. The volumetric response during direct exposure of an Opalinus Clay sample to supercritical CO2 revealed an initial swelling at pre-cracked zones and initiation of new micro-fissures at areas of direct contact with the anhydrous CO2 due to pore water evaporation. Advanced 3D image analysis showed an increasing CO2 uptake with time the elevated value of which after pressure release suggests potential CO2 trapping the material.

Eleni Stavropoulou and Lyesse Laloui

Status: open (until 27 Oct 2022)

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Eleni Stavropoulou and Lyesse Laloui

Eleni Stavropoulou and Lyesse Laloui


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Short summary
Shales are identified as suitable caprock formations for geolocigal CO2 storage thanks to their low permeability. In this work, small size shale samples are studied under field representative conditions with x-ray tomography. The geochemical impact of CO2 on calcite-rich zones is for the first time visualised, the role of pre-existing micro-fissures to the CO2 invasion trapping in the matererial is highlighted and the initiation of micro-cracks when in contact with anhydrous CO2 is demonstrated.