Insights into the interaction of a shale with CO2

Abstract. Caprock formations, such as shales, play a key role in 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–hydro–chemo–mechanical (THCM) phenomena that may threaten the long-term integrity of the caprock. 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 timescales (months/years) that are required for the various THCM processes to manifest. In this work, the long-term multiphysical interaction of the Opalinus Clay shale with liquid and supercritical CO2 is studied in 3D with live X-ray tomography. Three-dimensional analysis reveals the localised response of the coupled THCM processes that is often indistinguishable with conventional lab testing protocols. To improve spatial and temporal resolution while applying field-representative pressure and temperature conditions, small-sized samples are studied. Long-term injection of liquid CO2 resulted in significant fissuring of calcite-rich zones that were for the first time visualised and quantified from the X-ray images. Additionally, a re-arrangement of the pre-existing micro-fissures in the clay matrix was observed. The volumetric response during direct exposure of an Opalinus Clay sample to supercritical CO2 revealed an initial swelling at pre-fissured 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 in the caprock material with time, suggesting potential CO2 trapping in the material.