Insights into the interaction of a shale with CO ₂

Caprock formations, such as shales, play a key role in safe underground COinline-formula₂ storage since they serve as a hydromechanical barrier that prevents migration of the injected COinline-formula₂ to the surface. While their hydromechanical response is important to ensure their sealing capacity, interaction with the injected COinline-formula₂ 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 COinline-formula₂ 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 COinline-formula₂ 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 COinline-formula₂ revealed an initial swelling at pre-fissured zones and initiation of new micro-fissures at areas of direct contact with the anhydrous COinline-formula₂ due to pore water evaporation. Advanced 3D image analysis showed an increasing COinline-formula₂ uptake in the caprock material with time, suggesting potential COinline-formula₂ trapping in the material.