Evaluation of CO₂ sealing potential of heterogeneous Eau Claire shale

During geological carbon dioxide storage in deep saline aquifers, buoyant CO₂ tends to float upwards in the reservoir overlaid by a low permeable formation called a caprock. Caprocks should serve as barriers to potential CO₂ leakage that can happen through diffusion and permeation through faults, fractures or pore spaces. The leakage through intact caprock would mainly depend on its permeability and CO₂ breakthrough pressure and is affected by the heterogeneities in the material. Here, we study the sealing potential of a caprock from the Illinois Basin – Eau Claire shale, with sandy and clayey fractions distinguished via electron microscopy, grain/pore size analyses and surface area characterization. The direct measurements of permeability of sandy shale provide the values on the order of 10⁻¹⁵ m², while clayey specimens are three orders of magnitude less permeable. The CO₂ breakthrough pressure under in situ stress conditions is 0.1 MPa for the sandy shale and 0.4 MPa for the clayey counterpart – these values are higher than those predicted by the porosimetry methods performed on the unconfined specimens. Sandy Eau Claire shale would allow penetration of large CO₂ volumes at low overpressures, while the clayey formation can potentially serve as a caprock in the absence of faults and fractures.