Characterization of heterogeneous Opalinus Clay for CO2LPIE project at Mont Terri underground rock laboratory

ABSTRACT: The sealing potential of a caprock is critical for the long-term success of geologic carbon storage and can be influenced by lithological heterogeneities and structural discontinuities (e.g., bedding planes, fractures, and faults). CO2LPIE project investigates the field-scale sealing potential of heterogeneous Opalinus Clay formation under periodic CO2-water injection. Laboratory experiments are conducted on specimens representing the shaly and sandy facies extracted from potential injection sites. It appears that the sandy facies, due to their greater fraction of quartz and carbonate minerals, exhibit higher drained and unjacketed bulk moduli than the shaly facies. The sandy specimens display permeability a few times higher than the shaly counterparts due to differences in pore-scale characteristics. The permeability values of both facies remain well below the threshold required for an effective caprock. A power-law porosity-permeability relationship with sensitivity exponent values of 17-18 is identified, highlighting that even minor porosity variations can significantly change the fluid flow through Opalinus Clay. While existing models capture general permeability trends, they fail to represent this extreme sensitivity to porosity changes. To improve predictability at the field scale, future work will integrate these laboratory findings with numerical simulations and in-situ experiments, advancing the understanding of caprock integrity under CO2 injection conditions.