Long-term in-situ evidence for transient CO2 leakage in faulted caprock at Mont Terri: summary of results from the CS-D and CS-E experiments

Geological carbon storage (GCS) is essential for achieving net-zero emissions targets yet requires reliable long-term containment of injected CO₂ within storage formations. The sealing capacity of overlying caprocks is critical for ensuring storage integrity over geological timescales (Vialle et al. 2018). Faults within caprock formations represent potential pathways for CO₂ migration to shallower formations or the surface, making it essential to understand the coupled physical and chemical processes controlling fluid transport through faulted clay-rich formations. Fluid injection can perturb the in-situ stress field and potentially reactivate pre-existing faults, either through seismic or aseismic deformation (Zappone et al. 2021, and references therein). However, the long-term behavior of reactivated faults in clay-rich formations and particularly their capacity for self-sealing remains poorly understood. The Mont Terri rock laboratory (MTRL) in northwestern Switzerland provides unique access to the Opalinus Clay formation and its Main Fault, enabling controlled injection experiments at the decameter scale under realistic in-situ conditions. The recent FS-B experiment (2020) demonstrated fault reactivation mechanics and quantified self-sealing behavior through year-long monitoring (Guglielmi et al. 2025). Building on these findings, the CS-D and CS-E experiments (2019–2025) extended investigations to long-term, low-flow-rate injection of CO₂-rich brine, enabling comprehensive study of geochemical evolution and coupled hydro-mechanical processes over multi-year timescales (Zappone et al. 2021; Weber et al. 2023). This paper consolidates results from the five-year CS-D/CS-E experimental program, documenting the experimental setup, geochemical monitoring methodology, hydraulic behavior, and long-term mechanical response of the fault zone during CO₂-rich brine injection. We discuss implications for caprock integrity assessment and monitoring strategies for geological CO₂ storage.