Hydro-mechanical-chemical effects on flow properties of Opalinus Clay in CO2LPIE project

Abstract

This study investigates the coupled hydro-mechanical-chemcial behavior and multiphase flow properties of Opalinus Clay – a potential caprock candidate for geologic carbon storage. A comprehensive series of laboratory tests is conducted to support the CO₂ Long-term Periodic Injection Experiment (CO₂LPIE) project at the Mont Terri Underground Rock Laboratory, providing essential parameters for caprock characterization. Facies-dependent poroviscoelastic and transport properties are quantified: the sandy facies exhibit higher drained and unjacketed bulk moduli and permeability than the shaly facies, yet both facies display favorable long-term sealing potential with intrinsic permeability on the order of ∼10⁻²⁰ m² and breakthrough pressure of 2–4 MPa. Particular attention is given to the flow properties of the sandy facies under different testing scenarios including the experimental duration, pore pressure difference, fluid types, and saturation history. Long-term injection experiments highlight exponential permeability reduction driven by time-dependent compaction, which is effectively described by a poroviscoelastic model coupled with a power-law porosity-permeability relationship. In contrast, CO₂-rich water injection yields relatively stable permeability with only minor irreversible changes likely controlled by fluid-rock interactions, fluid affinity, and electrokinetic effects. Two-phase flow tests further reveal that CO₂ displaces water more effectively in the sandy facies, while CO₂ relative permeability is insensitive to lithological differences. These findings demonstrate that heterogeneous Opalinus Clay retains strong sealing integrity under coupled hydro-mechanical-chemical conditions and provide critical laboratory insights that complement ongoing in-situ monitoring within CO₂LPIE.