Coupled hydro-mechanical-chemical behavior of shale caused by CO2 injection in lab and pilot-scale experiments

This study investigates the coupled hydro-mechanical-chemical (HMC) 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 in Switzerland, 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 order of ~10^-20 m² and breakthrough pressure of 2-4 MPa. Particular attention is given to the transport properties of the sandy facies under different testing scenarios including the experimental duration, pore pressure difference, fluid types, and saturation history. Long-term tests 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. A hydro‐mechanical‐chemical coupling framework is employed to evaluate the time‐dependent response of fluid‐saturated rock subjected to CO₂ exposure. Carbonate mineral dissolution appears to play a key role in altering poroviscoelastic properties at experimental time scales of 3 to 5 weeks, so the HMC model is calibrated with the experimental data on limestones. The model predicts CO₂ injection‐induced porosity changes by accounting for the competing processes of chemical dissolution and time‐dependent compaction. 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. Overall, 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.