CO₂ Injection in Opalinus Clay at the Mont Terri CL-Experiment: Insights from Laboratory Experiments and Hydraulic-Geochemical Coupled Modeling

Carbon capture and storage (CCS) projects raise fundamental questions beyond technical performance, including how injected CO₂ behaves in the subsurface over long-time scales, how reliable model predictions are, and how experimental observations and simulations can be meaningfully combined. Addressing these questions requires not only process-based physical understanding, but also transparent modeling workflows, experimental validation, and effective collaboration across disciplines and institutions. In this contribution, we use the ongoing CO₂ Long-term Periodic Injection Experiment (CL-Experiment) at the Mont Terri Rock Laboratory in Switzerland as a central case study to illustrate how such integrated understanding can be developed. The core of the work is a numerical benchmark modeling study of CO₂ injection into the Opalinus Clay formation, using a two-dimensional axisymmetric representation of the injection system to investigate hydraulic propagation and coupled geochemical processes over a 20-year period. The simulations assume a fully water-saturated domain and single-phase injection at 3 MPa, using artificial porewater containing dissolved CO₂ corresponding to a partial pressure of 2 MPa. As part of a benchmark study, international teams use different numerical codes. Evaluation of the results enables a transparent assessment of model assumptions, sensitivities, and limitations, as well as model verification.To gain insights into CO₂–water–rock interactions, laboratory experiments were conducted using crushed Opalinus Clay from the in-situ sandy facies field site in an open system under controlled CO₂ conditions. Differences and consistencies between laboratory observations and numerical simulations are explicitly examined, highlighting key parameters and controlling processes that influence both model behavior and experimental responses.This study integrates numerical benchmarking, laboratory experiments, and interdisciplinary collaboration as a learning process to improve understanding of CO₂ storage in clay formations. Continuum-scale modeling shows that the CO₂ plume remains confined within approximately 1 m of the injection zone over 20 years (based on a cutoff concentration of 10 mmol/L), while CO₂-induced carbonate dissolution causes localized porosity increases within about 5 cm of the injection zone. At the laboratory scale, modeling indicates that carbonate reactions are the dominant factor on the pH evolution. However, strong spatial mineralogical heterogeneity observed in the in-situ samples limits the applicability of homogeneous batch-scale representations. For the international benchmark exercise, effective coordination relied on a hierarchical benchmarking strategy in which model complexity was increased stepwise by progressively introducing key variables and parameters. Together, the results of this study demonstrate the strength of coordinated benchmarking initiatives, and continuous exchange across disciplines, tools, and teams.