On the suitability of caprock reactivity studies for CO₂ storage: Insights from the Luna CCS field, Norwegian North Sea

The secure geological storage of CO₂ depends on the long-term integrity of caprock seals. Laboratory experiments and geochemical models are commonly used to assess reservoir and caprock reactivity, yet their ability to predict field-scale behaviour remains uncertain. We evaluate this reliability using samples from the Luna CCS field in the Norwegian North Sea. Reservoir sandstones and mudrock seals were characterised mineralogically and petrophysically, followed by high-pressure, high-temperature (300 bar, 120 °C) batch experiments and long-term (up to 10,000 years) PHREEQC modelling of CO₂–water–rock reactions. Both approaches indicate carbonate and feldspar dissolution and secondary clay and dolomite precipitation, producing minor porosity increases (<2% in reservoir; <4% in caprock). However, discrepancies in water–rock ratios, diffusion control, and surface area assumptions limit extrapolation to storage timescales. We demonstrate that such experiments and models are valuable for understanding reaction pathways but provide only qualitative constraints on caprock performance. For site assessment, first-order properties—capillary entry pressure, mineralogy, and mechanical strength—offer more robust indicators of seal integrity than laboratory reactivity tests.