CO2 mineralisation potential of the Leman Sandstone Formation, Southern North Sea

The Permian Rotliegend Leman Sandstone Formation in the Leman Gas Field is the target for the first geological carbon storage development in the United Kingdom. This study provides insight into the long-term fate of the injected CO2 in the depleted reservoir by investigating the formation’s potential for carbonate mineralisation. Our approach integrates petrographic analysis, X-ray diffractometry (XRD), and formation brine chemistry data with reactive transport modelling to simulate CO2-brine-rock reactivity over a 1000-year timescale. Model outputs indicate that dawsonite is the only stable carbonated mineral formed, thus making it the most important for CO2 mineral trapping in the Leman Sandstone Formation. The lack of formation of other potential secondary carbonates like calcite and magnesite reflects insufficient supply of the necessary divalent cations for their precipitation. Further evaluation reveals a low-reactivity, carbonate-free Leman Sandstone Formation with a significantly low mineral-trapping capacity of ~ 0.123 mol m-3 bulk rock (~ 5.4 g CO2 m-3 bulk rock), with negligible impact on porosity and overall reservoir quality. This limited capacity relates to an extremely low reaction rate linked to the slow dissolution of primary silicates and the limited availability of reactive cations. The findings have implications for long-term CO2 storage in the studied Leman Sandstone Formation, with mineral trapping playing a negligible role, and solubility trapping in reservoir brines and physical mechanisms apparently more important. This underscores the primary influence of host-rock mineralogy on mineral trapping mechanisms and why mineralogical compositions should be carefully examined when assessing reservoirs for secure long-term CO2 storage.