CO2 storage as clathrate hydrates in natural marine sands: Formation behaviour and microstructural insights from Raman and SEM analyses

The storage of carbon dioxide (CO 2 ) in the form of clathrate hydrates in marine environments represents a promising strategy for mitigating climate change. Clathrate hydrates are crystalline structures formed when CO 2 molecules are trapped within water cages under conditions of low temperature and high pressure, commonly found in marine environments. This study investigates the storage performance, dissociation kinetics and microstructural characteristics of CO 2 hydrates formed in natural sand from the Adriatic Sea using a 1 L isochoric reactor. Hydrate formation in three-phase systems was compared to pure water systems to isolate the role of the natural sand. Results show that sand significantly enhances storage performance, increasing gas uptake (from 7.7-18.2% up to 9.7-36.7%) and water conversion rate (from 1.3-2.8% up to 4.0-8.5%), particularly at lower pressures. Temperature remains the dominant operating parameter, while heterogeneous nucleation and improved heat dissipation given by the sand promote hydrate formation. Raman and SEM analyses reveal that natural sand decreases molecular disorder and lead to denser hydrate morphologies, supporting enhanced conversion efficiency. These findings demonstrate that the studied natural sand acts as kinetic and interfacial promoter of hydrate-based CO 2 storage, providing quantitative insights relevant for offshore CCS strategies. • Natural sand is quartz-rich and mainly mesoporous with low microporosity and 16 nm pores • Natural sand increases gas uptake, formation density and water conversion at all pressures. • CO 2 hydrates in sand enable diffuse storage with moderate density • CO₂ hydrates in sand form denser and more compact structures with higher stability. • Natural sand induces more ordered water structures, enhancing hydrate formation.