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

The storage of carbon dioxide (CO2) in the form of clathrate hydrates in marine environments represents a promising strategy for mitigating climate change. Clathrate hydrates are crystalline structures formed when CO2 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 CO2 hydrates formed in natural sand from the Adriatic Sea using a 1 L isochoric reactor. Hydrate formation in sand 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% to 18.2% up to 9.7–36.7%) and water conversion rate (from 1.3% to 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 CO2 storage, providing quantitative insights relevant for offshore CCS strategies.