Salinity-Sediment Interactions Governing CO₂ Hydrate Formation, Kinetics and Stability in Marine Environments

The storage of carbon dioxide in the form of gas hydrates represents a promising strategy for long-term CO₂ sequestration in marine environments. This work investigates the combined effects of salinity and natural sand on the formation, dissociation and stability of CO₂ hydrates under conditions representative of marine sediments. A systematic experimental comparison was carried out between binary gas-water systems and three-phase gas-water-sand systems, with and without salinity (3 wt % NaCl), at two initial pressures (25 and 35 bar) using a 1 L high-pressure reactor. The natural sand was sampled from the Adriatic Sea. It is formed mainly by quartz and is primarily mesoporous with limited microporosity. The particle size is mainly included in the range of 100-200 μm. The introduction of natural sand significantly mitigates salinity-induced inhibition, particularly at higher pressure, increasing hydrate density from 100.7 to 166.6 g_CO2/m³ and water utilization from 24.7% to 40.9% in saline systems. Kinetic analyses demonstrate that pressure controls the overall formation rate. At low pressure, sand acts mainly as an enabling factor in saline systems, while at high pressure, it induces a transition from continuous bulk growth to pore-controlled formation. Dissociation kinetics reflect the formation history and hydrate morphology. Salinity primarily controls the dissociation rates, while sand redistributes hydrate within the pore space, modifying the temporal pattern of CO₂ release. Equilibrium measurements indicate that natural sand shifts hydrate stability toward less restrictive conditions and partially compensates for salinity inhibition for the investigated grain-size range.