Tunable Strategies for Controlled CO₂ Hydrate Formation: Integrating Promoter-Driven Regulation and Cage Occupancy...

Hydrate-based carbon sequestration offers a promising pathway for offshore CCS by exploiting the complementary properties of structure I (sI) and structure II (sII) hydrates in different sedimentary domains. Here, we employed Diffuse Reflectance Infrared Fourier Transformations Spectroscopy (DRIFTS) and chemical potential calculations to quantify cage occupancies in binary 1,3-dioxane/CO₂ hydrates. At 8.5 °C and 3.6 MPa with 5.56 mol % dioxane, the large (5¹²6⁴) and small (5¹²) cage occupancies by CO₂ were determined to be 0.0810 and 0.7696, respectively. A numerical program was developed to calculate gas uptake, hydration number, density, and hydrate molar mass by incorporating cage occupancies and CO₂ solubility in saltwater. Stirring speed and gas-water ratio were optimized for hydrate formation in seamud, yielding an optimal stirring rate of 800 rpm and a gas-liquid ratio of 5. Kinetic experiments across 1,3-dioxane concentrations (5.56 and 2.86 mol %) and diverse kinetic promoters (SL-Na, l-Trp, l-Lys; 300-2000 ppm) reveal that 5.56 mol % 1,3-dioxane/CO₂ hydrate achieves a t₉₀ of 2.44 min and a gas uptake of 73.526 mmol/mol (millimoles of gas/mol of saltwater), while the optimal sI hydrate pathway was identified as 800 ppm of SL-Na, with a corresponding t₉₀ and uptake of 18.78 min and 53.719 mmol/mol, respectively. These findings highlight the potential of integrating cage occupancy insights with promoter optimization to design stratified sealing strategies, advancing safe and efficient offshore CO₂ sequestration.