High-pressure rheological signatures of CO2 hydrate slurries formed from gaseous and liquid CO2 relevant for refrigeration, pipeline transportation, carbon capture, and geological sequestration

Carbon dioxide hydrates are solid crystalline compounds made of water and carbon dioxide gas. Rheological characterization of carbon dioxide (CO2) hydrates is vital for advancing, optimising, and developing CO2 hydrate-based technologies. In this work, the rheology of the CO2 hydrate slurry has been investigated with and without surfactant, sodium dodecyl sulfate (SDS), using a modified Couette geometry at a constant temperature of 274.55 K and varying pressures (3.5, 5, and 6.5 MPa). It was observed that at higher pressures, dissolved CO2 had been actively engaged in nucleation and growth, unlike free gas at lower pressures. Steady-state viscosities and flow curves vary proportionately with driving force , exhibiting shear thinning behaviour in good agreement with the Cross-model criterion. With the addition of SDS, both the peak and steady-state viscosities experience a drop for all the experimental pressures. Moreover, the flow curves and yield stress of hydrate slurries reduce with surfactant concentration, exhibiting pseudoplastic behaviour. Viscoelastic properties manifest solid behaviour for hydrate slurries formed at a higher pressure of 5 and 6.5 MPa whereas liquid behaviour dominates at lower pressure of 3.5 MPa. Eventually, the presence of SDS has been observed to delay the CO2 hydrate dissociation (3.54 +/- 0.4 K/h for 2.5 h), signifying enhanced stability owing to surfactant blanketing/encircling the CO2 hydrate crystals. Thus, SDS facilitates CO2 hydrate flow under higher and lower pressures, administering an anti -agglomerating effect. This encourages its use in industrial applications like secondary refrigeration, cold storage, gas separation, and for improved hydrate slurry flowability in offshore pipelines.