Formation, growth and sintering of CO₂ hydrate crystals in liquid water with continuous CO₂ supply: Implication for subsurface CO₂ sequestration

The crystal growth of a CO₂ hydrate in liquid water with a continuous supply of CO₂ was visually investigated, while the high-pressure visual cell was connected to a water-flow loop filled with circulating CO₂-saturated water. The test section may be viewed as a simplified model of the subsurface with injected CO₂. The observation was continued for over 10 h, thereby revealing the morphology change of the hydrate crystals. The morphology of the initial hydrate crystals distinctly varied depending on the system subcooling temperature, ΔT_sub. At ΔT_sub < 1.5 K, polygonal flat-plate crystals were found. The morphology changed into columnar crystals at ΔT_sub ≈ 2.0 K and then dendritic crystals at ΔT_sub > 2.5 K. At any ΔT_sub, it was found that hydrate crystals grew and converted into spherical particles after an hour. The agglomeration of the crystals and the densification of the crystal layer were also observed. The hydrate crystals eventually became at least 0.5 mm in size. These are the features of the liquid phase sintering. If this sintering occurred in the pores in the subsurface, typically 100 micrometer or smaller, the hydrate crystals would occupy the pore space which would result in the decrease in porosity of hydrate-forming layer and in the change of geophysical properties.