Fractal analysis on CO2 hydrate-bearing sands during formation and dissociation processes with NMR

Injecting CO2 into submarine sediments to form hydrates is one of the potential methods of CO2 sequestration. The transition behavior of CO2 hydrates in porous media is of great practical significance. In this work, CO2 hydrate formation/dissociation in porous media was monitored in real time by a low-field magnetic resonance (MR) system, and a series of dynamic fractal dimensions of the pore space occupied by converted water during the hydrate formation/dissociation process were obtained based on the transverse relaxation time (T2) distributions. In general, the dimension of the converted water space increases with hydrate formation and decreases with the hydrate dissociation progress. A smaller particle size of porous media and a lower initial water saturation can promote hydrate formation, and the corresponding fractal dimension is higher during the hydrate formation process. There is a special status of the fractal period observed during the hydrate formation/dissociation process, and it is considered the temporally and spatially uniform distribution of hydrate crystal formation/dissociation inside the porous media. These results also indicate the relationships between the hydrate transition progress and the dynamic fractal dimension, which are useful for future works on pore-scale hydrate-bearing transitions during hydrate-based CO2 sequestration.