Phase Field Approaches to the Kinetic Modeling of Hydrate Phase Transitions
Bjørn Kvamme, Atle Svandal, Trygve Buanes, Tatyana Kuznetsova, 2009. "Phase Field Approaches to the Kinetic Modeling of Hydrate Phase Transitions", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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A phase field theory (PFT) with model parameters evaluated from atomistic simulations and experiments is applied for describing the nucleation and growth and the dissolution of CO2 hydrate in aqueous solutions under conditions typical to underwater natural-gas-hydrate reservoirs. We show that the size of the critical fluctuations (nuclei) is comparable to the interface thickness, and thus the PFT predicts a considerably lower nucleation barrier height and higher nucleation rate than the classical approach that relies on a sharp interface. The growth rates of CO2 hydrate corresponding to different growth geometries (planar, circular, and dendritic) have been determined. The predicted growth rates are consistent with experiments performed under similar conditions. An alternative phase approach, based on cellular automata, has also been formulated and applied to the same model systems. Time dependence for this approach is derived by relating the diffusivity to the interface thickness. For small times, the two approaches appear to give similar results but deviate significantly for larger time scales. Dissolution rates of the hydrate phase have been studied as a function of CO2 concentration in the aqueous solution. On the basis of a simple model of foreign particles, qualitative simulations were performed to describe hydrate formation in porous media. The Avrami-Kolmogorov exponent evaluated from these simulations varies substantially with the volume fraction occupied by the foreign particles.