Kinetics of solid hydrate formation by carbon dioxide: Phase field theory of hydrate nucleation and magnetic resonance imaging

Abstract

In the course of developing a general kinetic model of hydrate formation/reaction that can be used to establish/optimize technologies for the exploitation of hydrate reservoirs, two aspects of CO₂ hydrate formation have been studied. (i) We developed a phase field theory for describing the nucleation of CO₂ hydrate in aqueous solutions. The accuracy of the model has been demonstrated on the hard-sphere model system, for which all information needed to calculate the height of the nucleation barrier is known accurately. It has been shown that the phase field theory is considerably more accurate than the sharp-interface droplet model of the classical nucleation theory. Starting from realistic estimates for the thermodynamic and interfacial properties, we have shown that under typical conditions of CO₂ formation, the size of the critical fluctuations (nuclei) is comparable to the interface thickness, implying that the droplet model should be rather inaccurate. Indeed the phase field theory predicts considerably smaller height for the nucleation barrier than the classical approach. (ii) In order to provide accurate transformation rates to test the kinetic model under development, we applied magnetic resonance imaging to monitor hydrate phase transitions in porous media under realistic conditions. The mechanism of natural gas hydrate conversion to CO₂-hydrate implies storage potential for CO₂ in natural gas hydrate reservoirs, with the additional benefit of methane production. We present the transformation rates for the relevant processes (hydrate formation, dissociation and recovery).