A phase equilibrium model for hydrate in sediment accounting for pore capillary effect

Abstract

The hydrate phase equilibrium relationship is the one of the most fundamental issues in the hydrate related research area. In the procedure for developing a phase equilibrium model, it is critical to address the influence of pore capillary effect of sediment. This paper firstly compares the influence mechanisms of pore size distribution on water-gas suction in unsaturated soil and on hydrate-water capillary suction during hydrate phase transition, and finds that both influence mechanisms are similar to each other. With reference to the existing formula used for describing the relationship of water-gas suction and water saturation, such as V-G model, an expression for relating hydrate-water capillary suction to hydrate saturation is presented. Combined with the van der Waals-Platteeuw thermodynamic model, a modified phase equilibrium model for hydrate in sediment accounting for pore capillary effect is proposed. Furthermore, an approach for quantifying model parameters is developed. Compared to the existing models, the micro model parameters, such as pore diameter, are not involved in the proposed model. Instead, the model parameters are macro variables, and can be calculated from the measured phase equilibrium data of hydrate in sediment. Finally, part of experimental data is used to determine the model parameters, and the other part of experimental data is employed to verify the proposed phase equilibrium model. Through the comparison of the model predicated results and experimental data, it have been demonstrated that the proposed model is able to effectively predicate the phase equilibrium condition for hydrate in sediment. Note that, the proposed phase equilibrium model is needed to be verified in the following work because of shortage of relative experimental data.