考虑沉积物孔隙毛细效应影响的宏观相平衡模型

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

天然气水合物(以下简称水合物)的相平衡关系是水合物研究领域中的最基础的问题之一.沉积物的孔隙毛细效应对其中水合物的相平衡关系模型存在重要影响,应该在相平衡模型中进行考虑.本文对比分析了沉积物孔径大小分布特征对其非饱和状态下的气-水界面吸力和其中水合物相变过程中水合物-水界面吸力的影响机制,发现二者具有相似之处.据此,参考描述非饱和状态下沉积物的气-水界面吸力随水饱和度变化规律的V-G模型关系,建立了考虑相变过程中水合物-水界面吸力随水合物饱和度的变化规律的数学关系式.结合已有的van der Waals-Platteeuw热力学模型,建立了考虑沉积物毛细效应影响的宏观相平衡模型,并且给出了模型参数的确定方法.相比已有模型,该模型中的毛细效应势能项的参数不再涉及沉积物孔径大小等微观层面参数,而是通过有限的相平衡数据点来确定的宏观模型参数.最后,该模型预测了CH_4水合物和CO_2水合物在多孔介质中的相平衡条件,通过与试验数据点进行对比,说明该模型能有效预测沉积物中水合物的相平衡条件.

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.