Prediction of gas hydrates phase equilibrium in porous media – Pore size effect and thermodynamics approach

Abstract

Rapid growth in conventional energy consumption has drawn attention to renewable resources and unconventional fossil energy sources, such as Natural Gas Hydrates (NGHs). Comprehensive knowledge of the NGH accumulation conditions is the basis of exploration and exploitation. The mechanisms and factors, related with sediments and their matrix characteristics, and thermodynamic conditions affect the formation and dissociation of NGHs in sediments. Thermodynamic data on hydrate formation in porous media is limited due to the time-consuming and complex nature of obtaining experimental data. The use of thermodynamic models capable of predicting hydrate formation conditions is highly beneficial for studying gas extraction from hydrate reservoirs. In this research, previously published experimental data and modeling were analyzed. The impact of pore size on the formation of natural gas hydrates (NGHs) was modeled using two different thermodynamic approaches (fugacity and activity models), and their adaptation to experimental data was studied. The outcomes of the thermodynamic models revealed that the fugacity and activity model exhibited overall average absolute percent deviation (AAD%) 2.89 and 4.6 %, respectively. Minimum and maximum percentages of the average absolute deviation (AAD%) in fugacity model at 1.05 % and 5.5 %, respectively. Meanwhile, the activity model showed a minimum deviation of 1.67 % and a maximum deviation of 9.22 %. Additionally, it was observed that as the pore size diameter approaches approximately 100 nm, the equilibrium hydrate pressure in porous media and the equilibrium pressure in pure water become close. The surface tension of 0.039(J/m²) proposed by Uchida et al. leads to accurate modeling of methane hydrate in porous media.