Prediction Of Gas Hydrate Equilibrium Stability And Kinetic Nucleation In Porous Media

"Natural gas-hydrates are crystalline inclusion compounds with gas molecules (guest compounds) trapped within a host lattice formed by water molecules in an ice-like hydrogen-bonded framework. Natural gas-hydrates have the potential to become an important carbon-based resource addressing the increasing energy demand, and they pose a risk in terms of climate change. Accurate estimates of gas-hydrates global inventory, understanding of formation and dissociation processes of gas-hydrates, and evaluation of their environmental impact require models that accurately describe gas-hydrate stability in sediments and predict gas-hydrate kinetic nucleation processes. The hypothesis driving this work is that incorporation of selected sediment properties, i.e., surface energies and pore diameter, can lead to more accurate predictions of hydrate equilibrium, stability and nucleation in porous media. In this work, a model for gas-hydrate equilibrium in porous media was developed from basic thermodynamic principles and tested against available experimental data published in the scientific literature. The proposed model predicts reported experimental data with high accuracy for the range of pore sizes (3.4 ~ 24.75 nm) of different materials reported in the literature. It was found that the wettability of the pore surface affects the shape of the hydrate phase inside the pore and consequently influences the equilibrium pressures of gas-hydrates formed in porous media. A predictive macroscopic mathematical model describing the kinetic nucleation of gas-hydrates was developed based on Classical Nucleation Theory (CNT) in order to formulate correction factors for three types of interfaces mostly encountered in natural sediments (gas-liquid interface, liquid-solid interface and three-phase boundary lines). This approach, which incorporates the interfacial properties of sediments, can efficiently provide a fundamental understanding on the dependence of the formation mechanism of gas hydrates on a wide range of interfacial properties (wettability, substrate size, interfacial tension). The model predicts that hydrate nucleation is energetically favorable on confined surfaces with smaller contact-angle values, i.e., hydrophilic surfaces. Comparison between different types of interfaces leads to the conclusion that the nucleation of gas hydrates preferentially occurs in larger sediment pores. At the beginning of methane hydrate formation, for example, hydrate will preferentially nucleate at the gas-liquid interface. With the increase of hydrate volume or growth of the hydrate phase, the center of crystal growth moves towards the liquid-solid interface. In natural systems, gas hydrates form first on the concave liquid/solid interface and gas/liquid interface in sandstone sediments, gas/liquid interface and gas/liquid/solid triple boundary line in clay sediments and gas/liquid interface in pipeline with oil droplets. The inclusion of sediment properties in the model for gas-hydrate equilibrium in sediments predict experimental data within a margin of %AAD lower than 2%, a significant improvement upon previous modeling attempts. Additionally, the inclusion of sediment properties in the models for kinetic nucleation of gas hydrates result in mathematical models that capture the qualitative information obtained from examination of gas-hydrate core samples. Therefore, the hypothesis of the present work was proven."--Abstract.