61st Annual Report on Research 2016

This project seeks to provide a thorough characterization of the mechanical and thermal properties of gas hydrates through multiscale simulations. Gas hydrates are crystalline compounds in which hydrogen-bonded water molecule cages entrap gas molecules. They are important mainly because they store significant amounts of methane in the permafrost region and deep ocean sites; however, they also constitute a marine geohazard that can lead to landslides . They are also considered potential media for gas transportation due to their high energy density and relatively mild formation conditions. For these applications, the mechanical and thermal properties of gas hydrates are of paramount importance. Significant progress has been made at the nano- and micro-scale for the evaluation of the mechanical and thermal properties of hydrates, and work is in progress to better characterize the mechanical behavior at the micro-scale. At the nanoscale, key thermophysical properties of methane hydrate have been determined.

All simulations involved ab initio Density Functional Theory coupled to Molecular Dynamics (MD) as implemented in SIESTA. The heat capacity, compressibility, and thermal expansion coefficient have been quantified with their temperature and pressure dependence. The heat capacity of methane hydrate was found to be much higher than ice, and this can be attributed to the fewer degrees of freedom of motion in the latter. This was confirmed by the comparison of the mean square displacement in both. Compressibility data confirmed the dominance of water-water interactions only up to 100 MPa, after which the effect of methane in the cages becomes more pronounced. As for the thermal expansion coefficient, it was found lower in methane hydrate compared to ice which may be due to the more rigid internal structure, in terms of bond angles, of the former as revealed in our previous study. The stability and integrity of the cage structures was reflected by the mean square displacement and radial distribution functions under all conditions. Moreover, the analysis at the nanoscale has been extended to another class of gas hydrates. While methane hydrate takes on the sI structure, larger guest molecules form hydrates in the sII configuration. Specifically, propane, butane, ethane-methane, and propane-methane gas hydrates have been studied in terms of their lattice constant and bulk modulus using DFT. The lattice constant was found to increase with guest size, with double-guest hydrates showing a larger increase than single-guest hydrates, which has implications for the van der Waals and Platteeuw thermodynamic model for gas hydrates. Also, hydrogen bonds prove to be the most likely factor contributing to the resistance of gas hydrates to compression; bulk modulus was found to increase linearly with hydrogen bond density, resulting in a relationship that could be used predictively to determine the bulk modulus of various structure II gas hydrates. At the microscale, plastic deformation has been studied. The core structure and Peierls stress of edge and screw dislocations in methane hydrate were determined as a function of composition and pressure using molecular statics as implemented in LAMMPS. A coarse-grained potential was used, and the [100][010] slip system was considered. The edge dislocation had a much lower Peierls stress and stronger dependence on methane composition and pressure compared to screw dislocation. Both dislocations, however, exhibited wide spreading and dissociation at different stages of deformation. Finally, microscale simulations are currently being extended to study the dynamics of dislocations and point defects. Different force fields are compared, and different deformation modes are considered. These simulations are just being started. Gas hydrates have been heavily researched in the last few decades, and they have recently received commercial interest with Japan’s plan for mass production before 2018.

Thus, work on hydrates helps any researcher contribute to the booming industry of hydrates and plays a role in their realization as potential energy resources and gas transportation media, among other applications. Previous work on the strength of hydrates has been experimental where the effects of different defects have been lumped together. Also, previous work on thermophysical properties has been experimental, with its poorly reported accuracy, or based on classical molecular dynamics. This data is needed due to the scarcity and inconsistency of current literature data. This work aims to partly resolve this difficulty by using ab initio simulations and studying individual defects.

For the upcoming year, the aim is to finalize the study on defects in methane hydrate and to extend the analysis to other kinds of hydrates. A defect flow model will be formulated along with the different defect diffusion constants. sII gas hydrates will be studied for more applied properties. 2