Multiscale Piezoelasticity of Methane Gas Hydrates: From Bonds to Cages to Lattices

Abstract

For the past several decades, natural gas, also called methane gas, has been recognized as a green energy resource. It can generate energy by burning and produces fewer pollutants and carbon dioxide than coal and petroleum. For these reasons, it is also being viewed as a bridge fuel to renewable energy. Conservative estimates suggest that 80% of the naturally occurring hydrates on the planet contains natural gas. This vast amount translates to approximately twice the amount of energy stored in fossil fuels and oils. In addition, the special guest–host structure of gas hydrates provides a significant storage capacity, which makes the hydrate become a competitive candidate for carbon dioxide sequestration. However, the instability of hydrates during exploration becomes the biggest barrier for methane gas extraction and carbon dioxide sequestration. Thus, to overcome this barrier, this paper aims to investigate the stability limits and study the piezo effect of structure I (sI) methane gas hydrates at 0 K. This work investigates the structure, thermodynamics, and elasticity of sI methane hydrates subjected to pressure loads, at three scales, atoms, cages, and lattice, using the density functional theory in conjunction with homogenization methods and the theory of mixtures. The distribution functions of bond parameters are characterized in the hydrate system, which provides a novel understanding on the spread of values at the smallest scale. The roles of different types of cages at the mesoscale have been identified, such as continuous phase and disperse inclusion. At the continuum scale, the different deformation mechanisms are observed (affine and non-affine), which correspond to different fracture mechanisms (brittle and ductile) under tensile and compressive pressures, respectively. Taken together, the systematic atomic–cage–lattice multiscale characterization proves fruitful in linking processes that connect mechanical properties, cage geometry, and hydrogen bonding. The multiscale methodology can be generalized to other gas hydrates to improve the fundamental understanding and obtain engineering correlations.