Mechanical Loss in Polycrystalline Methane Hydrates

Abstract

The stability of natural gas hydrates in response to inevitable dynamic disturbances dictates the sustainability of the host sediments. Here we report the mechanical loss behaviors of polycrystalline methane hydrates under cyclic shearing via molecular dynamics simulations. Polycrystalline methane hydrates exhibit two distinct deformation behaviors, with elastoplastic and viscoelastic deformations upon high and low loading frequencies, respectively, which are influenced by grain size. Temperature and loading frequency affect the storage and loss moduli, in which the relations of mechanical loss - loading frequency are well-described by the Cross model for non-Newtonian fluids. The mechanical loss mechanisms of polycrystalline methane hydrates are dominated by diffusion, grain boundary sliding, massive cage transformations, and sI ↔ sII hydrate phase transitions. This study provides not only the mechanical dissipation properties of gas hydrates but also the molecular origins of mechanical loss, which is of critical importance for long-term evaluation of the stability of gas hydrate reservoirs under various disturbances.

Keywords: Polycrystalline hydrates, Mechanical Loss, Viscoelasticity, Cross model, phase transitions