Mechanical instability of monocrystalline and polycrystalline methane hydrates

Abstract

Despite observations of massive methane release and geohazards associated with gas hydrate instability in nature, as well as ductile flow accompanying hydrate dissociation in artificial polycrystalline methane hydrates in the laboratory, the destabilising mechanisms of gas hydrates under deformation and their grain-boundary structures have not yet been elucidated at the molecular level. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected crossover from strengthening to weakening in polycrystals. Upon uniaxial depressurisation, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilises the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insight not only into the metastable structures of grain boundaries, but also into unusual ductile flow with hydrate dissociation as observed during macroscopic compression experiments.

In Situ Mechanical Properties of Shallow Gas Hydrate Deposits in the Deep Seabed

“…Both the Young modulus and the strength of the MNGH are lower than these values. The MD simulation suggests that the Young modulus and strength decrease as the grain size increases (Wu et al, ), and this crystal grain‐size dependency has been confirmed in ice (Currier & Schulson, ) and other materials such as steel (Hall, ; Petch, ). According to these studies, the failure strength is in inverse proportion to the grain size of crystal.…”

Molecular dynamics simulation of sI methane hydrate under compression and tension

“…The stretching test was loading in the Z axis because the stress-strain relationships of sI hydrate were almost same in the X, Y and Z axes [30]. More specifically, the stretching test changed the Z dimension of the simulation box at a constant engineering strain rate of 1×10 7 /s at 200 K and 10 MPa.…”

“…This method has been successfully applied in the study of the thermal and interfacial properties of hydrate [27,28]. Ning et al [29,30] investigated the compressibility of CH 4 /CO 2 hydrate mixtures and mechanical instability of mono/poly-crystalline hydrate, recently, and revealed the mechanical behaviour of hydrate at the molecular level. This study is intended to investigate the changes in the microstructure of pure sI methane hydrate and the laws of stress-strain evolution under the condition of compression and tension by MD simulation.…”

Micromechanical tests of tetrahydrofuran hydrate using atomic force microscope

“…一般 [13] , E 2 =74 GPa [17] ), 微球的弹性变形可以忽略. 如果荷载 F=13 μN, μ 1 取值0.355 [13] , 弹性压入深度约为9 nm, 但 实测的压入深度为37~548 nm(图4(f)和5(b)), 压入深度 远大于弹性压入深度, 表明在压入过程中THF水合物 产生了塑性变形, 压入诱发的水合物相变 [18] 可能进一 步增强了这一塑性行为.…”