Grain-Size-Governed Shear Failure Mechanism of Polycrystalline Methane Hydrates

Henrik Andersen Sveinsson,Bin Fang,Anders Malthe-Sørenssen,Pinqiang Cao,Fulong Ning

doi:10.1021/acs.jpcc.1c00901

Abstract

The shear failure mechanism of polycrystalline gas hydrates is critical for understanding marine geohazards related to gas hydrates under a changing climate and for safe gas recovery from gas hydrate reservoirs. Since current experimental techniques cannot resolve the mechanism on a spatial and temporal nanoscale, molecular simulations can assist with proposing and substantiating nanoscale failure mechanisms. Here, we report the shear failure of polycrystalline methane hydrates using direct molecular dynamics simulations. Based on these simulations, we suggest two modes of shear behavior, depending on the grain sizes, d, in the polycrystal: grain-size-strengthening behavior with a d1/3 grain size dependence for small grain sizes and grain-size-weakening behavior for large grain sizes. Through the crossover from strengthening to weakening behavior, the failure mode changes from shear failure with a failure plane parallel to the applied shear to tensile failure with a failure plane lying at an angle with the applied shear, spanning a network of grain boundaries. The existence of such a change in mechanism suggests that the Hall–Petch breakdown in methane hydrates is due to a change from grain boundary sliding to tensile opening being the most important failure mechanism when the grain size increases.

Highlights

IntroductionKnown as clathrate hydrates, are ice-like crystalline host−guest compounds
Gas hydrates, known as clathrate hydrates, are ice-like crystalline host−guest compounds
We report the shear deformation and failure behaviors of polycrystalline methane hydrates with a range of grain sizes and the destabilization mechanisms elucidated by molecular dynamics simulations

Summary

Introduction

Known as clathrate hydrates, are ice-like crystalline host−guest compounds. Most gas hydrates are methane hydrates that are embedded in hydrate-bearing sediments Such sediments are prevalent under the seabed on continental margins and under Arctic tundra, where a gas supply and suitable thermodynamic conditions are provided.[2] Gas hydrates can form as plugs in oil production lines.[3] Over the last few decades, much attention has been directed toward hydrates as an energy resource[4,5] and their possible environmental impact.[6] Estimates of the global gas hydrate inventory vary by orders of magnitude, but a common and conservative estimate is approximately 1500 gigatons of carbon.[7] This estimate is an order of magnitude larger than current worldwide conventional natural gas reserves of approximately 120 gigatons of carbon (approximately 200 trillion m3 STP8)

Results

Discussion

Conclusion