Mechanical Properties Of Gas Hydrate Systems:Insight From Molecular Dynamics Simulations

Natural gas hydrate?NGH?is regarded as a potential alternative energy resource in the future,and it exhibits the common characteristics of abundant energy resources,wide range of distributions,low-carbon environmental protection,and so on.Therefore,NGH has become a hotspot in the field of earth science and energy.The safe and efficient exploitation of natural gas hydrates will suffer a lot of technical difficulties and challenges,including wellbore stability during drilling,sand production,deformation of casing,hydrate dissociation-induced geohazards,and other engineering geological problems.Behind these challenges above,mechanical instability of gas hydrate plays a key role.Therefore,understanding mechanical behavior and deformation mechanism of gas hydrate systems has an important scientific significance and practical application in ensuring the safe and efficient recovery and utilization of gas hydrates in the future.According to these above,upon ice,hydrate,and hydrate-bearing sediments in nature,the molecular models of gas hydrate systems,e.g.,monocrystals,bicrystals,polycrystals,and sediment-based methane hydrate systems and so on,are constructed.Under the guidance of both classic Newtonian mechanics theory and material mechanics theory,mechanical properties of gas hydrate systems are comprehensively investigated using the molecular dynamics simulations,and then the deformation molecular mechanisms of gas hydrate systems are clarified.Moreover,the micromechanical response characteristics of gas hydrate under local mechanical loads are revealed.The in-depth understanding of natural gas hydrate systems is obtained from the mechanical characteristics and micro-instability mechanism based on our molecular simulation results.Our knowledge provides a solid and reliable theoretical foundation for the characterization of mechanical properties of both pure gas hydrate and hydrate-bearing sediments in the future.Firstly,the mechanical behavior of monocrystalline methane hydrate and ice has been studied.The engineering strain rate,temperature,and occupancy of guest molecules in 5¹²6² cages greatly affect the mechanical properties of monocrystalline methane hydrate.For hexagonal ice?Ih?,it exhibits superior mechanical properties under the[0001]directional load.Remarkably,under the[0001]directional compression,one new solid phase of water ice forms from hexagonal ice.After both ice and hydrates fail,the plastic deformation of both crystals under compression is facilitated by grain boundary?GB?formation,GB sliding,amorphization,and recrystallization.Both crystals exhibit different plasticity with dislocation-free in monocrystalline methane hydrate yet dislocation activities in Ih.Under both tension and compression,the intrinsic differences in the mechanical properties of monocrystalline methane hydrate and monocrystalline Ih mainly result from the host-guest molecule interactions in methane hydrates and relative angles which tetrahedral hydrogen bonds make to the loading direction.Our results provide an important micro-theoretical basis for us to understand mechanical behavior of both ice and hydrates in nature.Secondly,the mechanical behavior and instability mechanism of bicrystalline hydrate systems,e.g.,ice-ice bicrystals,hydrate-ice bicrystals,hydrate-hydrate bicrystals,and polycrystalline hydrate systems,e.g.,ice polycrystals,hydrate polycrystals,ice-contained hydrate polycrystals,are investigated.The mechanical behavior of the bicrystalline hydrate systems is mainly determined by microstructures of grain boundaries,where membered rings,water cages,and disordered water structures locate.Under tension,hydrate-ice bicrystals,hydrate-hydrate bicrystals show brittle failure,initiating from the grain boundaries,but ice-ice bicrystals exhibit complex mechanical behavior;Under compression,the bicrystalline hydrate systems exhibit plastic failure characteristics.Subjected to shearing,all of the bicrystals exhibit the sawtooth-like mechanical stress response due to GB sliding.The mechanical behavior of polycrystalline hydrate systems is closely related to the three-dimensional networks of GB structures.The mechanical strengths of ice-contained polycrystals are mainly dominated by the proportions of hydrate-hydrate grain boundaries?HHGBs?,hydrate-ice grain boundaries?HIGBs?,and ice-ice grain boundaries?IIGBs?,resulting from that the mechanical strengths of the HHGBs and IIGBs are much higher than those of the HIGBs.For ice contents of 0%to 70%,the polycrystals are mechanically weakened,whereas for ice contents of 70%to 100%,they are strengthened.For the pure ice polycrystals and pure methane hydrate polycrystals,the mechanical strengths of ice polycrystals are significantly lower than these of pure methane hydrate polycrystals,which is consistent with previous laboratory test results.These results have enhanced the basic theory on the mechanical fracture behavior of both ice and hydrates,and they have provided the foundation for the experimental mechanical tests and practical applications of both ice and hydrates.Thirdly,to understand microscopic mechanical properties of gas hydrates under local mechanical loads and to guide microscopic characterization and testing of hydrate-bearing sediments in the future,the effects of indentation rate,indentation radius,and temperature on the mechanical response of methane hydrate under nanoindentation are investigated.Our results show that methane hydrates exhibit anomalous elastic recovery and plastic non-recovery under mechanical indentation.The mechanical deformation is accompanied by growth of hydrate crystal and memory effect.This deformation process is defined as the self-healing ability of methane hydrates.As the indentation rate increases,the self-healing ability of methane hydrates is weakened.In particular,the self-healing ability of methane hydrates is closely related to the indentation radius.These results clarify the local failure mechanism of methane hydrates under indentation and provide important theoretical support for the interfacial micromechanical experiments of hydrate-bearing sediments using atomic force microscope in the future.At last,systematic molecular dynamics simulations are carried out to illustrate the mechanical properties of sediment-based methane hydrates,e.g.,quartz-hydrate,kaolinite-hydrate,montmorillonite-hydrate,under tension and compression.Our results demonstrate that their mechanical stability is dominant by microstructures,which are made of the water molecules at the interface between sediments and methane hydrates,and mechanical properties of both the sediments and methane hydrates.Upon tension,results show that all sediment-based methane hydrates show brittle failure manner along the grain boundaries.Upon compression,the own mechanical properties of the corresponding sediments play a key role in the mechanical deformation of sediment-based methane hydrates.Our work firstly illustrates the influence of interaction between different components on the mechanical properties of the sediment-based methane hydrates systems.Our findings provide the important molecular structure information and the key basic knowledge for understanding the mechanical properties of methane hydrate-bearing sediments on Earth.