Mechanical properties of amorphous CO2 hydrates: insights from molecular simulations.

Understanding physicochemical properties of amorphous gas hydrate systems is of great significance to reveal structural stabilities of polycrystalline gas hydrate systems. Furthermore, amorphous gas hydrates can occur ordinarily in the nucleation events of gas hydrate systems. Herein, the mechanical properties of amorphous carbon dioxide hydrates are examined by means of all-atom classical molecular dynamic simulations. Our molecular simulation results reveal that mechanical strengths of amorphous carbon dioxide hydrates are evidently governed by temperatures, confining pressures, and ratios of water to carbon dioxide molecules. Notably, under compressive loads, amorphous carbon dioxide hydrates firstly exhibit monotonic strain hardening, followed by an interesting distinct phenomenon characterized by a steady flow stress at further large deformation strains. Furthermore, structural evolutions of amorphous carbon dioxide hydrates are analyzed on the basis of the N-Hbond DOP order parameter. These important findings can not only contribute to our understanding of the structural stabilities of amorphous gas hydrate systems, but also help to develop fundamental understandings about grain boundaries of gas hydrate systems.

A novel apparatus for modeling the geological responses of reservoir and fluid-solid production behaviors during hydrate production.

Gas hydrate is a promising alternative energy resource that undergoes complex phase changes and coupled geological responses during hydrate production. Insufficient knowledge of those coupled behaviors still challenge safe and efficient gas production from hydrate. Here, a novel experimental apparatus was developed to simulate the gas-water-sand production and to evaluate the related multifield and multiphase processes. The experimental apparatus is equipped with displacement, ultrasonic, and electrical resistivity sensors and gas/water flowmeters, and this apparatus can work up to a maximum loading stress of 25 MPa and a maximum pore pressure of 20 MPa over a temperature range from -20 to 50 °C. The hydrate production and sand production case were performed on a synthetic specimen with hydrate saturation of 12.8% by using multi-step depressurization. The pressure-temperature conditions, settlement, ultrasonic propagation, electrical resistivity, and permeability of hydrate reservoirs during production were simultaneously monitored to evaluate the geological characteristics and heat and mass transfer characteristics of the hydrate reservoir. The results indicated that the gas/water production mainly occurred during the first third of each depressurization period, and their production rates were low at the beginning. Flowing water mobilized the sand particles, and the addition of gas exacerbated the sand-particle migration. Interpretation of the coupled behaviors supported that the reservoir could maintain a temporary stable structure even when losing a certain amount of sand particles with no sand control methods; however, necessary sand-prevention approaches are wise to support long-term reservoir production operations. These laboratory insights would contribute to optimizing the field strategies for economical gas production from hydrate.

Effects of marine environments on methane hydrate formation in clay nanopores: A molecular dynamics study.

In nature, CH4 hydrates are mainly buried in marine sediments. The complex marine environments on the seafloor continuously affect hydrate formation. Herein, systematic molecular simulations have been performed to investigate CH4 hydrate formation in clay nanopore, mainly affected by several marine environmental factors, including seawater salinity, pressure and temperature. Simulation results show that these factors exert different effects on hydrate formation in the nanopore and the outside bulk solutions by affecting the mass transfer and phase separation inside and outside of the nanopore. Specifically, high salinity hinders the diffusion of CH4 molecules from nanopores into the outside bulk solutions, promoting hydrate formation in nanopore and inhibiting hydrate formation in bulk solution; salinity has a dual effect on hydrate formation in the whole system by changing the local CH4 concentration via the formation of the hydration of salt ions. High pressure favors the diffusion of CH4 molecules from nanopore into outside bulk solutions, promoting hydrate formation in bulk solution and inhibiting hydrate formation in nanopore; high pressure promotes hydrate formation at the nanopore throats by increasing CH4 concentration and reducing ion concentration therein. In contrast, temperature significantly affects hydrate formation in the system by causing phase separation, i.e. high temperature promotes the aggregation of CH4 molecules to form nanobubbles and inhibits hydrate formation. Under high temperature conditions, the nanobubble in the nanopore gradually decomposes, while the nanobubble in the outside bulk solution grows an extra-large cylindrical nanobubble. These molecular insights into the formation behavior of CH4 hydrates in clay nanopores are helpful for understanding the formation process of natural gas hydrates in marine sediments and the development and utilization of CH4 hydrates.

Effects of surface property of mixed clays on methane hydrate formation in nanopores: A molecular dynamics study.

HYPOTHESIS: Mixed clays (e.g. montmorillonite, illite and kaolinite) are ubiquitous in hydrate-bearing sediments under seafloor, and their surfaces inevitably affect the formation of natural gas hydrates therein. Nevertheless, the actual effects of clay surfaces on hydrate formation remain elusive. EXPERIMENTS: Systematic molecular dynamics simulations have been performed to investigate CH4 hydrate formation in mixed clay nanopores of montmorillonite, illite and kaolinite, to examine the effects of surface property and layer charges of mixed clays. FINDINGS: Simulation results indicate that the surfaces of mixed clays affect CH4 hydrate formation in the nanopores by changing the CH4 concentration (xCH4) and ion concentration (xions) in the middle region of the nanopores via surface adsorption for CH4, H2O and ions. Specifically, the surfaces of montmorillonite and illite, the siloxane and gibbsite surfaces of kaolinite show different affinities for adsorbing CH4, H2O and ions, which can significantly affect the xCH4 and xions in the interfacial and middle regions of the nanopores. Moreover, hydrate growth shows certain surface preference. These molecular insights into the effect of mixed clay surfaces on CH4 hydrate formation can help to understand the formation mechanism of natural gas hydrate in marine sediments.

Mechanical Instability of Methane Hydrate-Mineral Interface Systems.

Massive methane hydrates occur on sediment matrices in nature. Therefore, sediment-based methane hydrate systems play an essential role in the society and hydrate community, including energy resources, global climate changes, and geohazards. However, a fundamental understanding of mechanical properties of methane hydrate-mineral interface systems is largely limited due to insufficient experimental techniques. Herein, by using large-scale molecular simulations, we show that the mechanical properties of methane hydrate-mineral (silica, kaolinite, and Wyoming-type montmorillonite) interface systems are strongly dictated by the chemical components of sedimentary minerals that determine interfacial microstructures between methane hydrates and minerals. The tensile strengths of hydrate-mineral systems are found to decrease following the order of Wyoming-type montmorillonite- > silica- > kaolinite-based methane hydrate systems, all of which show a brittle failure at the interface between methane hydrates and minerals under tension. In contrast, upon compression, methane hydrates decompose into water and methane molecules, resulting from a large strain-induced mechanical instability. In particular, the failure of Wyoming-type montmorillonite-based methane hydrate systems under compression is characterized by a sudden decrease in the compressive stress at a strain of around 0.23, distinguishing it from those of silica- and kaolinite-based methane hydrate systems under compression. Our findings thus provide a molecular insight into the potential mechanisms of mechanical instability of gas hydrate-bearing sediment systems on Earth.

A testing assembly for combination measurements on gas hydrate-bearing sediments using x-ray computed tomography and low-field nuclear magnetic resonance.

Studying the pore-scale characteristics of gas hydrate-bearing sediments (GHBS) is very important for a deep understanding of (i) how fluid flows therein and (ii) the corresponding gas production. Micro X-ray computed tomography (X-CT) and low-field nuclear magnetic resonance (NMR) are often used independently to characterize the pore structure of GHBS. Here, we present a new testing assembly that combines X-CT scans and low-field NMR tests to determine the pore-scale characteristics of GHBS in situ. The main parts of the testing assembly are a removable core holder made of polyether ether ketone, an X-CT system, and a low-field NMR system. The core holder allows for independent pressure control for the formation/dissociation of gas hydrates, which is xenon hydrate here. X-CT scans and low-field NMR tests are conducted successively to obtain not only the hydrate pore-scale behavior but also the transverse relaxation time distributions of GHBS. Correlation analysis between the pore size distributions and the transverse relaxation time curves gives the transverse surface relaxivity of xenon hydrate-bearing sediments during hydrate dissociation. The results show that the hydrate pore occurs as a mixture of grain-coating, cementing, pore-filling, and patchy clusters in a gas-dissolved solution. The peak pore size at the maximum frequency ratio increases with decreasing hydrate saturation. In addition, the transverse surface relaxivity dependence on hydrate pore occurrences is in the range of 67.1-129.3 µm/s when the hydrate saturation is lower than 0.4. The combination measurements for GHBS have a promising potential in understanding the structure evaluation of pore space during gas recovery.

Mechanical creep instability of nanocrystalline methane hydrates.

Mechanical creep behaviors of natural gas hydrates are of importance for understanding the mechanical instability of gas hydrate-bearing sediments on Earth. Limited by the experimental challenges, intrinsic creep mechanisms of nanocrystalline methane hydrates remain largely unknown yet at the molecular scale. Herein, using large-scale molecular dynamics simulations, mechanical creep behaviors of nanocrystalline methane hydrates are investigated. It is revealed that mechanical creep responses are greatly dictated by internal microstructures of crystalline grain size and external conditions of temperature and static stress. Interestingly, a long steady-state creep is observed in nanocrystalline methane hydrates, which can be described by a modified constitutive Bird-Dorn-Mukherjee model. Microstructural analysis shows that deformations of crystalline grains, grain boundary diffusion and grain boundary sliding collectively govern the mechanical creep behaviors of nanocrystalline methane hydrates. Furthermore, structural transformation also appears to be important in their mechanical creep behaviors. This study provides new insights into understanding the mechanical creep scenarios of gas hydrates.

Mechanical Response of Nanocrystalline Ice-Contained Methane Hydrates: Key Role of Water Ice.

Water ice and gas hydrates can coexist in the permafrost and polar regions on Earth and in the universe. However, the role of ice in the mechanical response of ice-contained methane hydrates is still unclear. Here, we conduct direct million-atom molecular simulations of ice-contained polycrystalline methane hydrates and identify a crossover in the tensile strength and average compressive flow stress due to the presence of ice. The average mechanical shear strengths of hydrate-hydrate bicrystals are about three times as large as those of hydrate-ice bicrystals. The ice content, especially below 70%, shows a significant effect on the mechanical strengths of the polycrystals, which is mainly governed by the proportions of the hydrate-hydrate grain boundaries (HHGBs), the hydrate-ice grain boundaries (HIGBs), and the ice-ice grain boundaries (IIGBs). Quantitative analysis of the microstructure of the water cages in the polycrystals reveals the dissociation and reformation of various water cages due to mechanical deformation. These findings provide molecular insights into the mechanical behavior and microscopic deformation mechanisms of ice-contained methane hydrate systems on Earth and in the universe.

The effect of surfactants on hydrate particle agglomeration in liquid hydrocarbon continuous systems: a molecular dynamics simulation study.

Anti-agglomerants (AAs), both natural and commercial, are currently being considered for gas hydrate risk management of petroleum pipelines in offshore operations. However, the molecular mechanisms of the interaction between the AAs and gas hydrate surfaces and the prevention of hydrate agglomeration remain critical and complex questions that need to be addressed to advance this technology. Here, we use molecular dynamics (MD) simulations to investigate the effect of model surfactant molecules (polynuclear aromatic carboxylic acids) on the agglomeration behaviour of gas hydrate particles and disruption of the capillary liquid bridge between hydrate particles. The results show that the anti-agglomeration pathway can be divided into two processes: the spontaneous adsorption effect of surfactant molecules onto the hydrate surface and the weakening effect of the intensity of the liquid bridge between attracted hydrate particles. The MD simulation results also indicate that the anti-agglomeration effectiveness of surfactants is determined by the intrinsic nature of their molecular functional groups. Additionally, we find that surfactant molecules can affect hydrate growth, which decreases hydrate particle size and correspondingly lower the risk of hydrate agglomeration. This study provides molecular-level insights into the anti-agglomeration mechanism of surfactant molecules, which can aid in the ultimate application of natural or commercial AAs with optimal anti-agglomeration properties.

Influence of AFM Tip Temperature on THF Hydrate Stability: Theoretical Model and Numerical Simulation.

Atomic force microscopy (AFM) indentation is widely used to determine mechanical parameters of various materials. However, AFM tip may lead to phase transition of the cold sample in the region of contact area. It is a long-standing challenge that low-temperature phase-change materials (e.g., ice and hydrate) are hardly characterized by AFM, especially for clathrate hydrates. Here, with theoretical analysis and numerical simulation, we investigated the temperature influence of AFM tip on the tetrahydrofuran (THF) hydrate stability. At first, a steady-state model of heat conduction was established between a v-shaped probe and THF hydrate sample. The temperature of the tip was estimated at different laser spot positions and laser intensities. Through numerical simulation, the heat loss by air convection is less than 1% of the total laser heat, and the influence of ambient air on the AFM probe temperature can be neglected. Meanwhile, the local temperature in the region of contact area was also calculated at the THF hydrate temperature of 0°C, -10°C, -20°C, and -30°C. We found out that the AFM tip causes the cold THF hydrate to melt. The thermal melting thickness decreases with the reduction of laser intensity and THF hydrate temperature. On the contrary, it is positively correlated with the thickness of liquid-like layer on THF hydrate surface and is also linearly increased with the contact radius. This indicates that the thermal melting continues as the press-in depth of the tip into THF hydrate increases. The local temperature rises when the tip touches the THF hydrate. It is easier for THF hydrate to be melted by an external pressure. In addition, the proposed model may be useful for guiding force tests on low-temperature phase-change materials by the AFM indentation.

Nature-inspired entwined coiled carbon mechanical metamaterials: molecular dynamics simulations.

Entwining-induced robust natural biosystems show superior mechanical performances over their counterparts. However, the role played by topological entwinement in the mechanical properties of artificial nanohelixes remains unknown. Here, the tensile characteristics of nano-entwined carbon nanocoil (ECNC) metamaterials are explored by atomistic simulations. The simulation results show that ECNCs exhibit heterogeneous pre-stress distribution along the spiral surfaces. The predicted stretching stress-strain responses correlate with the topological nano-entwining and dimensionality. Topological analysis reveals that the collective stretching of the bond and bond angle on the inner hexagon edge of the coils characterizes both early and final elastic extensions, whereas the intermediate elasticity is exclusively attributed to the inner-edged hexagon-angular deformation. The ECNCs impart pronounced tensile stiffnesses to the native structures, surprisingly with a maximum of over 13-fold higher stiffness for one triple-helix, beyond the scalability of mechanical springs in parallel, originating from the nano-entwining mechanism and increase in bulkiness. However, the reinforcement in strengths is restricted by the elastic strain limits that are degraded in ECNCs owing to the steric hindrance effect. All metastructures show superelongation-at-break due to a successive break-vs.-arrest process. Upon plastic deformation, the localized reduction in the radii of ECNCs leads to the formation of carbyne-based networks.

Grain-Size-Controlled Mechanical Properties of Polycrystalline Monolayer MoS2.

Pristine monocrystalline molybdenum disulfide (MoS2) possesses high mechanical strength comparable to that of stainless steel. Large-area chemical-vapor-deposited monolayer MoS2 tends to be polycrystalline with intrinsic grain boundaries (GBs). Topological defects and grain size skillfully alter its physical properties in a variety of materials; however, the polycrystallinity and its role played in the mechanical performance of the emerging single-layer MoS2 remain largely unknown. Here, using large-scale atomistic simulations, GB structures and mechanical characteristics of realistic single-layered polycrystalline MoS2 of varying grain size prepared by confinement-quenched method are investigated. Depending on misorientation angle, structural energetics of polar-GBs in polycrystals favor diverse dislocation cores, consistent with experimental observations. Polycrystals exhibit grain-size-dependent thermally induced global out-of-plane deformation, although defective GBs in MoS2 show planar structures that are in contrast to the graphene. Tensile tests show that presence of cohesive GBs pronouncedly deteriorates the in-plane mechanical properties of MoS2. Both stiffness and strength follow an inverse pseudo Hall-Petch relation to grain size, which is shown to be governed by the weakest link mechanism. Under uniaxial tension, transgranular crack propagates with small deflection, whereas upon biaxial stretching, the crack grows in a kinked manner with large deflection. These findings shed new light in GB-based engineering and control of mechanical properties of MoS2 crystals toward real-world applications in flexible electronics and nanoelectromechanical systems.

Mechanical properties of monocrystalline and polycrystalline monolayer black phosphorus.

The mechanical properties of monocrystalline and polycrystalline monolayer black phosphorus (MBP) are systematically investigated using classic molecular dynamic simulations. For monocrystalline MBP, it is found that the shear strain rate, sample dimensions, temperature, atomic vacancies and applied statistical ensemble affect the shear behaviour. The wrinkled morphology is closely connected with the direction of the in-plane shear, dimensions of the samples, and applied ensembles. Particularly, small samples subjected to loading/unloading of the shear deformation along the armchair direction demonstrate a clear mechanical hysteresis loop. For polycrystalline MBP, the maximum shear stress as a function of the average grain size follows an inverse pseudo Hall-Petch type relationship under an isothermal-isobaric (NPT) ensemble, whereas under a canonical (NVT) ensemble, the maximum shear stress of polycrystalline MBP exhibits a 'flipped' behaviour. Furthermore, polycrystalline MBP subjected to uniaxial tension also exhibits a strongly grain size-dependent mechanical response, and it can fail by brittle intergranular and transgranular fractures because of its weaker grain boundary structures and the direction-dependent edge energy, respectively. These findings provide useful insight into the mechanical design of BP for nanoelectronic devices.

Mechanical instability of monocrystalline and polycrystalline methane hydrates.

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.