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

Mechanical Properties of Methane Hydrate: Intrinsic Differences from Ice

Understanding fundamental mechanical behaviors of ice-like crystals is of importance in many engineering aspects. Herein, mechanical characteristics of monocrystalline methane hydrate (MMH) and hexagonal ice (Ih) under mechanical loads are contrasted by atomistic simulations. Effects of engineering strain rate, temperature, crystal orientation, and occupancy of guest molecules on the mechanical properties of MMH are investigated. Results show that the engineering strain rate, temperature, and occupancy of guest molecules in 51262 cages greatly affect the mechanical strength and failure strain of MMH, whereas the effect of crystal orientation on the tensile response of MMH such as along the [100] and [110] directions is negligible. Particularly, the occupancy of guest molecules in 51262 cages primarily governs the mechanical strength and elastic limits of MMH. For Ih, it is tensile stiffer than that of MMH at 263.15 K and 10 MPa, and shows unique mechanical characteristics such as tension-induced stiffening and compression-induced remarkable softening under the [0001] directional load. Both crystals demonstrate brittle fracture behavior but different plasticity with dislocation-free in MMH yet dislocation activities in Ih. The intrinsic differences in the mechanical properties of MMH and monocrystalline Ih mainly result from the host–guest molecule interactions and relative angles which tetrahedral hydrogen bonds make to the loading direction. These mechanical characteristics present microscopic insights to understand the mechanical responses of naturally occurring and artificial synthetic gas hydrates

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

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

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.</p

Mechanical properties of bi- and poly-crystalline ice

A sound knowledge of fundamental mechanical properties of water ice is of crucial importance to address a wide range of applications in earth science, engineering, as well as ice sculpture and winter sports, such as ice skating, ice fishing, ice climbing, bobsleighs, and so on. Here, we report large-scale molecular dynamics (MD) simulations of mechanical properties of bi- and poly-crystalline hexagonal ice (Ih) under mechanical loads. Results show that bicrystals, upon tension, exhibit either brittle or ductile fracture, depending on the microstructure of grain boundaries (GBs), whereas they show ductile fracture by amorphization and crystallographic slips emitted from GBs under compression. Under shearing, the strength of bicrystals exhibits a characteristic plateau or sawtooth behavior drawn out the initial elastic strains. Nanograined polycrystals are destabilized by strain-induced amorphization and collective GB sliding. Their mechanical responses depend on the grain size. Both tensile and compressive strengths decrease as grain size decreases, showing inverse Hall-Petch weakening behavior. Large fraction of amorphous water structure in polycrystals with small grain size is mainly responsible for the inverse Hall-Petch softening. Dislocation nucleation and propagation are also identified in nanograined ice, which is in good agreement with experimental measurements. Beyond the elastic strain, a combination of GB sliding, grain rotation, amorphization and recrystallization, phase transformation, and dislocation nucleation dominate the plastic deformation in both bicrystals and polycrystals

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.Accepted Author ManuscriptEngineering Thermodynamic

Modeling thermodynamic properties of propane or tetrahydrofuran mixed with carbon dioxide or methane in structure-II clathrate hydrates

A sound knowledge of thermodynamic properties of sII hydrates is of great importance to understand the stability of sII gas hydrates in petroleum pipelines and in natural settings. Here, we report direct molecular dynamics (MD) simulations of the thermal expansion coefficient, the compressibility, and the specific heat capacity of C3H8, or tetrahydrofuran (THF), in mixtures of CH4 or CO2, in sII hydrates under a wide, relevant range of pressure and temperature conditions. The simulations were started with guest molecules positioned at the cage center of the hydrate. Annealing simulations were additionally performed for hydrates with THF. For the isobaric thermal expansion coefficient, an effective correction method was used to modify the lattice parameters, and the corrected lattice parameters were subsequently used to obtain thermal expansion coefficients in good agreement with experimental measurements. The simulations indicated that the isothermal expansion coefficient and the specific heat capacity of C3H8-pure hydrates were comparable but slightly larger than those of THF-pure hydrates, which could form Bjerrum defects. The considerable variation in the compressibility between the two appeared to be due to crystallographic defects. However, when a second guest molecule occupied the small cages of the THF hydrate, the deviation was smaller, because the subtle guest-guest interactions can offset an unfavorable configuration of unstable THF hydrates, caused by local defects in free energy. Unlike the methane molecule, the carbon dioxide molecule, when filling the small cage, can increase the expansion coefficient and compressibility as well as decrease the heat capacity of the binary hydrate, similar to the case of sI hydrates. The calculated bulk modulus for C3H8-pure and binary hydrates with CH4 or CO2 molecule varied between 8.7 and 10.6 GPa at 287.15K between 10 and 100 MPa. The results for the specific heat capacities varied from 3155 to 3750.0 J kg-1 K-1 for C3H8-pure and binary hydrates with CH4 or CO2 at 287.15K. These results are the first of this kind reported so far. The simulations show that the thermodynamic properties of hydrates largely depend on the enclathrated compounds. This provides a much-needed atomistic characterization of the sII hydrate properties and gives an essential input for large-scale discoveries of hydrates and processing as a potential energy source.Accepted Author ManuscriptEngineering Thermodynamic