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.@en

Knowledge on the kinetics of gas hydrate dissociation in microporous sediments is very important for developing safe and efficient approaches to gas recovery from natural gas hydrate (NGH) deposits. Herein, molecular dynamics (MD) simulations are used to study the dissociation kinetics in microporous sediments. The hydrate phase occupies a confined sandy nanopore formed by two hydroxylated silica surfaces with a buffering water layer between the hydrate and silica phase, meanwhile, this system is in contact with the bulk phase outside the pore. The hydrates in this sediment system dissociate layer-by-layer in a shrinking core manner. The released methane molecules aggregate and eventually evolve into nanobubbles, most of which are spherical cap-shaped on the hydroxylated silica surfaces. At high initial temperatures, a faster decomposition of the hydrate phase is observed, however, fewer methane molecules migrate to the bulk phase from the pore phase. These phenomena may occur because more methane molecules are released from the hydrate phase and facilitate the formation of nanobubbles with large heat injection; these nanobubbles can stably adsorb on the surface of silica and capture the surrounding methane molecules, thereby decreasing the number of methane molecules in the water phase. In addition, the injection speed of heat flow should be significantly increased at high dissociation temperatures when using the thermal stimulation method to extract gas from hydrates in tight sediments. This study provides molecular level insight into the kinetic mechanism of hydrate dissociation and theoretical guidance for gas production by thermal injection from sediments with low permeabilities.@en

Hydrate dissociation is often accompanied by the formation of nanobubbles. Knowledge of the effects of nanobubbles on hydrate dissociation is essential for understanding the dynamic behavior of the hydrate phase change and improving the gas production efficiency. Here, molecular dynamics simulations were performed to study the methane hydrate dissociation kinetics with and without a pre-existing methane nanobubble. The results show that the hydrate cluster in the liquid phase dissociates layer-by-layer. This process is shown to be independent of the temperature and nanobubble presence at the simulation conditions. Hydrate dissociation does not always lead to nanobubble formation because the supersaturated methane solution can be stable for a long time. A steep methane concentration gradient was observed between the hydrate cluster surface and the methane nanobubble, which can enhance the directional migration of methane and effectively minimize the methane concentration in the liquid phase, thereby increasing the driving force for the hydrate dissociation. Our findings indicate that the presence of a nanobubble near the hydrate surface does not decrease the activation energy of hydrate dissociation, but it can increase the intrinsic decomposition rate. The average hydrate dissociation rate is linearly correlated with the mass flow rate towards the nanobubble. The mass flow rate is determined by the nanobubble size and hydrate-nanobubble distance. Our findings contribute to the fundamental understanding of the dissociation mechanism of gas hydrates in the liquid phase, which is crucial for the design and optimization of efficient gas hydrate production techniques.@en

Knowledge on the kinetics of gas hydrate dissociation in clay pores at static and dynamic fluid conditions is a fundamental scientific issue for improving gas production efficiency from hydrate deposits using thermal stimulation and depressurization respectively. Here, molecular dynamics simulations were used to investigate poly- and mono-crystalline methane hydrates in Na-montmorillonite clay nanopores. Simulation results show that hydrate dissociation is highly sensitive to temperature and pressure gradients, but their effects differ. Temperature changes increase thermal instability of water and gas molecules, leading to layer-by-layer dissociation from the outer surface. Under flow conditions, laminar flow predominates in nano-pores, and non-Darcy flow occurs due to clay-fluid interactions. Viscous flow disrupts hydrogen bonding at the hydrate surface, enhancing kinetic instability of water. Grain boundaries of polycrystalline hydrates are less stable compared to bulk phases and preferentially decompose, forming new dissociation fronts. This accelerates dissociation compared to monocrystalline hydrates. Fracture occurs at the grain boundaries of polycrystalline hydrate in the fluid, resulting in separate hydrate crystal grains. This fracture process further accelerates hydrate dissociation. In flow systems, methane nanobubbles form in fluid and readily transport with fluid flow. Unlike surface nanobubbles at static conditions, these liquid nanobubbles exhibit mobility. The findings of this study can contribute to a better understanding of the complex phase transition behavior of hydrate in confined environment, and provide theoretical support for improving production control technology.@en

The trial production of natural gas hydrate reservoirs remains poor. Reasonable reservoir reconstruction, which can improve formation permeability, is an important approach to increasing the efficiency and enhancing production. In this work, water jet slotting is proposed to reconstruct an natural gas hydrate reservoir near a wellbore. The spatial slots formed by water jet slotting not only directly constitute high-permeability channels, but also generate disturbances to the surrounding in-situ sediment. Water jet slotting disturbances to nearby sediment was investigated using a three dimensional flow-structure coupling model to evaluate the proposed reconstruction method. The reservoir at the SH2 site in the Shenhu area of the South China Sea was used as the reference. A horizontal slotting arrangement along the vertical well was adopted. The results demonstrate that water jet slotting can change the primary stress state of the sediment around the wellbore, and generate a dominant stress relaxation zone and small stress concentration zone. Within the stress relaxation zone, the in-situ compressive stress was remarkably reduced or even transformed into tensile stress, accompanied by sediment displacement and volumetric expansion strain. This is conducive to loosening the sediment around the wellbore and improving the permeability characteristics. In addition, the influence of the water jet slotting parameters including slot radius, spacing, and number on disturbances to the nearby sediment was studied. Reservoir responses to water jet slotting under balanced and unbalanced bottom-hole pressures were compared and analyzed. This study provides a reference for natural gas hydrate reservoir reconstruction using water jet slotting.

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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.@en

Methane hydrate dissociation kinetics can be inhibited in NaCl solutions; however, this effect is reversed by promoting bubble formation that enhances dissociation. The negative and positive effects of inorganic salt injection on gas production from hydrate-bearing sediments are still controversial. Here, molecular dynamics simulations were performed to investigate the characteristics of NaCl solution invasion into hydrate-occupied nanopores and the effects on the confined hydrate dissociation kinetics. Two initial configurations comprising liquid and silica pore phases were studied with a low or high NaCl concentration, respectively. The results show that, under the simulation conditions, salt invasion decelerated hydrate dissociation within the silica pore as NaCl invasion into the pore is stepwise. Initially, few ions can diffuse into the pore phase, and gas nanobubbles form on the solid surface mainly via confinement and surface effects, independent of NaCl solution invasion. Subsequently, gradual salt diffusion immersed the residual hydrate in the salt solution and hindered hydrate decomposition until the dissociation finished. More ions could diffuse into the pore phase at the high NaCl concentrations with a low diffusion efficiency, leading to surface nanobubble growth toward the residual hydrate and somewhat accelerated hydrate dissociation. This severely hinders the escape of released methane from the pore. This study yields molecular-level insight into the origin of the negative effect of salt invasion on hydrate dissociation, which should be avoided during gas production from hydrate reservoirs with low permeabilities via salt injection combined with thermal stimulation.

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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.@en

Continuous Fractional Component Monte Carlo (CFCMC) and molecular dynamics (MD) simulations are performed to calculate the solubilities and self-diffusion coefficients of four light n-alkanes (methane, ethane, propane, and n-butane) in aqueous NaCl solutions as well as the thermodynamic properties of their corresponding hydrate crystals. Correction factors k_ij to the Lorentz-Berthelot combining rules for alkane groups (CH₃) and water are optimized (k_ij = 1.04) by fitting excess chemical potentials to experimental data at 1 bar and 298.15 K. Using these values of k_ij, we calculate the solubilities of the four alkanes in aqueous NaCl solutions with different molalities (0-6) mol/kg at different temperatures (278.15-308.15) K and pressures (1, 100, 200, 300) bar. The diffusion coefficients of the four alkanes in NaCl solutions (0-6) mol/kg are calculated at different temperatures (278.15-308.15) K and 1 bar and corrected for the finite-size effects. The lattice parameters of the corresponding hydrates with different guest molecules are computed using MD simulations at different temperatures (150-290) K and pressures (5-700) MPa. Isothermal compressibilities at 287.15 K and thermal expansion coefficients at 14.5 MPa for the corresponding hydrates are calculated. We present an extensive collection of thermodynamic data related to gas hydrates that contribute to a fundamental understanding of natural gas hydrate science.

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