Tectonically deformed coals (TDCs) exhibit significant potential as a natural hydrogen source due to their hydrogen-rich organic structures and unique mechanochemical reactivity developed during tectonic evolution. This study utilizes the reactive force field (ReaxFF) molecular dynamics simulations to investigate the mechanochemical reactions of coal macromolecules under shear stresses which are achieved by applying different dislocation velocities (1-10 m/s) between two parts of coal. It is found that massive gas can release during simulations and H2 is the predominant gaseous product. At a dislocation velocity of 9 m/s, H2 constitutes up to 88 % of the generated gas, with a production rate reaching up to 387 m3/(ton & sdot;ns). The H2 production rate exhibits a positive correlation with the dislocation velocities, exceeding those of other gaseous products by 1-2 orders of magnitude. The results reveal that TDC seams may represent a previously overlooked source of natural hydrogen.

Abstract Methane hydrogen isotope fractionation during hydrate growth is a critical factor that complicates gas source identification, particularly in diffusion gas hydrate reservoirs. Although previous hydrate synthesis experiments demonstrate small isotope fractionation, hydrate samples from the same region show significant isotopic composition discrepancies, such as those observed in the Shenhu area, South China Sea, so as to puzzle the gas source research. To address this, we employ molecular dynamics simulations for methane hydrate growth in a closed system to investigate the hydrogen isotope fractionation of methane. It is found that methane hydrogen isotopic heterogeneity arises spontaneously in the newly formed hydrate layers. Notably, the kinetic fractionation factor exhibits periodic fluctuations characterized by multiple reversals between H enrichment and D enrichment. Additionally, while keeping the fluctuations in isotopic compositions, cage types can cause a secondary isotope fractionation via adsorbing methane, making the large cages enrich H and deplete D relative to the small cages. This study provides a new perspective on the isotopic heterogeneity observed in hydrate samples from the same region, suggesting that such isotopic heterogeneity may be a general phenomenon during hydrate accumulation in closed systems and may not require different gas sources for forming hydrate reservoirs.

Seismic observations from the NASA InSight mission have sparked broad interest in the potential existence of a basal molten layer (BML) at the core-mantle boundary of Mars. Resolving the physical properties of this putative layer is crucial for understanding the thermal evolution and dynamo history of the planet. While previous studies have proposed a conductive state for the BML based on high reference viscosities of the solid mantle, the actual mechanism of heat transport-conduction or convection-remains unresolved due to the lack of constraints on key parameters, including density, viscosity, and thermal conductivity. Here, we employ molecular dynamics simulations to determine these properties of iron-rich silicate melts under deep-mantle conditions. Our results indicate that the gravitational stability of the BML requires at least similar to 30 wt% FeO, corresponding to a minimum melt density of similar to 4.0 g/cm3. This FeO concentration is significantly higher than that of the overlying Martian mantle, implying viscosities in the range of 0.01-0.4 Pa & centerdot;s, many orders of magnitude lower than the high viscosities assumed in previous models, and thermal conductivities of 2.3-2.7 W/m/K. Scaling analysis and geodynamic simulation reveal that when BML viscosity falls below 1019 Pa & centerdot;s, convection dominates heat transport and intensifies with decreasing viscosity, which strongly enhances core heat flux and accelerates core cooling, thereby influencing Martian mantle dynamics.

Natural gas hydrates represent a vast potential clean energy resource, and the CO2/CH4 replacement technique offers a promising strategy for their exploitation while enabling CO2 sequestration. However, the underlying replacement mechanism, particularly how CO2 enters the hydrate interior, remains unclear. Here, by employing microcanonical ensemble molecular dynamics simulations that inherently conserve the exothermic heat of replacement, a key physical aspect artificially dissipated in previous simulated ensembles, we investigated CO2/CH4 replacement behaviors in monocrystalline and polycrystalline CH4 hydrates. Our results demonstrate that grain boundaries (GBs) play a critical role in facilitating the CO2/CH4 replacement process. GBs act as permanent active pathways, allowing CO2 to penetrate deeply into the hydrate interior and sustaining the replacement. This process is driven by the abundance of non-standard cages within GBs, which exhibit transient lifetimes and higher molecular diffusivity. Furthermore, a cage transition analysis uncovers the cage transition pathways, indicating that large, mixed guest cages serve as intermediates during the conversion from CH4-filled to CO2-filled cages. These findings establish a "grain boundary diffusion and replacement" mechanism for CO2/CH4 replacement in gas hydrates, underscoring the crucial influence of microstructural defects on hydrate reactivity and replacement efficiency.

Natural gas hydrates are a significant potential energy source and key player in global geological climate dynamics, with our all understandings on both sides basing on hydrate stability conditions consisting of pressure (P) and temperature (T). Considering important significance of theoretical researches in two sides, whether there is another factor playing same role with P or T is worth being answered. Using molecular dynamics simulations, changes of non-clathrated water structures and gas bubbles during mechanical deformation processes of methane hydrates were investigated to characterize their destabilizing natures and the deformation is then identified as this new factor. Similar to P-T profile but at a new level of complexity, deformation also characterizes hydrates stability but in three-dimensional space form while combining with T, reflecting deeper influences of specific temperature values, hydrate purities and deformation modes on hydrates stability. Over two tensile and compressive deformations, the mechanisms responsible for hydrate destabilization vary, with stretching favoring formation of gas bubbles and compressing damaging internal hydrogen-bond structures. Once alternating, such two to-and-fro continuing deformations can act as a piston-like accelerator as well as one-way valve for dissociation of hydrates, destabilizing them rapidly. In summary, P and T act as the prerequisite of hydrate existences, but cannot accurately account for the influence of such dynamic events as earthquakes, ocean tides, and changes of ice shelves on dissociations. This research sheds a new light on the reasonable explanation to dynamic hydrate dissociations, e.g. some abnormal disappearances of reservoirs within appropriate P-T conditions. Stronger, current bottleneck of hydrate exploitation efficiency might also be broken down; likewise, the ultra-rapid dissociation performance during the terminal procedure of hydrate-based natural gas storage/transportation technology might achieve success.

A comprehensive understanding of the intricate water/gas two-phase flow in sedimentary pores is essential for accurately predicting gas production following the in-situ dissociation of natural gas hydrates, as it is crucial for optimizing resource extraction strategies. This study constructed three typical clay slit nanopore models with distinct wettability characteristics-hydrophilic, relatively hydrophobic, and Janus hybrid-wettability-and used molecular dynamics simulations to investigate the spatial distribution and transport dynamics of two-phase fluids under varying water saturation conditions. The results revealed a significant negative correlation between water saturation and gas relative permeability. When water saturation reaches a critical threshold, the water lock effect occurs, blocking gas flow. Pore wettability plays a key regulatory role in water/gas phase dynamics via influencing the formation pathways of water locks. In relatively hydrophobic pores, weaker solid-water interactions promote the rapid clustering of water molecules, forming water locks, while hydrophilic surfaces enable water lock formation through gradual thickening of the liquid film. In Janus pores with low water saturation, strong electrostatic interactions between oppositely charged pore walls facilitate the formation of discrete water bridge networks, maintaining "gas windows" that allow gas flow, although these windows eventually close as saturation increases. The lower the water saturation, the more favorable it is for gas transport; in contrast, hydrophilic pores exhibit higher gas transport efficiency. Our findings provide valuable molecular-scale insights into how wettability governs multiphase flow transport, offering a theoretical foundation for reservoir modification and seepage control in natural gas hydrate recovery.

The reactive force field (ReaxFF) molecular dynamics (MD) simulations were performed to mimic the two-phase equilibrium between silica melts and bulk water at conditions of 1473 K and 100-700 MPa. The water solubilities, H-bearing species, and dissolution-exsolution reactions in the melts are investigated. The main findings include: (1) The major H-bearing species in melts are -Si-O-H and molecular H2O; this agrees with previous views. In addition, we found that the content of H2O increases faster than that of -Si-O-H with increasing pressure. We also observed many minor H-bearing species at less than 1% in total, such as H, O-H, -Si-O(-H)-Si-, -Si-O-H-O-Si-, -Si-OH2, -Si-O-H-O-H, H3O, H-O-H-O-H, -Si-O-H-OH2, and (H2O)2. (2) The dissolving process of water has three steps: water molecules contact melts via H2O + -Si double left right arrow -Si-OH2, the -H2O groups dissociate subsequently via -Si-OH2 double left right arrow -Si-O-H + H, and then various H-bearing species reach equilibrium mainly via -Si-O-H + -Si double left right arrow -Si-O(-H)-Si-. Water exsolution from melts is exactly the reversal of the dissolving process. (3) The calculated water solubility increases from 0.82 wt % at 100 MPa to 4.37 wt % at 700 MPa. The values are on average 2.27 wt % (47%) lower than the experimental values [] but are consistent with a recent calculation using the classical MD simulations for hydrous rhyolite melts []. The present findings reveal reaction pathways for water dissolving in these melts and are helpful for the simulations of compositionally more complicated magmas and magmas containing various volatiles, including H2O, S, Cl, F, B, and P, for holistic understanding of volcanic and magmatic-hydrothermal processes.

Understanding elements uptake and release from minerals in source rocks is crucial for comprehending critical metals accumulation, yet the mechanisms and kinetics of element mobilization at the atomic scale remain mostly unknown. Here, we analyzed the distribution of cobalt (Co) in natural pyrite from a Cu-Co ore deposit and found that metals distribution is best described by steady-state diffusion with constant flux and concentration-dependent diffusivities, rather than transient-state diffusion with time-evolving concentrations. First-principles calculations and diffusion modelling further demonstrate that this diffusion is accelerated by vacancy pathways and is far more efficient than traditional vacancy-mediated lattice diffusion, with element transfer rates higher by almost two orders of magnitude. We conclude that steady-state lattice diffusion induced by vacancies in the presence of fluid can be an efficient mechanism promoting the preferential release of metals into ore fluids and the accumulation of metals during ore formation.

Surface wettability in geological sediments critically influences gas hydrate formation, yet the combined effects of wettability on gas migration and hydrate growth remain poorly understood. This study examines how surface wettability governs the dynamics of CO2 hydrate formation constrained through in situ nuclear magnetic resonance (NMR) spectroscopy and imaging across a spectrum of hydrophilic to hydrophobic conditions. Our experiments reveal that distinct hydrate formation patterns vary systematically with pore surface wettability. In hydrophobic pores, a hydrate shell forms rapidly at gas-water interfaces and restricts the diffusion of CO2, ultimately limiting the final hydrate saturation. Conversely, hydrophilic pores exhibit extended induction times, enabling sustained CO2 dissolution and, consequently, higher hydrate saturation. Time-resolved T 2 relaxation profiles reveal greater heterogeneity in gas-water-hydrate distribution within hydrophobic samples, where hydrates preferentially develop in a pore-filling morphology, while in hydrophilic samples, they predominantly exhibit a grain-coating morphology. These wettability-dependent formation patterns substantially influence sediment permeability, with hydrophobic pores showing a more pronounced reduction compared with hydrophilic conditions. We propose a conceptual model that explains these observations through the dynamic interplay among surface wettability, gas-water interface distribution, and subsequent hydrate growth patterns. These findings enhance our understanding of hydrate formation mechanisms in natural sediments, with important implications for the inhibition of secondary hydrate formations and geological carbon storage strategies, where formations are ubiquitous and intrinsically heterogeneous, in terms of structure and mineral surface properties.

Nucleation of multicomponent systems is a pervasive phenomenon in nature and is pertinent to a diverse array of scientific and industrial challenges. The nucleation mechanisms of immiscible multicomponent systems remain unclear. Here, gas hydrate is employed as a model system to study the nucleation of multicomponent systems. The effect of gas/liquid and solid/liquid interfaces on hydrate nucleation is examined through molecular dynamics simulations. The results demonstrate that gas hydrates tend to nucleate in the solution phase in the proximity of the gas/liquid interface at lower temperatures, which is controlled by mass transfer. As the temperature increases, the location of hydrate nucleation gradually shifts from the gas/liquid interface to the solid/liquid interface. We anticipate that the nucleation free energy barrier dominates the hydrate nucleation process at these conditions, making the heterogeneous nucleation with a lower free energy barrier more probable. The findings provide molecular insights into the mechanism and pathway underlying interface-induced gas hydrate nucleation. These insights will inform the development of the theory of gas hydrate nucleation, particularly in the context of heterogeneous nucleation.

AbstractThe intricate flow processes in nano‐pores pose limitations on the extraction of resources such as shale gas and gas hydrates. To observe water/gas two‐phase flow in nano‐pores, we employed molecular dynamics simulations on water/methane two‐phase flow in a hydrophilic SiO2 nanoslit, and obtained high‐quality data by applying the “pump method” and “nano‐manometer.” This study revealed the variation in phase distribution during flow process, and assessed the impact of water phase distribution on methane gas flow. We proposed the “Deformed Water Layer (DWL) model” based on physical mechanisms, which can precisely describe methane relative permeability and forecast the critical water saturation for forming water lock. Our results suggest a two‐stage transition in methane gas permeability with increasing water saturation within nano‐pores, governed by spatial deformation of water phase. This phenomenon underscores that maintaining a reduced groundwater saturation is imperative to facilitate superior gas permeability and enhance recovery efficacy.

Natural gas hydrates are not only substantial energy sources but also have significant applications in the chemical industry and other fields. Although investigating hydrate formation in sediment minerals is crucial for their development and utilization, the underlying hydrate formation mechanism remains unclear. Here, molecular simulations were conducted in systems incorporating hydrophobic and hydrophilic pores of different sizes to investigate methane hydrate formation processes. The findings suggest that, as the hydrophobic slit size increases, there is a larger number of dissolved methane after the system reaches a metastable equilibrium state. The probability of cage formation indicates that hydrate cages readily form on hydrophobic surfaces or in the solution phase near the solution/gas interface. The larger slits are preferred for hydrate nucleation, regardless of whether the surface is hydrophobic, with most initial nuclei located near the liquid/methane interface. However, the interface perturbation can lead to the movement and growth of hydrate nuclei near the solution/methane interface into the bulk solution phase. Additionally, hydrate can nucleate and grow on the hydrophobic surface, facilitated by the adsorbed methane molecules and nonstandard cages. Pores hinder methane storage capacity in the hydrate phase due to the confinement effect and the amorphous nature of the hydrate formed. These molecular-level findings enhance our understanding of hydrate formation in sedimentary environments and porous materials, benefiting the development of natural gas hydrates and the use of porous materials for gas storage and transportation.

Methane hydrate is a promising energy resource widely occurring in the world, but the formation and distribution of methane hydrate in marine sediments with complex minerals remains unclear. Due to the small pore space and fine-grained size of clay minerals, the contact relationship between methane hydrate and clay mineral surface under excess water has not been characterized fully, which is required to investigate the mineral effect on methane hydrate formation. In this study, cryo-SEM was applied to observe the distribution of methane hydrate synthesized in pure quartz sands and the quartz sands mixed with montmorillonite or illite separately. According to the results of the research, methane hydrate dispersedly forms in the pore space away from and on the surface of quartz under a local excess water condition. In addition, methane hydrate occurs away from montmorillonite in pores with excess water and contacts the edge of montmorillonite under a local excess-gas condition. Moreover, methane hydrate was only observed to form away from illite aggregates in the pore with residual water. Montmorillonite and illite influence the preferable distribution of methane hydrate to occur away from mineral surface in excess water condition. In the sediments composed of different minerals, methane hydrate forms stochastically in the pore space and grows up individually. As a result, the findings of this study significantly expand the understanding of the formation and distribution of methane hydrate in marine sediments and the prediction of physical properties of hydrate-bearing sediments, and provide insights into natural gas hydrate exploration and production.

The water/gas two-phase flow is a frequently encountered question in the percolation field, and is especially important for the exploitation of natural gas hydrates because their decomposition products are exactly liquid water and natural gas. We studied the water/methane two-phase flow in a hydrophilic cylindrical nanopore by performing molecular simulations, and obtained high-quality nanoflows under different water saturation (Sw) thanks to the newly established nano-manometer method to control pressure difference accurately. With increasing Sw, the methane flow decreases almost linearly until a sudden stop when Sw >= 0.52. The formation of the water lock accounting for this phenomenon is observed clearly, and the larger Sw, the earlier formation of the water lock as well as the longer water lock. Based on careful data analysis, a water lock model and its formation mechanism are newly proposed with two pieces of strong evidence - the continuous reduction of the surface area of the water/gas interface when the water lock forms and the existence of maximum thickness of water film for different Sw. Thus, the competition between the surface tension of the water/gas interface and the adsorption of the water/wall interface controls the development of the water lock. These findings are very helpful for un-derstanding the two-phase percolation and optimizing the gas production and water removal schemes during hydrate exploitation. In addition, the nano-manometer can be widely used in other nanoflow simulations for measuring the local pressure accurately.

How natural gas hydrates nucleate and grow is a crucial scientific question. The research on it will help solve practical problems encountered in hydrate accumulation, development, and utilization of hydrate related technology. Due to its limitations on both spatial and temporal dimensions, experiment cannot fully explain this issue on a micro-scale. With the development of computer technology, molecular simulation has been widely used in the study of hydrate formation because it can observe the nucleation and growth process of hydrates at the molecular level. This review will assess the recent progresses in molecular dynamics simulation of hydrate nucleation and growth, as well as the enlightening significance of these developments in hydrate applications. At the same time, combined with the problems encountered in recent hydrate trial mining and applications, some potential directions for molecular simulation in the research of hydrate nucleation and growth are proposed, and the future of molecular simulation research on hydrate nucleation and growth is prospected.

Although research on the relationship between coal functional groups and tectonic stress has made great progress in recent years, the evolution of the morphology and chemical structures of coal macromolecules under tectonic stress is still unclear. We used reactive force field molecular dynamics (ReaxFF MD) simulations to explore the gas generation mechanisms of high-volatile bituminous coal model under shear stress. Several interesting results were found. First, in the absence of frictional heat, shear stress can act on the macromolecular structures of coal directly and generate gases, such as CH4, CO2, H2O, H2, and CO. Second, under the action of shear stress, coal macromolecular structures will generate a variety of free radicals, such as center dot OH, center dot H, center dot CH3, and many small free radical fragments or monocyclic radicals. These detached free radicals may combine with each other or attach to other structures to promote further reactions. Third, in the entire macromolecular network, many components may participate in the reactions as "catalysts", providing conditions for the contact of various small free radicals. For the first time, we have obtained direct simulation evidence of the gas generation mechanisms of coal via mechanolysis instead of relying on frictional heat. These results are a beneficial supplement to understand the coalification process.

Gas hydrates have drawn considerable attention in the globe because of their importance in environment and energy field. Although the hydrates in nature and synthesized in the laboratory are polycrystalline materials, the structures of their grain boundaries, their stability, and the influence of the grain boundary structures on hydrate dissociation remain unclear. In this study, we conducted massive simulations of CH4 and CO2 hydrates to investigate these issues. We find the grain boundaries of polycrystalline natural gas hydrates exhibit complex cage structures featuring nonstandard cages and may contain small gas bubbles. The boundaries can be periodically connected by repeated cage motifs of defect cages containing 4 and 8-membered rings. Examining the thermal stability of these grain boundary structures, our results show that for polycrystalline CH4 hydrates, the dissociation process at the grain boundaries can be hindered, where annealing of the grain boundary structure can further enhance this effect. Consequently, polycrystalline CH4 hydrates are found to dissociate only when the temperature is somewhat above the bulk melting temperature, but not the case for polycrystalline CO2 hydrates. This indicates that the grain boundary structures and guest types both affect the thermal stability of polycrystalline hydrates at the grain boundaries. Moreover, we also investigated the dissociation process of polycrystalline hydrates, we find that CO2 molecules can greatly accelerate the dissociation of hydrates because CO2 molecules can induce the formation of gas bubbles and prefer to be in gas bubbles, and that the dissociation process initiates with the decomposition of 51262 cages.

By performing molecular dynamic simulations, we study the methane adsorption on rough graphite surfaces prepared by rotating the graphite (0001) face with different tilt angles from 0 degrees to 90 degrees. It is found that the methane adsorption on the surface with the tilt angle of 90 degrees is much weaker than that of the (0001) surface, and density profiles of the methane adsorption region show irregular shapes of adsorption peaks compared to the common regular peak on the graphite (0001) surface. With increasing the tilt angle from 0 degrees to 90 degrees, the average density of adsorption methane increases first, then decreases, and shows a maximum value at 5 degrees. These observations could be explained qualitatively with two controlling factors. One is the surface roughness affecting the contact area for methane adsorption, and the other is the layer-accumulated adsorption potential (LAAP), newly proposed, reflecting the actual surface-methane interactions with considering different inner structures of surfaces. We believe that the average adsorption density is determined by the LAAP intrinsically but that it can be enhanced by the surface roughness when the adsorption area increases due to the zigzag shape of surface. Anyway, the widely used graphite (0001) surface is too smooth and its structure is too ordered to represent the surfaces of organic nanopores in shale samples, and one should be cautious to use its adsorption density reported previously. This study is very helpful for understanding the gas adsorption mechanisms and has potential applications in many fields, such as the prediction for shale gas reserves, the research and development of nano materials, precise surface treatments, atomic layer deposition, and so on.

The commercial use of natural methane hydrate is hampered by several open questions that remain regarding hydrate formation. Here the authors comment on past interpretations and aim to provide a roadmap for developing a predictive theory of methane hydrate nucleation.