Molecular dynamics study on the nucleation of methane + tetrahydrofuran mixed guest hydrate

Transport Properties of Aqueous Methane Solutions and Blocking Behavior of Intelligent-Responsive Temporary Plugging Agent in a Switchable Nano-channel: A Dissipative Particle Dynamics Simulation Study

Intelligent-responsive temporary plugging agents (TPAs) have great potential in the field of oil and gas resource extraction due to their self-adaptability to the environment. However, the transport mechanism of oil and gas molecules, such as aqueous methane solution in intelligent-responsive TPA-modified nano-channels and the blocking behavior of TPA, have not been verified yet. In this work, dissipative particle dynamics simulations (DPD) are conducted to investigate the velocity distribution and the force characteristics of aqueous methane solutions under different driving velocities, as well as the blocking effect of TPA to the flow of solution. Simulation results indicate that aqueous methane solution primarily concentrates in the uncovered area of the TPA and diffuses into the TPA-covered area when the nano-channel is closed. The velocity distribution of the flowing solution in the open nano-channel follows a subparabolic pattern. Methane molecules in the closed nano-channel show sharp oscillations in velocity and force profiles, which can be mitigated by increasing the methane concentration. The designed TPA effectively blocks fluid flow but its head and tail are vulnerable to the shear forces from the fluid. This study enhances the understanding of the nanoflows in the wellbores during the extraction of oil and natural gas resources.

Molecular Dynamics Study of Clathrate-like Ordering of Water in Supersaturated Methane Solution at Low Pressure

Using molecular dynamics, the evolution of a metastable solution for “methane + water” was studied for concentrations of 3.36, 6.5, 9.45, 12.2, and 14.8 mol% methane at 270 K and 1 bar during 100 ns. We have found the intriguing behavior of the system containing over 10,000 water molecules: the formation of hydrate-like structures is observed at 6.5 and 9.45 mol% concentrations throughout the entire solution volume. This formation of “blobs” and the following amorphous hydrate were studied. The creation of a metastable methane solution through supersaturation is the key to triggering the collective process of hydrate formation under low pressure. Even the first stage (0–1 ns), before the first fluctuating cavities appear, is a collective process of H-bond network reorganization. The formation of fluctuation cavities appears before steady hydrate growth begins and is associated with a preceding uniform increase in the water molecule’s tetrahedrality. Later, the constantly presented hydrate cavities become the foundation for a few independent hydrate nucleation centers, this evolution is consistent with the labile cluster and local structure hypotheses. This new mechanism of hydrogen-bond network reorganization depends on the entropy of the cavity arrangement of the guest molecules in the hydrate lattice and leads to hydrate growth.

Designing Robust Superhydrophobic Materials for Inhibiting Nucleation of Clathrate Hydrates by Imitating Glass Sponges

Superhydrophobic surfaces are suggested to deal with hydrate blockage because they can greatly reduce adhesion with the formed hydrates. However, they may promote the formation of fresh hydrate nuclei by inducing an orderly arrangement of water molecules, further aggravating hydrate blockage and meanwhile suffering from their fragile surfaces. Here, inspired by glass sponges, we report a robust anti-hydrate-nucleation superhydrophobic three-dimensional (3D) porous skeleton, perfectly resolving the conflict between inhibiting hydrate nucleation and superhydrophobicity. The high specific area of the 3D porous skeleton ensures an increase in terminal hydroxyl (inhibitory groups) content without damaging the superhydrophobicity, achieving the inhibition to fresh hydrates and antiadhesion to formed hydrates. Molecular dynamics simulation results indicate that terminal hydroxyls on a superhydrophobic surface can inhibit the formation of hydrate cages by disordering the arrangement of water molecules. And experimental data prove that the induction time of hydrate formation was prolonged by 84.4% and the hydrate adhesive force was reduced by 98.7%. Furthermore, this porous skeleton still maintains excellent inhibition and antiadhesion properties even after erosion for 4 h at 1500 rpm. Therefore, this research paves the way toward developing novel materials applied in the oil and gas industry, carbon capture and storage, etc.

Mixed Hydrate Nucleation: Molecular Mechanisms and Cage Structures

The molecular-level details of the formation of mixed gas hydrates remain elusive despite their significance for a variety of scientific and industrial applications. In this study, extensive molecular simulations have been performed to examine the behavior of CH₄/H₂S mixed hydrate nucleation utilizing two different simulation setups varying in compositions and temperatures. The observed behavior exhibits similar phenomenology across the various systems once differences in nucleation rates and guest uptake are accounted for. We find that CH₄ is always enriched in the hydrate phase while the aqueous phase is enriched in H₂S. Even with H₂S as a minor component (i.e., 10% mole fraction), the system can mirror the overall nucleation kinetics of pure H₂S hydrate systems with CH₄-dominant nuclei. Through analyses of cages and their transitions, nonstandard cages, particularly those with 12 faces (e.g., 5¹⁰6²), have been found to be key intermediate cage types in the early stage of nucleation. Additionally, we present previously unreported cage types comprising heptagonal faces (e.g., 5⁹6²7¹) as having a significant role in the early-stage gas hydrate structural transitions.

Molecular Insights into the Effect of Nitrogen Bubbles on the Formation of Tetrahydrofuran Hydrates

In this work, a molecular dynamics simulation was conducted to study the microscopic mechanism of how nitrogen bubbles affect the formation of THF hydrates at the molecular level. The results obtained reveal that the nitrogen bubble can promote the formation of THF hydrates. In the system with a nitrogen bubble, more THF-filled cages were generated, and the crystal structure was more orderly. The promotion of nitrogen bubbles on hydrate crystallization comes from the dissolution of nitrogen molecules. Some of dissolved nitrogen molecules can be enclosed in small hydrate cages near the nitrogen bubble, which can serve as stable sites for hydrate crystal growth, resulting in the fact that THF-filled cages connected with N₂-filled cages are much more stable and have a long lifetime. The results in this work can help to understand the promotion effect of micro- and nano-air bubbles on the crystallization of THF hydrates.

Molecular behavior of hybrid gas hydrate nucleation: separation of soluble H2S from mixed gas

Soluble H₂S widely exists in natural gas or oil potentially corroding oil/gas pipelines. Furthermore, it can affect the hydrate formation condition, resulting in pipeline blockage; the nucleation mechanism from mixed gas including H₂S is still largely unclear. Molecular dynamics simulations were performed to reveal the effects of different initial mixed H₂S/CH₄ compositions on the hydrate nucleation and growth process. The geometric details of the nanobubbles and gas composition in the nanobubbles were analyzed; the size of the nanobubbles was found to decrease from 3.4 nm to 1.4 nm. With the increase in the initial H₂S proportion, the diameter of the nanobubbles decreased; more guest molecules were dissolved in the water, which improved the initial concentration of guest molecules in the water. A multi-site nucleation process was observed, and separate hydrate clusters could grow independently until the simulation box limited their growth due to high local H₂S concentration as a potential nucleation location. When the initial proportion of mixed gas approaches, H₂S preferred to occupy and stabilize the incipient cage. Moreover, 5¹², 4¹5¹⁰6², and 5¹²6² cages accounted for approximately 95% of the first hydrate cage. Nucleation rates were shown to increase from 4.62 × 10²⁴ to 9.438 × 10²⁶ nuclei cm^-3 s^-1. The present high subcooling and H₂S concentration provided a high driving force to promote mixed hydrate nucleation and growth. The proportion of cages occupied by H₂S increased with increasing initial H₂S proportion, but the largest enrichment factor of 1.38 occurred at 10% initial H₂S/CH₄ mixed gas.

Kinetics of Hydrate Formation and Dissociation in Coal at Different Temperatures Based on Impedance Method

In the process of coal mining, gas outburst is a challenge that must be prevented to guarantee mining safety. Forming gas hydrate in coal can reduce the original gas pressure and delay the concentrative outbursts of gas flow, which is one of the potential technologies to prevent gas outbursts in coal. In this work, we perform the formation and dissociation kinetics experiment of hydrate in the presence of coal and tetrahydrofuran (THF) at the temperature based on different geological conditions in China by means of the experimental device with the impedance measurement function. The results showed that the impedance change can qualitatively describe the kinetic characteristics of hydrate formation and dissociation in coal. The sudden change in pressure and system impedance during gas hydrate formation indicated the nucleation point at which hydrate formation started, by which the induction time can be acquired. Pressure and impedance suddenly changed at the same time, which implied that methane molecules and tetrahydrofuran (THF) molecules entered the hydrate phase at the same time. When the dissociation temperature increased to 303.15 K, the hydrate dissociation rate can be less affected by dissociation temperature if it continued to increase. This work highlights that gas hydrate formation in coal can effectively prevent gas outbursts.

Effect of Small Cage Guests on Dissociation Properties of Tetrahydrofuran Hydrates

It is well understood that tetrahydrofuran (THF) molecules are able to stabilize the large cages (5¹²6⁴) of structure II to form the THF hydrate with empty small cages even at atmospheric pressure. This leaves the small cages to store gas molecules at relatively lower pressures and higher temperatures. The dissociation enthalpy and temperature strongly depend on the size of gas molecules enclathrated in the small cages of structure II THF hydrate. A high-pressure microdifferential scanning calorimeter was applied to measure the dissociation enthalpies and temperatures of THF hydrates pressurized by helium and methane under a constant pressure ranging from 0.10 to 35.00 MPa and a wide THF concentration ranging from 0.25 to 8.11 mol %. The dissociation temperature of binary He + THF and methane + THF hydrates increases along with an increase in the THF concentration in the liquid phase at a fixed pressure (e.g., 30 MPa), reaching a maximum value of 280.8 and 312.8 K, respectively, at stoichiometric concentration (5.56 mol % THF), and then remains nearly constant for even higher THF concentrations (>5.56 mol %). The effect of gas occupancy in the small cages on the dissociation enthalpy of He + THF and methane + THF mixed hydrates was further examined by using molecular dynamics (MD) simulations. The dissociation enthalpy of the He-THF mixed hydrates is independent of pressure with an average of 5.68 kJ/mol H₂O over the pressure ranging from 0.10 to 30.0 MPa, consistent with the MD results of the He-THF mixed hydrates with low single occupancy (<23%) of helium molecules in the small cages. Consequently, the heat of adsorption of helium molecules in the small cages of the He-THF mixed hydrates is rather too weak to be identified. On the other hand, the dissociation enthalpy of the methane-THF hydrates increases from 9.11 to 10.01 kJ/mol H₂O along with an increase in methane pressure over the pressure ranging from 5.0 to 30.0 MPa, consistent with the MD results of the methane-THF mixed hydrates with full occupancy of methane molecules in the small cages. These findings provide important information for the design of a potential medium of gas storage and transportation.

Factors Promoting the Formation of Clathrate-Like Ordering of Water in Biomolecular Structure at Ambient Temperature and Pressure

Clathrate hydrate forms when a hydrophobic molecule is entrapped inside a water cage or cavity. Although biomolecular structures also have hydrophobic patches, clathrate-like water is found in only a limited number of biomolecules. Also, while clathrate hydrates form at low temperature and moderately higher pressure, clathrate-like water is observed in biomolecular structure at ambient temperature and pressure. These indicate presence of other factors along with hydrophobic environment behind the formation of clathrate-like water in biomolecules. In the current study, we presented a systematic approach to explore the factors behind the formation of clathrate-like water in biomolecules by means of molecular dynamics simulation of a model protein, maxi, which is a naturally occurring nanopore and has clathrate-like water inside the pore. Removal of either confinement or hydrophobic environment results in the disappearance of clathrate-like water ordering, indicating a coupled role of these two factors. Apart from these two factors, clathrate-like water ordering also requires anchoring groups that can stabilize the clathrate-like water through hydrogen bonding. Our results uncover crucial factors for the stabilization of clathrate-like ordering in biomolecular structure which can be used for the development of new biomolecular structure promoting clathrate formation.

The effects of ice on methane hydrate nucleation: a microcanonical molecular dynamics study

The NVE simulations realize the ice shrinking when methane hydrate nucleates both heterogeneously and homogeneously.

Unraveling Mixed Hydrate Formation: Microscopic Insights into Early Stage Behavior

The molecular-level details of mixed hydrate nucleation remain unclear despite the broad implications of this process for a variety of scientific domains. Through analysis of mixed hydrate nucleation in a prototypical CH₄/H₂S/H₂O system, we demonstrate that high-level kinetic similarities between mixed hydrate systems and corresponding pure hydrate systems are not a reliable basis for estimating the composition of early stage mixed hydrate nuclei. Moreover, we show that solution compositions prior to and during nucleation are not necessarily effective proxies for the composition of early stage mixed hydrate nuclei. Rather, microscopic details, (e.g., guest-host interactions and previously neglected cage types) apparently play key roles in determining early stage behavior of mixed hydrates. This work thus provides key foundational concepts and insights for understanding mixed hydrate nucleation.

Evidence from mixed hydrate nucleation for a funnel model of crystallization

The molecular-level details of crystallization remain unclear for many systems. Previous work has speculated on the phenomenological similarities between molecular crystallization and protein folding. Here we demonstrate that molecular crystallization can involve funnel-shaped potential energy landscapes through a detailed analysis of mixed gas hydrate nucleation, a prototypical multicomponent crystallization process. Through this, we contribute both: (i) a powerful conceptual framework for exploring and rationalizing molecular crystallization, and (ii) an explanation of phenomenological similarities between protein folding and crystallization. Such funnel-shaped potential energy landscapes may be typical of broad classes of molecular ordering processes, and can provide a new perspective for both studying and understanding these processes.