Molecular scale modeling approach to evaluate stability and dissociation of methane and carbon dioxide hydrates

Hydrate Technologies for CO2 Capture and Sequestration: Status and Perspectives

CO2 capture and sequestration based on hydrate technology are considered supplementary approaches for reducing carbon emissions and mitigating the greenhouse effect. Direct CO2 hydrate formation and CH4 gas substitution in natural gas hydrates are two of the main methods used for the sequestration of CO2 in hydrates. In this Review, we introduce the crystal structures of CO2 hydrates and CO2-mixed gas hydrates and summarize the interactions between the CO2 molecules and clathrate hydrate/H2O frames. In particular, we focus on the role of diffraction techniques in analyzing hydrate structures. The kinetic and thermodynamic properties then are introduced from micro/macro perspectives. Furthermore, the replacement of natural gas with CO2/CO2-mixed gas is discussed comprehensively in terms of intermolecular interactions, influencing factors, and displacement efficiency. Based on the analysis of related costs, risks, and policies, the economics of CO2 capture and sequestration based on hydrate technology are explained. Moreover, the difficulties and challenges at this stage and the directions for future research are described. Finally, we investigate the status of recent research related to CO2 capture and sequestration based on hydrate technology, revealing its importance in carbon emission reduction.

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.

Predicting mechanical properties of CO2hydrates: machine learning insights from molecular dynamics simulations

Understanding the mechanical properties of CO2hydrate is crucial for its diverse sustainable applications such as CO2geostorage and natural gas hydrate mining. In this work, classic molecular dynamics (MD) simulations are employed to explore the mechanical characteristics of CO2hydrate with varying occupancy rates and occupancy distributions of guest molecules. It is revealed that the mechanical properties, including maximum stress, critical strain, and Young's modulus, are not only affected by the cage occupancy rate in both large 51262and small 512cages, but also by the distribution of guest molecules within the cages. Specifically, the presence of vacancies in the 51262large cages significantly impacts the overall mechanical stability compared to 512small cages. Furthermore, four distinct machine learning (ML) models trained using MD results are developed to predict the mechanical properties of CO2hydrate with different cage occupancy rates and cage occupancy distributions. Through analyzing ML results, as-developed ML models highlight the importance of the distribution of guest molecules within the cages, as crucial contributor to the overall mechanical stability of CO2hydrate. This study contributes new knowledge to the field by providing insights into the mechanical properties of CO2hydrates and their dependence on cage occupancy rates and cage occupancy distributions. The findings have implications for the sustainable applications of CO2hydrate, and as-developed ML models offer a practical framework for predicting the mechanical properties of CO2hydrate in different scenarios.

Numerical Simulation on the Dissociation, Formation, and Recovery of Gas Hydrates on Microscale Approach

Investigations into the structures of gas hydrates, the mechanisms of formation, and dissociation with modern instruments on the experimental aspects, including Raman, X-ray, XRD, X-CT, MRI, and pore networks, and numerical analyses, including CFD, LBM, and MD, were carried out. The gas hydrate characteristics for dissociation and formation are multi-phase and multi-component complexes. Therefore, it was important to carry out a comprehensive investigation to improve the concept of mechanisms involved in microscale porous media, emphasizing micro-modeling experiments, 3D imaging, and pore network modeling. This article reviewed the studies, carried out to date, regarding conditions surrounding hydrate dissociation, hydrate formation, and hydrate recovery, especially at the pore-scale phase in numerical simulations. The purpose of visualizing pores in microscale sediments is to obtain a robust analysis to apply the gas hydrate exploitation technique. The observed parameters, including temperature, pressure, concentration, porosity, saturation rate, and permeability, etc., present an interrelationship, to achieve an accurate production process method and recovery of gas hydrates.

Promoting Effect of Ultra-Fine Bubbles on CO2 Hydrate Formation

When gas hydrates dissociate into gas and liquid water, many gas bubbles form in the water. The large bubbles disappear after several minutes due to their buoyancy, while a large number of small bubbles (particularly sub-micron-order bubbles known as ultra-fine bubbles (UFBs)) remain in the water for a long time. In our previous studies, we demonstrated that the existence of UFBs is a major factor promoting gas hydrate formation. We then extended our research on this issue to carbon dioxide (CO2) as it forms structure-I hydrates, similar to methane and ethane hydrates explored in previous studies; however, CO2 saturated solutions present severe conditions for the survival of UFBs. The distribution measurements of CO2 UFBs revealed that their average size was larger and number density was smaller than those of other hydrocarbon UFBs. Despite these conditions, the CO2 hydrate formation tests confirmed that CO2 UFBs played important roles in the expression of the promoting effect. The analysis showed that different UFB preparation processes resulted in different promoting effects. These findings can aid in better understanding the mechanism of the promoting (or memory) effect of gas hydrate formation.

Microscale Processes and Dynamics during CH4–CO2 Guest-Molecule Exchange in Gas Hydrates

The exchange of CH4 by CO2 in gas hydrates is of interest for the production of natural gas from methane hydrate with net zero climate gas balance, and for managing risks that are related to sediment destabilization and mobilization after gas-hydrate dissociation. Several experimental studies on the dynamics and efficiency of the process exist, but the results seem to be partly inconsistent. We used confocal Raman spectroscopy to map an area of several tens to hundreds µm of a CH4 hydrate sample during its exposure to liquid and gaseous CO2. On this scale, we could identify and follow different processes in the sample that occur in parallel. Next to guest-molecule exchange, gas-hydrate dissociation also contributes to the release of CH4. During our examination period, about 50% of the CO2 was bound by exchange for CH4 molecules, while the other half was bound by new formation of CO2 hydrates. We evaluated single gas-hydrate grains with confirmed gas exchange and applied a diffusion equation to quantify the process. Obtained diffusion coefficients are in the range of 10−13–10−18 m2/s. We propose to use this analytical diffusion equation for a simple and robust modeling of CH4 production by guest-molecule exchange and to combine it with an additional term for gas-hydrate dissociation.

Does Confinement Enable Methane Hydrate Growth at Low Pressures? Insights from Molecular Dynamics Simulations

Natural methane hydrates are estimated to be the largest source of unexploited hydrocarbon fuel. The ideal conditions for methane hydrate formation are low temperatures and high pressures. On the other hand, recent experimental studies suggest that porous materials, thanks to their confinement effects, can enable methane hydrate formation at milder conditions, although there has not been a consensus on this. A number of studies have investigated methane hydrate growth in confinement by employing molecular simulations; however, these were carried out at either very high pressures or very low temperatures. Therefore, the effects of confinement on methane hydrate growth at milder conditions have not yet been elaborated by molecular simulations. In order to address this, we carried out a systematic study by performing molecular dynamics (MD) simulations of methane water systems. Using a direct phase coexistence approach, microsecond-scale MD simulations in the isobaric-isothermal (NPT) ensemble were performed in order to study the behavior of methane hydrates in the bulk and in confined nanospaces of hydroxylated silica pores at external pressures ranging from 1 to 100 bar and a simulation temperature corresponding to a 2 °C experimental temperature. We validated the combination of the TIP4P/ice water and TraPPE-UA methane models in order to correctly predict the behavior of methane hydrates in accordance to their phase equilibria. We also demonstrated that the dispersion corrections applied to short-range interactions lead to artificially induced hydrate growth. We observed that in the confinement of a hydroxylated silica pore, a convex-shaped methane nanobuble forms, and methane hydrate growth primarily takes place in the center of the pore rather than the surfaces where a thin water layer exists. Most importantly, our study showed that in the nanopores methane hydrate growth can indeed take place at pressures which would be too low for the growth of methane hydrates in the bulk.