Lattice dynamics and heat transport in zeolitic imidazolate framework glasses.

The glassy state of zeolitic imidazolate frameworks (ZIFs) has shown great potential for energy-related applications, including solid electrolytes. However, their thermal conductivity (κ), an essential parameter influencing thermal dissipation, remains largely unexplored. In this work, using a combination of experiments, atomistic simulations, and lattice dynamics calculations, we investigate κ and the underlying heat conduction mechanism in ZIF glasses with varying ratios of imidazolate (Im) to benzimidazolate (bIm) linkers. The substitution of bIm for Im tunes the node-linker couplings but exhibits only a minor impact on the average diffusivity of low-frequency lattice modes. On the other hand, the linker substitution induces significant volume expansion, which, in turn, suppresses the contributions from lattice vibrations to κ, leading to decreased total heat conduction. Furthermore, spatial localization of internal high-frequency linker vibrations is promoted upon substitution, reducing their mode diffusivities. This is ascribed to structural deformations of the bIm units in the glasses. Our work unveils the detailed influences of linker substitution on the dual heat conduction characteristics of ZIF glasses and guides the κ regulation of related hybrid materials in practical applications.

Methane hydrate phase equilibrium considering dissolved methane concentrations and interfacial geometries from molecular simulations.

Natural gas hydrates, mainly existing in permafrost and on the seabed, are expected to be a new energy source with great potential. The exploitation technology of natural gas hydrates is one of the main focuses of hydrate-related studies. In this study, a large-size liquid aqueous solution wrapping a methane hydrate system was established and molecular dynamics simulations were used to investigate the phase equilibrium conditions of methane hydrate at different methane concentrations and interfacial geometries. It is found that the methane concentration of a solution significantly affects the phase equilibrium of methane hydrates. Different methane concentrations at the same temperature and pressure can lead to hydrate formation or decomposition. At the same temperature and pressure, in a system reaching equilibrium, the size of spherical hydrate clusters is coupled to the solution concentration, which is proportional to the Laplace pressure at the solid-liquid interface. Lower solution concentrations reduce the phase equilibrium temperature of methane hydrates at the same pressure; as the concentration increases, the phase equilibrium temperature gradually approaches the actual phase equilibrium temperature. In addition, the interfacial geometry of hydrates affects the thermodynamic stability of hydrates. The spherical hydrate particles have the highest stability for the same volume. Through this study, we provide a stronger foundation to understand the principles driving hydrate formation/dissociation relevant to the exploitation of methane hydrates.

Effects of ice and supercooled water on the metastability of methane hydrate: DSC analysis and MD simulations.

Methane hydrate (MH) has been viewed as a potential abundant clean energy resource worldwide. Its related technologies play important roles in applications of gas and energy storage, flow assurance of natural gas pipelines etc. Unlike the well-researched stability and decomposition of MH at temperatures above 273 K, the metastability of MH below the ice freezing point, i.e. the anomalous slow decomposition out of thermodynamically stable regions, remains to be unravelled. Studies regarding the influences of ice and supercooled water (SW) on the metastable properties of MH led to varied conclusions, i.e. the as-proposed self-preservation effect and metastable MH-SW-gas equilibrium. In this study, a series of DSC experiments were performed to investigate the thermal stability boundaries and the associated metastable behaviours of MH-ice-gas and MH-SW-gas samples in porous medium. The DSC analysis probed accurate thermal stabilities and characterized decomposition behaviors of the samples, contributing to the hypothesis of potential influences from SW and ice on the metastability of MH. MD simulations were also validated and performed. Active guest-host interactions by the SW layers between MH and gas phases were identified, suggesting probable microscopic configurations related to the metastability of the MH-SW-gas system. Indications of the DSC and MD simulation results call for future high-resolution in situ experimental validations.

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.

Behaviors of CO2 Hydrate Formation in the Presence of Acid-Dissolvable Organic Matters.

Carbon storage in the form of solid hydrate under seafloor has been considered to be promising for greenhouse gas control. Yet, open issues still remain on the role of the organic matters abundant in marine environments in the kinetics of hydrate formation; of particular interest is the involvement of the acid-dissolvable organic matters accompanying the acidification upon CO2 injection. In this work, the CO2 hydrate formation in the presence of the organic matters was in-situ monitored through the low-field nuclear magnetic resonance technique. It was found that the organic matters could kinetically promote the formation of CO2 hydrate; this effect was further enhanced by the sulfur-containing acid-dissolvable organic matters. Water in the large pores was preferentially consumed; the following water conversion facilitated by the organic matters would result in a fragmentation of the large pores into separated small pores isolated by the hydrate clusters. Consequently, a further enhancement of the gas-water contact is suggested as the existence of substantial hydrate patches could act as a mass transfer barrier. Our findings expand our understandings on the kinetics of CO2 hydrate formation in the presence of the organic matters and indicate the stability zone of gas hydrate a kinetically favorable geological setting for CO2 sequestration.

Molecular simulations on the stability and dynamics of bulk nanobubbles in aqueous environments.

Nanobubbles have attracted significant attention due to their unexpectedly long lifetimes and stabilities in liquid solutions. However, explanations for the unique properties of nanobubbles at the molecular scale are somewhat controversial. Of special interest is the validity of the Young-Laplace equation in predicting the inner pressure of such bubbles. In this work, large-scale molecular dynamics simulations were performed to study the stability and diffusion of nanobubbles of methane in water. Two types of force field, atomistic and coarse-grained, were used to compare the calculated results. In accordance with predictions from the Young-Laplace equation, it was found that the inner pressure of the nanobubbles increased with decreasing nanobubble size. Consequently, a large pressure difference between the nanobubble and its surroundings resulted in the high solubility of methane molecules in water. The solubility was considered to enable nanobubble stability at exceptionally high pressures. Smaller bubbles were observed to be more mobile via Brownian motion. The calculated diffusion coefficient also showed a strong dependence on the nanobubble size. However, this active mobility of small nanobubbles also triggered a mutable nanobubble shape over time. Nanobubbles were also found to coalesce when they were sufficiently close. A critical distance between two nanobubbles was thus identified to avoid coalescence. These results provide insight into the behavior of nanobubbles in solution and the mechanism of their unique stability while withstanding high inner pressures.

GII.4 Norovirus Protease Shows pH-Sensitive Proteolysis with a Unique Arg-His Pairing in the Catalytic Site.

Human noroviruses (NoVs) are the main cause of epidemic and sporadic gastroenteritis. Phylogenetically, noroviruses are divided into seven genogroups, with each divided into multiple genotypes. NoVs belonging to genogroup II and genotype 4 (GII.4) are globally most prevalent. Genetic diversity among the NoVs and the periodic emergence of novel strains present a challenge for the development of vaccines and antivirals to treat NoV infection. NoV protease is essential for viral replication and is an attractive target for the development of antivirals. The available structure of GI.1 protease provided a basis for the design of inhibitors targeting the active site of the protease. These inhibitors, although potent against the GI proteases, poorly inhibit the GII proteases, for which structural information is lacking. To elucidate the structural basis for this difference in the inhibitor efficiency, we determined the crystal structure of a GII.4 protease. The structure revealed significant changes in the S2 substrate-binding pocket, making it noticeably smaller, and in the active site, with the catalytic triad residues showing conformational changes. Furthermore, a conserved arginine is found inserted into the active site, interacting with the catalytic histidine and restricting substrate/inhibitor access to the S2 pocket. This interaction alters the relationships between the catalytic residues and may allow for a pH-dependent regulation of protease activity. The changes we observed in the GII.4 protease structure may explain the reduced potency of the GI-specific inhibitors against the GII protease and therefore must be taken into account when designing broadly cross-reactive antivirals against NoVs.IMPORTANCE Human noroviruses (NoVs) cause sporadic and epidemic gastroenteritis worldwide. They are divided into seven genogroups (GI to GVII), with each genogroup further divided into several genotypes. Human NoVs belonging to genogroup II and genotype 4 (GII.4) are the most prevalent. Currently, there are no vaccines or antiviral drugs available for NoV infection. The protease encoded by NoV is considered a valuable target because of its essential role in replication. NoV protease structures have only been determined for the GI genogroup. We show here that the structure of the GII.4 protease exhibits several significant changes from GI proteases, including a unique pairing of an arginine with the catalytic histidine that makes the proteolytic activity of GII.4 protease pH sensitive. A comparative analysis of NoV protease structures may provide a rational framework for structure-based drug design of broadly cross-reactive inhibitors targeting NoVs.

Effects of micro-bubbles on the nucleation and morphology of gas hydrate crystals.

Gas hydrate is usually regarded as a huge potential energy resource that has promising industrial applications in gas separation, storage, and transportation. Previous research studies have shown that a gas hydrate phase transition is mainly controlled by heat and mass transfer, while there are limited works on the mass transfer effects of gas micro-bubbles on hydrate crystallization. In this study, variations in the microscopic morphology of the hydrate crystal growth in a liquid-gas interface were observed using a microscope imaging system. The results indicated that the nucleation of the hydrate first tends to occur at the bubble surface. The cooling rates increased exponentially with the crystal growth rates and played an important role in the morphology of the hydrate crystal growth. In addition, the hydrate crystals tended to grow in the direction of the bubbles affected by the Ostwald ripening effects, which suggested that bubbling was an efficient measure to promote the application of hydrate-based technologies. In turn, reducing the concentration of the bubbles on the surface of the hydrate, inhibiting their generation, and enhancing the process of gas mass transfer in water around the hydrate surface were also conducive to further accelerate the decomposition of the hydrate, which may provide some guidance for the resource exploitation of gas hydrate.

In Situ Local Contact Angle Measurement in a CO2-Brine-Sand System Using Microfocused X-ray CT.

The wettability of porous media is of major interest in a broad range of natural and engineering applications. The wettability of a fluid on a solid surface is usually evaluated by the contact angle between them. While in situ local contact angle measurements are complicated by the topology of porous media, which can make it difficult to use traditional methods, recent advances in microfocused X-ray computed tomography (micro-CT) and image processing techniques have made it possible to measure contact angles on the scale of the pore sizes in such media. However, the effects of ionic strength, CO2 phase, and flow pattern (drainage or imbibition) on pore-scale contact angle distribution are still not clear and have not been reported in detail in previous studies. In this study, we employed a micro-CT scanner for in situ investigation of local contact angles in a CO2-brine-sand system under various conditions. The effects of ionic strength, CO2 phase, and flow pattern on the local contact-angle distribution were examined in detail. The results showed that the local contact angles vary over a wide range as a result of the interaction of surface contaminants, roughness, pore topology, and capillarity. The wettability of a porous surface could thus slowly weaken with increasing ionic strength, and the average contact angle could significantly increase when gaseous CO2 (gCO2) turns into supercritical CO2 (scCO2). Contact angle hysteresis also occurred between drainage and imbibition procedures, and the hysteresis was more significant under gCO2 condition.

High pressure micromechanical force measurements of the effects of surface corrosion and salinity on CH4/C2H6 hydrate particle-surface interactions.

In order to investigate the mechanism of gas hydrate deposition and agglomeration in gas dominated flowlines, a high-pressure micromechanical force (MMF) apparatus was applied to directly measure CH4/C2H6 hydrate adhesion/cohesion forces under low temperature and high pressure conditions. A CH4/C2H6 gas mixture was used as the hydrate former. Adhesion forces between hydrate particles and carbon steel (CS) surfaces were measured, and the effects of corrosion on adhesion forces were analyzed. The influences of NaCl concentration on the cohesion force between CH4/C2H6 hydrate particles were also studied for gas-dominated systems. It was observed that there was no measurable adhesion force for pristine (no corrosion) and corroded surfaces, when there was no condensed water or water droplet on these surfaces. With water on the surface (the estimated water amount was around 1.7 µg mm-2), a hydrate film growth process was observed during the measurement. CS samples were soaked in NaCl solution to obtain different extents of corrosion on surfaces, and adhesion measurements were performed on both pristine and corroded samples. The adhesion force was found to increase with increasing soak times in 5 wt% NaCl (resulting in more visual corrosion) by up to 500%. For the effect of salinity on cohesion forces, it was found that the presence of NaCl decreased the cohesion force between hydrate particles, and a possible explanation of this phenomenon was given based on the capillary liquid bridge model.

Experimental investigation on spontaneous counter-current imbibition in water-wet natural reservoir sandstone core using MRI.

Counter-current imbibition is a process whereby a wetting phase spontaneously imbibes into a porous media, displacing the non-wetting phase. This process is considered an important oil recovery mechanism during water flooding in fractured oil reservoirs. In this study, the dynamic process of counter-current imbibition for a natural reservoir sandstone core with an all-face-open boundary condition was monitored using magnetic resonance imaging (MRI). A series of images and relaxation time T1 spectra were acquired. The movement of water spontaneously entering the core sample while oil escapes, the spatial distribution of oil and water, and the in situ saturation change of oil and water in porous media can be accurately detected using MRI. MRI assists the direct evaluation of the basic mechanisms of imbibitions. Experimental results suggest the remaining oil was trapped in some large pores because of the capillary pressure, and the oil recovery in some large-pore regions is lower than that in some small-pore regions at the end of imbibition. Experimental findings show a close agreement between conventional material balance and oil recovery determined from MRI. The in situ oil recovery data agree well with the empirical models. The observations from MRI images could provide test cases to enable the development of mathematical models and to facilitate the evaluation of the proposed imbibition mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.

Water Contact Angle Dependence with Hydroxyl Functional Groups on Silica Surfaces under CO2 Sequestration Conditions.

Functional groups on silica surfaces under CO2 sequestration conditions are complex due to reactions among supercritical CO2, brine and silica. Molecular dynamics simulations have been performed to investigate the effects of hydroxyl functional groups on wettability. It has been found that wettability shows a strong dependence on functional groups on silica surfaces: silanol number density, space distribution, and deprotonation/protonation degree. For neutral silica surfaces with crystalline structure (Q(3), Q(3)/Q(4), Q(4)), as silanol number density decreases, contact angle increases from 33.5° to 146.7° at 10.5 MPa and 318 K. When Q(3) surface changes to an amorphous structure, water contact angle increases 20°. Water contact angle decreases about 12° when 9% of silanol groups on Q(3) surface are deprotonated. When the deprotonation degree increases to 50%, water contact angle decreases to 0. The dependence of wettability on silica surface functional groups was used to analyze contact angle measurement ambiguity in literature. The composition of silica surfaces is complicated under CO2 sequestration conditions, the results found in this study may help to better understand wettability of CO2/brine/silica system.

Microstructural characteristics of natural gas hydrates hosted in various sand sediments.

Natural gas hydrates have aroused worldwide interest due to their energy potential and possible impact on climate. The occurrence of natural gas hydrates hosted in the pores of sediments governs the seismic exploration, resource assessment, stability of deposits, and gas production from natural gas hydrate reserves. In order to investigate the microstructure of natural gas hydrates occurring in pores, natural gas hydrate-bearing sediments were visualized using microfocus X-ray computed tomography (CT). Various types of sands with different grain sizes and wettability were used to study the effect of porous materials on the occurrence of natural gas hydrates. Spatial distributions of methane gas, natural gas hydrates, water, and sands were directly identified. This work indicates that natural gas hydrates tend to reside mainly within pore spaces and do not come in contact with adjacent sands. Such an occurring model of natural gas hydrates is termed the floating model. Furthermore, natural gas hydrates were observed to nucleate at gas-water interfaces as lens-shaped clusters. Smaller sand grain sizes contribute to higher hydrate saturation. The wetting behavior of various sands had little effect on the occurrence of natural gas hydrates within pores. Additionally, geometric properties of the sediments were collected through CT image reconstructions. These findings will be instructive for understanding the microstructure of natural gas hydrates within major global reserves and for future resource utilization of natural gas hydrates.

Dependence of the formation kinetics of carbon dioxide hydrate on clay aging for solid carbon dioxide storage.

Clay-based marine sediments have great potential for safe and effective carbon dioxide (CO2) encapsulation by storing enormous amounts of CO2 in solid gas hydrate form. However, the aging of clay with time changes the surface properties of clay and complicates the CO2 hydrate formation behaviors in sediments. Due to the long clay aging period, it is difficult to identify the role of clay aging in the formation of CO2 hydrate in marine sediments. Here, we used ultrasonication and plasma treatment to simulate the breakage and oxidation of clay nanoflakes in aging and investigated the influence of clay aging on CO2 hydrate formation kinetics. We found that the breakage and oxidation of clay nanoflakes would disrupt the siloxane rings and graft hydroxyl on the clay nanoflakes. This decreased the negative charge density of clay nanoflakes and weakened the interfacial interaction of clay nanoflakes with the surrounding water. Therefore, the small clay nanoflakes enriched in hydroxyl would disrupt the surrounding tetrahedral water structure analogous to the CO2 hydrate, resulting in the prolongation of CO2 hydrate nucleation. These results revealed the influence of the structure-function relationship of clay nanoflakes with CO2 hydrate formation and are favorable for the development of hydrate-based CO2 storage.

Investigation on multi-parameter of hydrate-bearing synthetic sediment during hydrate replacement process: A in-situ X-ray computed tomography study.

Carbon sequestration can be achieved by carbon dioxide replacement in natural gas hydrate exploitation, which reducing greenhouse gas emissions and providing an effective solution to address climate change, while simultaneously protecting the environment and promoting sustainable energy development. Gas replacement can achieve gas exploitation, gas storage, and stability enhancement simultaneously. However, time-varying microstructure evolution of the hydrate-bearing sediment (HBS) during this process remain a large amount of uncertainty. In this study, with microfocus computer tomography, hydrate replacement process is realized using xenon gas to replace krypton hydrate. During this period, the initial hydrate saturation and effective confining pressure were 63 % and 1 MPa respectively, the results were obtained as follows: 1. Hydrate occurrence dynamically adjusted during replacement process due to the "barrier effect" and "diffusion effect". 2. Dissociated water migration occurred in the sediment, and this induced local hydrate enrichment temporarily and blockages, but the blockages were eventually dredged with the dissociation of the Kr hydrate. 3. The sphericity and surface roughness of the hydrate particles were slightly improved, the pore space connectivity was well enhanced, and both tortuosity and absolute permeability was better strengthened after replacement process, where the absolute permeability was increased by 225.23 %, though the blockage occurrence temporarily weakened this strengthener.

Orbital Electrowetting-on-Dielectric for Droplet Manipulation on Superhydrophobic Surfaces.

Electrowetting-on-dielectric (EWOD), recognized as the most successful electrical droplet actuation method, is essential in diverse applications, ranging from thermal management to microfluidics and water harvesting. Despite significant advances, it remains challenging to achieve repeatability, high speed, and simple circuitry in EWOD-based droplet manipulation on superhydrophobic surfaces. Moreover, its efficient operation typically requires electrode arrays and sophisticated circuit control. Here, a newly observed droplet manipulation phenomenon on superhydrophobic surfaces with orbital EWOD (OEW) is reported. Due to the asymmetric electrowetting force generated on the orbit, flexible and versatile droplet manipulation is facilitated with OEW. It is demonstrated that OEW droplet manipulation on superhydrophobic surfaces exhibits higher speed (up to 5 times faster), enhanced functionality (antigravity), and manipulation of diverse liquids (acid, base, salt, organic, e.g., methyl blue, artificial blood) without contamination, and good durability after 1000 tests. It is envisioned that this robust droplet manipulation strategy using OEW will provide a valuable platform for various processes involving droplets, spanning from microfluidic devices to controllable chemical reactions. The previously unreported droplet manipulation phenomenon and control strategy shown here can potentially upgrade EWOD-based microfluidics, antifogging, anti-icing, dust removal, and beyond.

CO2 hydrate formation and dissociation in cooled porous media: a potential technology for CO2 capture and storage.

The purpose of this study was to investigate the hydrate formation and dissociation with CO2 flowing through cooled porous media at different flow rates, pressures, temperatures, and flow directions. CO2 hydrate saturation was quantified using the mean intensity of water. The experimental results showed that the hydrate block appeared frequently, and it could be avoided by stopping CO2 flooding early. Hydrate formed rapidly as the temperature was set to 274.15 or 275.15 K, but the hydrate formation delayed when it was 276.15 K. The flow rate was an important parameter for hydrate formation; a too high or too low rate was not suitable for CO2 hydration formation. A low operating pressure was also unacceptable. The gravity made hydrate form easily in the vertically upward flow direction. The pore water of the second cycle converted to hydrate more completely than that of the first cycle, which was a proof of the hydrate "memory effect". When the pressure was equal to atmospheric pressure, hydrate did not dissociate rapidly and abundantly, and a long time or reduplicate depressurization should be used in industrial application.

Promoting Effect of Common Marine Cations on Hydrate Dissociation and Structural Evolution under a Static Electric Field.

Natural gas hydrate, a potential energy resource, is attracting worldwide attention. In this study, we propose a new method of hydrate dissociation which uses seawater and electrostatic fields (SE method) cooperatively. The hydrate molecular dissociation mechanism of gas hydrate is a key issue in studying the kinetic properties of gas hydrate using the SE method. Therefore, molecular dynamics simulations were used to investigate the thermodynamic properties and structural changes of methane hydrate (MH) in multiple kinds of salt solutions under an electrostatic field. The results show that the electric field can drive cations into the MH phase to form a series of random semiopen cages, which are essentially temporary and metastable. The variation in free energy indicates that it is more difficult for divalent cations to enter the hydrate phase than monovalent cations, meaning that the hydrate structures formed with divalent cations are more unstable. Then, the ion current occurred in the hydrate phase (called ion migration in this study), which greatly accelerated hydrate dissociation. In contrast, the promotion effect of cations with the same charge on MH dissociation is as follows: Sr2+ > K+ ≈ Na+ > Ca2+ ≈ Mg2+. In general, the presence of common marine cations enhanced the promotion effect of the electric field on gas hydrate dissociation.

Facilitation of Hydrate Dissociation and Structural Evolution by Major Marine Anions under Static Electric Fields.

Electric fields have been proven to be capable of significantly affecting the equilibrium state of hydrates. In this study, the thermodynamic properties and structural changes of methane hydrate (MH) in various anion solutions in an electric field at 0.7 V/nm were investigated by molecular dynamics simulations. The presence of anions significantly enhances the instability of methane hydrates under electric fields, leading to a staged dissociation process. First, the anions coexist with MH to form a temporary metastable structure under the action of an electric field. Then, the migration of anions causes the dissociation of nearby hydrates and the formation of flow channels in the hydrate layer, which leads to the complete dissociation of MH after a period. The promotive effects of F-, Br-, I-, and Cl- ions were close, while SO42- was relatively weak. The anions are still in hydration shells in the MH phase, but the structure of the hydration shells differs slightly from that in solution (the coordination numbers of I- and SO42- ions increased). The migration resistances of multiple anions to cross the surface of the hydrate layer are similar. However, inside the hydrate phase, the anions with a larger radius have a higher migration resistance. It is difficult for SO42- ions to migrate inside the hydrate phase, and they tend to form a metastable structure on the hydrate surface. Combining our previous studies, SrCl2 solution has the best hydrate promotion under an electric field environment.

Fractal analysis on CO2 hydrate-bearing sands during formation and dissociation processes with NMR.

Injecting CO2 into submarine sediments to form hydrates is one of the potential methods of CO2 sequestration. The transition behavior of CO2 hydrates in porous media is of great practical significance. In this work, CO2 hydrate formation/dissociation in porous media was monitored in real time by a low-field magnetic resonance (MR) system, and a series of dynamic fractal dimensions of the pore space occupied by converted water during the hydrate formation/dissociation process were obtained based on the transverse relaxation time (T2) distributions. In general, the dimension of the converted water space increases with hydrate formation and decreases with the hydrate dissociation progress. A smaller particle size of porous media and a lower initial water saturation can promote hydrate formation, and the corresponding fractal dimension is higher during the hydrate formation process. There is a special status of the fractal period observed during the hydrate formation/dissociation process, and it is considered the temporally and spatially uniform distribution of hydrate crystal formation/dissociation inside the porous media. These results also indicate the relationships between the hydrate transition progress and the dynamic fractal dimension, which are useful for future works on pore-scale hydrate-bearing transitions during hydrate-based CO2 sequestration.