Morphological and Compositional Characterization of Self‐Preserved Gas Hydrates by Low‐Vacuum Scanning Electron Microscopy

Ohno, Hiroshi; Nishimura, Okio; Suzuki, Kiyofumi; Narita, Hideo; Nagao, Jiro

doi:10.1002/cphc.201100079

Cited by 14 publications

(13 citation statements)

References 24 publications

(55 reference statements)

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“…The term "self-preservation" of gas hydrates was coined in Russia after researchers discovered that gas hydrate dissociation stopped soon after the onset of experiments when hydrate particles became coated with ice films. Later on, the results were corroborated and extended with data from more thorough experiments on structural changes in hydrates of methane and other gases capable of self-preservation [22][23][24][32][33][34][35][36][37][38][39][40][41][42]. The results were the basis for a unique technology of using hydrates as a gas storage and transportation media due to their very slow dissociation at <0 • C and 0.1 MPa [43][44][45][46].…”

Section: Introductionmentioning

confidence: 65%

Dissociation and Self-Preservation of Gas Hydrates in Permafrost

et al. 2018

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Gases releasing from shallow permafrost above 150 m may contain methane produced by the dissociation of pore metastable gas hydrates, which can exist in permafrost due to self-preservation. In this study, special experiments were conducted to study the self-preservation kinetics. For this, sandy samples from gas-bearing permafrost horizons in West Siberia were first saturated with methane hydrate and frozen and then exposed to gas pressure drop below the triple-phase equilibrium in the “gas–gas hydrate–ice” system. The experimental results showed that methane hydrate could survive for a long time in frozen soils at temperatures of −5 to −7 °C at below-equilibrium pressures, thus evidencing the self-preservation effect. The self-preservation of gas hydrates in permafrost depends on its temperature, salinity, ice content, and gas pressure. Prolonged preservation of metastable relict hydrates is possible in ice-rich sandy permafrost at −4 to −5 °C or colder, with a salinity of <0.1% at depths below 20–30 m.

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Section: Introductionmentioning

confidence: 65%

Dissociation and Self-Preservation of Gas Hydrates in Permafrost

et al. 2018

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“…Previous studies have shown that dissociation of metastable hydrate in the range 270 K < T < 273 K yields metastable supercooled water, , which converts into ice; therefore, ice formation during reformation could not be ruled out. It may also be possible that some of the reformed hydrates were also trapped inside ice sheet during the reformation. , Metastable hydrates are fundamentally identical to hydrates, and direct conversion of hydrate into ice was not feasible due to the high energy barrier to nucleate hexagonal ice from hydrates . Reformation was initiated due to disturbance caused by hydrate melting or gas and water migration that triggered the sudden onset of solidification due to the cavitation phenomenon .…”

Section: Resultsmentioning

confidence: 99%

“…It may also be possible that some of the reformed hydrates were also trapped inside ice sheet during the reformation. 79,26 Metastable hydrates are fundamentally identical to hydrates, and direct conversion of hydrate into ice was not feasible due to the high energy barrier to nucleate hexagonal ice from hydrates. 75 Reformation was initiated due to disturbance caused by hydrate melting or gas and water migration that triggered the sudden onset of solidification due to the cavitation phenomenon.…”

Section: T H I S C O N T E N T Imentioning

confidence: 99%

Pore-Scale Visualization of CH₄ Gas Hydrate Dissociation under Permafrost Conditions

et al. 2021

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In this study, we demonstrate the effectiveness of combined production technique involving depressurization and thermal stimulation for gas production from CH4 gas hydrates in subzero temperature range between -3℃ to 0℃. CH4 gas hydrate phase transitions during formation, depressurization, re-formation, self -preservation, thermal stimulation stage were visualized using a high-pressure, water-wet, silicon-wafer micromodel with pore network of actual sandstone rock.A set of eight experiments were performed in which CH4 gas hydrate was formed at a constant pressure between 60 -85 bar and constant temperature between 0 °C -4°C. CH4 gas hydrate was then dissociated at constant system temperature between -3 °C to -2 °C by pressure depletion to study the effect of hydrate and fluid saturation on dissociation rate, self-preservation, and risk of ice formation. The dissociation rate and behaviour were heavily affected by the total hydrate saturation and initial hydrate distribution in the pore space. Additionally, the amount of produced CH4 gas was limited below 0 °C due to the rapid formation of ice from the liquid water that was liberated from the initial hydrate dissociation. The liberated CH4 gas was therefore immobilized and trapped by the formed ice and could not be produced without thermal stimulation. Thermal stimulation removed the blockage of pore space caused by ice and secondary hydrate formation and enhanced gas production. Visual observation showed self-preserved hydrates in metastable state dissociated before ice below subzero temperature providing experimental evidence of recently discovered methane leaking from gas hydrate deposits due to global warming. The results highlight the influence of heterogeneity in hydrate distribution and total saturation on the hydrate dissociation behaviour below 0 ℃ temperature. Micromodel observation provides direct insights into hydrate dissociation, self-preservation, fluid migration, gas coalescence, ice and secondary hydrate formation at pore scale below subzero temperature.

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“…According to the data of ref , 0.5 mm particles of natural gas hydrate were preserved for 2 weeks. As it has been shown recently, an ice shell around a decomposing hydrate particle does not form in all cases. , During the decomposition of argon hydrate, the formed ice is released as a crust, but in the case of krypton hydrate, a mixture of hydrate microparticles and ice is formed.…”

Section: Introductionmentioning

confidence: 95%

“…As it has been shown recently, an ice shell around a decomposing hydrate particle does not form in all cases. 37,38 During the decomposition of argon hydrate, the formed ice is released as a crust, but in the case of krypton hydrate, a mixture of hydrate microparticles and ice is formed. Many studies of the influence of external factors on the selfpreservation of hydrates are presented in the literature.…”

Section: Introductionmentioning

confidence: 99%

Decomposition Kinetics and Self-Preservation of Methane Hydrate Particles in Crude Oil Dispersions: Experiments and Theory

et al. 2019

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The self-preservation phenomenon may be used for storage and transportation of natural gas in the form of gas hydrate. In this work, decomposition kinetics and self-preservation of methane hydrate dispersions in two types of crude oils were studied. Hydrate particle sizes in both dispersions did not exceed 30 μm. The experiments were performed at constant temperatures from −4 to −20 °C without preliminary deep-freezing of the samples. The kinetic curves of the methane hydrate decomposition were recorded as depending on methane pressure in the autoclave vs time. In the experiments with the first dispersion, it was shown that at the decomposition degree of 50%, the hydrate decomposition rates decreased by 1–2 orders of magnitude in comparison to the rates at the start of the decomposition process; therefore, hydrate self-preservation took place. Estimated thickness of the ice shell providing self-preservation did not exceed 1.3 μm at the decomposition degree of 50%. Self-preservation was less pronounced in experiments with the second dispersion. It may be explained by the presence of the surfactant in the second dispersion, which was added to stabilize the water-in-oil emulsion from which the dispersion was obtained. Possible mechanisms and the effect of various factors on the efficiency of self-preservation are discussed in this work. A mathematical model of hydrate decomposition process taking into account self-preservation of hydrate particles in the dispersion is suggested. A time-dependent (decreasing) diffusion coefficient of methane in ice shell was used in the model.

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Morphological and Compositional Characterization of Self‐Preserved Gas Hydrates by Low‐Vacuum Scanning Electron Microscopy

Cited by 14 publications

References 24 publications

Dissociation and Self-Preservation of Gas Hydrates in Permafrost

Dissociation and Self-Preservation of Gas Hydrates in Permafrost

Pore-Scale Visualization of CH₄ Gas Hydrate Dissociation under Permafrost Conditions

Decomposition Kinetics and Self-Preservation of Methane Hydrate Particles in Crude Oil Dispersions: Experiments and Theory

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Morphological and Compositional Characterization of Self‐Preserved Gas Hydrates by Low‐Vacuum Scanning Electron Microscopy

Cited by 14 publications

References 24 publications

Dissociation and Self-Preservation of Gas Hydrates in Permafrost

Dissociation and Self-Preservation of Gas Hydrates in Permafrost

Pore-Scale Visualization of CH4 Gas Hydrate Dissociation under Permafrost Conditions

Decomposition Kinetics and Self-Preservation of Methane Hydrate Particles in Crude Oil Dispersions: Experiments and Theory

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Pore-Scale Visualization of CH₄ Gas Hydrate Dissociation under Permafrost Conditions