In this study, the melting of a porous methane hydrate driven to self-preservation conditions is investigated experimentally and modelled by adapting/extending the classical Stefan problem of phase change with a moving free boundary to the two-component (water+guest molecules) system of interest here. Experimentally, the very slow buildup of a layer of a supercooled meltwater is observed by optical microscopy at the boundary of the porous hydrate initially facing the gas phase, whose thickness increases as the square root of time, a typical signature of a Stefan-like melting process. As shown by Stefan's model, this very slow buildup is the expression of temperatures within the melting hydrate and meltwater that remain extremely close to the imposed outside temperature: the melting process is quasi-isothermal. The (supercooled) liquid water is supersaturated with the guest (methane) molecule, which pops up as a bubble when the meltwater layer thickness exceeds a few hundreds of microns; this gas bubble expels the liquid water and the melting process starts again at the hydrate/gas interface, etc. The guest (methane) dissolved in this layer, which is monitored by microRaman spectroscopy, exhibits a concentration gradient from a high value at the melting hydrate front to the (low) equilibrium concentration at the meltwater/guest interface. The Stefan problem is solved analytically for a semi-infinite hydrate, and numerically by a finite difference method. The melting process is driven by mass transfer effects rather than being controlled by heat transfer, as has also been observed with other hydrate phase change processes.

Transmission microscopy and microRaman imaging are used to investigate the morphologies, growth process, and porous structure of methane hydrate promoted by two prototypical surfactants, sodium dodecyl sulfate (SDS) and dioctyl sulfosuccinate sodium salt (AOT). Large ranges of surfactant concentrations and subcooling are considered from 100 to 1500 ppmw and 1 to 10 K. Gas hydrates are generated on the guest/water interface as hollow crystallites. These crystallites expand into the aqueous phase from their (open) basis on the interface until they detach from it to form a slurry and ultimately a porous structure, whose pores are filled with the aqueous phase. The morphologies and growth rates of those crystallites and the structure (porosity and pore sizes) of the porous medium are characterized as a function of subcooling and surfactant formulation. Under low subcooling (approximate to 1 K), the hydrate crystallites are hollow cylinders, whose diameters decrease with increasing surfactant concentration. Under low-to-moderate subcooling (from 2 to 4 K), the crystallites are hollow spikes or cones, growing at a rate that increases with subcooling. Under high subcooling (>4 K), tiny (micronic or submicronic) crystallites are massively produced at the interface. The porosity and pore sizes of the resulting porous structure decrease with increasing subcooling, down to values in the range of 40-50% and a few microns for the highest subcooling investigated (approximate to 10 K). Rather than altering the kinetics of hydrate formation, the surfactant prevents the aggregation of hydrate crystallites on the interface as an impermeable crust and/or at the rear of the interface, thus allowing the back-flow of water and the continuous production of hydrate crystallites. In other words, the surfactant ensures the flowability of the slurry of hydrate crystallites and its removal away from the interface. These phenomena are primarily driven by the interfacial properties of the water/gas/hydrate system.

The temperatures and pressures of the three-phase equilibria between liquid water (Lw), gas hydrate (H)and a guest-rich phase (a vapor, V, or a condensed liquid, Lw) and between Lw, ice (I), and V, are determined experimentally by monitoring the disappearance of the hydrate or ice phases near the meniscus between the water-rich and guest-rich phases when temperature or pressure are varied slowly. This monitoring is carried out by optical microscopy used in the transmission mode with or without crossed polarizers, the latter serving to identify the presence or absence of hexagonal (birefringent) ice. The guest molecule (or hydrate-former) taken as an example is CO2. The triple lines corresponding to Lw-H-V, Lw-I-V and Lw-H-Lw equilibria are determined together with their intersections, i.e., the lower quadruple point Q1 (Lw-I-H-V coexistence) and upper quadruple point Q2 (Lw-H-V-Lw coexistence). The metastable extension of the three-phase line Lw-H-V for temperatures and pressures below those of Q1 is also determined. A Clausius-Clapeyron treatment of this line and its metastable extension shows that a similar dissociation process exists, whether the dissociation is to supercooled liquid water or not.

Methane hydrates (MHs) are considered an alternative energy resource but also a potential source of geo-hazards and climate change. The physical/mechanical properties of gas hydrate-bearing sandy sediments are strongly dependent on the distribution of hydrates within the pore space. The purpose of this study is to investigate morphologies and pore-habits of MHs formed in sandy sediments by means of experiments that probe a wide range of scales, from the pore scale – using Synchrotron X-Ray Computed Tomography (SXRCT) and optical microscopy – to the core scale, through mechanical property measurements. The same synthetic sands are used, in which MHs are generated successively under excess gas and excess water conditions. At the macroscopic (core) scale, MH pore habits are inferred by comparing the measured sonic wave velocities to velocities calculated from rock physics models and further assessed via triaxial compression tests. Furthermore, Magnetic Resonance Imaging is used to investigate the kinetics of MH formation and distribution along the core height. The pore habits and MH morphologies are directly visualized at the pore (grain) scale by SXRCT and, with still better spatial and temporal resolution, by transmission optical microscopy, revealing some more complex morphologies than in the hydrate pore habits commonly admitted.

There are mutual benefits in storing H-2 in sedimentary reservoirs jointly with another gas serving as a cushion gas, such as the CO2 of a carbon capture and storage (CCS) operation or the natural gas of seasonal storage or left in a depleted hydrocarbon reservoir. When H-2 occupies the crest of the reservoir, the presence of either gas is beneficial to the other. H-2 reinforces the sealing efficiency of the caprock due to its very favorable interfacial properties with respect to brine and rock-forming minerals. H-2 storage safety and capacity are also increased with cushion gases such as CO2, which alleviate the buoyancy pressure at the top of the gas column. The potential drawback of this storage scheme is gas/gas mixing, which can, however, be strongly reduced if, by an appropriate choice of well completion and placement, H-2 is positioned in the upper zones of the reservoir, and its injection rate is kept below a critical value corresponding to the incipient fingering instability of the H-2/cushion gas mixing zone. This value, which depends on reservoir permeability, the dip angle of the mixing front, and how density varies with viscosity in the mixing front, turns out to be well above practical injection rates. Therefore, dispersive mixing is the only cause of front spreading, which is acceptable for not-too-heterogeneous reservoirs. The mutual benefits identified in this study are the strongest when the cushion is made up of dense CO2, which suggests that the crest of offshore CO2 storage reservoirs is a good candidate for H-2 storage. (c) 2024 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Surfactants present in tiny amounts in the aqueous phase are known to be efficient gas hydrate promoters; yet, the promotion mechanisms are still not fully understood. Understanding and directing those mechanisms is key to the implementation of gas-hydrate-based applications such as gas storage and separation, secondary refrigeration or water treatment, and desalination. In this work, the growth at the water/gas interface and the porous structure of surfactant-promoted methane hydrate are observed by optical microscopy and Raman imaging in glass capillaries used as optical cells. Hollow crystals are continuously generated and expelled from the methane/water meniscus into the water or surfactant solution, where they ultimately form the skeleton of a porous medium filled with the solution. Unprecedented information is gathered over a range of scales from the molecular scale (crystal structure and cage filling) to the mesoscale (crystal morphologies, growth habits and pore sizes) and macroscale (rates and amounts of water and gas converted into hydrate and hydrate porosity). Following an early steady-state growth regime, a sudden order-of-magnitude increase of the conversion rate occurs, which is related to gaseous methane microbubbles being directly incorporated across the meniscus in the aqueous solution and later converted to methane hydrate. An assessment and comparison are made of the mechanisms and performance of two common anionic surfactants known to be efficient gas hydrate promoters, SDS (sodium dodecyl sulfate) and AOT (dioctylsulfosuccinate sodium or AerosolOcTyl). AOT provides a quicker but more limited conversion into hydrate than SDS, suggesting that it is more appropriate for continuous flow processes while SDS is better suited for gas storage applications. Raman spectra reveal that cage filling by methane of structure I methane hydrate is not affected by surfactants.

There are mutual benefits in storing H 2 ${\rm H}_2$ in sedimentary reservoirs jointly with another gas serving as a cushion gas, such as the CO 2 ${\rm CO}_2$ of a carbon capture and storage (CCS) operation or the natural gas of seasonal storage or left in a depleted hydrocarbon reservoir. When H 2 ${\rm H}_2$ occupies the crest of the reservoir, the presence of either gas is beneficial to the other. H 2 ${\rm H}_2$ reinforces the sealing efficiency of the caprock due to its very favorable interfacial properties with respect to brine and rock-forming minerals. H 2 ${\rm H}_2$ storage safety and capacity are also increased with cushion gases such as CO 2 ${\rm CO}_2$ , which alleviate the buoyancy pressure at the top of the gas column. The potential drawback of this storage scheme is gas/gas mixing, which can, however, be strongly reduced if, by an appropriate choice of well completion and placement, H 2 ${\rm H}_2$ is positioned in the upper zones of the reservoir, and its injection rate is kept below a critical value corresponding to the incipient fingering instability of the H 2 / cushion ${\rm H}_2{/{\rm cushion}}$ gas mixing zone. This value, which depends on reservoir permeability, the dip angle of the mixing front, and how density varies with viscosity in the mixing front, turns out to be well above practical injection rates. Therefore, dispersive mixing is the only cause of front spreading, which is acceptable for not-too-heterogeneous reservoirs. The mutual benefits identified in this study are the strongest when the cushion is made up of dense CO 2 ${\rm CO}_2$ , which suggests that the crest of offshore CO 2 ${\rm CO}_2$ storage reservoirs is a good candidate for H 2 ${\rm H}_2$  storage. © 2024 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Venet et al., reported unprecedented, bundled growth of approximate to 1-10mm long, hollow gas hydrate fibres, diameter approximate to 1-10lm, on an activated carbon close to a guest-water interface, circumventing the hydrate growth bottleneck. Here, in situ video microscopy traces growth (2-3 mu ms(-1) at 0.5 degrees C) to surface macropores of similar diameter, further exhibited by confocal and scanning electron microscopies. X-ray radiography shows fluid exchange to a depth of approximate to 50-100 mu m in a total porous fraction 40% (gravimetric porometry). Computed X-ray microtomography reveals a belt of organised peripheral porosity of aligned approximate to 5 mu m pores. These structures explain the origin of the comb or whalebone-like appearance of the hydrate. Whereas hydrate promotion by porous media is usually assumed to proceed inside the materials, like tiny hydrate storage warehouses, we find that the substrate behaves more like a 'lean' or 'just in time' production line, with hydrate extruded as fast as it is produced near the surface. (C) 2022 Elsevier Ltd. All rights reserved.

Joule–Thomson (JT) and isentropic expansion coefficients describe the temperature change induced by a pressure variation under isenthalpic and isentropic conditions, respectively. They are commonly used to model a variety of processes in which either fluid compression or expansion is involved. While a lot of work has been devoted to inferring the JT coefficient from an equation of state when the fluid is a single phase, little attention has been paid to multiphase fluids, where phase equilibrium has to be taken into account; previous work has only addressed the construction on the JT inversion curve. In the present paper, we describe and implement an approach to calculate these two coefficients for multi-component fluid systems, including when they form two different phases, liquid, and vapor, in thermodynamic equilibrium. The only ingredients are an equation of state and expressions for the ideal part of the specific heats of the fluid components. We make use of cubic equations of state, but any thermodynamic model can be used in the proposed framework. Calculations conducted with typical geofluids, some of them containing CO2, show that these coefficients are discontinuous at phase boundaries (where enthalpy and entropy variations exhibit angular points), as expected with any thermodynamic quantity built from first-order derivatives of state functions, and cannot be simply inferred from the coefficients of the liquid and vapor phases.

Unprecedented massive growth of fibrous cyclopentane hydrate on an activated carbon was shown recently to circumvent the mass-transfer bottleneck caused by hydrate crust at guest-host interfaces. Here, we generalize to other carbons and to porous silicas, confirming hydrate formation in surface macropores. Experiments with hydrophilic or -phobic porous substrates and with single glass capillary model pores elucidate the growth process. Surprisingly, wettability is of indirect importance. Detailed observation of unlimited single fibers with the better optical access and well-defined geometry of the glass model pores reveals the growth mechanism: crystal dewetting at pinned guest-host interfaces, in surface macropores in the case of porous substrates, where hydrophobicity serves only indirectly, to conduct the guest to the active interface. Fiber growth is closely analogous to the dewetted Bridgman process observed decades ago in microgravity. "Micro" or negligible gravity is provided here by the small size of the pores compared to the capillary length. Transposing the theory to the present system, we correlate fiber diameter and dynamic contact angles on the fiber and the pore wall, which in turn depend on the growth rate, hence on the supercooling. The model prescribes for generalization to further combinations of guest and porous substrates.

We show how insights into the porous structure of surfactant-promoted gas hydrate can be obtained from two types of experiments that probe very different length scales. On the one hand, the observation of an existing porous hydrate being imbibed with the aqueous surfactant solution gives information as to the porosity - inferred from the volume increase - and the average pore size - inferred from the imbibition rate. On the other hand, direct pore visualization and determination of the fluid and hydrate contents at the micron scale are obtained by optical microscopy and Raman microspectroscopy. We observe a porosity in the range of 60-70% and pore sizes of about 20-30 mu m for a porous methane hydrate formed from a 500 ppmw SDS (sodium dodecyl sulfate) solution under moderate subcooling. (C) 2021 Elsevier Ltd. All rights reserved.

Understanding the mechanisms involved in the formation and growth of methane hydrate in marine sandy sediments is crucial for investigating the thermo-hydro-mechanical behavior of gas hydrate marine sediments. In this study, high-resolution optical microscopy and synchrotron X-ray computed tomography were used together to observe methane hydrate growing under excess gas conditions in a coarse sandy sediment. The high spatial and complementary temporal resolutions of these techniques allow growth processes and accompanying redistribution of water or brine to be observed over spatial scales down to the micrometre—i.e., well below pore size—and temporal scales below 1 s. Gas hydrate morphological and growth features that cannot be identified by X-ray computed tomography alone, such as hollow filaments, were revealed. These filaments sprouted from hydrate crusts at water–gas interfaces as water was being transported from their interior to their tips in the gas (methane), which extend in the µm/s range. Haines jumps are visualized when the growing hydrate crust hits a water pool, such as capillary bridges between grains or liquid droplets sitting on the substrate—a capillary-driven mechanism that has some analogy with cryogenic suction in water-bearing freezing soils. These features cannot be accounted for by the hydrate pore habit models proposed about two decades ago, which, in the absence of any observation at pore scale, were indeed useful for constructing mechanical and petrophysical models of gas hydrate-bearing sediments.

Video microscopy reveals a novel morphology and growth process of a gas hydrate, promoted here by a porous activated carbon substrate astride a guest-host interface. Surface pores of the substrate continuously extrude hollow cyclopentane hydrate fibers, fed by flow of the guest through the substrate. Growth continues even when it pushes the substrate away from the interface, fed by flow of either fluid through the fibers themselves, so that fiber growth is limited here by the size of the setup. This remarkable process overcomes the gas hydrate mass-transfer bottleneck, which in quiescent systems limits the conversion of water and guest to a thin impervious hydrate crust between the two phases.

Glass micro-capillaries are the simplest yet most versatile, robust, practical and cheap microfluidic devices.Their small size and high optical quality favour detailed investigation under the optical microscope.Here we first review some of their applications, such as determining contact angles and the observation of tenuous wetting films under harsh conditions of pressure and temperature.We further explore how an optical cusp formed by reflection off the inner wall of a glass capillary may be used to monitor the refractive index of its fluid content.Finally, we illustrate how the above advantages may be put to use in the study of extremophile microorganisms, for example in recreating under the microscope the conditions prevailing on the ocean floors. RÉSUMÉ :Les microcapillaires de verre sont les outils microfluidiques les plus simples, et né anmoins ils sont polyvalents, robustes et bon marché .Leur petite taille et grande qualité optique favorisent leur utilisation sous le microscope optique.Nous passons en revue quelques unes de leurs applications, comme la dé termination d'angles de contact et l'observation de films de mouillage en conditions sé vè res de pression et tempé rature.Nous explorons ensuite comment la caustique formé e par ré flexion sur la paroi interne du capillaire est relié e à l'indice de ré fraction du contenu fluide.Finalement, nous illustrons comment ces avantages peuvent servir à l'é tude d'organismes extré mophiles, par exemple en recré ant sous le microscope les conditions des fonds océ aniques.

Methane hydrate is widely distributed in the pores of marine sediments or permafrost soils, contributing to their mechanical properties. Yet the tensile properties of the hydrate at pore scales remain almost completely unknown, notably the influence of grain size on its own cohesion. Here we grow thin films of the hydrate in glass capillaries. Using a novel, contactless thermal method to apply stress, and video microscopy to observe the strain, we estimate the tensile elastic modulus and strength. Ductile and brittle characteristics are both found, dependent on sample thickness and texture, which are controlled by supercooling with respect to the dissociation temperature and by ageing. Relating the data to the literature suggests the cohesive strength of methane hydrate was so far significantly overestimated.

We designed and implemented an experimental methodology to investigate gas hydrate formation and growth around a water-guest meniscus in a thin glass capillary, thus mimicking pore-scale processes in sediments. The glass capillary acts as a high-pressure optical cell in a range of supercooling conditions from 0.1 °C, i.e., very close to hydrate dissociation conditions, to ∼35 °C, very near the metastability limit. Liquid or gaseous CO2 is the guest phase in most of the experiments reported in this paper, and N2 in a few of them. The setup affords detailed microscopic observation of the roles of the key parameters on hydrate growth and interaction with the substrate: supercooling and substrate wettability. At low supercooling (less than 0.5 °C), a novel hydrate growth process is discovered, which consists of a hollow crystal originating from the meniscus and advancing on the guest side along the glass, fed by a thick water layer sandwiched between the glass and this crystal.

We examine the asymptotic behavior of the equilibrium ratios (Ki) near the convergence locus in the pressure-temperature plane. When the Equation of State (EoS) is analytical, which is the case of most EoS of engineering purpose, Ki tends towards unity or, equivalently, its logarithm lnKi tends to zero, according to a power ½ of the distance to this locus. As a consequence, if lnKi is expressed as a linear combination of pure component parameters with coefficients only depending on mixture phase properties (i.e., reduction parameters), these coefficients obey a similar power law. Deviations from the ½ power law are thus fairly limited for lnKi and for the reduction parameters (at least in the negative flash window between the convergence locus and the phase boundaries), which can be exploited to speed up flash calculations and for quickly determining approximate saturation points and convergence pressures and temperatures. The chosen examples are representative synthetic and natural hydrocarbon mixtures, as well as various injection gas-hydrocarbon systems.

We design and implement a simple and versatile droplet-based millifluidic method for investigating nucleation and growth processes in crystal-forming aqueous systems. It consists in generating and storing in a transparent capillary a train of identical and regularly-spaced droplets of an aqueous phase in a carrier oil phase, and then in video-monitoring crystal nucleation and subsequent growth and melting events as temperature and/or pressure are varied. Compared to previous investigations, the novelty is the possibility of working with aqueous solutions containing dissolved gas under controlled pressure, thus opening the way to gas hydrate studies. In the absence of dissolved gas, i.e., at ambient pressure, we observe ice nucleation to be weakly promoted by titanium oxide and montmorillonite particles, and strongly promoted by silver iodide, in agreement with literature results. Ice nucleation is also promoted when the carrier oil is more wetting towards the capillary, which is the case for fluorinated oil as compared to n-hexane. With cyclopentane, a hydrate-former, as the carrier oil, and dissolved CO2, also a hydrate-former and a "help gas" for cyclopentane hydrate formation, we find evidence for hydrate nucleation along with that of ice, and monitor the different solid phases as temperature varies. (C) 2018 Published by Elsevier Ltd.