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Natural gas hydrates, a promising clean energy resource, hold substantial potential. Porosity plays a crucial role in hydrate systems by influencing formation processes and physical properties. To clarify the effects of porosity on hydrate elasticity, we examined methane hydrate formation and its acoustic characteristics. Experiments were conducted on sediment samples with porosities of 23%, 32%, and 37%. P- and S-wave velocities were measured to assess acoustic responses. Results show that as hydrate saturation increases, sample acoustic velocity also rises. However, high-porosity samples consistently exhibit lower acoustic velocities compared to low-porosity samples and reach a lower maximum hydrate saturation. This behavior is attributed to rapid pore filling in high-porosity samples, which blocks flow pathways and limits further hydrate formation. In contrast, hydrate formation in low-porosity sediments progresses more gradually, maintaining clearer pore channels and resulting in relatively higher hydrate saturation. Higher porosity also accelerates the shift of hydrates from cementing to load-bearing morphologies. These findings underscore porosity's significant influence on hydrate formation and provide insights into observed variations in hydrate saturation and acoustic velocity across different experimental conditions. [ABSTRACT FROM AUTHOR]

Attenuation is an essential reservoir property, and understanding its mechanism in gas hydrate-bearing sediments is important for predicting gas hydrate saturation. Natural gas hydrates mainly accumulate in coarse-grained sands or fine-grained clays with different morphologies, and the attenuation characteristics of these gas hydrate-bearing sediments are inconsistent. Many rock-physics models have been proposed in recent years to elucidate the attenuation mechanisms of gas hydrate occurrence. There are two types of attenuation models for gas hydrates in sands. One of these models is based on the three-phase Biot theory and contains multiple mechanisms that describe the contact effects between hydrate and sediment grains, such as grain cementation and squirt flows caused by microcracks. This model can reasonably reproduce the enhanced attenuation observed with increasing hydrate saturation in sonic-logging data. Attenuation described in the effective grain model is dominated by the viscous flow between the water in hydrate pores and free water. These types of models agree with the seismic attenuation observed in gas hydrates in sands. Moreover, the attenuation model of gas hydrates in clay is formulated by first establishing an attenuation model for background sediments, which employs the effective grain model to characterize attenuation in clay minerals; then, the properties of pure gas hydrate and the effects of hydrate occurrence on sediments are quantified. This model has also been successfully applied to seismic reflection data in the seismic frequency band. The above models have achieved significant progress in attenuation studies on gas hydrate-bearing sediments, and they have aided the quantitative interpretation of observed attenuation in field data and attenuation-related constraints for gas hydrate saturation. However, the application of these models is limited to certain extents: the attenuation predicted by the gas hydrate-bearing sands model failed to match that observed in the field data. Additionally, the gas hydrate-bearing clays model does not consider fracture shapes, and its performance has not been verified at the ultrasonic frequency band. To improve the accuracy and applicability of these models, the relationship between hydrate morphology and attenuation must be elucidated, and further rock-physics studies based on field data must be conducted.

Natural gas hydrates, a promising clean energy resource, hold substantial potential. Porosity plays a crucial role in hydrate systems by influencing formation processes and physical properties. To clarify the effects of porosity on hydrate elasticity, we examined methane hydrate formation and its acoustic characteristics. Experiments were conducted on sediment samples with porosities of 23%, 32%, and 37%. P- and S-wave velocities were measured to assess acoustic responses. Results show that as hydrate saturation increases, sample acoustic velocity also rises. However, high-porosity samples consistently exhibit lower acoustic velocities compared to low-porosity samples and reach a lower maximum hydrate saturation. This behavior is attributed to rapid pore filling in high-porosity samples, which blocks flow pathways and limits further hydrate formation. In contrast, hydrate formation in low-porosity sediments progresses more gradually, maintaining clearer pore channels and resulting in relatively higher hydrate saturation. Higher porosity also accelerates the shift of hydrates from cementing to load-bearing morphologies. These findings underscore porosity’s significant influence on hydrate formation and provide insights into observed variations in hydrate saturation and acoustic velocity across different experimental conditions.

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Natural gas hydrates (NGH) stored in submarine deposits are a promising energy resource, Yet, the deterioration in sediment strength can trigger geological disasters due to drilling-induced hydrate dissociation. Hence, an indepth investigation on geo physical-mechanical performance of gas hydrate-bearing sediments (GHBS) is crucial for recovery hydrates safely and efficiently. This paper provides a comprehensive assessment of the research progress on formation conditions, intrinsic properties, and mechanical responses of GHBS. The key findings have been presented: gas composition, inhibitors and promoters alter hydrate formation by modifying the thermodynamic equilibrium of temperature and pressure. Also, we identified the key determinants of porosity of GHBS and revealed the correlation between permeability, hydrate saturation, and hydrate morphology. Moreover, we highlighted the differences in mechanical behavior between hydrate-free sediments and GHBS along with their underlying mechanisms. Furthermore, we examined the methods for GHBS preparation as well as the employed test apparatuses, providing critical insights into the limitations and recommendations. By synthesizing data from existing literature, we conducted a comprehensive analysis of the dependence of mechanical parameters of GHBS on factors such as hydrate saturation, effective confining stress, and temperature, and discussed the mechanical responses subjected to various hydrate dissociation methods. Finally, we offer a perspective for future research to focus on the micro-scale aspects, heterogeneous distribution, and long-term stability of GHBS. The discerned patterns and mechanical mechanisms are expected to guide the improvement of predictive model for geo physical-mechanical behavior of GHBS and establish a reference for developing effective strategies for recovery hydrates. [ABSTRACT FROM AUTHOR]

The GMGS6 hydrate expedition discovered hydrates in the Quaternary channel sand by logging and coring for the first time in the Qiongdongnan Basin, with an average sand grain size of ~ 70 μm and sandy content over 80%. Hydrate saturation exceeds 80%, and the P-wave velocity exceeds 3500 m/s. The quantitative link between hydrate saturation and velocity is not clear due to the complex hydrate morphologies. In this study, we analyzed the possible hydrate morphologies regarding the variation in velocity, resistivity, and permeability versus hydrate saturation based on in situ permeability measurements, pressure core degassing testing, and well logs, which indicates that both cementing and frame-supporting behaviors occur within the pores. Using the cementing model alone underestimates saturation, while the frame-supporting model overestimates saturation. As a result, the cementation model and three-phase Biot equation are jointly used to quantitatively invert the hydrate saturation based on the least-squares principle. The inverted results agree with the saturation from pressure cores and resistivity, and indicate that the prevalence of cementing hydrates greatly increases the velocity and strength of the hydrate-bearing sands, which is consistent with laboratory observations of the "gas excess" scene from previous studies. With the continuous supply of gas-bearing fluids, more hydrates cement sediment particles and previously formed hydrates, occupying most of the pore space and consuming the free and bound water in the pore space. This study thus provides a new method for estimating the saturation in hydrate-bearing sands, which is also important for estimating the geomechanical behavior of hydrate-bearing sands. [ABSTRACT FROM AUTHOR]

Hydrogen (H 2) is recognized as a promising energy carrier for the future world, owing to its high energy density and zero carbon emissions. H 2 storage in the form of solidified hydrates represents an emerging economic-viable and eco-friendly technology for large-scale application. Thermodynamic hydrate promoters (THPs) enhanced H 2 hydrate formation at mild pressure conditions by forming sII or sH hydrates. However, the formation pathway and promotion mechanism of H 2 -THP sII hydrates are yet to be elucidated. In this study, the composition of H 2 -THF/H 2 -DIOX sII hydrates was analyzed in both 5.56 mol% THF and DIOX systems by high-pressure μ-DSC. The well-designed kinetic experiments coupled with morphology observation were conducted to reveal the key stage for H 2 -THF/H 2 -DIOX hydrates formation at pressures from 8.3 MPa to 18.3 MPa. Moreover, Raman spectroscopy was employed to validate the proposed formation pathway and the cage occupancy ratio of H 2 and THP molecules. Both pure THP hydrates and H 2 -THP hydrates were identified by high-pressure μ-DSC. Formation of H 2 -THP sII hydrates requires a specific condition that both H 2 and THP molecules simultaneously occupy the small and large cages of sII hydrates for 5.56 mol% THP. Based on the Raman spectroscopy, the ratio of H 2 molecules in the 512 small cages to THP molecules in the 51264 large cages increased with pressure. The experimental results contribute to a fundamental understanding of the role of THP in promoting H 2 -THP hydrate formation. The findings guide the adoption of effective THPs with optimal concentrations and contact patterns for hydrate-based H 2 storage technology. [Display omitted] • The pathway of sII H 2 hydrate formation in the presence of THF and DIOX was examined. • DIOX and H 2 -DIOX hydrate mixtures were identified by high pressure micro-DSC. • Cage occupancy ratio of thermodynamic promoter and H 2 molecules were obtained by Raman. • Thermodynamic promoter and H 2 molecules simultaneously enclathrated in sII hydrate cages. • Implications drawn on large-scale hydrate-based H 2 storage. [ABSTRACT FROM AUTHOR]

Natural Gas Hydrates (NGH) is considered a vast unconventional energy source that holds significant promise in addressing future energy demands. In Shenhu area (located at northern slope of the South China Sea, SCS), there has been conducted a series of further studies of NGH such as exploration, drilling, and twice field production testing. The lithological characteristics of cores from marine sediments and their influence on methane hydrate (MH) formation are relatively unknown and merits further investigation. In this study, we conducted a chain of lithological characterization on the core sediments recovered from Shenhu W17 well, the coring well which located near the 1st NGH field production in SCS. The core sediments are classified as clayey-silt, its median grain size is 6.91 μm, comprising primarily clay minerals, quartz, and calcite. According to X-ray diffraction analysis, the main content of clay minerals is illite-smectite layers, illite, chlorite, and kaolinite, respectively. Porosity of the core sediments is 32.5% and the permeability is 7.8 mD based on mercury intrusion tests. Four types of primary pores are identified based on SEM and QEMSCAN analysis: intergranular pore, intercrystalline pore, intragranular pore, and pores associated with marine fossils. Moreover, the influence of the recovered core sediments (0–40 wt%) on MH formation kinetics were examined with morphology observed to elucidate the MH-core sediments interaction. The induction time was reduced significantly to ∼20 min in the presence of SCS core sediments. A two-stage MH formation behavior is observed with a maximum gas uptake of 134.1 V g · V w −1: (a) an initial MH formation at the gas-liquid interface with MH upward growth and fine-grain sediments migration; and (b) a second stage of significant MH growth with layered formation of MH and core sediments. The capillary channels of MH formed facilities the migration of core sediments, which in turn provides additional gas-liquid contact area for MH formation. The study provides valuable insights on the role Shenhu core sediments in MH formation, which is essential for understanding the spatial heterogeneity of NGH in reservoir and for designing suitable production strategy. [Display omitted] • The mineral composition and pore structure are characterized for Shenhu W17 core sediments. • Various types of pores are identified by SEM and QEMSCAN analysis. • Effect of core sediments on the kinetics and morphology of CH 4 hydrate formation is examined. • The migration of sediments facilities CH 4 hydrate growth resulting in a two-stage growth behavior. [ABSTRACT FROM AUTHOR]

Absolute and effective permeability are two very important petrophysical parameters that govern the production of gas from hydrate-bearing sediments. In the present study, an attempt is made to estimate the permeability from well log data using a theoretical approach, which is validated by comparing the obtained results with the core-derived values. The log data of the well NGHP-02-16B in Krishna–Godavari basin is used for the purpose of computing the permeability, and the core data from the same site are used for validation. The absolute permeability in the reservoir estimated using the Timur method ranges from 0.1 to 100 mD, and matches well with the core sample permeability. It is also demonstrated that the hydrate saturation and the existing hydrate morphology in pore spaces of the sediments play a significant role in the computation of effective permeability. The computed P-wave velocities reveal that the hydrates occur within the pore spaces of the sediments with hydrate saturation of 44–90%. The effective permeability of the hydrate-bearing sediments obtained by the Masuda model with a permeability reduction exponent (N = 2.5) agrees well with the core-derived permeability. The coating of the grain surfaces by the interspace hydrate within the pore is confirmed by comparison and normalization of effective permeability obtained from the Masuda model. The present study infers that the Masuda model is the most accurate and can be reliably used in the absence of core data for the computation of permeability of hydrate-bearing sediments in the vicinity of the study area. [ABSTRACT FROM AUTHOR]

ABSTRACT: Innovative gas hydrate-based applications and hydrate flow assurance problems of oil & gas pipelines have enabled new opportunities to develop and deploy biocompatible or green low-dosage hydrate promoters and inhibitors, respectively. In this context, we evaluate the performance of a biocompatible additive, lecithin (extracted from egg yolk), for the formation kinetics of binary cyclopentane (CP)-carbon dioxide (CO2) hydrates simulating to the structure-II hydrate of natural gas. A high-pressure visual autoclave is employed in this study to map the morphological observations with the gas hydrate kinetic data. Multiple experiments are performed to examine the inhibition or promotional effect of lecithin in saline (3.0 wt % NaCl) and non-saline water systems at low pressure of 1.0 MPa while utilizing the various CP content (1.5, 3.0, and 6.0 mol%) and 500 to 5000 ppm of lecithin. Moreover, we evaluate the effect of salinity and lecithin on the thermodynamic hydrate equilibrium conditions. The results imply that the presence of lecithin retards the hydrate formation kinetics. However, no shift in the equilibrium frontier is observed. Importantly, we propose a chemical potential based kinetic model by accounting the influence of additives, vapor-liquid interfacial area, temperature and pressure on gas hydrate formation. This model has closely interpreted the experimental data of hydrate former uptake during binary hydrate formation in the presence and absence of lecithin. Our findings highlight the potential of engaging lecithin for hydrate inhibition to mitigate the flow assurance issues and also offer comprehensive insights toward integrated hydrate-based carbon capture and seawater desalination approach.

The present study proposes a new elasto-plastic constitutive model that considers different types of hydrates in pore spaces. Many triaxial compression tests on both methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have been carried out over the last few decades. It has been revealed that methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have different strength and dilatancy properties even though they have the same hydrate contents. The reason for this might be due to the different types of hydrate morphology. In this study, therefore, the effect of the hydrate morphology on the mechanical response of gas-hydrate-bearing sediments is investigated through a model analysis by taking into account the different hardening rules corresponding to each type of hydrate morphology. In order to evaluate the capability of the proposed model, it is applied to the results of past triaxial compression tests on both methane hydrate-containing and carbon dioxide hydrate-containing sand specimens. The model is found to successfully reproduce the different stress–strain relations and dilatancy behaviors, by only giving consideration to the different morphology distributions and not changing the fitting parameters. The model is then used to predict a possible range in which the maximum deviator stress can move for various hydrate morphology ratios; the range is defined as the strength-band. The predicted curve of the maximum deviator stress obtained by the constitutive model matches the empirical equations obtained from past experiments. It supports the fact that the hydrate morphology ratio changes with the total hydrate saturation. These findings will contribute to a better understanding of the relation between the microscopic structures and macro-mechanical behaviors of gas-hydrate-bearing sediments.

In recent years, gas hydrate technology has been considered to be a promising method for CO 2 capture and transport. Nevertheless, the development of the gas hydrate technology is hindered by the slow hydrate formation. In this work, the effects of concentration, initial pressure and gas-liquid ratio on the CO 2 hydrate formation kinetics in the compound system of environmentally friendly additive l -methionine (L-met) coupled with low-dose multi-walled carbon nanotubes (MWCNTs) were investigated for the first time. The results showed that L-met coupled with MWCNTs could effectively promote the CO 2 hydrate formation, and the promotion effect was related to the MWCNTs concentration. At 4.0 MPa, compared to the 0.1250 wt% L-met single system, the compound system with the MWCNTs concentration of 0.0270 wt%∼0.0720 wt% increased the CO 2 gas consumption (G sum) by an average of 6.37 %, and the induction time (t in) of CO 2 hydrate formation were shortened by an average of 36.03 %. The optimal concentration of MWCNTs was 0.0450 wt%. Meanwhile, the increase of the pressure and gas-liquid ratio could effectively accelerate the kinetics of CO 2 hydrate formation. The promotion effect of the pressure increasing was better than that of the gas-liquid ratio increasing. The pressure increasing could result in the maximum increase of 39.75 %, 122.83 %, and 69.85 % in G sum , initial gas consumption rate (N 10), and the gas to hydrate conversion (GTH), respectively; and the highest reduction of t in was 83.72 %. Furthermore, the hydrate morphological observations revealed that CO 2 hydrate initially nucleated in the inner of L-met solution, and the hydrate particles in the compound system of L-met coupled with the MWCNTs were porous snowflakes. The wall climbing effect induced by capillary forces was further explained, and the presence of L-met enhanced the mass transfer capacity of CO 2 hydrates. The practical use of fast hydrate formation technology for CO 2 capture and sequestration has been facilitated by this work. • L-met and MWCNTs could synergistically promote CO 2 hydrate formation. • Rapid CO 2 hydrate formation can be realized in a minimum time of 9.00 min. • The gas to hydrate conversion can be improved by a maximum of 50.49 %. • 0.1250 wt%L-met mixed with 0.0450 wt%MWCNTs is selected as the optimal concentration. • The hydrate particle was smoother and more uniform surface and porous in the compound system. [ABSTRACT FROM AUTHOR]

Fine-grained sediments are widely distributed in naturally occurring hydrate-bearing sediments (HBS). However, the effects of silty and clayey minerals on the kinetics of methane hydrate (MH) formation and distribution are less well understood than in sandy sediments. In this study, a series of experiments were designed, which involves the kinetics and morphological observations to investigate the MH formation in clayey silty sediments with mass fractions of illite ranging from 0 to 50 wt%. The evolution of MH accumulation and distribution were analyzed based on temperature and electrical resistance measurements. The experimental results showed that the mass fraction of illite has a critical effect on MH nucleation, formation rate and distribution within the sandy sediment. The effect of illite on the gas uptake rate is primarily observed in the early MH formation stages, in which the MH formation rate in the system with 10 wt% illite exhibits approximately 1.66 times higher than that of pure sandy sediment. However, as the illite mass fraction increases from 20 wt% to 40 wt%, the MH formation rate decreases, only to increase significantly when the mass fraction reaches 50 wt%, which may be due to changes in the sediment skeletal structure. In the pure sandy system, MH primarily accumulates in the upper layer of the sediment. As the illite mass fraction increases, MH content in the lower layer of the sediment gradually increases. In morphological observations, several new cracks appeared after MH formation in highly silty and clayey sediments, increasing the MH formation rate. The electrical resistance of sediments exhibits a close relationship with hydrate saturation, and it basically increases proportionately with hydrate saturation until certain inflection points. After these points, the electrical resistance shows a significant increase. Moreover, the hydrate saturation at the inflection points tends to increase with higher illite mass fractions due to a more dispersed water distribution. • Methane hydrate formation, accumulation, and distribution in silty-clayey sediments are investigated. • Methane hydrate distribution is detected by temperature, and electrical resistance and visual observation. • Methane hydrate formation exhibits different improvement degrees and influencing mechanisms under different illite contents. • High illite content enhances the methane hydrate formation rate and content in the lower layer of the sediment. • Several new cracks are observed after methane hydrate formation in highly silty-clayey sediments. [ABSTRACT FROM AUTHOR]

Physical properties of hydrate‐bearing sediments are often correlated with hydrate saturation with little or no information on hydrate distribution uniformity in the specimens. This study focuses on water redistribution and sediment skeleton shift depending on various hydrate formation conditions in unsaturated systems, as well as on the resulting hydrate distribution patterns. Using X‐ray computed tomography, we investigate the factors such as fines content and the pressure‐temperature path on mass migration during carbon dioxide hydrate formation. The experiments show water migration, preferential hydrate formation toward the core periphery, localized patchy hydrate distribution, and sediment particle movement toward the core center. Sediment particle movement can be impeded in densely packed specimens. The overall mass migration due to hydrate formation can be significantly suppressed by adding 5% by mass of kaolinite. Hydrate formation initiated by pressurization and then cooling causes less mass migration than the cases where hydrate is formed using cooling followed by pressurization or pressurizing frozen cores followed by heating methods. Freezing can induce water migration and particle pushing in a similar manner as hydrate formation. Image analyses show that the pressure‐temperature path and the rates of heat transfer during hydrate nucleation and growth govern the uniformity of hydrate distribution in sediments. Key Points: Hydrate formation causes mass movement in sediments mainly by phase change, water migration, and soil skeleton deformationThermal boundary and temperature gradient dominate mass migration and hydrate distribution in excess‐gas systemsLow‐temperature gradient across the specimen and 5% by weight of kaolinite in sands suppress mass migration during hydrate formation [ABSTRACT FROM AUTHOR]

[Display omitted] • Secondary hydrate formation (SHF) are studied both experimentally and numerically. • A new kinetic model integrates hydrate pore-scale morphology and the SHF characteristics. • Heterogeneous manner of hydrate formation are quantified in Cubic Hydrate Simulator. • The SHF significantly exacerbates the hydrate formation heterogeneity. The challenge of secondary hydrate formation (SHF) is frequently encountered in hydrate formation and dissociation experiments. However, there is currently a lack of modeling theory of SHF kinetics in the field and laboratory tests. In this work, the hydrate formation experiments concerning the SHF are carried out in a cubic hydrate simulator (CHS), where the pressure profiles and temperature spatial distributions are measured. A new kinetic model of hydrate formation integrating hydrate pore-scale morphology and SHF characteristics is developed. The newly developed model with unified kinetic parameters is employed in the Tough + hydrate (T + H) simulator to duplicate the experiment processes numerically, which achieves excellent agreement with the experimental data. The results show that the evolutions of the spatial distributions of the temperature and hydrate saturation behave in heterogeneous manners in the hydrate formation processes. This also reveals that the SHF can significantly exacerbate the hydrate heterogeneity in the CHS. Two additional experiments and comparisons with currently available models have validated the feasibility and accuracy of the new model. This work provides a reliable and adaptable model for describing the entire lifecycle of the hydrate formation kinetics in the porous media. [ABSTRACT FROM AUTHOR]

Oceanic hydrate-based CO 2 sequestration (HBCS) holds great promise for achieving carbon neutrality. However, the presence of inorganic salts, particularly MgCl 2 besides NaCl, in seawater can significantly impact the formation rate and the stability of CO 2 hydrate. In this study, experimental investigations were conducted to examine the thermodynamics, kinetics, and the resulting morphological features of CO 2 hydrate in the presence of MgCl 2 , covering mass fractions ranging from 0 to 5.0 wt%. The experimental findings reveal that MgCl 2 exerts a thermodynamic inhibitory effect with its inhibitory capacity increasing with higher mass fractions. The solubility model of CO 2 in MgCl 2 solution was modified, demonstrating a gradual weakening of CO 2 solubility as MgCl 2 mass fraction increases. Additionally, the growth kinetics of CO 2 hydrate decreases with increasing MgCl 2 mass fraction. Regarding CO 2 hydrate morphology, it was observed that at low mass fractions of MgCl 2 (<1.0 wt%), a dense hydrate film rapidly formed at the gas-liquid interface after CO 2 hydrate nucleation, hindering the further conversion of CO 2 into hydrate. Conversely, at higher mass fractions (>3.0 wt%), CO 2 hydrate exhibits a more porous and slurry-like structure, facilitating more gas-liquid contact and mass transfer, thereby enhancing the conversion of CO 2 into hydrate. During hydrate dissociation, a salt-removal effect associated with CO 2 hydrate formation was observed, leading to the accumulation of concentrated electrolyte (MgCl 2) and facilitating CO 2 hydrate dissociation. These findings have implications for understanding the CO 2 hydrate formation and dissociation in the presence of MgCl 2 relevant in the subsea environment and can contribute to the development of effective hydrate-based CO 2 sequestration strategies. [Display omitted] • Measured phase equilibria of CO 2 hydrate in the presence of 0–5.0 wt% MgCl 2. • Increasing MgCl 2 reduced CO 2 solubility in H 2 O by 22 % for 5.0 wt% MgCl 2. • Increasing MgCl 2 yielded sluggish CO 2 formation kinetics but enhancement observed at 1.0 wt% MgCl 2. • CO 2 hydrate morphology transits from surface film growth to slurry with increasing MgCl 2. • CO 2 hydrate dissociation triggered by concentrated MgCl 2 after CO 2 hydrate formation. [ABSTRACT FROM AUTHOR]

A better understanding of the effect of methane hydrate morphology and saturation on elastic wave velocity of hydrate‐bearing sediments is needed for improved seafloor hydrate resource and geohazard assessment. We conducted X‐ray synchrotron time‐lapse 4‐D imaging of methane hydrate evolution in Leighton Buzzard sand and compared the results to analogous hydrate formation and dissociation experiments in Berea sandstone, on which we measured ultrasonic P and S wave velocities and electrical resistivity. The imaging experiment showed that initially hydrate envelops gas bubbles and methane escapes from these bubbles via rupture of hydrate shells, leading to smaller bubbles. This process leads to a transition from pore‐floating to pore‐bridging hydrate morphology. Finally, pore‐bridging hydrate coalesces with that from adjacent pores creating an interpore hydrate framework that interlocks the sand grains. We also observed isolated pockets of gas within hydrate. We observed distinct changes in gradient of P and S wave velocities increase with hydrate saturation. Informed by a theoretical model of idealized hydrate morphology and its influence on elastic wave velocity, we were able to link velocity changes to hydrate morphology progression from initial pore‐floating, then pore‐bridging, to an interpore hydrate framework. The latter observation is the first evidence of this type of hydrate morphology and its measurable effect on velocity. We found anomalously low S wave velocity compared to the effective medium model, probably caused by the presence of a water film between hydrate and mineral grains. Key Points: We observe the evolution of methane hydrate morphology in porous media by 4‐D X‐ray CT imaging and laboratory geophysical experimentsX‐ray CT shows that hydrate morphology evolves from an initial pore‐floating, to pore‐bridging, to a final interpore hydrate frameworkWe found anomalously low S wave velocity probably caused by the presence of water films between hydrate and host grains [ABSTRACT FROM AUTHOR]

With the help of a sapphire high pressure reactor， experiments of water cut， the concentration of anti-agglomerant ( Span80) and muddy silt on the place of NGH formation， and the morphologies of NGH formation were carried out and analyzed. The result shows the position of hydrate formation contains droplet surface，gas-liquid-solid interface，and gas-solid interface ( except the condition of high water cut) . With the increase of the concentration of Span80，the emulsion becomes more stable and the position of hydrate formation is transferred from gas-liquid-solid interface to droplet surface， meanwhile， hydrate form is changed firstly from bulk to fractal aggregation and then particle，resulting in the formation of stable hydrate slurry. Additionally，it was observed that hydrate particles were formed with and without sand，and that the wall-attached hydrate layer presented a sandwich structure. The rolling and colliding implantations of hydrate particles were also observed experimentally. Sand aggregation was caused by hydrate particle implantation and the carrying sand effect. The analysis of hydrate formation and aggregation morphology can provide technical guidance for the study of particle aggregation and jam in the pipeline. [ABSTRACT FROM AUTHOR]

Adsorption-hydration hybrid (AHH) technology combining methane adsorption and hydrate formation has attracted extensive attention for methane storage/transportation. This work explored the AHH kinetic properties in the hydrophobic metal-organic framework ZIF-8, using a visualization platform, to elucidate the relationship between adsorption and hydrate formation. A two-stage adsorption process of methane physisorption and hydrate formation-adsorption occurred, in which hydrate fibers and hydrate chunks were formed successively. To investigate the effects of water content (Rw) and pressure on AHH kinetic properties, the mass transfer coefficient of methane and kinetic constant of hydrate formation in two stages were calculated, respectively. Increasing Rw from 0.3 to 1.0 resulted in a significant decrease of 68.2% and 89.8% in the mass transfer coefficient of methane in the first stage and the kinetic constant of hydrate formation in the second stage, respectively. The higher Rw hindered hydrate fiber formation by impeding methane diffusion within the pore space, while raising the initial pressure promoted methane diffusion. Specifically, when the pressure was increased from 5.3 MPa to 7.0 MPa, the methane physisorption rate and hydrate formation rate could increase by up to 1.77 and 1.35 times, respectively. Furthermore, it was found that the decrease of physisorption rate further reduced the hydrate formation rate, revealing the relationship between physisorption and hydrate formation. A combined analysis of initial pressure and water content suggested that the inhibitory effect of Rw on mass transfer outweighed the promoting effect of initial pressure. [Display omitted] • Mechanism of AHH was first studied from the perspective of kinetics. • Two stages consisting of physisorption and hydrate formation-adsorption were found. • The hydrate nucleates rapidly at 7.8 min due to the promoting of physisorption. • The decrease of physisorption rate further reduces the hydrate formation kinetics. • Higher water content reduces AHH kinetics and inhibits the hydrate fibers formation. [ABSTRACT FROM AUTHOR]

During the exploitation of subsea natural gas hydrate (NGH) deposits, the muddy silt from marine sediment is carried by the fluid inside the wellbore. Furthermore, NGH is readily formed inside the wellbore when the fluid is in a hydrate formation area. Once NGH has formed inside the wellbore, the exploitation operation will be hindered, or shut down, due to the blockage. Understanding the kinetic characteristics and morphology of NGH formation is important to prevent its occurrence. To this end, a high-pressure autoclave system was designed and constructed in this work. Experiments were conducted to determine the mechanism of sand aggregation, the effect of sand on the kinetic characteristics and the morphology of hydrate formation. Additionally, models of hydrate particle formation with sand and sand aggregation, and structure of wall-attached hydrate layer growth were proposed. The results showed that sand could promote the growth of hydrate and the wall-attached hydrate layer. Additionally, it was observed that hydrate particles were formed with and without sand, and that the wall-attached hydrate layer presented a sandwich structure. The rolling and colliding implantations of hydrate particles were also observed experimentally. Sand aggregation was caused by hydrate particle implantation and the carrying sand effect. Hydrate particle formation with sand can be divided into four stages: nucleation, surface growth, shell formation, and shell growth. The sand aggregation process can also be divided into four stages: hydrate film formation, rupture of hydrate film, particle aggregation, and hydrate layer sintering. For structure of wall-attached hydrate layer growth, the growth front of the hydrate layer was concave-upward. [ABSTRACT FROM AUTHOR]

The permeability of the methane gas hydrate-bearing sand, which reflects the hydrate saturation and morphology of the sediments, significantly influences the rate of gas production. In this study, we formulated permeability reduction models for cubic packs of cylinders and spheres and random sphere packs with the grain-coating (GC) and pore-filling (PF) hydrates. Our models were based on the Kozeny–Carman equation. Three assumptions were made: (1) electrical tortuosity could be used in place of hydraulic tortuosity, (2) the effect of hydrate saturation on the shape factor could be ignored, and (3) the presence of an overlapping surface area of the GC hydrate could also be ignored. To confirm the validity of these assumptions, we conducted a series of computational fluid dynamics simulations of particle packs with the GC and PF hydrates generated by the discrete element method. Assumptions (2) and (3) caused the simulated normalized permeability of the GC hydrate to diverge from the proposed models because the simulated hydraulic tortuosity and shape factor of the PF hydrate were different from their analytical counterparts. However, because these differences canceled out at low hydrate saturations, the simulated normalized permeability of the PF hydrate agreed well with the proposed model. This agreement disappeared as the hydrate saturation increased because the cancellation effect atrophied. We compared the prediction accuracy of the proposed models with that of existing models in our simulations and with published experimental results. The proposed models agreed well with the simulation results and the experimental data. We showed that the parameters of the proposed model had a physical meaning: the average size of small clusters of PF hydrates. Analyzing the values of this parameter, we found that the PF type was the dominant morphology in natural core samples used in previous experimental studies. [ABSTRACT FROM AUTHOR]

Methane hydrates (MHs) formation involves the crystallization process of a hybrid system between methane and water. Former studies focus more on macroscopic but lack of visualization and temporal resolution, therefore, microfluidic device was used in this paper. Similar to the icing process, with the influence of supercooling effect, the hybrid system can be easily trapped in a metastable state. Under this circumstance, crystallization between methane and water molecules will not easily appear spontaneously, significantly extending the induction time. Therefore, artificial approaches are needed during the hydrate formation processes. In this work, based on microfluidic chips, a high-pressure visible device was designed and 2 kinds of perturbation methods were employed during the experiments. Both methods caused disturbance to the hybrid system, breaking the metastable state and achieving hydrate formation inside the microfluidic chips of the different pore structures. The results showed that hydrate formation in microfluidic chips require phase equilibrium state and perturbation in the regions with crystal nuclei. Perturbation was needed in hydrate formation under microfluidic chips and disturbance caused by constant pressure flow in the random pore structure is the most effective method. The repeated movement of methane-water phase played a significant role in the hydrate reformation process. Due to the heat conduction of hydrate formation and dissociation, the movements of the methane phase, water phase, and hydrate phase repeatedly appeared in the pore structure, and this behavior inside the pores directly caused hydrate reformation. • Microfluidic chip was applied to analyze the phase transition. • Temporal resolution during hydrate formation was investigated. • Disturbance was applied to facilitate hydrate formation in different situations. • Perturbation and gas-water interface were connected to induction time during hydrate formation. • Hydrate reformation process was discovered and analyzed. [ABSTRACT FROM AUTHOR]

In this paper, we present the kinetics of CO 2 hydrate formation in presence of additives at different operating conditions that result in the formation of pure sI hydrate, pure sII hydrates and a mixture of sI and sII hydrates. Visual observations of different hydrates formed are presented with the associated CO 2 uptake achieved under different experimental conditions. We observe a striking contrast in hydrate formation behavior observed for CH 4 hydrate and CO 2 hydrate in presence of tetrahydrofuran (THF) under similar diving force and operating conditions. Based on our experiments, it can be inferred that hydrate formation kinetics in presence of the THF is highly influenced by the type of guest gas. [ABSTRACT FROM AUTHOR]

[Display omitted] • Phase equilibria of CO 2 + DIOX hydrate for C DIOX between 0.05 mol% and 5.56 mol% • The kinetics of CO 2 + DIOX hydrate is quantified with morphology observation. • Phase separation of DIOX/H 2 O above 3.1 MPa weakens the promotion effect of DIOX. • A comprehensive evaluation of DIOX in the application of hydrate-based CO 2 storage. To reduce anthropogenic CO 2 emissions for the mitigation of climate change require novel CCUS solutions. Hydrate-based CO 2 sequestration (HCS) is a novel carbon-neutrality technology that aims to store CO 2 in solid hydrate form with long-term stability. However, imminent issues exist for the application of HCS in terms of demanding thermodynamic conditions, slow formation kinetics, and low CO 2 gas uptake. These challenges necessitate the quest for an efficient and eco-friendly CO 2 hydrate promoter. In this study, 1,3-dioxolane (DIOX) as a low-toxicity CO 2 hydrate promoter was systematically examined. The phase equilibria, cage occupancy, and the kinetics of binary CO 2 + DIOX hydrate were measured for DIOX concentrations (C DIOX) varying from 0.05 mol% to 5.56 mol%. It was confirmed that DIOX is a dual-function promoter for CO 2 hydrate, but its promotion effect is weakened for C DIOX between 0.60 mol% and 1.00 mol% and for all C DIOX at relatively high pressure. The CO 2 uptake in the hydrate phase increases with C DIOX above 2.00 mol% and is the highest for C DIOX = 5.56 mol% (57.08 ± 6.39 mmol/mol). Based on the morphology observation, hydrate transits from ice-like to slurry to mushy-like and finally to snow-like with increasing C DIOX. Interestingly, we observed that the aqueous phase separates into two phases (i.e., DIOX-rich and H 2 O-rich) at pressure above 3.09 MPa, which explains the gradual loss of the promotion effect. The results of our study provide a comprehensive evaluation on DIOX as a possible promoter for CO 2 hydrate in HCS application. [ABSTRACT FROM AUTHOR]

Gas hydrate plug formation is a major concern in oil and gas exploitation efforts, wherein line blockages can pose major safety, economic, and environmental risks. Kinetic hydrate inhibitors (KHIs) are a promising class of hydrate management chemicals, which are potentially cleaner, cheaper, and greener than traditional thermodynamic hydrate inhibitors (THIs). Therefore, understanding the effects that KHIs have on hydrate particles is vital to their application. In this study, polyvinylpyrrolidone (PVP), a model KHI, was investigated at ultralow concentrations to determine its effect on the properties of hydrates and elucidate when nucleation and growth inhibition begins. It was found that PVP can adsorb at the hydrate particle surface to reduce interparticle force by 40–54 %. Low concentration PVP continues to affect interparticle forces at prolonged contact times, reducing forces at 30-minutes to 1-hour of contact by 20–40 % and reducing sintering rate. PVP also reduces film growth rates by 30–50 % depending on the concentration of PVP in the water phase. The onset of major nucleation and growth effects was observed to occur at 0.01 wt% PVP in the water phase, two orders of magnitude below concentrations typically employed in hydrate management. It was discovered that low dosage PVP can cause major morphological changes to the hydrate particles in both the short and long term, which can influence interparticle forces and particle agglomeration, and may serve as a morphological screening tool for KHIs. A proposed mechanism for the observed morphology changes explains how the heterogenous adsorption of chemicals at the particle surface can lead directly to the newly observed particle morphology. The results presented in this paper show that ultralow concentrations of KHIs (0.0005 wt%) can have combined effects on the interfacial activity and crystal growth and morphology of hydrates, showing KHIs to be a dual function inhibitor of both interparticle interactions and hydrate growth. These results can inform KHI applications from industrial flow assurance to carbon dioxide transport for unimpeded carbon capture and sequestration. [Display omitted] • Interparticle hydrate forces at short contact times are reduced by 40–54 % with ultralow levels of PVP (ul-PVP). • Sintering forces at contact times up to an hour are decreased by 20–40 % with ul-PVP. • Hydrate film growth rates are retarded by 30–50 % and long and short-term morphology changes are induced with ul-PVP. • Nucleation and growth inhibition begins at concentrations as low as 0.01 wt% PVP. [ABSTRACT FROM AUTHOR]

• Hydrate synthesis and decomposition occur simultaneously throughout phase transition process. • Hydrate growth habit is not monotonous change in fractured-porous medium sandstone at different stages of phase transition. • Hydrate occurences in fractured-porous medium differs from that in porous matrix system. • The preferential occurence of hydrates in fracture leads to significantly permeability changes. Natural gas hydrate, as an efficient and clean energy resource, are naturally distributed in porous and fractured-porous medium. With the most recent development of advanced micro-scale imaging techniques, hydrate habits evolution, hydrate occurrences, and pore structure evolution as well as seepage characteristics during hydrate phase transition in porous hydrate-bearing sediments have been studied extensively at pore scale. However, there are few studies on gas hydrates in fractured-porous sediment. In this work, xenon hydrate phase transition experiment by excess-gas method is carried out in a fractured sandstone core with in-situ micro computed tomography (micro-CT) scanning to explore the evolution of hydrate habits and physical parameters of the host sediment. The results indicate that hydrate-bearing sediment is a dynamic equilibrium system as hydrate synthesis and decomposition occur simultaneously at each moment of hydrate phase transition induced by pressure change. The hydrate occurrences in fractured hydrate reservoirs include contiguous-sheet, clustered and isolated, which are slightly different from that of porous hydrate formation; and the contiguous-sheet hydrate is the occurrence that dominantly determines the seepage characteristics of fractured hydrate-bearing sediments. In addition, the logic diagram for hydrate growth paths in fractured-porous medium is presented for the first time. These findings are significant for detailed understanding of pore-scale hydrate distribution throughout phase transition process and provide theoretical basis for precise modeling of permeability in host sediments. [ABSTRACT FROM AUTHOR]

To gain in-depth understanding of natural gas hydrate behavior it is necessary to identify key parameters that affect their formation, distribution and destabilization within sediments. Hydrate formation kinetics in porous media is amongst the aspects which deserve important considerations as it may provide useful information on the formation history and the formation mechanisms of natural gas hydrate accumulations. Yet, it is at its early stage. In this paper, experiments on methane hydrate formation and dissociation in porous media are reported and discussed. The first part of this work is devoted to the investigation of the kinetics of methane hydrate formation within silica sand using a custom-design apparatus. The latter is suitable for investigating small hydrate-bearing cores. The influence of the methane injection flow-rate is examined, and then a straightforward method is proposed to quantify the amount of hydrate-bound gas. In the second part, three mixtures of clays and sand are used as geologic matrix to study the influence of clay content on the hydrate morphology for a predetermined amount of injected water. Visual observations showed that the morphology shifts from disseminated through massive to moussy hydrates with increasing proportion of clays. [ABSTRACT FROM AUTHOR]

[Display omitted] • A comparison of the promoting effect of SDS and L -Met on CO 2 hydrate formation. • Morphology of CO 2 hydrate in the presence of SDS and L -Met with stirring. • A calculation method quantifying CO 2 distribution in gas, liquid and hydrate phase. • Verification of L -Met promoting effect in repeated tests and at various G-L ratios. The continuously increasing CO 2 emissions since industrialization has sparked widespread concerns on environmental and climate change issues. Carbon sequestration and storage technologies (e.g., hydrate-based CO 2 sequestration) present promising potential for the disposing of excessive CO 2 in geological media. High gas uptake capacity and rapid CO 2 hydrate formation kinetics are required for the application of such a technology. In this study, we compared the kinetic promotion effects of sodium dodecyl sulfate (SDS) and L -methionine (L -Met) on CO 2 hydrate with the objective of identification of an effective and eco-friendly CO 2 hydrate kinetic promoter. The experimental results suggested that L -Met (0.1 wt%) promotes CO 2 hydrates formation significantly with a gas uptake in CO 2 hydrate five times more than SDS at the same concentration. The hydrate morphology observations indicated that the wall-climbing hydrates associated with the addition of L -Met induced by the capillary driving force offer a possible explanation for the difference in the observed kinetics. A novel calculation method was developed to quantify the partition of CO 2 in gas, liquid and hydrate phases. Increasing the initial gas–liquid ratio promotes CO 2 stored in the hydrate phase as solid rather than in the liquid phase as dissolution. We further demonstrated the reusability of L -Met in five consecutive cycles of CO 2 hydrate formation and dissociation. It was assessed that L -Met as a CO 2 hydrate kinetic promoter is superior to SDS and can be employed as an efficient, reliable and eco-friendly kinetic promoter for the hydrate-based CO 2 sequestration technology. [ABSTRACT FROM AUTHOR]

Understanding the mechanical response of hydrate-bearing marine sediments (HBMSs) prior to gas production enables engineers to forecast risks associated with large-scale methane extraction from silty hydrate reservoirs in the South China Sea. To this end, this study remoulded HBMSs containing hydrate with pore-spacing or cementing morphologies and then designed a series of drained shear tests on these materials. The results show that the HBMSs containing hydrate in cementing morphology present greater strength behaviours than and similar deformation characteristics to those containing hydrate in pore-spacing morphology. The increase in effective confining pressure magnifies the enhancement effect of hydrate on the stiffness and strength of marine sediments, which is more pronounced for the hydrate in pore-spacing morphology. The cohesion of marine sediments decreases and the internal friction angle increases as the hydrate forms, while the cohesion and internal friction angle show a slight decrease when the hydrate morphology transforms from cementing to pore-spacing. The results are expected to motivate future studies of the strength behaviours of silty hydrate reservoirs. [Display omitted] • Pore-spacing hydrate are re-formed in cores drilled from the South China Sea. • Cementing hydrate shows higher strength and similar deformation than pore-spacing. • Effective stress affects the pore-spacing effect of hydrate on marine sediments. • Cohesion of sediment reduces and internal friction angle increases as hydrates form. • Sediments with pore-spacing hydrate show lower cohesion and internal friction angle. [ABSTRACT FROM AUTHOR]

Understanding the engineering properties of hydrate-bearing sands (HBS) is the key to assessing the safety during the exploitation of natural gas hydrates. The formation and presence of hydrates in pore spaces are complicated, yet the HBS specimens with different hydrate morphologies have not been precisely tested due to the difficulty in controlling the hydrate formation in the laboratory. In this study, the discrete element method (DEM) is used to create HBS models containing pore-filling, cementing, load-bearing, grain-coating, and patchy hydrates. A series of HBS specimens with different hydrate morphologies and hydrate saturations ranging from 0 to 40% are tested by simulating biaxial compression tests. The numerical results show that the shear strength is slightly but the secant modulus is significantly influenced by the hydrate morphology. The shear strength and secant modulus of HBS increase with hydrate saturation regardless of the hydrate morphology. Further relationships are established between the micro mechanical analyses of the evolution of bonds and contact-type-related contributions and the macro mechanical properties of HBS. These DEM results can provide lower and upper limits for HBS, which are beneficial for further understanding the mechanical responses of HBS with complex hydrate morphologies. [ABSTRACT FROM AUTHOR]

The present study proposes a new elasto-plastic constitutive model that considers different types of hydrates in pore spaces. Many triaxial compression tests on both methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have been carried out over the last few decades. It has been revealed that methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have different strength and dilatancy properties even though they have the same hydrate contents. The reason for this might be due to the different types of hydrate morphology. In this study, therefore, the effect of the hydrate morphology on the mechanical response of gas-hydrate-bearing sediments is investigated through a model analysis by taking into account the different hardening rules corresponding to each type of hydrate morphology. In order to evaluate the capability of the proposed model, it is applied to the results of past triaxial compression tests on both methane hydrate-containing and carbon dioxide hydrate-containing sand specimens. The model is found to successfully reproduce the different stress–strain relations and dilatancy behaviors, by only giving consideration to the different morphology distributions and not changing the fitting parameters. The model is then used to predict a possible range in which the maximum deviator stress can move for various hydrate morphology ratios; the range is defined as the strength-band. The predicted curve of the maximum deviator stress obtained by the constitutive model matches the empirical equations obtained from past experiments. It supports the fact that the hydrate morphology ratio changes with the total hydrate saturation. These findings will contribute to a better understanding of the relation between the microscopic structures and macro-mechanical behaviors of gas-hydrate-bearing sediments. [ABSTRACT FROM AUTHOR]

Visual evidences to understand the interactions between hydrate decomposition and heat/mass transfer are currently lacking. This study proceeds from the hydrate morphology to visualize the interactions between depressurization-induced hydrate decomposition and heat/mass transfer from different scales. Reactor-scale hydrate distribution evolution shows that the dominant influencing factor of hydrate decomposition transforms from heat transfer to mass transfer. More importantly, pore-scale visual evidences suggest that the mass transfer of gas shows significant effects on hydrate morphology evolution. Specifically, the limited gas diffusion in liquid phase could lead to the hydrate morphology evolution from patchy pore-filling to "grain-bridging" during hydrate decomposition. The combination of grain-bridging hydrate together with the water layer that wraps the hydrate is termed as "hydrate bridge" in this work. It is also worth noting that the grain-bridging hydrate could accelerate fluid flow in pores according to our seepage simulation results. These findings provide visual evidences for variations in physical properties of hydrate-bearing sediments during hydrate decomposition. Since physical properties of hydrate-bearing sediments play important roles in hydrate decomposition, the hydrate morphology evolution characteristics analyzed here are valuable for hydrate exploitation in field tests. [Display omitted] • Interactions between heat/mass transfer and hydrate decomposition are visualized. • The definitions of "grain-bridging" hydrate and "hydrate bridge" have been proposed. • Mass transfer of gas shows significant effects on hydrate morphology evolution. • Hydrate morphology evolution leads to abnormal change of physical properties. [ABSTRACT FROM AUTHOR]

To evaluate the stability of seabed ground during gas hydrate exploitation, a constitutive model that can appropriately estimate the mechanical properties of methane hydrate-bearing sediments must be established. A better understanding of the stress–dilatancy characteristics is crucial in modeling the stress–strain behavior of methane hydrate-bearing sediments under complex loading paths. This study presents a compressive analysis of the effects of several factors on the stress–dilatancy behavior of methane hydrate-bearing sediments. The dilatancy behavior of methane hydrate-bearing specimens synthetized in the laboratory and natural hydrate-bearing samples acquired in the field are compared. Results show that the hydrate saturation, temperature, porosity, and hydrate formation pattern affect the peak stress ratio, dilatancy rate, and critical state stress ratio of methane hydrate-bearing sediments. However, the effect of effective confining pressure on the stress–dilatancy curve is minimal. Different hydrate formation methods yield different hydrate morphologies; consequently, the dilatancy behavior of methane hydrate-bearing sediments is altered significantly. Differences observed between synthetized specimens and natural samples may originate from various hydrate morphologies. The stress–dilatancy relationship of methane hydrate-bearing sediments is interpreted and described using Rowe's theory. Furthermore, the dilatancy behavior of methane hydrate-bearing sediments and cemented sands under similar test conditions is compared. • Hydrate saturation, temperature, and hydrate formation pattern affected the stress-dilatancy curves of hydrate-bearing sediments. • The stress-dilatancy curves for hydrate-beaing sediments could be modelled using Rowe's equation. • The varying tendency for stress-dilatancy curves for hydrate-bearing sediments are similar. [ABSTRACT FROM AUTHOR]

The efficiency and safety of natural gas hydrate exploitation is significantly affected by the occurrence and distribution of hydrate and the seepage behaviors of gas and water in hydrate bearing sediments. We present the results from xenon hydrate formation and dissociation experiments with different salinities using quartz etching microfluidic chips. Direct visualization of hydrate growth morphology is obtained with the help of industrial grade charge coupled device (CCD) camera coupled with a microscope module. The results show that the hydrate growth process in microfluidic chip can be divided into four stages, which are the hydrate inducing stage, the primary hydrate nucleation stage, the rapid hydrate nucleation stage and the hydrate nucleation maturation stage, respectively. Different hydrate morphologies (dendritic, intestine-like) and aggregation patterns (grain-coating, grain-cementing) are observed during the different stage of hydrate formation. The average value of all 219 contact angles of water phase on gas-hydrate interface is 44.03°, which indicates the hydrate surface is hydrophilic. And the hydrophilicity of the hydrate surface is impaired by the increase of brine salinity, owing to the loss of attraction between the molecules caused by the salt-removing effect during the hydrate formation process. The effective water permeability in hydrate bearing microfluidic chip decreases with the decrease of the pore size and is further affected by the heterogeneity. The permeability reduction caused by the hydrate formation is more pronounced in the microfluidic chips with larger initial pores. The hydrate formation kinetics is significantly inhibited by the coupling effect of the tortuosity, the pore size and the salinity and resulted in drastic reduction in hydrate saturation. • Hydrate morphologies and aggregation patterns in microfluidic chip were observed. • Contact angle at water-gas-hydrate interface and the wettability of hydrate surface were quantified. • Effects of pore structure heterogeneity and salinity on hydrate formation, wettability and permeability were analyzed. [ABSTRACT FROM AUTHOR]

The properties of gas hydrate bearing sediments (GHBS) depend on their changing morphology and wettability characteristics, which are in turn important for efficiency and safety issues during production. We present the results of xenon hydrate formation experiments using quartz sand packs at different salinities. Direct 3D visualization of hydrate distribution in the pore space was obtained with the help of high-resolution micro-computed tomography (CT). Four phases, xenon hydrate, xenon gas, brine and quartz sands were segmented and labelled in the raw micro-CT scanned images. A 3D reconstruction reveals typical pore-scale patterns of hydrate occurrence (pore-filling, grain-coating) and shows that hydrate preferentially aggregates at the gas-water interface, reflecting greater local access to xenon gas molecules. The hydrate cluster shape factor ranges from 2 to 10, indicating that xenon hydrate forms with a complex and irregular microstructure, consistent with random growth nature of hydrate. The hydrate saturation is found to decrease with salinity under excess-gas condition, which we interpret in terms of the hydrate formation kinetics to indicate that gas hydrate prefers to grow into the gas phase at the gas-water interface. A total of 270 local contact angles of water at the gas and hydrate interfaces in pores were measured with a micro-CT image analysis method to obtain information on the wettability variation of hydrate surfaces. The average was calculated to be 47.6°, indicating that xenon hydrate surface is hydrophilic. Our analysis of the xenon hydrate morphology provides a new method for revealing hydrate growth behavior at pore-scale. The wettability characterization offers key parameters for accurate numerical simulation of multiphase flow prediction and seepage behavior in GHBS. • Micro-CT scanning is used to acquire the 3D structure of gas hydrate bearing sediments. • Hydrate morphology is visualized at pore scale and the shape factors of hydrate clusters are calculated. • The local contact angles at three-phase interfaces are measured and show a normal distribution. [ABSTRACT FROM AUTHOR]

Numerous models have been developed for prediction of gas hydrate saturation based on the microstructural relationship between gas hydrates and sediment grains. However, quantification of hydrate saturation and morphology from elastic properties has been hindered by failing to account for complex hydrate distributions. Here, we develop a generalized effective medium model by applying the modified Hashin-Shtrikman bounds to a newly developed cementation theory. This model is validated by experimental data for synthetic methane and tetrahydrofuran hydrates. Good comparison of model predictions with experimental measurements not only reveals its ability to merge the results of contact cementation theory and effective medium theory, but also indicates its feasibility for characterizing complex morphologies. Moreover, the results of inverting acoustic measurements quantitatively confirm that for synthetic samples in "excess-gas" condition gas hydrates mainly occur as a hybrid-cementing morphology with a low percentage of pore-filling morphology, whereas for pressure-core hydrate-bearing sediments in natural environments they exist as matrix-supporting and pore-filling morphologies with a very low percentage of hybrid-cementing morphology. The hydrate saturations estimated from sonic and density logs in several regions including northern Cascadia margin (Integrated Ocean Drilling Program Expedition 311, Hole U1326D and Hole U1327E), Alaska North Slope (Mount Elbert test well) and Mackenzie Delta (Mallik 5L-38), are comparable to the referenced hydrate saturations derived from core data and resistivity, and/or nuclear magnetic resonance log data, confirming validity and applicability of our model. Furthermore, our results indicate that ~8% hybrid-cementing, ~33% matrix-supporting and ~59% pore-filling hydrates may coexist in the fine-grained and clay-rich marine sediments on the northern Cascadia margin, whereas ~10% hybrid-cementing, ~54% matrix-supporting and ~36% pore-filling hydrates may coexist in the coarse-grained and sand-dominated terrestrial sediments of the Alaska North Slope and Mackenzie Delta. • A modified cementation theory is developed by introducing generalized pressure-dependent normalized contact-cemented radii. • A generalized effective medium model is proposed to merge the effective medium theory and cementation theory. • Modeling and inversion schemes are proposed to quantify hydrate saturation and morphology from laboratory and well-log data. • Hydrates mainly grow as matrix-supporting form (~54%) in sands and as pore-filling form (~59%) in clay-rich marine sediments. [ABSTRACT FROM AUTHOR]

Gas hydrates often exhibit several microscopic morphologies within the host sediments, which subsequently affect their elastic responses. Hence, good knowledge of hydrate morphology is essential for better understanding elastic responses of gas hydrate reservoirs and accurately estimating hydrate saturation. To detect possible hydrate morphology and quantify the hydrate saturation and other reservoir parameters (e.g., free gas saturation, porosity, clay content) directly from well logs or seismic data, we have developed a new rock physics inversion scheme based on two types of templates honoring the hydrate morphology. Through analysis of the developed rock physics templates, we found that gas hydrate reservoirs have different elastic behaviors for different hydrate morphologies. Ignoring the impact of hydrate morphology is insufficient in modeling the elastic properties of hydrate-bearing sediments and predicting hydrate saturation. Results of Mount Elbert test well at North Slope of Alaska show that hydrates are mainly presented as pore-filling form and the reservoir parameters estimated from 3D elastic template inversion are comparable to the references derived from core data or other well logging interpretations. Based on the attribute template inversion scheme, the results suggest that hydrates mainly occur as matrix-supporting or matrix-inclusion morphology and the hydrate saturation ranges from 20% to 40%, and free gas saturation varies from 0% to 10% for uniform distribution and from 10% to 35% for patchy distribution near the bottom simulating reflectors (BSR) at the Makran Accretionary Prism, Arabian Sea. • Attribute and elastic rock physics templates are constructed for different hydrate morphologies using theoretical models. • The effect of gas hydrate morphology on elastic properties is investigated in detail. • Reservoir parameters of gas hydrate-bearing sediments are estimated by using 3D elastic rock physics template. • AVO attribute templates allow for diagnosing hydrate morphology and quantifying hydrate saturation. [ABSTRACT FROM AUTHOR]