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Graphical abstract Highlights • The N 2 O hydrate formation behavior in porous sediments is followed through MRI. • A sufficient contact between gas and water is crucial for hydrate formation. • A moderate initial P followed by a pressurization are suggested for sequestration. Abstract Increasing nitrous oxide (N 2 O) emissions and their potential impacts on the ozone layer are of worldwide concern. Hydrate-based sequestration technology is considered to be a promising method for N 2 O control. This study investigated the conditions required for N 2 O sequestration through forming hydrates. Magnetic resonance imagining (MRI) was used to capture and quantify N 2 O hydrate formation in porous media simulated by fine quartz sands. The effect of the sequestration conditions on hydrate formation were investigated, involving pressure (1.5, 2, and 2.5 MPa), temperature (275.15, 276.15, and 277.15 K) and initial water saturation (25.06, 33.23, 42.13, and 50.81%). The results indicated the inhomogeneous formation and distribution of N 2 O hydrates in pore spaces. Initial water saturation and pressure were found to significantly impact the hydrate distribution during the initial formation stage. Moreover, N 2 O hydrates formed more rapidly in sediments with a lower water saturation and temperature, indicating that sufficient contact between gas and water and a higher driving force are crucial for abundant hydrate formation. A higher initial water saturation could help improve the sequestration efficiency, which would play a crucial role in locating the sequestration site. This work could provide some insights of the mechanism of N 2 O sequestration via hydrate formation in sediments, making contributions to potential applications in field tests. [ABSTRACT FROM AUTHOR]

The purpose of this work is to develop a permeability estimation method for porous media. This method is based on an improved capillary bundle model by introducing some pore geometries. We firstly carried out micro-CT scans to extract the 3D digital model of porous media. Then we applied a maximum ball extraction method to the digital model to obtain the topological and geometrical pore parameters such as the pore radius, the throat radius and length and the average coordination number. We also applied a random walker method to calculate the tortuosity factors of porous media. We improved the capillary bundle model by introducing the pore geometries and tortuosity factors. Finally, we calculated the absolute permeabilities of four kinds of porous media formed of glass beads and compared the results with experiments and several other models to verify the improved model. We found that the calculated permeabilities using this improved capillary bundle model show better agreement with the measured permeabilities than the other methods. [ABSTRACT FROM AUTHOR]

The geological sequestration of carbon dioxide (CO2) is considered a promising technology for CO2 emissions. However, accurate estimation of the storage capacity for CO2 remains a challenge owing to the effect of capillary pressure variation. The CO2-salty aqueous contact angle and sub-core scale heterogeneity are the major factors that influence capillary pressure. This paper reports the results of CO2/brine two-phase flow experiments in porous media by using an X-ray computed tomography (CT) technique at 40°C and 8 Mpa and examines the strong influence of wettability and sub-core scale heterogeneities on the spatial distribution of CO2. Gas injection experiments were conducted at flow rates of 0.1 mL/min, 0.2 mL/min, and 0.5 mL/min, and the porous media were packed with feldspar, quartz, or dolomite. This study indicates that at the core scale, the wettability of porous media is a crucial factor in determining CO2 saturation. Capillary fingering and low CO2 saturation occurred with high water wettability. Sub-core scale heterogeneities controlled the CO2 distribution and saturation variation along the porous media, particular that with small pore sizes. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd [ABSTRACT FROM AUTHOR]

Natural gas hydrates dissociate via an endothermic process. One of the key requirements for any production technique is to supply the heat necessary for this dissociation. In this study, first, a microwave stimulation model for the production of gas from methane hydrate sediment is developed, which includes mass transport, energy conversion and conservation, and intrinsic kinetic reactions as the governing equations. In addition, the theoretical mixing rule of Lichtenecker and Rother is introduced for calculating the average dielectric data of the sediment containing methane hydrates, which affects the penetration of microwaves into the sediment. Next, simulations are performed for investigating gas production, as well as effects of initial water saturation, initial hydrate saturation, and sediment thermal properties induced by microwave stimulation. Moreover, the energy efficiency ratio is employed in the simulation. The simulation results show that microwave stimulation provides timely energy conversion sufficient for promoting the dissociation of hydrates, with rapid, continuous gas production. Temperature gradients caused by the decrease of the microwave penetration depth appear in the reservoir, leading to a rapid dissociation rate in the upper part of the sediment. The energy efficiency ratio for all simulations ranges between 3.752 and 6.452. Hydrate saturation and the specific heat capacity of porous media are two factors that significantly affect energy efficiency. High hydrate saturation contributes to a rapid gas generation rate and a long rapid gas generation lasting time, leading to a high energy efficiency. A low specific heat capacity of porous media can decrease the loss of heat to the sediment, increase the gas generation rate, and improve energy efficiency. Furthermore, high initial water saturation can cause a rapid decrease in the microwave penetration depth and lead to large temperature gradients in the sediment. The thermal conductivity of porous media mainly affects gas generation in the latter period of hydrate dissociation. Macroscopic heat conduction in the sediment decreases the temperature gradients in the reservoir and promotes homogeneous hydrate dissociation. [ABSTRACT FROM AUTHOR]

Because of the clean burning and minimal pollution generation of natural gas hydrates, they are viewed as premium fuel energy. The gas and water flow is highly dependent on the seepage properties of the hydrate sediments; therefore, the permeability is a key parameter for estimating the gas production of hydrate reservoirs. Pore network models combined with X-ray computed tomography (CT) were proposed to analyze the index properties and percolation characteristics of porous media containing hydrates in our previous study. This study extends our previous investigation into the influences of particle size and porosity on permeability. The results suggest the porosities of hydrate-bearing porous media formed by different-size particles, as calculated by both the pore network models and the volumetric method, are in agreement. Moreover, the hydrate-bearing porous media formed by larger-size particles has larger porosity, smaller capillary pressure, and larger absolute permeability. In addition, under the same degree of water saturation, larger porosity induced a larger water-phase relative permeability, but a smaller gas-phase relative permeability. [ABSTRACT FROM AUTHOR]

It is of great importance to investigate gas and water flow characteristics to better understand the geological CO2 sequestration process. The interfacial tension between CO2 and brine and the wettability of reservoir rocks are the most important parameters for two-phase flow in porous media that have a significant influence on the capacity of CO2 storage. In this paper, we present a set of interfacial tension data between CO2 and a sodium chloride (NaCl) solution and the contact angle data between CO2, NaCl and quartz by using a visual experimental method at multiple pressure, temperature and salinity conditions. We also performed simulations of CO2 and NaCl solution two-phase flow in quartz bead-packed beds by introducing interfacial tension and contact angle data into the pore network model to evaluate the relative permeability and capillary pressure. We found that the phase alternation of CO2 from a gas to its supercritical state has a significant influence on the wettability of pore surfaces and thus governs the capillary trapping mechanism. This indicates that CO2 sequestration efficiency in a saline aquifer will be greatly affected while CO2 is in its different phase states. The simulation results showed that a pore network model is available for the study of CO2 and brine two-phase flow characteristics in porous media. © 2015 American Institute of Chemical Engineers Environ Prog, 34: 1756-1762, 2015 [ABSTRACT FROM AUTHOR]

The permeability of hydrate-bearing porous media is one of the most important factors in predicting fluid flow behavior, fluid distribution, and gas performance during hydrate exploitation. The wettability of porous media greatly affects the relative permeability of two-phase gas-water mixtures. In this paper, we extend our previous investigation into the effects of the wettability of porous media containing hydrates on their seepage properties using pore network models combined with X-ray computed tomography (CT). The simulation results show that in a uniform wetting system, increasing the contact angle reduces the wettability of hydrate-bearing porous media, increases the relative permeability of the water phase, and decreases the relative permeability of the gas phase at a given water saturation. In addition, increasing the diameter of quartz sand particles notably enhances these changes in the gas-water phase relative permeability. In a fractional-wetting system, increasing the fractional wettability and connectivity effectively reduces the residual gas saturation. Moreover, the capillary pressure distributions are highly dependent on the fractional wettability and quartz sand size. [ABSTRACT FROM AUTHOR]

In this work, magnetic resonance imaging (MRI) was employed to observe the in-situ formation and dissociation of methane hydrates in porous media. Methane hydrate was formed in a high-pressure cell with controlled temperature, and then the hydrate was dissociated by thermal injection. The process was photographed by the MRI, and the pressure was recorded. The images confirmed that the direct visual observation was achieved; these were then employed to provide detailed information of the nucleation, growth, and decomposition of the hydrate. Moreover, the saturation of methane hydrate during the dissociation was obtained from the MRI intensity data. Our results showed that the hydrate saturation initially decreased rapidly, and then slowed down; this finding is in line with predictions based only on pressure. The study clearly showed that MRI is a useful technique to investigate the process of methane hydrate formation and dissociation in porous media. [ABSTRACT FROM AUTHOR]

The induction time of hydrate formation is an important parameter during gas exploitation, which allows secondary natural gas hydrates to form, and in the improvement of hydrate-based technologies. The effects of the memory effect, subcooling, and the particle size of the porous media on the induction time for tetrahydrofuran (THF) hydrate formation were investigated in this study. Six test tubes were used as reactors when forming the hydrates in porous media. The experimental results indicated that that the memory effect and the use of extreme sub-cooling as a strong driving force can reduce the induction time effectively. Extreme sub-cooling can strengthen the driving force for hydrate formation. The relationship between the induction time and subcooling is less pronounced when the THF hydrate forms from hydrate-dissociated water. The average induction time for the formation of the THF hydrate from hydrate-dissociated water was 27.5 min in BZ-06 when using a 3 mol % THF solution. The induction time can be controlled easily when the driving force increases to a threshold value. The average induction time decreases as the particle size increases. Porous media with larger particle sizes can control the induction time and reduce the stochastic nature of the induction time. [ABSTRACT FROM AUTHOR]

A clear understanding of two-phase flows in porous media is important for investigating CO geological storage. In this study, we conducted an experiment of CO/brine flow process in porous media under sequestration conditions using X-ray CT technique. The flow properties of relative permeability, porosity heterogeneity, and CO saturation were observed in this experiment. The porous media was packed with glass beads having a diameter of 0.2 mm. The porosity distribution along the flow direction is heterogeneous owing to the diameter and shape of glass beads along the flow direction. There is a relationship between CO saturation and porosity distribution, which changes with different flow rates and fractional flows. The heterogeneity of the porous media influences the distribution of CO; moreover, gravity, fractional flows, and flow rates influence CO distribution and saturation. The relative permeability curve was constructed using the steady-state method. The results agreed well with the relative permeability curve simulated using pore-network model. [ABSTRACT FROM AUTHOR]

Gas hydrates can be free floating in the sediment matrix; contacting, but do not cement; actually cementing and stiffening the sediment. The microstructure and growth of gas hydrates (CH 4 and CO 2 ) in porous media were observed in situ by Magnetic resonance imaging (MRI). The experimental results confirmed the random nucleation of gas hydrates in porous media. The results show that hydrates are preferentially formed in the pore spaces. Hydrates then occupy the pores of porous media gradually, but do not cement the adjacent glass beads in the final stage. Gas hydrates are believed growing as free-floating model in porous media. [ABSTRACT FROM AUTHOR]

The study of formation and dissociation of CO hydrate in porous media was characterized by magnetic resonance imaging (MRI) system in in situ conditions. This work simulated porous media by using glass beads of uniform size. The growth and dissociation habit of CO hydrate was observed under different temperature and pressure conditions. The induction time and the hydrate saturation during the growth and dissociation process in different sizes of porous media were obtained by using the MRI signal intensity. The results indicate that hydrate growth rate and the induction time are affected by the size of porous media, pressure, and degree of supercooling. There are three hydrate growth stages, i.e., initial growth stage, rapid growth stage and steady stage. In this study, the CO hydrate forms preferentially at the surface of vessel and then gradually grows inward. The hydrate tends to cement the glass beads together and occupies the pore gradually. As the hydrate decomposes gradually, the dissociation rate increases to the maximum and then decreases to zero. [ABSTRACT FROM AUTHOR]

Abstract: Tetrahydrofuran (THF) hydrate has long been used as a substitute for methane hydrate in laboratory studies. This article investigated the formation and dissociation characteristics of THF hydrate in porous media simulated by various-sized quartz glass beads. The formation and dissociation processes of THF hydrate are observed using magnetic resonance imaging (MRI) technology. The hydrate saturation during the formation is obtained based on the MRI data. The experimental result suggests that the third surface has an effect on hydrate formation. THF hydrate crystals lean to form on the glass beads and in their adjacent area as well as from the wall of the sample container firstly. Furthermore, as the pore size diminishes, or as the formation temperature decreases, the nucleation gets easier and the formation processes faster. However, the dissociation rate is mostly dependent on the dissociation temperature rather than on the pore size. [Copyright &y& Elsevier]

The flow characteristics during decomposition of hydrate-bearing sediments are the most critical parameters for the gas recovery potential from natural gas hydrate reservoirs. The absolute and relative permeability and the flow field distribution during the decomposition process of hydrate-bearing porous media synthetically created by glass beads are in-situ measured by using magnetic resonance imaging. The absolute permeability value increased slowly, then became stable after the decomposition amount was 50%. The relative permeability change curve is a typical X-shaped cross curve. As the hydrate decomposed, the relative permeability values of the two phases increased, the range of the two-phase co-infiltration zone increased with the increase of relative permeability at the endpoint, and the coexistence water saturation decreased. At the beginning of the decomposition, (hydrate content 100% to 70%), the relative permeability of methane and water rose rapidly from 22% to 51% and from 58% to 70%, respectively. When the amount of the remaining hydrate was less than 50%, the relative permeability curve of the hydrate-bearing glass beads almost kept unchanged. During the hydrate decomposition process, the velocity distribution was very uneven and coincided with the porous media structure. [ABSTRACT FROM AUTHOR]

Wettability design is of crucial importance for the optimization of multiphase flow behaviour in gas diffusion layers (GDLs) in fuel cells. The accumulation of electrochemically-generated water in the GDLs will impact fuel cell performance. Hence, it is necessary to understand multiphase displacement to design optimal pore structures and wettability to allow the rapid flow of gases and water in GDLs over a wide saturation range. This work uses high-resolution in situ three-dimensional X-ray imaging combined with a pore network model to investigate the breakthrough capillary pressure and water saturation in GDLs manufactured with different mass fractions of polytetrafluoroethylene coating: 5, 20, 40, and 60%, making them more hydrophobic. We first demonstrate that the pore network extraction method provides representative networks for the fibrous porous media examined. Then, using a pore-network flow model we simulate water invasion into initially gas-filled fibrous media, and analyze the effect of wettability on breakthrough capillary pressure and water saturation. With an appropriate pore-scale characterization of wettability, a pore network model can match experimental results and predict displacement behaviour. • Pore network modelling is extended to study fibrous materials. • The model estimates water breakthrough pressure accurately. • The model is extended to study primary imbibition and drainage in mixed-wet media. • A representative wettability can be determined to match experiment. [ABSTRACT FROM AUTHOR]

To investigate the gas production from methane hydrate-bearing sediments, the gas production processes from methane hydrate in porous media using depressurization, two-cycle warm-water injection and a combination of the two methods were characterized in this study. The methane hydrates were formed in porous media with various initial hydrate saturation ( S hi ) in a pressure vessel. The percentage of gas production, rate of gas production, and energy efficiency were obtained and compared using the three methods. The driving force of the hydrate dissociation at different stages of depressurization was analyzed and ice formation during the gas production was observed. For the two-cycle warm-water-injection method, the percentage of gas production and the energy efficiency increased with increasing of S hi . However, due to the large amount of warm water needed to heat the porous media at the dissociation site, the percentage of gas production was lower than the other two methods under the same experimental conditions. The experimental results proved that the combined method had obvious advantages for hydrate exploitation over the depressurization and warm-water-injection method in terms of the energy efficiency, percentage of gas production and average rate of gas production, and with increasing of S hi , the advantages are enhanced. For the S hi of 51.61%, the percentage of gas production reaches 74.87%, which had increments of 18.63% and 31.19% compared with the depressurization and warm-water-injection methods. The energy efficiency for the combined method were 31.47, 49.93 and 68.13 for S hi of 31.90%, 41.31% and 51.61%, respectively. [ABSTRACT FROM AUTHOR]

• Overhead arrangement of temperature-controlled piping makes the hydrate in-situ generator suitable for Nano-CT. • The in-suit depressurization and seepage lead to different hydrate redistribution patterns. • Formation of new "column cementing" forms by hydrates after the end of seepage. The spatial distribution characteristics of natural gas hydrates in porous media and its phase transition mechanism under different environmental conditions are the key bases for achieving efficient exploitation. In this study, the evolution of natural gas hydrate morphology in porous media under depressurization and seepage was observed using X-ray computed tomography (CT). The experimental results show that after the depressurization, the hydrate near the depressurization point is preferentially decomposed. The resulting memory water migration induces the hydrate to show a bottom-up, center-to-side re-distribution pattern. In addition, the gas seepage accompanying the gas production process is also worthy of attention. The heat carried by the seeping gas destabilizes the hydrate and provides a forced convective environment. This results in the initially cemented hydrate being decomposed and re-distributed in the opposite direction to the gas flow, forming a new "column cemented" structure. This study clarifies the mechanism of the influence of gas–water transport on the hydrate distribution pattern during natural gas hydrate exploitation, which is of great significance for the development of natural gas hydrate exploitation strategies. [ABSTRACT FROM AUTHOR]

CO 2 carbon sequestration in marine gas hydrate occurrence areas is a potential way to mitigate the continued growth of CO 2 in the atmosphere. The organic matters of marine sediment can affect the phase equilibrium conditions of CO 2 hydrate in salt system, and the organic salt system in pore medium has effect on the nucleation growth of CO 2 hydrate. The organic matters on the surface of the sediment is mainly composed of lipids, proteins, carbohydrates, and unsaturated hydrocarbon organic compounds, and these polar functional groups weaken the inhibitory effect of salt ions on hydrates through coordination bonds and Ca ion complexation. The nucleation kinetics of CO 2 hydrate under organic salt system in pore medium in marine environment is an important parameter determining the carbon sequestration of hydrate. In addition, experimental results show that clay surface organic matters (CSOM) can reduce the surface tension of pore water, and thus pore water migration occurs during hydrate growth and expansion. The basic background in our findings will bring a much better recognition of phase equilibrium conditions among the offshore marine sediment CO 2 sequestration process and further assess the location of potential marine CO 2 storage sites. • Hydrate-based CO 2 sequestration is utilized for stable sequestration. • Phase equilibrium and nucleation growth of CO 2 hydrate in organic/salt systems are investigated. • Coordination between organics and Ca2+ weakened the inhibitory effect of salt ions on hydrates. • Changes in phase equilibrium play effective role in CO 2 hydrates nucleation growth in pore media. [ABSTRACT FROM AUTHOR]

Extensive efforts have been made regarding gas hydrate sample reconstruction in the laboratory for a better understanding and development of natural gas resources. Magnetic resonance imaging (MRI) is a useful method for directly observing the reconstruction of methane hydrate, yet relevant studies remain limited. In this study, a 9.4-T 400-MHz MRI instrument was employed to investigate CH 4 hydrate formation in porous media involving various initial water saturation levels and sand diameters. Pressure histories and MRI signal variations were monitored to discuss the process of CH 4 hydrate growth, and the three main formation stages of induction, rapid growth, and slow formation were determined. Furthermore, the liquid water performance in MRI micro-images was analyzed to predict the characteristics of CH 4 hydrate formation. The results indicated that CH 4 hydrate formed in a spatially and temporally random manner and that pore plugging occurred owing to the residual water encased in grown hydrate. Additionally, phase saturations, water conversion percentages, and formation rates were defined to evaluate the effect of sand diameter and initial water saturation on CH 4 hydrate formation. With the reduction in the diameter of quartz glass beads from 400 μm to 100 μm, the average hydrate formation rate increased from 0.0010 min−1 to 0.0034 min−1, respectively. When the initial water saturation decreased to the optimized value (0.22 in this study), the water conversion percentage and hydrate saturation increased. [ABSTRACT FROM AUTHOR]

An understanding of the nucleation and growth mechanism of methane hydrate in porous space is essential for exploitation and application of hydrates, but the mechanism is yet to be clarified. Magnetic resonance imaging (MRI) was employed to visually analyze the spatial and temporal formation behavior of methane hydrate in a porous media. Detailed information about the water distribution, initial nucleation sites, and hydrate growth was obtained, in addition to MRI images. The results demonstrated that the water molecules distributed in the vertical direction preferred the middle slice of a porous medium sample, and the decrease in the number of molecules in the middle slice and on both sides of the slice was similar during hydrate formation. The formation process are quite different in selected horizontal slices, which were contributed to the various distribution of water and gas in pore spaces and the randomness of methane hydrate formation. The extension of these predicted results could have important implications for optimizing the formation processes of gas hydrate in hydrate-based technologies. [ABSTRACT FROM AUTHOR]

Natural gas hydrates are a potential future energy modality with the advantages of clean burning and large resource reserves and have attracted worldwide attention. Natural gas hydrates formation and dissociation impact the skeletal structure and mechanical properties of porous media sediments. In addition, there is a synergistic effect between the migration of fine clay particles in porous media and hydrate behavior. In this study, methane hydrate was examined in-situ using a low-field nuclear magnetic resonance system in the presence of two suspensions of clay particles with different stability. The results showed that clay particles impacted methane hydrate formation and dissociation and the pore structure of porous media. Hydrate nucleated preferentially in large pores, causing them to split into smaller pores; the presence of clay particles, especially illite, improved the water conversion rate. The results indicated that water content in large pores increased after hydrate dissociation but was discontinuous among different pores. When illite was present, the distribution was more continuous; when montmorillonite was present, the water distribution of the large pores was similar to that of the original state. This work increases our understanding of the kinetics, water migration, and pore structure alteration of methane hydrate formation and dissociation in sediments with clay particles and provides support for the safe and efficient development of natural gas hydrates. • Clay particles impacted methane hydrate formation and dissociation • Hydrate nucleated preferentially in large pores • Clay particles, especially illite, improved the water conversion rate • Water content in large pores increased after hydrate dissociation but was discontinuous among different pores [ABSTRACT FROM AUTHOR]

[Display omitted] • The max total interaction potential between the montmorillonite particles (358 k b T) was 8.3 times that of the illite particles (43 k b T) in seawater. • The presence of clay particles and especially illite particles could promote CO 2 hydrate formation. • The pore structure would be changed during hydrate formation and dissociation. CO 2 storage in marine sediments in the form of solid hydrates with high storage capacity and stability is a promising approach for the control of greenhouse gas pollution. Specifically, clay is much abundant in marine minerals, and multiple forces exist between fine clay particles suspended in seawater, significantly affecting the hydrate formation and dissociation behavior. This study investigates the phase transition performance of CO 2 hydrates with or without montmorillonite and illite particles using low-field nuclear magnetic resonance. According to the DLVO theory, the total interaction potential between the montmorillonite particles reached 358 k b T, with that between illite particles being only 43 k b T. It was found that water in large pores was preferentially consumed during hydrate formation, with clay particles significantly promoting the conversion of water into hydrate. In seawater-montmorillonite and seawater-illite solutions, a higher number of water molecules was detected in the small and medium pores, respectively. Notably, the initial pore structure changed upon hydrate dissociation, with more medium pores formed. This effect was enhanced in the seawater-montmorillonite solutions and weakened in seawater-illite solutions. The findings could further clarify the formation and dissociation kinetics of CO 2 hydrates in the presence of clay particles, as well as the geological structure evolution and safety of CO 2 hydrate storage under the seafloor. [ABSTRACT FROM AUTHOR]

The hydrate reformation that occurs in natural gas hydrate (NGH) exploration reduces mining efficiency and safety. To elucidate the hydrate formation/reformation characteristics during NGH exploration, methane hydrate (MH) was formed/reformed in two different modes to simulate mining of NGH sediment. The effects of residual water, residual MH and methane flow rate on MH reformation in a porous medium were investigated experimentally. Magnetic resonance imaging (MRI) was used to analyze MH saturation and distribution in the porous medium. In reformation, a positive correlation exists between the hydrate saturation (S h ) increment and MH dissociation. Moreover, the percentage of reformation S h increment increases with the methane injection rate. That demonstrates MH dissociation by depressurization improves the contact area of gas-liquid and enhance the nucleation rate, which contributes to hydrate reformation. In addition, the residual S w and MH reformation rate maximum (R max ) are positively correlated in the rapid-reformation period. According to MRI images, crack-like pathways exist in the porous medium after MH dissociates completely in the first experimental mode. However, constantly flowing methane in the MH reformation process can render the water distribution uniform after MH dissociation in the second experimental mode. That means the methane flow affects the capillary force distribution then further influences the pore water distribution in porous medium. [ABSTRACT FROM AUTHOR]

As a potential alternative strategic energy source, and for their potential impact on global climate, natural gas hydrates have garnered worldwide attention. This study explored the measurement of effective thermal conductivity of methane hydrate-bearing sediments and evaluated existing models for the prediction of effective thermal conductivity. A thermistor-based method combined with Micro-CT observations was employed in the determination of the effective thermal conductivity of porous matrix materials with various hydrate and water saturation levels and physical characteristics. The effects of sample component characteristics, including the volume content of hydrate and water, phase conversion, and properties of porous materials, on the effective thermal conductivity of hydrate-bearing sediments were systematically evaluated. The effective thermal conductivity positively correlated with hydrate saturation, water content, and the thermal conductivity of porous media. In addition, the effective thermal conductivity slightly increased with hydrate dissociation, indicating an increasing heat transfer capacity during gas production. Existing prediction models were evaluated using our measured results, and a hybrid model combining the parallel and series models was proposed with expanded applicability to the scope of our research. The feasibility of the proposed model was also verified in a comparison with previous research. The results of this study are important for future investigation of the actual thermal properties of hydrate-bearing sediment and the understanding of the heat transfer mechanism during gas production. Furthermore, the results can provide guidance in the selection of an optimal technique for gas production from hydrate deposits at the field scale. [ABSTRACT FROM AUTHOR]

• Aerogels can be easily fabricated via cross-linking vermiculite and MXene. • The pore structures and surface compositions of aerogels can be finely tuned. • Monolithic aerogel has excellent CO 2 storing capacity by boosted hydrate formation. • Degree of water saturation plays the most vital role in hydrate formation. Gas hydrate provides an ideal way for CO 2 capture and storage using water by forming cages via an environment-friendly and energy-efficient hydrate formation process. However, the practical utilization and upscaling of hydrate-based gas storage are impeded by the slow formation kinetics of gas hydrate due to the limited multiphase interface for mass transfer and reaction. Herein, we demonstrated a simple strategy to fabricate composite aerogels assembled of natural vermiculite and MXene nanosheets as the ideal substrate for boosting CO 2 hydrate formation. The structure and surface compositions of vermiculite/MXene composite aerogels were analyzed by XRD, FT-IR, SEM, and XPS. It shows that monolithic pore structures and surface functional groups can be finely tuned by controlling the mass ratio of vermiculite and MXene, leading to an outstanding CO 2 storage capacity of 136.9 v/v (corresponding to 0.121 mol CO 2 /mol water) with enhanced hydrate formation kinetics. The degree of water saturation plays the most vital role in controlling formation kinetics and gas storage capacity. This work provides a reliable method to synthesize aerogels for boosting CO 2 storage via enhanced hydrate formation and sheds light on the structure performance relationship of porous materials for enhancing gas hydrate formation. [ABSTRACT FROM AUTHOR]

Hydrate-based desalination has attracted considerable attention as an innovative desalination process without extra pollution; however, the slow kinetics of hydrate formation and the difficulty of separating solid hydrate from liquid brine hinder the industrialization of this technology. Hydrates exhibit water absorption effects and can be formed above silica sand beds to promote separation. Unfortunately, the mechanism underlying the self-driven and continuous directional transport of water during hydrate formation is unclear. In this study, the spatial and temporal formation behavior of hydrate in quartz glass beads were observed using a nuclear magnetic resonance system. The results revealed that not all types of guest molecules capable of forming hydrates exhibited the hydrate water absorption effect. The interfacial tension between the hydrate and bound water provided capillary driving force for water migration. Large particle sand had larger pore channels for water migration and low initial water saturation will inhibited water migration. The results of this study improve the understanding of water migration during hydrate formation in porous media. Our findings have significant potential for the application in desalination, sludge dewatering processes, and gas capture. • Not all types of hydrate guest molecules exhibit the hydrate water absorption effect. • Mechanism underlying the self-driven and continuous directional transport of water is analyzed. • Gases with low solubility tend to nucleate at the gas–liquid interface. • The capillary force of water in a porous structure drives water migration. • Sands with large particle sizes have large pore channels, which promote migration. [ABSTRACT FROM AUTHOR]

The induction time for the formation of hydrates is an important parameter in hydrate-based technology and natural gas hydrate exploitation. The influences of effective factors on induction time are not explained clearly in the current. To obtain data on the induction time for hydrate formation and re-formation processes, experimental studies were carried out for methane hydrates formed in porous media. The vessel used is divided into six compartments of the same size. The temperatures of different regions in the vessel can be measured during experiments. Six T-type thermocouples and a pressure transducer (Nagano Keiki, Japan) were connected to the vessel to measure the temperature (T) and pressure (P), respectively. Forty-eight experimental cycles were performed at 6 MPa and at temperatures from 273.65 K to 276.65 K. The influences of the memory effect, initial water saturation, and hydrate dissociation temperature and dissociation time on the CH 4 hydrate formation/reformation induction time are studied. The experimental results showed that there is a memory effect when the experimental temperature is greater than 275.15 K at 6 MPa and that the memory effect reduces the hydrate formation induction time. Long dissociation times and high dissociation temperatures eliminate the memory effect. Low experimental temperature led to a short induction time, and the influence of experimental temperature on the induction time was reduced when the experimental temperature was less than 275.65 K. The initial saturation also affected the induction time. The induction time for the hydrate formation decreased as the initial water saturation increased from 30% to 70%. The measured induction times were correlated to an induction time model developed based on the nucleation theory. The results obtained in this work indicate that hydrate formation in porous media is heterogeneous nucleation. [ABSTRACT FROM AUTHOR]

Gravity drainage characteristics are important to improve our understanding of gas–liquid or liquid–liquid two-phase flow in porous media. Stable or unstable displacement fronts that controlled by the capillary force, viscous force, gravitational force, etc., are relevant features of immiscible two-phase flow. In this paper, three dimensionless parameters, namely, the gravity number, the capillary number and the Bond number, were used to describe the effect of the above mentioned forces on two-phase drainage features, including the displacement front and final displacing-phase saturation. A series of experiments on the downward displacement of a viscous fluid by a less viscous fluid in a vertical vessel that is filled with quartz beads are performed by using magnetic resonance imaging (MRI). The experimental results indicate that the wetting properties at both high and low capillary numbers exert remarkable control on the fluid displacement. When the contact angle is lower than 90°, i.e., the displaced phase is the wetting phase, the average velocity V f of the interface of the two phases (displacement front velocity) is observably lower than when the displaced phase is the non-wetting phase (contact angle higher than 90°). The results show that a fingering phenomenon occurs when the gravity number G is less than the critical gravity number G ’ = Δμ /μ g . Moreover, the higher Bond number results in higher final displacing-phase saturation, whereas the capillary number has an opposite effect. [ABSTRACT FROM AUTHOR]

Molecular diffusion has been considered to be an underlying mechanism for many oil recovery processes. Reliable estimation of the molecular diffusion coefficient as a transport property is therefore important for CO 2 -enhanced oil recovery. In the present work, the dynamic processes of CO 2 diffusion in bulk n-decane and n-decane saturated porous media were investigated using the micro-focus X-ray CT (micro-CT) scanning technique. CO 2 diffusion was visually and quantitatively analyzed by interpreting the CO 2 concentration with grayscale CT images. Next, local CO 2 diffusion coefficients, varying with time and position, were calculated from concentration profiles based on Fick's second law. The results showed that the local diffusion coefficients in bulk n-decane demonstrate an exponential function of diffusion distance and time. The total diffusion coefficients in bulk oil under pressure from 1 to 6 MPa and temperature at 29 °C and 35 °C were calculated. The results showed that the initial pressure has a strong influence on the diffusion coefficient, i.e., high CO 2 initial pressure leads to high CO 2 diffusivity in oil. Experiment results in n-decane saturated porous media showed that the CO 2 local diffusion coefficient decreases gradually along the diffusion path with time until reaching a stable state. The total diffusion coefficients in n-decane saturated porous media were smaller than those in bulk oil under the same pressure and temperature conditions because the diffusion path is more complicated than in bulk oil. It is demonstrated that the pathways of porous media impede CO 2 mass transfer and decrease the diffusion coefficient. [ABSTRACT FROM AUTHOR]

Natural gas hydrates (NGHs) are a promising energy source with huge reserves. The dissociation characteristics of NGHs need to be clarified further for developing safe and efficient technology for its recovery. In this study, Classes 1 and 2 NGH deposits were simulated by forming methane hydrate (MH) in porous media, and MH dissociation induced by depressurization was investigated using magnetic resonance imaging (MRI). MRI showed the liquid water distribution, which was used to analyze MH formation and dissociation. The vessel pressure was also measured during the experiments, which was compared with the MRI mean intensity of liquid water. MH dissociation processes were measured and analyzed under different backpressures, from 2.2 to 2.8 MPa. It was observed that liquid water hindered methane gas output during gas production; hydrate dissociation caused the movement of some liquid water, which usually led to fluctuations in MRI signal intensity. The experimental results also indicated that the MH dissociation pattern was affected by heat transfer; although a larger depressurization range led to faster dissociation, the average dissociation rate was controlled by heat transfer. [ABSTRACT FROM AUTHOR]

The study of immiscible fluid displacement between aqueous-phase liquids and non-aqueous-phase liquids in porous media is of great importance to oil recovery, groundwater contamination, and underground pollutant migration. Moreover, the attendant viscous, capillary, and gravitational forces are essential to describing the two-phase flows. In this study, magnetic resonance imaging was used to experimentally examine the detailed effects of the viscous, capillary, and gravitational forces on water–oil flows through a vertical straight capillary, bifurcate channel, and monolayered glass-bead pack. Water flooding experiments were performed at atmospheric pressure and 37.8 °C, and the evolution of the distribution and saturation of the oil as well as the characteristics of the two-phase flow were investigated and analyzed. The results showed that the flow paths, i.e., the fingers of the displacing phase, during the immiscible displacement in the porous medium were determined by the viscous, capillary, and gravitational forces as well as the sizes of the pores and throats. The experimental results afford a fundamental understanding of immiscible fluid displacement in a porous medium. [ABSTRACT FROM AUTHOR]

The sequestration of CO 2 in natural gas hydrate reservoirs is an attractive idea for both yielding CH 4 and reducing CO 2 emissions. To investigate the kinetics of CO 2 –CH 4 gas mixture hydrate formation and dissociation, two types of CO 2 –CH 4 gas mixtures were applied to form hydrates in porous media with different porosities. The process of hydrate formation and dissociation was monitored in situ by magnetic resonance imaging (MRI) in this study. Hydrate saturation was calculated quantitatively using the mean intensity ( MI ) of each image. The results showed that the hydrates first formed in the centre of the porous media and then grew along the axis of the vessel. The hydrate saturation and the initial water saturation were linearly correlated when the initial water saturation changed from 5% to 40%. By comparing the formation process of two different CO 2 –CH 4 gas mixtures, the formation rate of the 79.8% CO 2 /20.2% CH 4 gas mixture hydrate was faster. The MRI data gave excellent information on the spatial distribution of water in the porous media during hydrate growth and dissociation. [ABSTRACT FROM AUTHOR]

Theseparation of CO2from fuel gas (CO2/H2) as hydrates was studied. In this investigation, the effectsand mechanism thereof of the additive mixture (1, 2, 3, and 4 mol% tetrahydrofuran (THF), with 1000 mg/L sodium dodecyl sulfate) onthe thermodynamic and kinetic properties of the hydrate in porousmedia were measured using an isochoric method, keeping the volumeconstant. The experimental results show that an increasing THF concentrationincreases the driving force for hydrate formation and decreases thehydrate induction time. The Langmuir constants of H2andCO2showed that H2may occupy the small cavitiesof s-II hydrate in the H2–CO2–THF–H2O system. The presence of THF results in a drastic decreaseof the hydrate phase equilibrium pressure. Higher THF concentrationscorrespond to lower hydrate phase equilibrium pressures, but the decreasein pressure with concentration slows when the THF concentration exceeds3 mol %. An improved thermodynamic model was used to predict the hydratephase equilibrium, and the calculations agreed well with the experimentaldata. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics and stability conditions of carbon dioxide (CO2) hydrate formation are crucial for hydrate-based CO2 capture and storage. The effects of a mixture of additives (THF/SDS) on CO2 hydrate formation and dissociation in porous media have been investigated experimentally using a graphic method. The Gibbs phase rule is used to analyze the experimental p–T curves. Hydrate formation processes can be divided into two cases, depending on the CO2 initial state. The experimental results showed that 1000mg/L SDS is the best additive and concentration for CO2 hydrate formation among those studied in this investigation due to its shorter induction time and resultantly higher hydrate saturation than those of other concentrations. The presence of 3mol% THF dramatically decreased the hydrate phase equilibrium pressure. The hydrate equilibrium temperature is 291.55K in the aqueous phase with 3mol% THF and 0mg/L SDS, which is the highest equilibrium temperature at 3.04MPa observed in this investigation. The experimental results also showed that “pseudo-retrograde” behavior exists at nearly 3.00MPa with all SDS concentrations. An improved model is used to predict the phase equilibrium conditions for CO2 hydrates in glass beads in the presence of THF, in which the mechanical equilibrium of force between the interfaces in a hydrate–liquid–vapor system is considered. [Copyright &y& Elsevier]

• The water conversion rate in large pores of porous media was higher than that in small pores. • Gas injection from top to bottom was beneficial to the hydrate-based gas storage and transportation. • The kinetics of methane hydrate formation in size-varied porous media was explored. • The contribution of 2nd hydrate growth to total gas consumption exceeds 33.7%. Using porous media to store and transport natural gas by the hydrate method can significantly improve gas-liquid contact and increase the hydrate formation rate. Therefore, it is particularly important to understand the characteristics of hydrate formation in porous media. In this study, the formation characteristics of hydrate in porous media with different particle sizes were quantitatively investigated by low-field 1H NMR from the microscopic scale to macroscopic characteristics. The experimental results show that hydrate always tends to grow in the larger pore space area caused by the heterogeneity of the porous media, and the water conversion rate in large pores is significantly higher than that in small pores. Due to the water accumulation under the action of gravity, the distribution of hydrate shows obvious spatial heterogeneity, and the lower part is more prone to clogging. Analysis of the macroscopic characteristics of hydrate formation shows that the induction time of hydrate has no obvious relationship with the particle size of the porous media, while the formation rate and gas consumption of hydrate increase with the decrease of the particle size of porous media. The gas consumption and formation rate of hydrate in BZ01 increase by 16.85% and 174.7% respectively compared with those in BZ02/04. In addition, due to gas diffusion – hydrate film rupture – capillary force, hydrate growth has the characteristics of 2nd growth, resulting in more than 33.7% of the final gas consumption. [ABSTRACT FROM AUTHOR]

CO 2 flooding has been proven to be an effective method to improve oil recovery, and the interface characteristics of the displacement front directly affect the oil recovery and CO 2 utilization efficiency in reservoirs. In this study, we aimed to investigate the in situ interfacial behavior during CO 2 flooding using high-resolution magnetic resonance imaging (MRI). A series of CO 2 flooding tests were carried out using upward and downward injection methods consisting of different rates (28, 56 and 112 pore volume (PV)/day) in porous media. The unstable CO 2 front is indicated to significantly impact the gas sweep efficiency, which is not favorable for oil recovery and leads to low energy utilization efficiency. For downward injection, subordinate displacement of CO 2 was observed after breakthrough, which can improve oil recovery. The optimum oil recovery and CO 2 utilization factor were obtained under upward injection at a low rate in the immiscible state, which was controlled by the stable interface behavior. From the perspective of CO 2 storage, miscible flooding improved the storage volume by more than 25% compared to immiscible flooding. For high permeability oil reservoirs, the oil recovery and gas sequestration can be synthetically optimized by adjusting the injection rates and phase state. Image 1 • MRI data can directly reflect the fluid interfacial behavior in porous media. • A stable displacement interface benefits oil recovery and CO 2 utilization factor. • A secondary flooding phenomenon was observed for the first time. • Correlation of CO 2 -oil mixing layer and oil recovery was proposed. [ABSTRACT FROM AUTHOR]

Hydrate cold storage technology has been intensively researched in recent years and plays an important role in the macro-control of energy. This paper reviews the diversity and variability of hydrate cold storage media and the new hydrate cold storage system. The diversity is embodied by the types of hydrate cold storage media, which include alkane hydrates, freon hydrates, CO 2 hydrates, water-soluble organic hydrates, and mixed hydrates. In mixed hydrates, the different components in the mix media play different roles in the process of hydrate formation. The variability is reflected in the differences in the external environments (environmental compatibility: ozone depletion potential and global warming potential) and the multiple reinforcement methods of hydrate nucleation. The reinforcement methods including mechanical methods, external field functions, and additives (mainly surfactants, nanoparticles, porous materials, and thermodynamic accelerators) are compared and discussed for optimisation of the hydrate formation conditions. The operational characteristics and application advantages of the new cold storage systems with different hydrate media are summarised. The environmental impact, energy efficiency, life cycle assessment, and commercial and industrial possibilities of the hydrate cold storage system are discussed. Finally, this review provides a comprehensive outlook of hydrate cold storage from an application perspective. • The characteristics of different hydrate cold storage media are compared and analyzed. • Reinforcement methods are summarised from the perspective of application. • The latest applications of hydrate cold storage are analyzed and are introduced. • Feasibility of hydrate cold storage system is evaluated comprehensively. • The key problems in the application of hydrate cold storage are mentioned. [ABSTRACT FROM AUTHOR]