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Safety issues are always a major concern in the oil and gas transportation facilities. Equipment damages are frequently encountered due to solid deposition such as gas hydrate deposition. A fast and efficient detection of the location, length, and rate of the accumulating blockage will significantly help relieve the potential risk. Most existing pressure wave‐based models suffer the difficulty to properly predict the blockage percentage arising from the ignorance of the wave attenuation. In the present work, an attenuation model to describe the transportation of the pressure pulse wave in gas is developed; the effects of waveform distortion and absorption as a result of the nonlinear effect and viscous dissipation are collectively considered for the first time. A simplified procedure to couple the wave attenuation in the model is proposed. The results show that the model can remarkably improve the prediction accuracy of blockage percentage by reducing the errors from −9.0% to −4.2%. Moreover, the attenuation process of the pressure pulse wave is determined to consist of three stages. The effect of waveform distortion on amplitude mainly occurs in the second stage, when our proposed model shows an improved prediction. The performance of the proposed model will help the early warning of the blockage in the pipelines and effectively avoid the potential injury and financial loss. [ABSTRACT FROM AUTHOR]

Abstract The permeability of porous media containing gas hydrates is of crucial importance in predicting the fluid flow behavior and gas production capacity during the exploitation of gas hydrate resource. Gas hydrates are generally unevenly distributed in the natural samples, the inhomogeneity of which could have significant impacts on the permeability properties. So here we extend our study on the permeability of heterogeneous hydrate-bearing porous media using pore network models combined with X-ray computed tomography (CT). The effects of varying pore/throat radii, non-even distribution of hydrates and fluid flow directions on the absolute and relative permeability are discussed. It is indicated that the rather high water saturation would significantly impact the gas permeation, which is not favorable for gas production. The microstructures of pores and throats would likely differ even under similar porosities; some pores may not have sufficient throats connected to them, limiting the fluid flow. The absolute permeability could also significantly vary with flow directions; the direction favorable for gas flow may not be true for water flow. This finding can be of help in case a preferential gas production is more required rather than water in the field test. Highlights • The effect of inhomogeneity on permeability of hydrate-bearing sediments is studied. • The micro-connection of pores and throats would differ even under similar porosities. • Some pores may not have sufficient throats connected to them, limiting the fluid flow. • The direction favorable for gas flow may not be true for water flow. [ABSTRACT FROM AUTHOR]

The enormous potential as an alternative energy resource has made natural gas hydrates a material of intense research interest. Their exploration and sample characterization require a quick and effective analysis of the hydrate-bearing cores recovered under in situ pressures. Here a novel Pressure Core Ultrasonic Test System (PCUTS) for on-board analysis of sediment cores containing gas hydrates at in situ pressures is presented. The PCUTS is designed to be compatible with an on-board pressure core transfer device and a long gravity-piston pressure-retained corer. It provides several advantages over laboratory core analysis including quick and non-destructive detection, in situ and successive acoustic property acquisition, and remission of sample storage and transportation. The design of the unique assembly units to ensure the in situ detection is demonstrated, involving the U-type protecting jackets, transducer precession device, and pressure stabilization system. The in situ P-wave velocity measurements make the detection of gas hydrate existence in the sediments possible on-board. Performance tests have verified the feasibility and sensitivity of the ultrasonic test unit, showing the dependence of P-wave velocity on gas hydrate saturation. The PCUTS has been successfully applied for analysis of natural samples containing gas hydrates recovered from the South China Sea. It is indicated that on-board P-wave measurements could provide a quick and effective understanding of the hydrate occurrence in natural samples, which can assist further resource exploration, assessment, and subsequent detailed core analysis. [ABSTRACT FROM AUTHOR]

• An anisotropy of gas hydrate growth was in-situ characterized through X-ray technique. • Themotactic habit of hydrate growth towards a low temperature region was proposed. • The hydrate morphology can be controlled via regulating the heat transfer scheme. Promoting hydrate formation kinetics is considered a major challenge hindering the application of hydrate-based techniques. Surfactants are thus widely used for a better gas solution and water conversion. Yet significant issues involving an uncontrolled morphology and foaming are encountered. In this work, a unique anisotropic behavior of gas hydrate formation was identified through in-situ X-ray technique with the rate of vertical growth 5 times higher than the lateral. This was ascribed to the varying driving force of hydrate nucleation at the wall surface where the reaction heat could be more easily dissipated. Consequently, a thermotactic habit of gas hydrate growth was proposed: the nucleation and growth would preferentially proceed towards a low temperature region to release the reaction heat. Hence through regulating the temperature and thereby the heat transfer scheme one can design the behavior and morphology of hydrate formation. Specifically, a surprisingly fast hydrate formation was observed when freezing the surfactant-containing solution prior to warming-up and pressurization. The melting ice consuming large amount of heat enabled a sequential transformation of water into hydrate tracing the thawing front. This thermotactic behavior was also consolidated in a more macroscopic process. The findings could provide new insights into the kinetics of gas hydrate formation coupling the heat-associated phase transition; the results could also be of help in the up-scale of the hydrate-based techniques where a concentrated transformation is highly required. [ABSTRACT FROM AUTHOR]

Thermal properties of gas hydrate-bearing sediments directly govern the heat transfer process during hydrate decomposition which couples with phase transitions and multiphase flows. The effective thermal conductivity of a multiphase system represents the composite capacity to conduct heat. Here we report on point heat source measurements of the effective thermal conductivity of methane hydrate-bearing sediments through a thermistor-based method combining with X-ray CT observations. Methane hydrates were formed at different saturations, with various initial water contents, and in porous matrices simulated by grains with differing thermal conductivities. It is indicated that the effective thermal conductivity of sediments negatively correlated with the hydrate saturation, while an increase of initial water contents and thermal conductivity of grains has a positive impact on the elevation of the effective thermal conductivity. Moreover, the effective thermal conductivity was found to slightly increase with the proceeding of hydrate decomposition. Typical effective medium models were evaluated with the measurements of this study, and a hybrid fitting model combining three forms of self-consistent models was proposed, with the optimal weighting parameters determined via the genetic algorithm. The effective prediction of the measurements in this work and results in literatures corroborates the feasibility of the model. This study could help in understanding the evolutions of sediment thermal properties during gas production and their effects on large-scale hydrate decomposition when expanded to field scale tests. [ABSTRACT FROM AUTHOR]

Gas hydrates are considered as a potential strategic energy source for sustainable development. The thermal properties of hydrate-bearing sediments govern the hydrate dissociation behavior and gas production process that accompany phase transformation and multiphase flow. This paper presents a thermistor-based measuring method to obtain the effective thermal conductivity of tetrahydrofuran hydrate-bearing sediments. The effects of different porosities, hydrate saturations and porous materials on the effective thermal conductivity were investigated. The porosity and the hydrate saturation were obtained using an X-ray CT (computed tomography) apparatus. The findings indicated that the effective thermal conductivity of hydrate-bearing sediments increased from 0.6468 W/(m K) to 0.7318 W/(m K) with porosity decreasing from 42.5% to 37.2%. Increasing hydrate saturations from 0% to 100% decreased the effective thermal conductivity from 0.7876 W/(m K) to 0.7318 W/(m K). Additionally, existing effective medium correlations were examined using the experimental data. The results showed that none of the existing correlations can suitably predict the measured data. Therefore, a hybrid correlation was proposed by optimizing the weighting parameters of the Parallel correlation and the Series correlation using the PIKAIA genetic algorithm. The agreement of the fitting correlation with the experiments is given, and the effective prediction of other researchers' work confirms the feasibility of our correlation. [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]

Abstract: Over the past century, fossil fuels have provided the majority of China's energy. However, their extensive utilization leads to a shortage and environmental pollution. Recently, submarine and permafrost gas hydrate deposits have been investigated as a possible clean and sustainable energy source by governmental institutions, research organizations, and energy industries in China. The primary objective of this paper is to review the potential studies pertaining to gas hydrate exploration and resource assessment, the safe and efficient exploitation of gas hydrates and the basic properties of gas hydrates. To date, there are over 20 institutions and organizations in China committed to gas hydrate investigation, among which the Guangzhou Marine Geological Survey (GMGS) and the Chinese Academy of Geological Sciences (CAGS) etc. primarily focus on gas hydrate exploration research, while the China National Offshore Oil Corporation (CNOOC) Research Center, Guangzhou Institute of Energy Conversion (GIEC) and China University of Petroleum-Beijing (CUPB) etc. concentrate on gas hydrate mining technologies. In this paper, the occurrence and exploration of gas hydrates in both permafrost regions and the continental slope of China have been determined from numerous research contributions and are presented. Moreover, the latest progress in gas hydrate fundamental studies, including hydrate phase equilibria, hydrate formation mechanisms, hydrate thermal physical properties and the acoustics and resistivity characteristics of gas hydrates are briefly reviewed, and relevant data are gathered and compared. Emphasis is also placed on gas hydrate mining technologies and gas production using depressurization methods, thermal stimulation methods or other methods. Furthermore, the security of natural gas hydrate-bearing sediments during gas production and the environmental impacts of gas hydrate are identified. With additional financial and political support and advanced research facilities, research on gas hydrates in China is progressing rapidly but is still in its early developing stage, thus, future work should be undertaken with greater diligence. [Copyright &y& Elsevier]

Vibrational behavior and host-guest interaction of C 7 D 14 molecule (the most representative large molecular guest hydrate) in 51268 cage of structure H hydrate. [Display omitted] • The Raman spectral confusion of 512 cage and 435663 cage have been clarified by Density Functional Theory and in situ Raman spectra, which provides a theoretical basis for the accurate determination of sH hydrate. • The stretching vibration Raman spectrum of C 7 D 14 in the 51268 cages of sH is reported for the first time, which provides a basis for regulating the formation of macromolecular hydrate. • A high precision Raman spectral resolution method of Raman wavenumber for multi-hydrocarbon supramolecular was raised. • Repulsion and attraction among guest and host molecules cooperatively stabilizes the cluster structure. • Raman spectral calibration equation for hydrate under different temperature and pressure was proposed. The formation of complex multi-guest clathrate hydrates, which are more stable than single-guest ones, has expanded the clathrate hydrate stability phase space. Multi-guest clathrate hydrate may lead to the formation of sI, sII, and sH structures, introducing a variety of possible guest occupancy and crystal composition; the occupancy of sH cages in particular remains poorly understood. We report the occupancy of methane (CH 4) in the 512 and 435663 cages of sH measured via in situ Raman spectrum and density functional theory, identifying the corresponding peak position for the C–H stretching mode at 2909.80 cm−1 and 2912.40 cm−1, respectively. The in situ Raman spectrum of C 7 D 14 in the 51268 cages of sH is also reported for the first time. Vibrational-mode analysis shows that repulsion and attraction among guest and host molecules cooperatively stabilizes the cluster structure. In addition, our work provides an accurate method of distinguishing sI and sH hydrates by in situ Raman spectroscopy, offering a spectroscopic basis for the study of crystal transformation dynamics in polymorphous clathrate hydrates. [ABSTRACT FROM AUTHOR]

Laboratory research based on the physical characteristics of unconsolidated sediments containing gas hydrate was carried out on hydrate-bearing sediment samples recovered from China National Offshore Oil Corporation (CNOOC) drilling expeditions in 2015 and 2017 at the station of LW3 in the northern part of the South China Sea. We herein have measured the gas composition, index properties, surface characteristics, thermal properties, and mechanical properties of these sediment cores drilled at different seafloor depths. Based on the measurement results, more than 99% of the gas composition is methane gas. The hydrate-bearing sediments recovered from Shenhu Area are non-diagenetic rock, the fraction of grain size which less than 40 μm was more than 90%. The results could provide useful guidance for potential sand-control system operations. Such small and consolidated particles indicated that the application of a novel solid fluidization mining method is viable and promising. The fine-grained layers were found to be mostly self-filtering and internally stable for the seepage flow during production. Gas hydrate mostly occurs in vein-type, and bioclastics consisting of marine planktonic diatoms and foraminifera were also found. The mineral components of the sediments were mostly quartz, clay minerals like illite and some feldspar like albite. The thermal conductivities changed from 1.4 to 1.77 W/(mK), which suggests that the isolated depressurization methods could exacerbate the problem of gas hydrate reformation or ice generation; thus, heat injection in wellbores should be combined for gas hydrate production strategies. The low liquid limit (38%) and plastic limit (24%) imply that stability of the production platform should be taken into account for avoiding submarine landslide impacts. The results of mechanical property analyses indicated that hydrate-bearing sediments at deeper sites have a relatively higher strength at hydrate layer of 120–133 mbsf in the Shenhu Area of the South China Sea. • Physical properties of HBS recovered from the Shenhu area in the South China Sea were analyzed using different methods. • The comparison of characteristics of gas hydrate samples with other various hydrate regions was summarized. • The guidance of this hydrate sediments properties for future gas hydrate exploitation was discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] Gas hydrates have gained great interest in the energy and environmental field as a medium for gas storage and transport, gas separation, and carbon dioxide sequestration. The presence of small doses of surfactants in the aqueous phase has been reported to enhance hydrate formation; however, the underlying mechanisms remain poorly understood. Thus, in situ high-resolution X-ray computed tomography measurements were performed to monitor the upward water migration and the resulting hydrate nucleation and growth. It was found that the presence of hydrate crystals at the gas–liquid–solid contact line triggered the enhanced growth of hydrates on the reactor wall. A time delay was observed between the disappearance of the bulk water reservoir and its transformation into hydrate. The lower interfacial tension between the hydrate surface and the solution facilitated its adsorption onto the reactor wall once a thin film of hydrate nucleated on the solid wall surface. These hydrate layers present on the reactor wall were found to be porous, wherein the porosity decreased with increased subcooling. These fundamental results will be of value in understanding the mechanism of hydrate growth in the presence of surfactants and its potential application in hydrate-based technologies. [ABSTRACT FROM AUTHOR]

Gas hydrates have been considered promising in gas storage, gas separation, and water desalination. The hydrates' technical application is affected by the long kinetics time. Of interest is gas hydrates show strong facilitation in dissociated water containing a high concentration of micro-nanobubbles. Here, it was discovered that the nano-particles present in the dissociated water were identified as nano-bubbles, characterized by nanoparticle track analyzer and infrared spectrum. The presence of bubbles increased the nucleation probability, likely attributed to the provision of nucleation sites. Results indicated that the ability of facilitation was related to the nanobubble types: the bubble solution with the same guest molecules exhibited similar facilitation to dissociated water; specifically, the average induction time was shortened by 27.48 %, the nucleation probability was increased by 50 % compared to deionized water. Moreover, the time hydrate filled the field of view with high bubble concentration was reduced by 60 % compared with low bubble concentration. The facilitation effect mechanism could be related to the presence of gas-dense regions inside the bubbles, based on the Raman results. This finding may offer valuable insights for the application of hydrate-based energy storage technology and shed light on the potential role of bubbles in causing memory effects. • Nano-bubbles are generated as a result of gas hydrate dissociation. • These bubbles have the potential to facilitate hydrate nucleation, with the specific bubble type influencing the process. • The bubbles play a role in promoting hydrate growth, reducing the growth time by 60 %. • The existence of high-pressure zones within the bubbles may be confirmed. [ABSTRACT FROM AUTHOR]

Several trial productions have been conducted on marine natural gas hydrate reservoirs, demonstrating its enormous potential as an alternative energy source. Nonetheless, sustained and efficient production still remains challenging, with crucial issues on low gas production efficiency and blockages. The large amount of hydrate decomposition and resulting heat absorption can lead to prolonged subzero conditions close to the well; this can readily induce ice formation and blockage. This study aims to investigate the issue of ice blockages caused by insufficient energy supply during the recovery process. A systematic analysis of the energy consumption and ice blockage quantification at each stage of the traditional multi-stage direct depressurization and constant speed depressurization methods is conducted. A hybrid optimized scheme is proposed, which enhances energy utilization efficiency by appropriately utilizing reservoir sensible heat during the initial stage of production. Additionally, a multi-stage constant speed depressurization method is introduced in the later stages to mitigate production fluctuations and reduce blockage duration. Results showed a 69.5 % increase in energy efficiency and an average 67.1 % decline in gas blockage duration with comparable gas productivity. The proposed method could be a viable solution to mitigate challenges of icing blockages and fluctuating production during trial tests of marine hydrate reservoirs, achieving improved energy efficiency and sustainable production. • Ice blockage in hydrate reservoirs reduced production efficiency during direct and constant-speed depressurization methods. • Proposed a hybrid method to address gas production fluctuations caused by ice blockage through energy consumption analysis. • Multi-stage analysis revealed 70% energy efficiency improvement and 67% reduced ice blockage duration with the hybrid method. [ABSTRACT FROM AUTHOR]

• 3D xenon hydrate shell meshes on water particle and ice sphere created from micro XCT. • Time-dependent hydrate shell thickness and inner/outer diameter were determined. • Novel 3D mesh method can quantify heterogeneous hydrate shell growth. Gas hydrates have gained increasing attention in energy and environmental fields and have potential applications in gas storage and transport, carbon dioxide sequestration, and flow assurance. Nevertheless, the kinetics of hydrate formation are still not well understood. In situ high-resolution X-ray computed tomography measurements were performed to monitor xenon hydrate formation on water droplets and ice spheres. For the first time, three-dimensional thickness meshes were used to quantify and visualize the kinetics of hydrate formation. The evolution of the hydrate morphology was investigated, and the time-dependent kinetic parameters were obtained, including the hydrate shell thickness and inner and outer diameters and the effective volume fraction of hydrate particles. The results indicate that the formation of gas hydrates undergoes an initial reaction-controlled stage followed by a mass-transfer-limited growth stage. For hydrate formation from a water droplet with an initial diameter of 1.66 mm, the hydrate shell thickness was approximately 30 μm and the effective volume fraction of hydrate particles was approximately 11% at 12 h after hydrate formation began. The standard deviation of the shell thickness, which indicates the surface roughness of the hydrate shell, increased with time for hydrate formation from water droplets. The results presented in this study renew our knowledge on the kinetics of hydrate formation, which is essential for their use in flow assurance and other hydrate-related technologies. [ABSTRACT FROM AUTHOR]

The detailed knowledge of the low thermal conductivity of host–guest compounds is essential to improve our fundamental understanding of heat conduction in complex solids and develop high-performance thermoelectric materials. In this Letter, the intrinsic ultralow thermal conductivity (0.44 ± 0.06 W m − 1 K − 1 in 140–190 K) of the tetrahydrofuran (THF) clathrate hydrate is characterized by the time-domain thermoreflectance technique. The underlying heat conduction mechanism is further investigated by non-equilibrium molecular dynamics simulations. We find that trapped THF molecules do harmonic motions and behave as parts of a crystalline structure, thus playing negligible roles in thermal conductivity reduction. The large unit cell and complex cage-like host structure dominate the low thermal conductivity of the THF hydrate. [ABSTRACT FROM AUTHOR]

The vast hydrocarbon resources (oil, gas, and gas hydrate) in deepwater areas are considered one of the most significant resources of the future. However, the dangerous flow patterns and blockage accidents caused by hydrates threaten the safety and efficiency of the transportation process. A mutual interaction exists between hydrate growth and multiphase flow. In this study, hydrate formation and gas–slurry two–phase flow tests were conducted in a pure water system with a constant pressure (4 MPa) and various liquid loadings (40–80%) in a visual flow loop. It was found that the hydrate growth process in the dynamic system included bubble promotion, subcooling promotion, and mass transfer inhibition. Meanwhile, two atypical slug patterns (rapid conversion slug and deposition slug) in the vertical upward flow were determined, which corresponded to the promotion and inhibition regions. In the rapid conversion slug flow, the churn segments replaced the original Taylor bubbles when the liquid loading was less than 60%. In the deposition slug flow, slurry shedding phenomena was observed. In addition, the hydrate particle deposition behavior was found to inhibit hydrate growth and cause flow pattern shifts, but did not trigger the differential pressure surge. The gas isolation caused by the liquid plug and energy dissipation due to the collision between hydrate particles and the tube wall may be responsible for the sudden increase in pressure drop. Furthermore, the feasibility of predicting hydrate blockage using the differential pressure method was explored. The experiment results showed that this warning method was effective when the liquid loading was below 76%. And an effective warning can improve the transport capacity by approximately 34%. • The hydrate formation and flow behaviors are complicated in deepwater risers. • Hydrate formation and multiphase flow behaviors were observed by a visual flow loop. • Three regions of growth kinetics and two atypical slug patterns were determined. • The differential-pressure hydrate blockage warning method was proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Maximum energy storage density (57.4 kWh/m3) of hydrate cold storage was obtained. • Increasing the solution concentration boost the cold charge capacity by 2–3 times. • Rapid dilution of dissociated solution was key to enhancing the cold discharge. • Self-regulation of driving force was relied on temperature and concentration. Low energy storage density, intermittent phase changes, and heat transfer barriers have posed significant challenges in the implementation of hydrate energy storage systems. Based on the heterogeneous nucleation mechanism for tetrabutylammonium bromide (TBAB) hydrate phase change energy storage, a novel cold storage system with internally circulating gas disturbance was constructed for energy evaluation and thermodynamic evolution. The hydrate cold storage efficiency, response time, dynamic driving force, unique force pattern of the coils and guest molecule diffusion were analyzed for the first time. The bubbles generated by gas internal circulation provided numerous nucleation sites for hydrate formation, efficiently promoting the energy storage process. The disturbances caused by the bubbles also rapidly transmitted the energy generated by the phase change. A hydrate storage density of 57.4 kWh/m3 for this system was the maximum among known hydrate storage systems. Notably, a change in solution concentration (from 10 wt% to 40 wt%) resulted in a 2–3-fold increase in the cold charge capacity. Rapid dilution of local solutions and rapid release of cold were the ways to promote the cold discharge. Less damage to the stress structure of the coil was due to the loose accumulation of hydrate particles under gas disturbance. The microbubbles generated by the gas disturbance enabled heterogeneous nucleation and heat-mass transfer in the hydrate cold storage. The gas disturbance-based hydrate energy storage process holds significant guiding value for various applications such as refrigerated transport, building cooling systems, and grid peak shaving. [ABSTRACT FROM AUTHOR]

Increasing the efficiency of gas production from marine gas hydrate still faces significant problems. Based on the geological survey of marine reservoirs, gas hydrates are generally associated with underlying free gas layers with high temperature and water saturation. Here the effect of this gas layer and its characteristics on the gas and water production from the hydrate layer are studied. It was found that a large temperature difference existed between the two layers during gas production due to the different heat demand and consumption in each layer. Slower gas production rate was observed in case of dense distribution of hydrates in the hydrate-bearing layer and the resulting sluggish gas permeation and enormous heat supply. Notably, this was not present when the free gas layer was more water-saturated. The resulting maximum gas production rate could be effectively enhanced by 41 % at most as well. This was attributed to the increased sensible heat available from the pore water and the interbedded heat exchange. It was thus indicated that a hydrate reservoir with an underlying water-rich layer could be more favorable for gas production in terms of heat supply. • Joint exploitation through pre-embedding compartment device and vertical well. • A significant temperature difference between the two layers during exploitation. • Highly water-saturated underlying shallow gas layer enhance hydrate decomposition. [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]

The gas production from marine gas hydrate has always been plagued by low productivity; the complex geological environment poses a significant obstacle to its commercialization. Horizontal wells are thus getting increasing attention due to their advantages in facilitating pressure propagation. Here a pre-embedded dual horizontal well was used to probe its effects in the local temperature distribution and gas production behavior. By doubling the number of boreholes, the flow channels of gas were increased enlarging the decomposition zone. Specifically, this was found to raise the reservoir temperature from −1 °C in a single well to approximately 0 °C. The temperature decline was more moderate due to a weakened Joule-Thomson effect. Consequently, almost 90% of the cumulative gas yield was produced before the lowest temperature occurred, compared to ∼30% in the single vertical well case. This indicates that the gas production from a dual well case was proceeding at a relatively higher temperature, potentially benefitting an enhanced gas production efficiency. An enlarged depressurization region was thus suggested in the field test for a controlled temperature decline to make more use of the sensible heat of the reservoir. • Simulation of horizontal wells by the scheme of pre-embedding. • Increase the minimum temperature during gas production by the dual horizontal wells. • Multiple gas flow paths can alleviate the rapid drop in reservoir temperature. [ABSTRACT FROM AUTHOR]

Nanobubbles have attracted wide scientific attention due to their large number of potential applications in nanotechnology, and they are ubiquitous during natural gas hydrates (NGHs) mining which is one of the strategic energy resources. Molecular dynamic simulations were performed to investigate CH 4 nanobubble characteristics during NGHs dissociation. The nucleation mechanism and evolution characteristics of nanobubbles vary with hydrate dissociation temperature. The higher the temperature, the earlier the bubble nucleation, the more small methane clusters are produced, and the more complex the evolution path of nanobubbles, the initial position of bubble nucleation is closer to the decomposition interface. The gas reservoir can provide another possibility for methane molecules to overcome solubility barriers to avoid the formation of nanobubbles at lower dissociation temperature. CH 4 nanobubble evolution was found to be governed by the gas supersaturation and the diffusion of CH 4 molecules. The self-diffusion coefficient of CH 4 molecules near the decomposition interface area is small due to the adsorption effect of the incomplete cages, nanobubbles are more likely to form outside the adsorption zone where the CH 4 diffusion coefficient is larger at a lower temperature. A direct proof of speeding up of hydrate dissociation by nanobubble growth was given. The acceleration of dissociation is later than nanobubble growth and the accumulation of nanobubbles on dissociation interface may have a negative influence. The new findings on nanobubble evolution may help design and optimize nanobubble formation techniques in nanotechnology and efficient gas hydrate production protocols. • CH 4 diffusion coefficient increases with distance from the decomposition interface. • CH 4 nanobubble evolution was governed by gas supersaturation and diffusion. • A direct proof on speeding up of hydrate dissociation by nanobubble growth was given. • A mechanism for nanobubble nucleation during hydrate dissociation was proposed. [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]

More than 65% of the worldwide natural gas is transported through pipelines. Gas hydrate formation and blockage in the pipeline are commonly encountered, causing significant problems in safe and efficient transportation. Yet the instrument simulating the high-pressure multiphase flow and demonstrating the hydrate blocking behavior at flowing conditions is not available. In this work, a high-pressure visual flow loop is developed to detect and observe the hydrate nucleation, growth, and deposition during the multiphase flow. The pressure limit is designed to be 8 MPa, and the temperature can go down to −20 °C. This device allows a direct observation of the gas-liquid flow regime during transportation; investigations on the initiation and evolution of hydrate blockage in the dead-leg section, U-shaped region, and rising pipes are now possible. Sensors are distributed along the pipelines to detect the pressure and temperature change; the differential pressure is used to effectively detect the degree of hydrate blockage, which increases upon hydrate formation. The visual flow loop could also help examine the effect of shut-down and restart of the pump on the flow behavior. The system has been verified feasible demonstrating the flow behavior and predict hydrate blockage. The developed device would help understand the mechanism of hydrate blockage in the transportation pipelines and provide guidance to efficiently avoid such problems. [ABSTRACT FROM AUTHOR]

In gas capture/storage and energy storage, tetrabutylammonium bromide (TBAB) hydrates have been regarded as superior carriers owing to their unique molecular selectivity and high latent heat. Hydrate conversion amount is a significant parameter in evaluating gas selection and cold storage characteristics. This study proposes a method for calculating hydrate conversion amount proposed based on the correlation between electrical conductivity and solution concentration. The correlation index (R2) between electrical conductivity and TBAB solution concentration was above 0.99 at concentrations less than below 5 wt%. The conversion amounts of different hydrate types were obtained using a unique hybrid calculation method. The differences in hydrate conversion amount, conversion ratio, and memory effect were compared at different mass fractions. The results indicated that the hydrate morphology and accumulation patterns were influenced by the TBAB solution ionization degree. Regular hexagonal TBAB hydrate crystals were observed at a low ionization degree. The maximum conversion ratio of TBAB hydrate reached 96% (1 °C/30 wt%). The variation trend of TBAB hydrate phase equilibrium temperature was consistent with that of the electrical conductivity of the solution. The investigation of the memory effect revealed that the phase change process of TBAB hydrate is more predictable in high-concentration initial solutions. [Display omitted] • Regular hexagonal hydrate was observed for the first time. • Insufficient ionization of TBAB slowed hydrate conversion. • TBAB hydrate phase equilibrium temperature was consistent with ionization degree. • Temperature, induction factor, and residual solution volume affected hydrate conversion. [ABSTRACT FROM AUTHOR]

An accurate description of the reservoir characteristics is essential to the precise prediction of gas production from nature gas hydrate (NGH) deposits. In most numerical studies related to large-scale onshore or offshore NGH production cases, the reservoir permeability was usually assumed to be homogeneous. In this work, a multi-layered heterogeneous-permeability reservoir model (including a hydrate-bearing layer, a three-phase layer, and a free gas layer) was established, and the gas production behavior was analyzed by using the TOUGH + HYDRATE simulator. It is found that hydrate and free gas tend to be scatteredly distributed in this reservoir model. Reservoirs with heterogeneous permeability have better gas production performance than homogeneous ones. The 10-year cumulative gas production from the heterogeneous reservoir with the heterogeneity degree of 0.4μ (622 × 104 m3) is 15.19% higher than that from the homogeneous reservoir (540 × 104 m3). Moreover, the layered production results show that the free gas layer contributes the most to gas production among the three layers. The effect of heterogeneity on gas production in high-permeability reservoirs is more obvious than that in low-permeability reservoirs. Therefore, the permeability heterogeneity of the reservoir, especially the permeability heterogeneity of the free gas layer, should be considered in the long-term prediction of NGH production. • A multi-layered permeability heterogeneity hydrate reservoir model was built. • Gas production behaviors and evolution of reservoir characteristics were analyzed. • The sensitivity of permeability heterogeneity was discussed. • Gas production in heterogeneous reservoirs was better than in homogeneous ones. • Heterogeneity should be especially concerned in reservoirs with free gas. [ABSTRACT FROM AUTHOR]

Among deep-sea oil and gas transportation, kinetic inhibitors of gas hydrates are widely added in transportation pipelines to prevent gas hydrates from clogging pipelines. Unfortunately, the micro-and nano-sized clay particles will enter the wellbore to adsorb the practical components of the inhibitors, thereby weakening the inhibitory effect of the inhibitors on hydrates. This work shows that 1:1-type layered silicates can only adsorb inhibitors through the surface, while 2:1-type layered silicates can adsorb inhibitors through both surface and intercalation. The clay particles with adsorbed inhibitors tend to be aggregated and settled because the surface charge density and the potential energy between the particles are reduced. Meanwhile, the inhibitors adsorbed on the surface of the clay lose their inhibitory effect on gas hydrates. Therefore, the suppression effect of layered silicate minerals on inhibitors should be considered in clay-rich oil and gas transportation pipelines. • Inhibitory effect of different clays on polyvinyl caprolactam hydrate was studied. • The inhibitory effects were weakened in the presence of clas due to adsorption. • The inhibitor adsorption ability of 2:1 clay is stronger than that of 1:1 clay. [ABSTRACT FROM AUTHOR]

• Saturates, aromatics, and resins were extracted from the crude oil from the SCS. • Saturates accelerated the rates of both hydrate nucleation and growth. • The more polar aromatics and especially resins retarded hydrate formation kinetics. Gas hydrate has been a major problem for flow assurance due to its potential to cause pipelines clogging inside subsea multiphase transportation systems. Of special interest is that polar components in crude oil should play a significant role in the hydrate formation kinetics, but the detailed understanding of their interaction remains unclear. Here, the individual component in the crude oil collected from the South China Sea was for the first time separated through the saturates, aromatics, resins, and asphaltenes (SARA) fractionation method. The promotion effect of saturates on hydrate nucleation and growth, and the surprising inhibition effect of aromatics and resins were discovered. Specifically, the average induction time was shortened by 45.13 % with the addition of the saturates; while the aromatics and resins would prolong the induction time by 2.20 and 20.83 times, respectively. Results also indicated that gas flow direction determined the behavior of hydrate growth; hydrates preferentially nucleated at the cell walls with a higher subcooling. It was further illustrated that the average growth rate of hydrate films was enhanced by 32.65 % with the addition of the saturates; while it was reduced by 54.01 % and 84.52 % with aromatics and resins, respectively. Our findings provided the first knowledge on the effect of single polar component in crude oil on hydrate formation kinetics; natural fraction such as resins was also suggested for a potential application in preventing hydrates plugging in pipelines. [ABSTRACT FROM AUTHOR]

[Display omitted] • An approach of warm brine injection to promote CH 4 /CO 2 replacement is proposed. • Warm brine injection provides a 2.0 times enhancement of the cumulative gas yield. • A potential risk of hydrate reformation is alleviated by increasing salinity. • Gas production is insensitive to injected heat as the significant heat loss. • Brine injection scheme should be optimized based on energy harvest and CO 2 storage. CH 4 /CO 2 replacement for natural gas hydrates (NGHs) exploitation is a promising method for CO 2 geological sequestration and energy recovery simultaneously. However, the puzzles of low replacement efficiency and slow reaction rate caused by the mass transfer obstacle of gas exchange are fatal bottlenecks for field application of gas replacement method. Therefore, we propose a method that uses warm brine injection during CH 4 /CO 2 replacement process to break the barrier of CO 2 diffusion and enhance CH 4 recovery as well as CO 2 storage. Benefiting from the synergistic influence of salt effect and thermal stimulation as well as water flow erosion, warm brine injection provides three dimensionally connected channels in hydrate for subsequent mass transfer, improving CH 4 /CO 2 replacement in the deep layer of hydrate. CO 2 sequestration efficiency of the newly proposed method reaches 76.46%, and the maximum amounts of CH 4 recovery is nearly treble than that achieved by single CO 2 replacement method. Notably, a potential risk of secondary hydrate formation could occur upon the pressure surge and fluid migration attached to warm brine injection, which could be effectively alleviated by increasing salinity to destabilize hydrate lattices by reducing water activity. The gas production is insensitive to the injected heat as the significant heat loss during the transportation of injected brine in pipelines and thermal diffusion through boundaries. The introduction of free water increases the complexity of the reactions in hydrate reservoirs, and the formulation of brine injection regimes should be synergistically optimized based on energy harvest, energy efficiency and CO 2 sequestration. This work extends the previous knowledge on CH 4 /CO 2 replacement and shows important practical significance for future field studies of NGHs exploitation and CO 2 geological sequestration. [ABSTRACT FROM AUTHOR]

• A new hydrate formation promoter was selected for methane storage. • The R141b was confirmed to improve the equilibrium condition of methane hydrate. • Thermodynamic characteristics of mixture hydrate was analysed. • Mechanism of mixture hydrate nucleation was proposed. • Gas storage procedure was optimized. Solidified natural gas is a promising alternative for improving natural gas storage owing to its safe and environmentally friendly properties. However, harsh conditions limit its application. In this study, the R141b was used as a thermodynamic promoter to enhance methane hydrate formation under mild conditions. Addition of R141b to the water system significantly shifted the phase equilibrium boundary to lower pressures and higher temperatures, in which the R141b-CH 4 hydrate can be formed at 0.1 MPa and 287.2 K. Notably, the final pressure of the cell after hydrate formation was near the atmospheric pressure even at an initial pressure of 5.384 MPa, suggesting that the R141b-CH 4 mixed hydrate had milder storage conditions and lower costs than liquefied natural gas (approximately 110 K). The higher initial pressure significantly reduces the induction time and increases gas storage. The mechanism of unusual kinetic characteristics of the R141b-CH 4 hydrates that were observed can be attributed to the different nucleation behaviour of CH 4 , R141b, and R141b-CH 4 hydrates. This study provides evidence of a new promoter using the solid hydrate method under mild conditions that can be alternatively used for the storage and transport of natural gas, thereby increasing the supply of clean energy without the high economic costs and safety issues. [ABSTRACT FROM AUTHOR]

• Presence of fine particle could block the pores and hinder gas production. • Water content determines the behavior of fine particle aggregation. • Distribution pattern of fine particles directly affects the gas production efficiency. Natural gas hydrates are widely distributed in silty marine sediments containing fine particles less than 75 μm in size. The effects of WC (water content) on the behavior of fine particle migration and their role in gas and water production do not appear well understood. This work found that the WC slightly affected the gas production pattern in uniform sandy reservoirs. Yet upon a mixture of coarse and fine particles, the fine particles would instantly aggregate at the nearby pore throat reducing the reservoir permeability and prolonging the production by more than 3 times. Surprisingly, 16% of the fine particles at the edge and 19% at the middle were found to migrate with the fluid flow upon increasing water content, resulting in a less hindered initial gas production but a following more complete blockage or even production failure. This behavior can be consolidated in the scenario of a sand-kaolin mixture. The findings could provide new knowledge on the particle migration behavior upon gas and especially water production and guide the strategy design on dealing with the sand problem in the field test. [ABSTRACT FROM AUTHOR]

There have been several field tests worldwide to recover natural gas from marine gas hydrate reservoirs. The resulting intrinsic characteristics of the sediments are recognized as the major challenge hindering the large-scale gas production; of particular interest is the heterogeneous spatial distribution of natural gas hydrates in the sediments. Open issues still remain on mimicking the locally varying reservoir and revealing its role in the production process. Here in this work, reservoirs with a heterogeneous distribution of gas hydrates were prepared by controlling the uneven distribution of moisture in the fine natural sediments drilled from the South China Sea; special interest was put in the effects of the well location and depressurization schemes on the gas production performance. The results showed that the production efficiency in the heterogeneous reservoirs was significantly weakened compared with the homogeneous case; this was ascribed to the ununiform distribution of methane hydrates and pore fluids and a resulting locally different energy demand. Consequently, the production behavior showed a strong dependence on the depressurization location. A wellbore arranged in the layer with a high permeability can help improve the gas production efficiency by up to 38.25%. This yet resulted in an uneven temperature distribution; the rapid gas production and methane hydrate dissociation decreased the reservoir temperature to −3.33 °C. Further applying a cycling and step-wise depressurization could raise the minimum reservoir temperature up to 1.12 °C. Whereas, relieving temperature decline only through consuming the sensible heat in the reservoir and the surroundings was found to seriously weaken the gas production rate; an external heat supply was thus suggested in the hydrate-abundant regions for efficient gas production. Our findings expand the understanding of the spatially varying gas production behavior from the vertically heterogeneous hydrate-bearing fine marine sediments and may guide the operation of well location and depressurization schemes for field production in similar reservoirs. [Display omitted] • Heterogeneous hydrate samples were prepared by unevenly distributing the moisture. • Production behavior from the vertically heterogeneous hydrate reservoir was studied. • The optimized wellbore location would increase the production efficiency by 38.25%. • Heat supply was suggested in the hydrate-rich area to compensate the sensible heat. [ABSTRACT FROM AUTHOR]

• The long-term joint production from hydrate and shallow gas layer provides a 4.1 times enhancement of the cumulative gas yield. • The regulated layer-dependent scheme effectively alleviated the interbedded pressure difference by ∼70%, avoiding the interlayer instability. • An energy return on investment (EROI) analysis was carried out to evaluated the optimized joint production method. Recent field tests to recover natural gas from marine gas hydrate reservoirs in Japan and China have exhibited worldwide attention to this strategic energy form; significant challenges still remain in improving the accumulative gas yield for a better economic efficiency. According to the geological survey in marine hydrate reservoirs, there exists a concomitant free gas layer underlying the gas hydrate reservoir. Consequently, here we propose a new scheme to jointly produce the gases from hydrate layer and its underlying shallow gas layer. It was found that a dual horizontal well deployment respectively in the corresponding two layers could contribute to a 4.1 times cumulative gas yield comparing that of a single vertical well scenario. This implies a remarkable enhancement of gas productivity via making full use of the gases in the shallow gas layer. Notably, a potential risk of interlayer failure could occur upon the great interbedded pressure difference (maximum 10.7 MPa) arising from the varying behavior of pressure propagation in the layers. This can be effectively alleviated by separately controlling the depressurization scheme in different layers; a mild step-wise depressurization was suggested in the more permeable shallow gas layer while the hydrate layer with a lower permeability should experience a sharper pressure drop. This was found beneficial reducing the interbedded pressure difference by about 70% without intervening the gas production. An energy return on investment (EROI) analysis showed a promising positive energy harvest of our optimized layer-dependent pressure scheme. It could be therefore a potential method to be applied in the field tests in the South China Sea to improve the economic efficiency without disturbing the reservoir stability. [ABSTRACT FROM AUTHOR]

• 3D morphologies and time-dependent parameters of the hydrate shell are obtained. • Water is more mobile than gas in hydrates in the diffusion-controlled growth stage. • Protrusions growth on the outer surface of a hydrate shell is due to water permeation. • Effective diffusion coefficients of gas and water in hydrate are determined. Understanding the transport properties of water and guest molecules in dense hydrate structures is pivotal in controlling their formation and stability; this is nevertheless hindered by challenges in characterizing the time-varying diffusion and permeation process. This paper reports experimental results on the mass-transfer characteristics of water and guest molecules across hydrate shells during their formation from water droplets. The time-dependent 3D morphologies and parameters of hydrate shells were in-situ obtained via X-ray computed tomography to reveal the transport mechanism in hydrates. It was found that in the mass-transfer-limited stage, hydrate growth occurred primarily at the hydrate-gas interface, indicating a more mobile characteristic of water molecules in those hydrate shells than gas. The resulting outward permeation of water led to the growth of hydrate protrusions on the outer surface of the hydrate shell, suggesting a transition of the water transport schema through the hydrate shell from diffusion to permeation. Consequently, a diffusion-based shell growth model was developed to quantify the diffusivity of gas and water molecules in hydrates. Combining the measurements in the temperature range of 275.15–283.15 K, the effective diffusion coefficient of gas through the hydrates was estimated to be in the range of 1.34 × 10−14 − 1.90 × 10−13 m2/s, with that of interstitial water in the range of 2.87 × 10−12 − 3.83 × 10−11 m2/s. These results are of fundamental value in developing an improved understanding of the kinetics of hydrate formation from gas-water interfaces, which is essential in the optimization of hydrate-based techniques. [ABSTRACT FROM AUTHOR]

[Display omitted] • CH 4 /CO 2 replacement behaviors in natural marine sediments are investigated. • CO 2 diffusion is weakened by clayey sediment during replacement process. • NGH reservoir with moderate water content has excellent potential for replacement. • Optimized P-T conditions are crucial to improve CH 4 recovery from hydrates. The use of CH 4 /CO 2 replacement from hydrate bearing sediments for CH 4 recovery and CO 2 storage is an alternative option to mitigate energy shortage and global warming. Fine marine sediments are highly attractive for abundant gas hydrate reserves and tremendous CO 2 sequestration potential. However, the CH 4 /CO 2 replacement regularity previously obtained from coarse sands may not be suitable for fine marine sediments because of their distinct differences in physical properties. In this study, fine natural marine sediments obtained from the South China Sea were used as porous media to investigate the CH 4 /CO 2 replacement characteristics. The results indicated that the pressure and the temperature were the main controlling factors affecting the replacement efficiency. The content of initial methane hydrate and water in reservoir had a more significant effect on CO 2 storage than CH 4 recovery. The gas exchange kinetics in hydrates presented in fine marine sediments were significantly different from those in coarse grained sediments. CH 4 /CO 2 replacement in fine marine sediment seemed to be inhibited by weak CO 2 diffusion as peculiarities of the sediment including fine grain size, clay swelling and high proportion of bound water. According to the observed experimental results, the pressure and temperature conditions should be comprehensively optimized to enhance the mass transfer effect and improve economic benefits. This work provided greater insights into future marine NGH exploitation and contributed to carbon sequestration with applying CH 4 /CO 2 replacement method. [ABSTRACT FROM AUTHOR]

Pressure oscillation controlled CH 4 /CO 2 replacement in methane hydrates to enhance CH 4 recovery and CO 2 storage. [Display omitted] • The oscillated pressure control is proposed to enhance CH 4 /CO 2 replacement. • Inner replacement cause CH 4 hydrate decomposition and mixed hydrate regeneration. • Optimized depressurization is crucial to improve CH 4 production and CO 2 storage. The recovery of CH 4 from natural gas hydrates via CH 4 /CO 2 replacement is a highly attractive method for achieving the recovery of CH 4 and storing CO 2. However, although extensive simulation and experimental studies have been conducted, applying CH 4 /CO 2 replacement is currently hampered by a lack of in-depth understanding about the replacement mechanism and the insufficient replacement efficiency and rate achieved. Therefore, to enhance CH 4 recovery and CO 2 storage, we propose a method that uses pressure oscillation during CH 4 /CO 2 replacement. The effects of depressurization pressure, depressurization time, and depressurization frequencies were analyzed in this study. The characteristics of pressure and temperature were analyzed during the depressurization process, and the decomposition and regeneration of hydrates relating to the oscillation pressure were observed on a macro level. The results show that this newly proposed combined method effectively improves CH 4 /CO 2 replacement, and the maximum amounts of CH 4 recovered and the CO 2 storage efficiency are nearly double than those achieved without pressure oscillation. CH 4 /CO 2 replacement is promoted by the pressure oscillation mechanism by breaking the balance of the hydrate layer, which results in the dissociation of CH 4 hydrates and finally forms a CO 2 /CH 4 mixed hydrate layer with free water. The results of this study provide a valuable method that can be used to enhance CH 4 recovery and CO 2 sequestration. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrate formation induced by cold seeps is first investigated by MRI. • Local heat release by hydrates formation leads to the dissociation of nearby hydrates. • Cold seeps with high gas flow rate and pressure are more valuable for hydrate resource surveys. Gas hydrates could occur as outcrop resources overspreading on the seafloor, coexisting with bubbling gas in activated cold seeps. This involves a complicated process of hydrate nucleation and accumulation in the sedimentary matrices as well as in the bulk water with an underlying gas plume. In addition, the escaping methane gas could also be a major concern in terms of its greenhouse effect. Consequently, the behaviors of hydrate formation in the sediments with a successive gas flow to mimic the cold seeps were monitored through magnetic resonance imaging technique in this work. It was found that the initial hydrate nucleation could occur universally throughout the sedimentary matrices resulted from an enhanced gas–water contact upon the gas flow. Specially, the substantial heat released upon hydrate formation could even locally trigger dissociation of adjacent hydrates. A faster hydrate formation was observed under higher gas flow rates and formation pressures, consolidating the dominant role of a sufficient gas–water contact in the hydrate formation. Nevertheless, large amount of residual water could still remain till the very end of the formation process; the subsequent reaction would be significantly limited by the gas diffusion barrier effect of the existing hydrate shell. The findings could provide insights into the kinetic process of hydrate formation in the cold seeps and expand our knowledge on the diverse occurrence of gas hydrate under the sea. [ABSTRACT FROM AUTHOR]

[Display omitted] • Inhibitors are used to prevent gas hydrates from clogging pipelines. • Effects of clay particles on polyvinylcaprolactam hydrate inhibition were studied. • Clay particles adsorbed the inhibitor, and the particle surface potential changed. • In the presence of clay particles, the inhibitory effects were weakened. Kinetic hydrate inhibitors are widely used during upstream activities in the oil industry to prevent gas hydrates from clogging pipelines. Yet their performance in the pipelines transporting natural gas from marine environment is largely veiled where clay particles and their adsorbability are generally present. In this study, it was found that the fine clay particles from a hydrate rich area in the South China Sea are majorly composed of layered silicate with a negative surface charge. Consequently, the clay particles would adsorb the inhibitor through Van der Waals attractions and ion–dipole interactions. The resulting weakened surface potential of the particles would trigger an aggregation of the clays together with the inhibitors attached on their surfaces. This will thereby result in less active components in the solution, significantly weakening the inhibition effect. It is therefore suggested that the effects of clay-rich conditions on the performance of the inhibitors should be carefully considered for an efficient dosage in the marine environments. [ABSTRACT FROM AUTHOR]

• Metabolic molecules significantly promote hydrate formation. • Protein macromolecules could act as kinetic inhibitor of hydrate nucleation. • Water in the macropores is preferentially consumed during hydrate formation. Numerous efforts have been made to mimic the natural marine environments to understand the behavior of hydrate formation and accumulation in the sediments under seafloor. Yet it is found that the role of organic matters in the kinetics of hydrate formation is largely veiled, which should be much enriched as a result of the activities of the marine ecosystem. Therefore in this study, the kinetic effects of protein macromolecules and metabolic small molecules on CH 4 hydrate formation were examined; the organic compounds were extracted from the samples gathered from the hydrate reservoir located in the South China Sea. A total of 2058 species classified into 10 types of protein macromolecules and metabolic molecules were identified. Low-field nuclear magnetic resonance technique was used to in-situ monitor the hydrate formation process. The results showed that the metabolic molecules could remarkably promote hydrate formation, with more water converted into hydrate within a shorter time. Proteins have been widely studied to inhibit hydrate formation; here it is for the first time confirmed that the proteins extracted from the natural samples drilled in the hydrate reservoir could act as kinetic inhibitor, lowering the possibility of hydrate nucleation. It was further indicated that hydrate formation majorly consumed the water in the macro pores; this was followed by a diffusion-limitation process with the hydrate shell acting as mass transfer barrier slowing down the water conversion. The water in the micropores was yet found to be very difficult to participate in the reaction, potentially resulting from the unavailable gas-water contact. The findings could provide insights into the kinetics of hydrate formation in the presence of organic matters abundant in natural sediments and expand the current understandings on the occurrence of natural gas hydrate under marine environments. [ABSTRACT FROM AUTHOR]

• Initial gas production rate is enhanced by combining fracturing with depressurization. • Increasing fracturing length accelerates the average gas production rate. • Timely and sufficient boundary heat transfer is necessary for the continual hydrate dissociation. • Hydrate dissociation pattern is visually presented and systemically discussed. Most of the existing field tests of gas recovery from hydrate-bearing sediments suffer from the difficulty in pressure propagation, leading to a low productivity and energy efficiency. Fracturing has shown enormous potential in the shale gas production; thus in this study, we introduced this technique into the laboratory-scale gas hydrate production. The performance of gas production was investigated through numerically analyzing the gas hydrate dissociation behavior under different fracturing patterns. The results indicate that fracturing can significantly facilitate the pressure propagation in the hydrate sediments with a low intrinsic permeability during depressurization, thereby contributing to a better gas production performance. Fracture depth plays a critical role in promoting the gas production efficiency; the maximal enhancement ratio of average production rate by fracturing could attain 13.1%. Moreover, the contribution of reservoir's sensible heat in hydrate dissociation is limited in the core with a low permeability; timely and sufficient heat supply is thus important for the successive gas production. The findings of this study illustrate the effects of fracturing on enhancing the gas production efficiency of depressurization-induced gas hydrate exploitation; this will provide important guidance for its potential application in the field test of marine and permafrost hydrate reservoir where low permeability is commonly encountered and an enhancing technique is much required. [ABSTRACT FROM AUTHOR]

The kinetics of bulk hydrate formation is of crucial importance in the application of hydrate-based technique involving energy storage, desalination and flow assurance. Various procedures have been found to significantly impact the kinetics of hydrate formation; yet with few studies touching the effects of a weak electric field. A voltage across the solution could likely affect the activity and orientation of the molecules inside, thereby playing a potential role in the hydrate nucleation, crystal growth and even diffusion of water and guest molecules. In this study, the effect of a weak electric field on the induction time, bulk surface characteristics and growth rate of THF (tetrahydrofuran) hydrate were investigated by controlling the voltage across the two electrodes placed on the two sides of the reactor. The results showed that an increasing voltage intensity could result in a longer induction time of hydrate formation. The memory effect was fading at an increasing voltage intensity, indicating that the relic components helping the fast formation could be destroyed by the electric field. The following formation under the electric field can be identified by a neat zigzag surface of hydrate generated from the periphery of the reactor. The formation rate of THF hydrate was found to increase with a rising voltage intensity during the hydrate growth period. The results could be of help in the application of hydrate-based techniques in industries where a fast and successive hydrate growth is expected. Image 1 • An increasing electric intensity could result in a longer induction time. • The memory effect was fading upon an increasing voltage intensity. • Hydrate growth could be slightly promoted by the electric field. [ABSTRACT FROM AUTHOR]

• The step-wise depressurization contributes to reducing the initial water production. • There exists a water-productive stage accounting for 47% of the total production. • A strong dependency of the water yield on the production pressure is identified. • The thermal stimulation contributes to more than 30% decline of water yield. There have been several trial production tests carried out from marine natural gas hydrate reservoir recently, showing its great potential as an alternative source of energy. Yet an unsustainable production with low productivity and short duration is generally encountered. The marine hydrate reservoirs are mostly highly water-saturated; the resulting water production behaviors remain largely unclear. In this work, the gas and specially water production from a water-saturated reservoir were investigated. The role of water acting as a diffusion barrier of gas was determined: a higher water yield will significantly improve the following gas production. The step-wise depressurization was found to help relieve the initial water production compared with the straightforward depressurization scenario. A high-water-production stage was for the first time identified, accounting for ~47% of the total water production. A further controlled depressurization with finer steps in this stage could enhance the gas productivity by at most 31%; yet its effect on controlling water production was limited. The cumulative water yield depended much on the overall degree of depressurization, regardless of the number of steps. In order for an enhanced gas production under a regulated water yield, thermal stimulation is introduced in the high-water-production stage. This is found to effectively contribute to an optimized water producing process and an at least 30% decline of water yield under comparable gas productivity. The proposed combination method could be applied in the field tests from water-saturated marine reservoir to achieve a high gas-water ratio and a thereby improved energy and economic efficiency. [ABSTRACT FROM AUTHOR]

• Significant difference in production behavior from fine and coarse grain is found. • Blockage problem could be induced by accumulation of fine natural sands. • The proposed step-wise depressurization can alleviate the temperature drop. • The sensible heat of the reservoir plays a dominant role in the gas production. There have been several field tests around the world to recover natural gas from gas hydrate reservoirs, yet they mostly suffer from the low productivity, sand problems and poor economic efficiency. The ultralow permeability of the sediments and an insufficient heat supply act as major barriers. Intense investigations have been carried out to study the gas production behavior from lab-synthesized hydrate-bearing coarse sands; yet the porous matrices could differ significantly from the natural sediments. Few studies have been found to focus on the role of fine natural sediments in the gas recovery performance. In this study, natural marine sediments from the South China Sea are used as the porous matrices, and gas is produced through an optimized stepwise depressurization. Significant differences in the gas production behaviors from fine natural sediments and coarse grains are found. The fine marine sediments may induce the blockage of the production well, even with protection measures; this problem will result in an uncontrolled pressure drop and gas output. Moreover, the stepwise depressurization efficiently contributes to remitting the temperature drop of the reservoir, increasing the minimum temperature from −0.5 ℃ upon a straight pressure drop to 0.27 ℃ upon a step-wise depressurization. This method can therefore help prevent the ice generation during hydrate dissociation. Additionally, a finer pressure gradient of 0.5 MPa will enhance the hydrate dissociation rate by 18.92% compared with the 2 MPa step scenario. The average gas production rate is determined to positively correlate with the Stefan number, which indicates a dominant role of the sensible heat of the reservoir in the efficient gas production. The findings could be applied in the field tests of gas hydrates to avoid ice generation and achieve a stable and continuous gas production behavior. [ABSTRACT FROM AUTHOR]