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Evaluation of wave impacts on submarine landslides is an essential element in geohazard studies. The slight desaturation of sediments (due to dissociation of gas hydrates) has been found to adversely impact the slide of the sloping seabed in the Fraser River Delta in Canada. In this study, to investigate the role of wave action on the slide of partially saturated seabed slopes, an integrated FEM model is developed. Despite most earlier studies that used a simplified decoupled undrained analysis, in this article, a more realistic model for coupled flow-and-deformation processes (within the sediments) and fluid-seabed interaction is utilized. Linear wave theory and Biot's poroelasticity for the fluid and seabed domains are considered, respectively, and continuity of flux and traction is enforced along the interface of the media. The instability of the sloping seabed is investigated using strength reduction finite element method (SRFEM) with Mohr-Coulomb failure criterion. The limitation of limit equilibrium methods in the evaluation of submarine landslides is shown through comparison with SRFEM analyses where partly-dynamic and quasi-static idealizations of seabed response are considered. Finally, the adverse impacts of slight desaturation on seabed instability are assessed, and the reduction of the stability number with seabed steepness is presented. [ABSTRACT FROM AUTHOR]

The kinetic characteristics governing methane conversion into solid hydrates within seabed sediments influence hydrate accumulation and resource distribution, which is significant for the evaluation of global natural gas hydrate reserves and the implication of hydrate development. However, the influence of sediment particle size on methane hydrate formation kinetics remains unclear. In this study, simulating in situ conditions of the seafloor, the kinetics behaviors of CH 4 hydrate formation within saline porous sediments were investigated. The experimental results indicate that the methane hydrate formation rate does not exhibit a strictly unidirectional change with changed quartz sand particle size. The unidirectionality and magnitude of hydrate formation rate change are simultaneously influenced by both quartz sand particle size and water saturation levels. The different effective gas-liquid contact area is the primary factor underlying the occurrence of these phenomena. However, for more porous clay sediments with greater specific surface area, the theory of effective gas-liquid contact area becomes inapplicable. The addition of clay to quartz sand sediment inversely inhibits the methane hydrate formation, which is caused by water absorption. This work can advance the comprehension of the intricate interplay between hydrate growth kinetics and the attributes of sedimentary hosts. • CH 4 hydrate formation rate does not exhibit a strictly unidirectional change with varying quartz sand particle size. • The unidirectionality of hydrate formation rate change are simultaneously influenced by particle size and water saturation. • Effective gas-liquid contact area determines hydrate formation rate rather than quartz sand particle size. • The inherent water-absorbing quality of porous clay instead results into reduced CH 4 hydrate formation kinetics. [ABSTRACT FROM AUTHOR]

Mechanical behavior of hydrate‐bearing sediments is critical for well stability, reservoir deformation, and sea floor settlement during gas production. Current understanding on the mechanism of hydrate strengthening hosting sediments relies on conceptual models based on idealistic pore‐scale assumptions. Yet pore‐scale study is rare and limited to hydrate‐bearing sediments formed with surrogate guest molecules rather than methane. We present for the first time pore‐scale triaxial test results of methane hydrate‐bearing sediments. Besides the traditional stress‐strain relationship, we further explored (1) sand particle crushing and (2) pressure‐temperature dependent strength variations and (3) creep of hydrate‐bearing sediments. Results show that as hydrate enables the sand skeleton to bear additional loads, the potential of sand crushing upon hydrate dissociation also increases. Strength of hydrate‐bearing sediments decreases as pressure‐temperature condition approaches hydrate phase boundary. Hydrate‐bearing sediments creep and heal with time. The new observations suggest additional complications to be considered during gas production. Plain Language Summary: The presence of solid hydrate in sediment pores is known to enhance the strength of hosting sediments, similar to the effect of ice in frozen sediments. However, our understanding on the mechanism of increased strength still relies on simplified conceptual models based on idealistic pore‐scale assumptions. Direct visualization of pore‐scale hydrate behavior can greatly advance our knowledge on this issue, yet studies are rare and conducted with surrogate gas hydrates due to challenges (1) to maintain the stability of hydrate under high pressure and low temperature, (2) to visualize the specimen with high resolution while the specimen is housed inside high‐pressure core holders, and (3) to distinguish methane hydrate from other pore constituents in CT images. We overcome these issues and conducted three experiments on specimens with different hydrate saturations. Our results highlight that the presence of hydrate strengthens the sediments by (1) providing supports for the original sediment skeleton and (2) forming hydrate skeletons to directly bear the load, especially when the hydrate saturation is high. In addition, our results show massive sand crushing, temperature‐pressure‐dependent specimen strength, creeping, and healing of hydrate‐bearing sediments. Key Points: Higher hydrate saturation increases the load‐bearing capacity of sediments, hence the potential of sand crushingHydrate‐bearing sediments degrade gradually as the pressure‐temperature approaches hydrate phase boundary and can fail catastrophicallyHydrate‐bearing sediments creep, therefore induce reservoir deformation with time and meanwhile heal to partially regain strength [ABSTRACT FROM AUTHOR]

Gas hydrates can form more or less at the same time as seafloor sediment. They can have the effect of significantly stiffening and strengthening deep‐ocean sediments. Subsequent increases in situ temperature or decreases in pressure may trigger hydrate dissociation, leading to large reductions in the strength and stiffness of the sediment and possible seafloor instability. Gas hydrate dissociation not only removes cementing. It also releases freshwater and significant amounts of trapped gas that are dependent on multiple factors such as type of sediment, available pore space, hydrate morphology, and hydrate saturation. The presence of pock marks in areas of known seabed instability suggests that hydrate dissociation may have been a factor in triggering failure at these locations. Having reviewed the mechanisms by which the strength and stiffness of seabed sediment may be changed during dissociation, this paper reports the results of laboratory testing to evaluate the effects of loss of hydrate cement on strength and stiffness, for a range of sand‐sized materials with differing particle size, specific surface area, and particle shape, using a laboratory gas hydrate triaxial apparatus. The results suggest that both the strength and the stiffness of hydrate‐cemented granular materials are affected significantly by the specific surface available for hydrate cementation and, to a certain extent, by the particle shape. Uniform coarse granular sediments of lower specific surface area can suffer significant loss of stiffness and strength upon hydrate dissociation, changing the sediment from dilative to contractive. Finer‐grained sediments appear less affected by dissociation. Key Points: The effect of methane hydrate cement on the strength and stiffness of four different sands was determined from laboratory testsMeasurements were made using very small strain probe tests during hydrate formation (using the excess gas method), and at failureParticle size, sphericity, and platyness have considerable and complex effects on the stiffness and strength of hydrate‐bearing sediment [ABSTRACT FROM AUTHOR]

Deep-sea fans have been proposed to act as carbon sinks, rapid deposition driving shallow methanogenesis to favor net storage within the gas hydrate stability zone (GHSZ). Here, we present new evidence of widespread gas venting from the GHSZ on the upper Amazon deep-sea fan, together with analyses of the first samples of gas hydrates recovered offshore NE Brazil. Multibeam water column and seafloor imagery over an 18,000-km2 area of the upper Amazon fan reveal 53 water column gas plumes, rising from venting features in water depths of 650-2600 m. Most gas vents (60%) are located along seafloor faults that record the ongoing gravitational collapse of the fan above deep décollements, while others (40%) are located in water depths of 650-715 m within the upper edge of the GHSZ. Gas compositions from hydrates recovered in vents at three locations on and north of the fan indicate biogenic sources (dominantly methane with 2-15% of CO2; δ13C from − 81.1 to − 77.3‰), whereas samples from vents adjacent to the fan proper include possible thermogenic contributions (methane 95%, CO2 4%, and ethane 1%; δ13C - 59.2‰). These results concur with previous findings that the upper edge of the GHSZ may be sensitive to temporal changes in water temperatures, but further point to the importance of gas escape from within areas of gas hydrate stability. Our results suggest the role of fluid migration along pathways created by faulting within rapidly deposited passive margin depocenters, which are increasingly recognized to undergo gravitational collapse above décollements. Our findings add to evidence that gas can escape from sediments to the sea in areas where gas hydrates are stable on passive margins, and suggest the need of further studies of the dynamics of deep-sea depocenters in relation to carbon cycling. [ABSTRACT FROM AUTHOR]

Natural gas hydrate is a potential source of methane which needs to be extracted from under the sea bed. For the economic recovery of methane from natural gas hydrates, production approaches such as depressurization, thermal stimulation, and inhibitor injection are being investigated. However, studies involving hydrate-bearing clayey sediments and recovery of methane from such reservoirs are rare. This work investigates in detail the potency of hydrate dissociation methods such as depressurization by constant rate gas release, thermal stimulation and the combination of two for energy recovery from hydrate bearing clayey sediments underlying a free gas zone. Pure water and two different mud samples containing 3 and 5 wt% of bentonite were used for methane hydrate formation and dissociation studies. Thermodynamic study of methane hydrate in the presence of bentonite clay was also conducted for the above two concentrations. No considerable effect of clay on the inhibition or promotion of methane hydrate formation was observed. Initially, methane hydrate formation has been investigated using pure water, 3 and 5 wt% bentonite mud at an initial hydrate formation pressure of 8 MPa and at a temperature of 278.15 K. Subsequently, methane hydrate dissociation experiments were carried out using depressurization, thermal stimulation and their combination. The effect of the rate of gas release on hydrate dissociation by depressurization was investigated using two different rates of 10 mL/min and 20 mL/min. Thermal stimulation experiments were carried out for ΔT = 15 K at the rate of 7.5 K/hr and the results on methane recovery were recorded. The detailed investigation shows that the combination of the two methods is more efficient for methane production than the standalone method in clayey hydrate reservoir. This study provides important insights into the hydrate production methodology from clayey hydrate reservoirs. [ABSTRACT FROM AUTHOR]

We investigated the effect of microscopic distribution modes of hydrates in porous sediments, and the saturation of hydrates and free gas on the elastic properties of saturated sediments. We simulated the propagation of seismic waves in gas hydrate-bearing sediments beneath the seafloor, and obtained the common receiver gathers of compressional waves (P-waves) and shear waves (S-waves). The numerical results suggest that the interface between sediments containing gas hydrates and free gas produces a large-amplitude bottom-simulating reflector. The analysis of multicomponent common receiver data suggests that ocean-bottom seismometers receive the converted waves of upgoing P- and S-waves, which increases the complexity of the wavefield record. [ABSTRACT FROM AUTHOR]

Highlights: [•] Six cores from topographic highs of the eastern Sea of Japan were examined. [•] The cores were from areas with gas hydrate on the sea floor and in sediments. [•] Traces showed submarine landslides during and after the last glacial maximum. [•] Low pressure and warming caused gas hydrate dissociation and slope failure. [•] Seismic activity must also be considered. [Copyright &y& Elsevier]

Recent estimations suggest that vast amounts of methane are locked in the Arctic Ocean bottom sediments in various forms of gas hydrates. A potential feedback from a continued warming of the Arctic region is therefore the release of methane to the atmosphere. This study addresses the relationship between a warming of the Arctic ocean and gas hydrate stability. We apply a theoretical model that estimates the base of the gas hydrate stability zone in the Arctic Ocean considering different bottom water warming and sea level scenarios. We model the present day conditions adopting two different geothermal gradient values: 30 and 40°C/km. For each geothermal gradient value, we simulate a rise and a decrease in seafloor temperature equal to 2°C and in sea level equal to 10 m. The results show that shallow gas hydrates present in water depths less than 500 m would be strongly affected by a future rise in seafloor temperature potentially resulting in large amounts of gas released to the water column due to their dissociation. We estimate that the area, where there could be complete gas hydrate dissociation, is about 4% of the area where there are the conditions for gas hydrates stability. [ABSTRACT FROM AUTHOR]

Abstract: Simultaneous CO2 sequestration and CH4 production has been considered as a possible way to form and restore CO2 hydrates in sediments of natural gas hydrates in seabeds. The phase equilibrium data for the upper-quadruple (H–W–L–V) phase states of CO2CH4 mixed hydrates are needed for the application of the replacement of CH4 hydrate with CO2. In this paper, the upper-quadruple (Q2) phase equilibrium properties of CO2CH4 mixed hydrates were investigated by using a visual experimental apparatus in the temperature range of 273.16–297.15K and pressure up to 10MPa and verified by calculation with thermodynamic models. The coexisting of upper-quadruple phase region was identified in 4.46–8.4MPa, 283.09–287.9K and 0–0.225 methane mole fractions. It is found that the range of temperature and pressure for Q2 phase coexistence increases with the methane concentration until the retrograde of the quadruple phase is reached. The start and end melting points of the CO2CH4 mixed hydrates in the upper quadruple states were tested in constant volume condition. The tested results had also been compared with the predictions with a combination of the thermodynamic models for the mixed gas hydrate and CO2CH4 vapor–liquid equilibrium. The existence of the Q2 phase equilibrium region and its extension are clarified. [Copyright &y& Elsevier]

Abstract: Gas hydrate systems offshore western Svalbard are extensive and include the whole Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200–1300m is pierced with fluid flow features. Seafloor pockmarks vary in size up to 700m in diameter. Pockmarks are generally larger at the eastern start of the Vestnesa Ridge than at its western termination. A recent cruise with R/V Jan Mayen discovered methane flares in the water column above the pockmark field at the onset of the Vestnesa Ridge. Over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800m. It is still unclear what has triggered the increase in gas expulsion from seafloor sediments. High-resolution 3D seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems. At the western end of the Vestnesa Ridge, sub-seafloor fluid flow features resemble well-described chimney structures. However, the seismic expression of the active fluid flow features is much broader, much more chaotic, dome-shaped and is not very similar to a typical chimney structure. The Vestnesa Ridge gas-hydrate and free-gas system occurs within few kilometers of a mid-oceanic ridge and transform fault, which makes this gas-hydrate system unique on Earth. The close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas-hydrate and free-gas system. The high heat flow together with the high tectonic activity of this region, a thick sedimentary cover, a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free-gas system leading to high variability in gas supply, gas migration and gas hydrate build up and dissociation. [Copyright &y& Elsevier]

The article presents information on available tools and advancement in pressure core technology for undertaking various oceans drilling projects for recovery of hydrates bearing sediments. The pressure core technology for characterization of hydrate bearing sediments requires coring, manipulation and testing under pressure and temperature (P-T) conditions within the ocean bed. The pressure core system derives preservation of sediments and production of natural gas.

The passive eastern Indian margin is rich in gas hydrates, as inferred from the wide-spread occurrences of bottom-simulating reflectors (BSRs) and recovery of gas hydrate samples from various sites in the Krishna Godavari (KG) and Mahanadi (MN) basins drilled by the Expedition 01 of the Indian National Gas Hydrate Program (NGHP). The BSRs are often interpreted to mark the thermally controlled base of gas hydrate stability zone (BGHSZ). Most of the BSRs exhibit moderate to typically higher amplitudes than those from other seismic reflectors. We estimate the average geothermal gradient of ∼40°C/km and heat flow varying from 23 to 62 mW/m in the study area utilizing the BSR's observed on seismic sections. Further we provide the BGHSZ where the BSR is not continuous or disturbed by local tectonics or hidden by sedimentation patterns parallel to the seafloor with a view to understand the nature of BSR. Since, gas hydrate bearing sediment has higher electrical resistivities than that of the host sediment, we estimate two levels of gas hydrates saturations up to 25% in the depth interval between 70 to 82, and less than 20% in the depth interval between 90 to 104 meter below the seafloor using the resistivity log data at site 15 of NGHP-01. [ABSTRACT FROM AUTHOR]

The existence of gas-hydrates in marine sediments increases the seismic velocity, whereas even a small amount of underlying free-gas reduces the velocity considerably. The change in velocities against the background (without gas-hydrates and free-gas) velocity can be used for identification and assessment of gas-hydrates. Traveltime inversion of identifiable reflections from large offset multi channel seismic (MCS) experiment is an effective method to derive the 2-D velocity structure in an area. We apply this method along a seismic line in the Kerala-Konkan (KK) offshore basin for delineating the gas-hydrates and free-gas bearing sediments across a bottom simulating reflector (BSR). The result reveals a four layer 2-D shallow velocity model with the topmost sedimentary layer having velocity of 1,680-1,740 m/s and thickness of 140-190 m. The velocity of the second layer of uniform thickness (110 m) varies from 1,890 to 1,950 m/s. The third layer, exhibiting higher velocity of 2,100-2,180 m/s, is interpreted as the gas-hydrates bearing sediment, the thickness of which is estimated as 100 to 150 m. The underlying sedimentary layer shows a reduction in seismic velocity between 1,620 to 1,720 m/s. This low-velocity layer with 160-200 m thickness may be due to the presence of free-gas below the gas-hydrates layer. [ABSTRACT FROM AUTHOR]

Abstract: Submarine gas hydrates are a major global reservoir of the potent greenhouse gas methane. Since current assessments of worldwide hydrate-bound carbon vary by one order of magnitude, new technical efforts are required for improved and accurate hydrate quantifications. Here we present hydrate abundances determined for surface sediments at the high-flux Batumi seep area in the southeastern Black Sea at 840m water depth using state-of-the art autoclave technology. Pressure sediment cores of up to 2.65m in length were recovered with an autoclave piston corer backed by conventional gravity cores. Quantitative core degassing yielded volumetric gas/bulk sediment ratios of up to 20.3 proving hydrate presence. The cores represented late glacial to Holocene hemipelagic sediments with the shallowest hydrates found at 90cmbsf. Calculated methane concentrations in the different cores surpassed methane equilibrium concentrations in the two lowermost lithological Black Sea units sampled. The results indicated hydrate fractions of 5.2% of pore volume in the sapropelic Unit 2 and mean values of 21% pore volume in the lacustrine Unit 3. We calculate that the studied area of ~0.5km2 currently contains about 11.3kt of methane bound in shallow hydrates. Episodic detachment and rafting of such hydrates is suggested by a rugged seafloor topography along with variable thicknesses in lithologies. We propose that sealing by hydrate precipitation in coarse-grained deposits and gas accumulation beneath induces detachment of hydrate/sediment chunks. Floating hydrates will rapidly transport methane into shallower waters and potentially to the sea–atmosphere boundary. In contrast, persistent in situ dissociation of shallow hydrates appears unlikely in the near future as deep water warming by about 1.6°C and/or decrease in hydrostatic pressure corresponding to a sea level drop of about 130m would be required. Because hydrate detachment should be primarily controlled by internal factors in this area and in similar hydrated settings, it serves as source of methane in shallow waters and the atmosphere which is mainly decoupled from external forcing. [Copyright &y& Elsevier]

The article offers information on the accumulation of gas hydrates at the bottom of the sea during sequential deposits of sendiments with different transport properties. It states that geophysical studies have revealed more than 200 hydrate manifestations in the World Ocean, mainly in the offshore zones where favorable conditions for the generation of biogenic and thermogenic gas are realized. It mentions that accumulation of gas hydrates occurs during the deposition, burying, and compaction of sediments. It highlights that filtration in the sediment medium is the most powerful mechanism of gas supply into the stable zone of gas hydrates.

Two seismic sections offshore Arauco and Coyhaique, Chile, have been analysed to better define the seismic character of hydrate-bearing sediments. The velocity analysis was used to estimate the gas-phase concentration, which can serve to correlate hydrate presence to the geological features. The velocity model allowed us to recognise the hydrate layer above the bottom simulating reflector (BSR), and the free gas layer below it. The velocity field is affected by strong lateral variation, showing maximum (above the BSR) and minimum (below the BSR) values in the southern sector. Here, highest gas hydrate and free gas concentrations were calculated (15% and 2.7% of total volume respectively). The estimated geothermal gradient ranges from 35 to 95°C/km. In the northern sector, the highest gas hydrate and free gas concentrations are 15% and 0.2% of total volume respectively, and the geothermal gradient is uniform and equal to about 30°C/km. [ABSTRACT FROM AUTHOR]