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With the increasing maturity of ocean bottom seismometer technology in gas hydrate exploration, more and more researchers apply ocean bottom seismometer exploration technology for offshore oil exploration. However, due to the special observation mode of ocean bottom seismometer, it is impracticable to process ocean bottom seismometer data using traditional data processing methods, such as velocity analysis and pre‐stack time migration. This manuscript proposed a new velocity analysis method for ocean bottom seismometer data, which obtains more accurate root‐mean‐square velocity than the existing method. Then we deduced the Kirchhoff pre‐stack time migration formula for ocean bottom seismometer data. Two models demonstrate the correctness of the velocity analysis and migration methods. Finally, the two methods were applied to the actual ocean bottom seismometer data, and the obtained migration profile is consistent well with the profile of towed streamer data in the nearby area. [ABSTRACT FROM AUTHOR]

Fluid conduits play important roles in the dynamic evolution processes of marine gas hydrate systems (GHS), but detailed analysis of these specific roles has been inadequate. In this paper, various types of gas hydrate-related fluid conduits are analysed by examining abundant global case studies mainly using seismic data, and the interaction of these fluid conduits with the GHS is analysed. Results show that faults and gas chimneys are the most common fluid conduits, accounting for ~80% of the analysed GHS-related conduits. Other fluid conduits include permeable dipping strata, polygonal faults, mud/salt diapirs, mud volcanoes, and unconformity surfaces. Two groups of fluid conduits can be classified according to their roles for the GHS. Group 1 fluid conduits mainly exist underneath the GHS which transported deeper gas upwards to the GHS, providing gas source to form gas hydrates or the underlying free gas zone (FGZ). Group 2 fluid conduits are less in quantity compared to group 1, accounting for ~28% of the analysed conduits and often located at shallower depth within the GHS. When the FGZ is critically pressured due to the continuous accumulation of free gas or gas hydrates dissociate, group 2 fluid conduits may be triggered or reactivated, channelling free gas to the shallower layers or even the seabed. Group 1 fluid conduits mainly play a positive role in the formation of the GHS, while group 2 fluid conduits may play a role in affecting the seabed ecosystem or even climate change due to the gas escape to the seabed or even the sea water. Generally, this classification method can help better understand the roles of fluid conduits for the GHS, which should be widely applicable in other gas hydrate study areas. [ABSTRACT FROM AUTHOR]

Natural gas hydrates are crystalline solids found in sediments beneath the seabed and permafrost, which significantly affect the mechanical behavior of sediments. The effect of hydrates often varies for different sediments, which has been captured in some laboratory studies. But the particle-scale information involved is obscure and the underlying mechanisms often rely on assumptions. In this study, discrete element method (DEM) was used to improve understanding of the hydrate effect on the mechanical behavior of different sands. Triaxial compression tests were carried out using DEM on the digital hydrate-bearing sands (HBS) with different grain sizes. Simulation results shows that the large-grain samples often yield earlier and tend to strain-hardening behavior, while hydrate occurrence induces strain-softening behavior. In terms of stress-strain response and porosity changes, the hydrate strengthening on the small-grain samples is much greater than the large-grain samples. Particle-scale observations indicate that particle rotation usually occurs in regions with small particle displacement and is more intense on shear slip surfaces. Hydrate bonds are mainly subjected to tensile failure, and large grain size may hinder bond shear failure. Hydrates restrict the development of radial fabrics and enhance the stress-induced anisotropy, rendering a more stable framework for sediments. • Large-grain sediments often yield earlier and tend to strain-hardening behavior. • Hydrate strengthens small-grain sediments much greater than large-grain sediments. • Significant particle rotation occurs on the shear slip surface. • Hydrate enhances the stress-induced anisotropy in sediments. [ABSTRACT FROM AUTHOR]

Long offset seismic reflection data reveal the presence of four Bottom Simulating Reflectors (BSR0-3) within folded sediments of the Tuapse Trough, along the NE margin of the Eastern Black Sea Basin (EBSB). Multiple BSRs are observed in other sites worldwide, however, their origin and formation mechanisms are still debated. Here, we investigate the formation mechanisms of the EBSB multiple BSRs based on their seismic character and on their physical properties derived from reflected and refracted arrival seismic velocities. Seismic reflection data are downward continued to enhance refracted arrivals. A 2D travel-time velocity model of the sub-seabed, using combined travel-times from non-downward-continued reflected and downward-continued refracted signals, shows variations in the physical properties at the BSRs and nearby sediments. The P-wave velocity (V P) increase of 1.55–1.72 km/s between the seafloor and BSR0 (258 mbsf) reflects normal compaction trends in sediments, whereas the V P of 1.75–1.83 km/s between BSR0 and BSR1 (360 mbsf) is higher than that expected for sediments at that depth. Beneath BSR1, a V P decrease from 1.83 km/s to 1.61 km/s occurs within a 70-80 m-thick layer including BSR2 (395 mbsf) and extending to BSR3 (438 mbsf). Beneath BSR3, V P increases. Based on an analytical model linking seismic velocity to physical properties, these V P trends can be explained by a gas hydrate saturation from 0 to 2% between the seafloor and BSR0, reaching 4 ± 2% just above BSR1. A free gas saturation of up to 20–25% is estimated within the low-velocity zone between BSR1 and BSR3. BSR1 likely represents the present-day base of the gas hydrate stability zone (BGHSZ), which aligns with the theoretical BGHSZ assuming a geothermal gradient of 26–30 °C/km. Based on seismic polarities and results from travel-time analysis and rock physics modelling, we suggest that hydrate dissociation and recycling processes may explain the negative polarity of BSR2 and BSR3, which are still visible due to the presence of relict gas, and inferred higher gas hydrate saturations close to the present-day base of the stability zone at BSR1. Also, structural and stratigraphic controls seem to have favoured focused free gas flow and hydrate formation at the top of an anticlinal structure, thus likely controlling multiple BSR generation in the EBSB. • A 2D seismic reflection profile reveals the presence of four multiple BSRs at the crest of folded sediments in the Eastern Black Sea Basin (EBSB). • Downward-continuation of the seismic profile is used to perform 2D travel-time tomography , helping to constrain seismic velocities at the BSRs. • Combined travel-time analysis and rock physics modelling provide an estimate of gas hydrate and free-gas saturations at the observed BSRs. • Methane recycling mechanisms may explain an enriched pore-floating gas hydrate between BSR0 and BSR1, and a free-gas zone between BSR1 and BSR3. • The tectono-stratigraphic settings of the EBSB favoured a focused gas and gas hydrate accumulation at the crest of the folded structure. [ABSTRACT FROM AUTHOR]

This article describes the combustion behavior of combustible gases as they are released from the clathrate cages of a hydrate. Gas hydrates (clathrates) are ice-like crystalline solids that encapsulate guest gas molecules. It has become known that a significant methane storehouse is in the form of methane hydrates on the sea floor and in the arctic permafrost. There is great interest in this large fuel storehouse, particularly how to extract the methane from the clathrates. One of the unique features of methane clathrate is that it is flammable, despite being 85% water – fiery ice. While methane clathrates are the most prevalent in nature, other combustible gas hydrates (notably, propane and hydrogen) also have potential energy technology implications. In addition, carbon-dioxide hydrates have been proposed as a potential post-combustion greenhouse gas sequestration strategy, and there is a wide range of separation technologies and thermal management that take advantage of the unique thermodynamic and kinetic features of hydrate formation. To better understand the important implications of direct utilization of fuel clathrates and the related potential environmental consequences of CO 2 hydrates, we describe the state-of-the-art knowledge regarding the formation and structure of gas hydrates, and the combustion behavior of flammable gas hydrates. The combustion studies involve determining the rate of ice melt and water evaporation during the hydrate burn, as well as the interesting phenomenon of self-healing, where the hydrates stop burning by forming an ice sheet on their surface. Experimental results are used to estimate the heat transfer from the flame into the hydrate and to calculate the amount of energy released to sustain the flame. This article provides the reader with a comprehensive understanding of the basics and the subtleties of hydrates and their combustion, thereby explaining the true meaning of fiery ice. [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]

This study extends the seismic dynamics of two-phase/three-phase poroelastic sediments in coupled ocean-sediment-substrate systems to gas hydrate-bearing sediments. The ocean sediment floor is described as a finite-thickness porous medium with two solids and a compressible viscous fluid (porous skeleton, gas hydrate, and fluid), which sits on an elastic seabed. In the coupled system of seawater, gas hydrate-bearing sediment layer and seabed, analytical solutions for the P wave reflection and transmission coefficients are derived. Two key factors are derived: displacement amplitude and magnification factor. The relationship among two key factors with incident angle, frequency, sediment thickness, cementation parameter, contact parameter, temperature and saturation in the sedimentary layer containing gas hydrate is analyzed. The research shows that when the P wave is vertically incident (θ ip = 0°), the incident frequency has a greater influence on the vertical displacement amplitude, and when the P wave is grazing incident (θ ip = 90°), the influence is not observed. The displacement of the sedimentary layer fluctuates in different degrees with the increase of the layer thickness. When the temperature increases, the saturation is higher, that is, the hydrate content is lower, and the influence of the contact parameters gradually disappears. [ABSTRACT FROM AUTHOR]