Marine sands highly saturated with gas hydrates are potential energy resources, likely forming from methane dissolved in pore water. Laboratory fabrication of gas hydrate‐bearing sands formed from ...dissolved‐phase methane usually requires 1–2 months to attain the high hydrate saturations characteristic of naturally occurring energy resource targets. A series of gas hydrate formation tests, in which methane‐supersaturated water circulates through 100, 240, and 200,000 cm3 vessels containing glass beads or unconsolidated sand, show that the rate‐limiting step is dissolving gaseous‐phase methane into the circulating water to form methane‐supersaturated fluid. This implies that laboratory and natural hydrate formation rates are primarily limited by methane availability. Developing effective techniques for dissolving gaseous methane into water will increase formation rates above our observed (1 ± 0.5) × 10−7 mol of methane consumed for hydrate formation per minute per cubic centimeter of pore space, which corresponds to a hydrate saturation increase of 2 ± 1% per day, regardless of specimen size.
Key Points
Hydrate formation from methane dissolved in water is efficient and rapid
Dissolving methane into water is the rate‐limiting step in hydrate formation
In the lab or field, focus on water take‐up of methane to assess formation rate
Methane gas hydrates, crystalline inclusion compounds formed from methane and water, are found in marine continental margin and permafrost sediments worldwide. This article reviews the current ...understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate‐bearing sediments. Formation phenomena include pore‐scale habit, solubility, spatial variability, and host sediment aggregate properties. Physical properties include thermal properties, permeability, electrical conductivity and permittivity, small‐strain elastic P and S wave velocities, shear strength, and volume changes resulting from hydrate dissociation. The magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate‐bearing sediments to changes in mechanical, thermal, or chemical boundary conditions. These predictions are vital for mitigating borehole, local, and regional slope stability hazards; optimizing recovery techniques for extracting methane from hydrate‐bearing sediments or sequestering carbon dioxide in gas hydrate; and evaluating the role of gas hydrate in the global carbon cycle.
Bulk properties of gas hydrate-bearing sediment strongly depend on whether hydrate forms primarily in the pore fluid, becomes a load-bearing member of the sediment matrix, or cements sediment grains. ...Our compressional wave speed measurements through partially water-saturated, methane hydrate-bearing Ottawa sands suggest hydrate surrounds and cements sediment grains. The three Ottawa sand packs tested in the Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI) contain 38(1)% porosity, initially with distilled water saturating 58,31, and 16% of that pore space, respectively. From the volume of methane gas produced during hydrate dissociation, we calculated the hydrate concentration in the pore space to be 70,37, and 20% respectively. Based on these hydrate concentrations and our measured compressional wave speeds, we used a rock physics model to differentiate between potential pore-space hydrate distributions. Model results suggest methane hydrate cements unconsolidated sediment when forming in systems containing an abundant gas phase.
Gas hydrate is an ice‐like form of water and low molecular weight gas stable at temperatures of roughly −10°C to 25°C and pressures of ~3 to 30 MPa in geologic systems. Natural gas hydrates sequester ...an estimated one sixth of Earth's methane and are found primarily in deepwater marine sediments on continental margins, but also in permafrost areas and under continental ice sheets. When gas hydrate is removed from its stability field, its breakdown has implications for the global carbon cycle, ocean chemistry, marine geohazards, and interactions between the geosphere and the ocean‐atmosphere system. Gas hydrate breakdown can also be artificially driven as a component of studies assessing the resource potential of these deposits. Furthermore, geologic processes and perturbations to the ocean‐atmosphere system (e.g., warming temperatures) can cause not only dissociation, but also more widespread dissolution of hydrate or even formation of new hydrate in reservoirs. Linkages between gas hydrate and disparate aspects of Earth's near‐surface physical, chemical, and biological systems render an assessment of the rates and processes affecting the persistence of gas hydrate an appropriate Centennial Grand Challenge. This paper reviews the thermodynamic controls on methane hydrate stability and then describes the relative importance of kinetic, mass transfer, and heat transfer processes in the formation and breakdown (dissociation and dissolution) of gas hydrate. Results from numerical modeling, laboratory, and some field studies are used to summarize the rates of hydrate formation and breakdown, followed by an extensive treatment of hydrate dynamics in marine and cryospheric gas hydrate systems.
Key Points
Gas hydrates are widespread, store large amounts of carbon, and are susceptible to degradation under warming ocean‐atmosphere conditions
Gas hydrate formation and breakdown rates are generally limited by mass transport and heat transport processes, respectively
Hydrate dissolution, not dissociation, is the more common breakdown process; sufficient dissolved gas in adjacent fluids is necessary for hydrate to persist
Physical property measurements of sediment cores containing natural gas hydrate are typically performed on material exposed, at least briefly, to non‐in situ conditions during recovery. To examine ...the effects of a brief excursion from the gas‐hydrate stability field, as can occur when pressure cores are transferred to pressurized storage vessels, we measured physical properties on laboratory‐formed sand packs containing methane hydrate and methane pore gas. After depressurizing samples to atmospheric pressure, we repressurized them into the methane‐hydrate stability field and remeasured their physical properties. Thermal conductivity, shear strength, acoustic compressional and shear wave amplitudes, and speeds of the original and depressurized/repressurized samples are compared. X–ray computed tomography images track how the gas‐hydrate distribution changes in the hydrate‐cemented sands owing to the depressurizaton/repressurization process. Because depressurization‐induced property changes can be substantial and are not easily predicted, particularly in water‐saturated, hydrate‐bearing sediment, maintaining pressure and temperature conditions throughout the core recovery and measurement process is critical for using laboratory measurements to estimate in situ properties.
We used ultrasonic pulse transmission to measure compressional, P, and shear, S, wave speeds in laboratory‐formed polycrystalline ice Ih, sI methane hydrate, and sII methane‐ethane hydrate. From the ...wave speed's linear dependence on temperature and pressure and from the sample's calculated density, we derived expressions for bulk, shear, and compressional wave moduli and Poisson's ratio from −20 to −5°C and 22.4 to 32.8 MPa for ice Ih, −20 to 15°C and 30.5 to 97.7 MPa for sI methane hydrate, and −20 to 10°C and 30.5 to 91.6 MPa for sII methane‐ethane hydrate. All three materials had comparable P and S wave speeds and decreasing shear wave speeds with increasing applied pressure. Each material also showed evidence of rapid intergranular bonding, with a corresponding increase in wave speed, in response to pauses in sample deformation. There were also key differences. Resistance to uniaxial compaction, indicated by the pressure required to compact initially porous samples, was significantly lower for ice Ih than for either hydrate. The ice Ih shear modulus decreased with increasing pressure, in contrast to the increase measured in both hydrates.
Methane released from seafloor seeps contributes to a number of benthic, water column, and atmospheric processes. At seafloor seeps within the methane hydrate stability zone, crystalline gas hydrate ...shells can form on methane bubbles while the bubbles are still in contact with the seafloor or as the bubbles begin ascending through the water column. These shells reduce methane dissolution rates, allowing hydrate‐coated bubbles to deliver methane to shallower depths in the water column than hydrate‐free bubbles. Here, we analyze seafloor videos from six deepwater seep sites associated with a diverse range of bubble‐release processes involving hydrate formation. Bubbles that grow rapidly are often hydrate‐free when released from the seafloor. As bubble growth slows and seafloor residence time increases, a hydrate coating can form on the bubble's gas‐water interface, fully coating most bubbles within ∼10 s of the onset of hydrate formation at the seafloor. This finding agrees with water‐column observations that most bubbles become hydrate‐coated after their initial ∼150 cm of rise, which takes about 10 s. Whether a bubble is coated or not at the seafloor affects how much methane a bubble contains and how quickly that methane dissolves during the bubble's rise through the water column. A simplified model shows that, after rising 150 cm above the seafloor, a bubble that grew a hydrate shell before releasing from the seafloor will have ∼5% more methane than a bubble of initial equal volume that did not grow a hydrate shell after it traveled to the same height.
Plain Language Summary
Methane is the primary component of natural gas. Processes that affect the formation, consumption, and redistribution of methane in natural settings have significant environmental consequences, such as ocean acidification and greenhouse warming. In water deeper than 500–600 m, methane bubbles can acquire shells of gas hydrate, a crystalline solid made of methane and water molecules. Here, we study how the process of hydrate shell formation alters the fate of methane that naturally bubbles up from the seafloor. We find that bubbles taking longer than ∼10 s to release at seafloor that is within the local methane hydrate stability often grow hydrate shells before rising into the water column. As bubbles ascend through the water column, the hydrate shells slow down bubble dissolution and gas exchange, allowing more methane to be delivered to shallower water depths.
Key Points
In deepwater marine environments, some methane bubbles emerging from the seafloor acquire gas hydrate shells before they release
Gas hydrate shells form when bubble release rates from seeps within the hydrate stability zone are slower than ∼10 s per bubble
Within 1.5 m of rise, most bubbles grow shells, with initially shelled bubbles having ∼5% more gas than initially clean bubbles
Thermal conductivity, thermal diffusivity and specific heat of sI methane hydrate were measured as functions of temperature and pressure using a needle probe technique. The temperature dependence was ...measured between −20°C and 17°C at 31.5 MPa. The pressure dependence was measured between 31.5 and 102 MPa at 14.4°C. Only weak temperature and pressure dependencies were observed. Methane hydrate thermal conductivity differs from that of water by less than 10 per cent, too little to provide a sensitive measure of hydrate content in water-saturated systems. Thermal diffusivity of methane hydrate is more than twice that of water, however, and its specific heat is about half that of water. Thus, when drilling into or through hydrate-rich sediment, heat from the borehole can raise the formation temperature more than 20 per cent faster than if the formation's pore space contains only water. Thermal properties of methane hydrate should be considered in safety and economic assessments of hydrate-bearing sediment.
Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star. Current physical models posit that shocked expanding gas from the nova shell can produce ...x-ray emission, but emission at higher energies has not been widely expected. Here, we report the Fermi Large Area Telescope detection of variable γ-ray emission (0.1 to 10 billion electron volts) from the recently detected optical nova of the symbiotic star V407 Cygni. We propose that the material of the nova shell interacts with the dense ambient medium of the red giant primary and that particles can be accelerated effectively to produce π° decay γ-rays from proton-proton interactions. Emission involving inverse Compton scattering of the red giant radiation is also considered and is not ruled out.
Biomimetics seeks to elucidate the biochemical and structural principles governing functionality in biological systems and exploit them in the design of synthetic materials. Although the lure has ...been great, breakthroughs have been rare. The overwhelming complexity of chemical compositions and structural motifs in biology renders the task of “reverse engineering” a formidable challenge. The challenge is amplified by the inherent limitations in probing the effects of a single test variable, keeping others fixed. Here a novel methodology is presented for probing the effects of various transition metals on the hardness and stiffness of a single biological system, notably the jaws of the marine polychaete annelid
Nereis virens. Not only is the exceptionally high efficacy of Zn, Cu, and Mn in hardening demonstrated, but also that metal selection is not restricted to that which occurs naturally in the targeted system.
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