Hydrate Stability Field Research Articles

A joint electromagnetic and seismic study of an active pockmark within the hydrate stability field at the Vestnesa Ridge, West Svalbard margin

AbstractWe acquired coincident marine controlled source electromagnetic (CSEM), high‐resolution seismic reflection and ocean‐bottom seismometer (OBS) data over an active pockmark in the crest of the southern part of the Vestnesa Ridge, to estimate fluid composition within an underlying fluid‐migration chimney. Synthetic model studies suggest resistivity obtained from CSEM data can resolve gas or hydrate saturation greater than 5% within the chimney. Acoustic chimneys imaged by seismic reflection data beneath the pockmark and on the ridge flanks were found to be associated with high‐resistivity anomalies (+2–4 Ωm). High‐velocity anomalies (+0.3 km/s), within the gas‐hydrate stability zone (GHSZ) and low‐velocity anomalies (−0.2 km/s) underlying the GHSZ, were also observed. Joint analysis of the resistivity and velocity anomaly indicates pore saturation of up to 52% hydrate with 28% free gas, or up to 73% hydrate with 4% free gas, within the chimney beneath the pockmark assuming a nonuniform and uniform fluid distribution, respectively. Similarly, we estimate up to 30% hydrate with 4% free gas or 30% hydrate with 2% free gas within the pore space of the GHSZ outside the central chimney assuming a nonuniform and uniform fluid distribution, respectively. High levels of free‐gas saturation in the top part of the chimney are consistent with episodic gas venting from the pockmark.

Authigenesis of magnetic minerals in gas hydrate-bearing sediments in the Nankai Trough, offshore Japan

Gas hydrate occurrence is one of the possible mechanisms invoked for iron sulfide formation. A high-resolution rock magnetic study was conducted in IODP Expedition 316 Hole C0008C located in the Megasplay Fault Zone of the Nankai Trough, offshore Japan. In this particular zone, no bottom simulating reflectors (BSR), indicating the base of the gas hydrate stability field, have been identified. Two hundred and eighteen Pleistocene samples were collected from 70 to 110 m CSF in order to document the changes in the concentration, grain size, and rock magnetic parameters of magnetic minerals, through the gas hydrate-bearing horizons. Two different populations of magnetic grains are recognized in the pseudosingle domain range. Three types of magnetic mineral assemblages are identified: iron oxides (magnetite), ferrimagnetic iron sulfides (greigite and pyrrhotite), and their mixture. Greigite and pyrrhotite are authigenic and constitute six layers, called IS1–IS6. IS1, IS3, IS4, and IS6 are associated with pore water anomalies, suggesting the occurrence of gas hydrates and anoxic conditions. IS2 and IS5 are probable gas hydrates horizons, although there is no independent data to confirm it. The remaining intervals are mainly composed of detrital iron oxides and paramagnetic iron sulfides. Two scenarios based on different diagenetic stages are proposed to explain the variations in the magnetic properties and mineralogy over the studied interval. The results suggest that rock magnetism appears useful to better constrain the gas hydrate distribution in Hole C0008C, and counterbalances the low resolution of pore water analyses and the absence of a BSR.

Gas migration into gas hydrate‐bearing sediments on the southern Hikurangi margin of New Zealand

AbstractWe present multichannel seismic data from New Zealand's Hikurangi subduction margin that show widespread evidence for gas migration into the field of gas hydrate stability. Gas migration along stratigraphic layers into the hydrate system manifests itself as highly reflective segments of dipping strata that originate at the base of hydrate stability and extend some distance toward the seafloor. The highly reflective segments exhibit the same polarity as the seafloor reflection, indicating that localized gas hydrate precipitation from gas‐charged fluids within relatively permeable layers has occurred. High‐density velocity analysis shows that these layer‐constrained gas hydrate accumulations are underlain by thick (up to ~500 m) free gas zones, which provide the source for focused gas migration into the hydrate layer. In addition to gas being channeled along layers, we also interpret gas migration through a fault zone into the field of hydrate stability; in this case, a low‐velocity layer within the hydrate stability zone extends laterally away from the fault, which might indicate that gas‐charged fluids have also migrated away from the fault along strata. At this site, where both dipping strata and faulting seem to influence fluid migration, we observe anomalously high velocities at the base of hydrate stability that we interpret as concentrated gas hydrates. Our results give insight into how shallow fluid flow responds to permeability contrasts between strata, fault zones, and perhaps also the gas hydrate system itself. Ultimately, these relationships can lead to gas migration across the base of hydrate stability and the precipitation of concentrated hydrate deposits.

Record of methane emissions from the West Svalbard continental margin during the last 23.500 yrs revealed by δ13C of benthic foraminifera

The values of δ13C in benthic foraminifera have been measured in a gas-hydrate-bearing sediment core collected from an area of active methane venting on the Vestnesa Ridge (West Svalbard continental margin) to reconstruct the local history of methane emissions over the past 23.500yrs BP. The chronostratigraphic framework of the core has been derived from AMS 14C dates and biostratigraphic analysis. While foraminifera from some intervals have δ13C within the normal marine range (0 to −1‰), five intervals are characterized by a much lower δ13C, as low as −17.4‰. These intervals are interpreted to record the incorporation of 13C-depleted carbon in the presence of methane emissions at the seafloor during biomineralization of the carbonate foraminiferal tests and subsequent secondary mineralization. Methane emission events (MEE) occur from the Last Glacial Maximum (LGM) to the Holocene, with the most prominent one, in terms of δ13C depletion, predating the Bølling–Allerød Interstadial (GI-1 in the Greenland ice core record). The lack of correlation between the values of δ13C and δ18O, however, appears to preclude warming of bottom waters as the principal control on methane release. Rather, it seems likely that methane release is a consequence of episodicity in the supply of gas to the hydrate system and in the processes that enable methane gas to migrate through the hydrate stability field to the seabed, or of other geological processes still under debate.

Hydrate petroleum system approach to natural gas hydrate exploration

Natural gas hydrate (NGH) is a solid crystalline material composed of water and natural gas (primarily methane) that is stable under conditions of moderately high pressure and moderately low temperature found in permafrost and continental margin sediments. A NGH petroleum system is different in a number of important ways from conventional petroleum systems related to large concentrations of gas and petroleum. The critical elements of the NGH petroleum system are: (1) a gas hydrate stability zone (GHSZ) in which pressure and temperature lie within the field of hydrate stability, creating a thermodynamic trap of suitable thickness for NGH concentrations to form; (2) recent and modern gas flux into the GHSZ along migration pathways; and (3) suitable sediment host sands within the GHSZ. These elements have to be active now and in the recent geological past. Exploration in continental margin sediments includes basin analysis to identify source and host sediment likelihood and disposition, potential reservoir localization using existing seismic analysis tools for locating turbidite sands and estimating NGH saturation, and deposit characterization using drilling and logging. Drilling has validated first-order seismic analysis techniques for identifying and quantifying NGH using rock physics mechanical models.

Assessment of gas hydrate stability zone and geothermal modeling of BSR in the Andaman Sea

Wide-spread bottom simulating reflectors (BSRs) are observed along available multichannel seismic profiles covering an area of about 290km2 in the Andaman Sea. The seismic data shows that the BSR occurs at places where water depth exceeds 1000m, and is identified by cross-cutting relationships with the dipping reflectors. The BSR that represents the base of gas hydrate stability field can be used to infer the gas hydrate stability thickness, which ranges between ∼518m to ∼861m depending on water depths. In situ measurement at site 17 during the Indian National Gas Hydrate Program (NGHP) Expedition-01 shows very low geothermal gradient 19±2°C/km. A conductive model was used to determine geothermal gradients from BSRs, which is calculated and varying between 10°C/km to 40°C/km. The low geothermal gradient is responsible for the deepest BSR or gas hydrate stability zone (GHSZ) in the Andaman region and in the world. The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.

Spatial distribution of gas hydrates from high-resolution seismic and core data, Woolsey Mound, Northern Gulf of Mexico

Integration of new jumbo piston cores with high-resolution seismic reflection data provides the first documentation on the subsurface distribution of gas hydrates at a deep-water cold seep. The study area, Woolsey Mound is a carbonate–hydrate mound of thermogenic origin, in the northern Gulf of Mexico where salt tectonics dominate the regional structure. Two typologies of unconventional seismo-acoustic data were used in this study: 1) surface-source deep-receiver (SSDR) data and 2) AUV-borne chirp sub-bottom profiler data. Correlation of the coring results with seismic interpretations supports the hypothesis that distinctive seismic brightening (high frequency scattering) present in the SSDR records may indicate the presence of solid hydrates. This suggests that such unconventional seismic survey may be prospective for mapping gas hydrates in complex deep-water settings. Coring results revealed the nature of the shallow gas hydrate system to be dominated by fine-grained sediments. Gas hydrates were found exclusively in fracture porosity in the vicinity of a major active fault. We present a model for Woolsey Mound where shallow gas hydrates are systematically distributed along segments of faults intersected by transit of thermogenic hydrocarbons. This dual nature of the faults being both gas hydrate reservoirs and gas migration routes suggests a very dynamic hydrate stability field for this site.

Ocean circulation promotes methane release from gas hydrate outcrops at the NEPTUNE Canada Barkley Canyon node

The NEPTUNE Canada cabled observatory network enables non‐destructive, controlled experiments and time‐series observations with mobile robots on gas hydrates and benthic community structure on a small plateau of about 1 km2 at a water depth of 870 m in Barkley Canyon, about 100 km offshore Vancouver Island, British Columbia. A mobile Internet operated vehicle was used as an instrument platform to monitor and study up to 2000 m2of sediment surface in real‐time. In 2010 the first mission of the robot was to investigate the importance of oscillatory deep ocean currents on methane release at continental margins. Previously, other experimental studies have indicated that methane release from gas hydrate outcrops is diffusion‐controlled and should be much higher than seepage from buried hydrate in semipermeable sediments. Our results show that periods of enhanced bottom currents associated with diurnal shelf waves, internal semidiurnal tides, and also wind‐generated near‐inertial motions can modulate methane seepage. Flow dependent destruction of gas hydrates within the hydrate stability field is possible from enhanced bottom currents when hydrates are not covered by either seafloor biota or sediments. The calculated seepage varied between 40–400 μmol CH4 m−2 s−1. This is 1–3 orders of magnitude higher than dissolution rates of buried hydrates through permeable sediments and well within the experimentally derived range for exposed gas hydrates under different hydrodynamic boundary conditions. We conclude that submarine canyons which display high hydrodynamic activity can become key areas of enhanced seepage as a result of emerging weather patterns due to climate change.

Potential effects of obliquity change on gas hydrate stability zones on Mars

Methane hydrate dissociation due to obliquity-driven temperature change has been suggested as a potential source of atmospheric methane plumes recently observed on Mars. This work uses both equilibrium and time-dependent models to determine how geothermal gradients change on Mars as a result of obliquity and predict how these changes affect gas hydrate stability zones (HSZs). The models predict that the depth to the HSZ decreases with increasing latitude for both CO2 and CH4 hydrate, with CO2 hydrate occurring at shallower depths than CH4 hydrate over all latitudes. The depth of the HSZ increases as surface temperatures warm and decreases as surface temperatures cool with changing obliquity, with the largest change in HSZ volume predicted near the equator and the poles. Therefore, changes in the depth to the HSZ may cause hydrate dissociation near the equator and poles as the geothermal gradient moves in and out of the hydrate stability field over hundreds of thousands of years. Sublimation of overlying ice containing diffused methane could account for recent observations of seasonal methane plumes on Mars. In addition, near-surface gas hydrate reservoirs may be preserved at mid-latitudes due to minimal changes in surface temperature with obliquity over geologic time scales. Comparisons of the predicted changes in the HSZ with hydrate dissociation and diffusion rates reveal that metastable hydrate may also remain in the near subsurface, especially at high latitudes, for millions to billions of years. The presence of methane hydrate in the near subsurface at midlatitudes could be an important analytical target for future Mars missions, as well as serving as a source of fuel for future spacecraft.

Pressurized laboratory experiments show no stable carbon isotope fractionation of methane during gas hydrate dissolution and dissociation

The stable carbon isotopic ratio of methane (δ(13)C-CH(4)) recovered from marine sediments containing gas hydrate is often used to infer the gas source and associated microbial processes. This is a powerful approach because of distinct isotopic fractionation patterns associated with methane production by biogenic and thermogenic pathways and microbial oxidation. However, isotope fractionations due to physical processes, such as hydrate dissolution, have not been fully evaluated. We have conducted experiments to determine if hydrate dissolution or dissociation (two distinct physical processes) results in isotopic fractionation. In a pressure chamber, hydrate was formed from a methane gas source at 2.5 MPa and 4 °C, well within the hydrate stability field. Following formation, the methane source was removed while maintaining the hydrate at the same pressure and temperature which stimulated hydrate dissolution. Over the duration of two dissolution experiments (each ~20-30 days), water and headspace samples were periodically collected and measured for methane concentrations and δ(13)C-CH(4) while the hydrate dissolved. For both experiments, the methane concentrations in the pressure chamber water and headspace increased over time, indicating that the hydrate was dissolving, but the δ(13)C-CH(4) values showed no significant trend and remained constant, within 0.5‰. This lack of isotope change over time indicates that there is no fractionation during hydrate dissolution. We also investigated previous findings that little isotopic fractionation occurs when the gas hydrate dissociates into gas bubbles and water due to the release of pressure. Over a 2.5 MPa pressure drop, the difference in the δ(13)C-CH(4) was <0.3‰. We have therefore confirmed that there is no isotope fractionation when the gas hydrate dissociates and demonstrated that there is no fractionation when the hydrate dissolves. Therefore, measured δ(13)C-CH(4) values near gas hydrates are not affected by physical processes, and can thus be interpreted to result from either the gas source or associated microbial processes.

Gas hydrate within the Winona Basin, offshore western Canada

In western Canada gas hydrates have been thought to exist primarily in the Cascadia accretionary prism off southern Vancouver Island, British Columbia (BC). We present evidence for the existence of gas hydrate in folds and ridges of the Winona Basin up to 40 km seaward from the foot of the continental slope off northern Vancouver Island. The occurrence of a bottom-simulating reflector (BSR) observed in a number of vintage seismic reflection profiles is strongly correlated to faulted, and folded sedimentary ridges and buried folds. The observed tectonic structures of the Winona Basin are within the rapidly evolving Juan de Fuca – Cascadia – Queen Charlotte triple junction off BC. Re-processing of multi-channel data imaged mildly to strongly deformed sediments; the BSR is confined to sediments with stronger deformation. Changes in the amplitude character of sediment-reflections above and below the depth of the base of gas hydrate stability zone were also used as an indicator for the presence of gas hydrate. Additionally, regional amplitude and frequency reduction below some strong BSR occurrences may indicate free gas accumulations. Gas hydrate formation in the Winona Basin appears strongly constrained to folds and ridges and thus correlated to deeper-routed fluid-advection regimes. Methane production from in situ microbial activities as a source of gas to form gas hydrates, as proposed to be a major contributor for gas hydrates within the accretionary prism to the south, appears to be insufficient to produce the widespread gas hydrate occurrences in the Winona Basin. Potential reasons for the lack of sufficient in situ gas production may be that sedimentation rates are 5–100 times higher than those in the accretionary prism so that available organic carbon moves too quickly through the gas hydrate stability field. The confinement of BSRs to ridges and folds within the Winona Basin results in an areal extent of gas hydrate occurrences that is a factor of five less than what is expected from regional gas hydrate stability field mapping using water-depth (pressure) as the only controlling factor only.

Hydrogeology, Chemical and Microbial Activity Measurement Through Deep Permafrost

Little is known about hydrogeochemical conditions beneath thick permafrost, particularly in fractured crystalline rock, due to difficulty in accessing this environment. The purpose of this investigation was to develop methods to obtain physical, chemical, and microbial information about the subpermafrost environment from a surface-drilled borehole. Using a U-tube, gas and water samples were collected, along with temperature, pressure, and hydraulic conductivity measurements, 420 m below ground surface, within a 535 m long, angled borehole at High Lake, Nunavut, Canada, in an area with 460-m-thick permafrost. Piezometric head was well above the base of the permafrost, near land surface. Initial water samples were contaminated with drill fluid, with later samples <40% drill fluid. The salinity of the non-drill fluid component was <20,000 mg/L, had a Ca/Na ratio above 1, with δ(18) O values ∼5‰ lower than the local surface water. The fluid isotopic composition was affected by the permafrost-formation process. Nonbacteriogenic CH(4) was present and the sample location was within methane hydrate stability field. Sampling lines froze before uncontaminated samples from the subpermafrost environment could be obtained, yet the available time to obtain water samples was extended compared to previous studies. Temperature measurements collected from a distributed temperature sensor indicated that this issue can be overcome easily in the future. The lack of methanogenic CH(4) is consistent with the high sulfate concentrations observed in cores. The combined surface-drilled borehole/U-tube approach can provide a large amount of physical, chemical, and microbial data from the subpermafrost environment with few, controllable, sources of contamination.

A pressure core based characterization of hydrate-bearing sediments in the Ulleung Basin, Sea of Japan (East Sea)

[1] The physical characteristics of hydrate-bearing sediments sampled by pressure coring from the Ulleung Basin in the Sea of Japan (East Sea) were investigated using an instrumented chamber capable of testing recovered natural sediments that have never left the methane hydrate stability field. The heterogeneous distribution of segregated hydrate veins and lens structures in sediments results in highly variable geophysical and geomechanical properties. The scaled production test was conducted by controlled depressurization of pressure cores while dissociation and gas production were concurrently monitored using various sensors in the instrumented chamber. The hydrate saturation was estimated to be ∼19.5% in the pore space. Data show a sharp reduction in sediment shear and bulk stiffnesses during hydrate dissociation. Relatively fast gas migration was observed, probably along high-conduction planes left behind as hydrate veins dissociated. The spatial distribution of hydrates in sediment was analyzed based on 3-D image processing. The phenomena relevant to the production test and sampling effects during pressure coring are discussed.

CH4‐CO2 replacement in hydrate‐bearing sediments: A pore‐scale study

The injection of CO2 into CH4 hydrate‐bearing sediments causes the release of CH4 and the formation of CO2 hydrate within the CH4 hydrate stability field. CH4‐CO2 replacement allows for the recovery of an energy source, CH4, while trapping CO2. In this study, we monitor pore‐scale changes in electrical resistance and relative stiffness during CH4 hydrate formation, CH4‐CO2 replacement, and hydrate dissociation; experiments are also observed using high‐resolution time‐lapsed photography. Results show that CH4‐CO2 replacement occurs locally and gradually so that the overall hydrate mass remains solid and no stiffness loss should be expected at the sediment scale. Other experimental results confirm the slow diffusion of CH4 through the hydrate shell that forms between water and gas; this may allow for the coexistence of gas‐hydrate‐water phases for long periods of time.

Parametric study of the physical properties of hydrate‐bearing sand, silt, and clay sediments: 2. Small‐strain mechanical properties

The small‐strain mechanical properties (e.g., seismic velocities) of hydrate‐bearing sediments measured under laboratory conditions provide reference values for calibration of logging and seismic exploration results acquired in hydrate‐bearing formations. Instrumented cells were designed for measuring the compressional (P) and shear (S) velocities of sand, silts, and clay with and without hydrate and subject to vertical effective stresses of 0.01 to 2 MPa. Tetrahydrofuran (THF), which is fully miscible in water, was used as the hydrate former to permit close control over the hydrate saturation Shyd and to produce hydrate from dissolved phase, as methane hydrate forms in most natural marine settings. The results demonstrate that laboratory hydrate formation technique controls the pattern of P and S velocity changes with increasing Shyd and that the small‐strain properties of hydrate‐bearing sediments are governed by effective stress, σ′v and sediment specific surface. The S velocity increases with hydrate saturation owing to an increase in skeletal shear stiffness, particularly when hydrate saturation exceeds Shyd≈ 0.4. At very high hydrate saturations, the small strain shear stiffness is determined by the presence of hydrates and becomes insensitive to changes in effective stress. The P velocity increases with hydrate saturation due to the increases in both the shear modulus of the skeleton and the bulk modulus of pore‐filling phases during fluid‐to‐hydrate conversion. Small‐strain Poisson's ratio varies from 0.5 in soft sediments lacking hydrates to 0.25 in stiff sediments (i.e., subject to high vertical effective stress or having high Shyd). At Shyd ≥ 0.5, hydrate hinders expansion and the loss of sediment stiffness during reduction of vertical effective stress, meaning that hydrate‐rich natural sediments obtained through pressure coring should retain their in situ fabric for some time after core retrieval if the cores are maintained within the hydrate stability field.

Properties and phenomena relevant to CH4‐CO2 replacement in hydrate‐bearing sediments

The injection of carbon dioxide, CO2, into methane hydrate‐bearing sediments causes the release of methane, CH4, and the formation of carbon dioxide hydrate, even if global pressure‐temperature conditions remain within the CH4 hydrate stability field. This phenomenon, known as CH4‐CO2 exchange or CH4‐CO2 replacement, creates a unique opportunity to recover an energy resource, methane, while entrapping a greenhouse gas, carbon dioxide. Multiple coexisting processes are involved during CH4‐CO2 replacement, including heat liberation, mass transport, volume change, and gas production among others. Therefore, the comprehensive analysis of CH4‐CO2 related phenomena involves physico‐chemical parameters such as diffusivities, mutual solubilities, thermal properties, and pressure‐ and temperature‐dependent phase conditions. We combine new experimental results with published studies to generate a data set we use to evaluate reaction rates, to analyze underlying phenomena, to explore the pressure‐temperature region for optimal exchange, and to anticipate potential geomechanical implications for CH4‐CO2 replacement in hydrate‐bearing sediments.

Reduced sulfur–carbon–water systems on Mars may yield shallow methane hydrate reservoirs

Methane clathrate hydrate reservoirs capped by overlying permafrost have been proposed as potential sources of atmospheric methane plumes on Mars. However, the surface flux of methane from hydrate dissociation is limited by the diffusion rate of methane through the overlying ice. Assuming hydrates underlay the entire plume footprint, the maximum diffusion path length is expected to be less than 15 m, depths too shallow to stabilize pure methane hydrates under Mars geothermal and lithostatic conditions at low to mid latitudes. Therefore, pure methane hydrates confined within permafrost could not produce methane surface fluxes of the magnitude observed near the equator. However, the addition of 10% H 2S, a secondary gas commonly associated with methane production on Earth, expands the hydrate stability field, with clathrates expected within 10 m of the surface at the equator and at depths less than 1 m at higher latitudes. This indicates that H 2S would also be expected to be released as well as methane if the plumes have a confined hydrate reservoir source.

Numerical studies of gas hydrate systems: sensitivity to porosity-permeability models

The formation of sub-seafloor gas hydrates in marine environments can be described as a coupled transport and thermodynamic process inside a host sediment matrix undergoing structural evolution. The transport processes are driven by the sedimentary load and induced overpressure gradients, controlled by sediment permeability. In order to accurately model the resulting fluid flow profile, the decrease of sediment permeability during hydrate precipitation has to be taken into account, which affects both the transport of solutes and sediment compaction. In this paper, we investigate how total hydrate abundance is affected by regions of low permeability which deflect the flow field in their vicinity. For this purpose, a two-dimensional numerical hydrate system model was set up which permits to quantify this effect in scenarios where changes in water depth cause lateral variations of the thickness of the hydrate stability field, as well as of hydrate saturation and sediment permeability. The microscopic structure of gas hydrate crystals in the host sediment matrix defines the evolution of the permeability reduction during hydrate formation. Grain-coating precipitates have a stronger tendency to clog flow paths through pore throats than do pore-filling precipitates. Our results clearly show that these pore-scale processes affect the large-scale flow field and hydrate abundance. The sensitivity depends on the model geometry and, for a 5° slope of the seafloor, 4.1% relative difference is predicted for the hydrate saturation according to different porosity-permeability relationships.

Methane hydrate dissolution rates in undersaturated seawater under controlled hydrodynamic forcing

The dissolution of in-situ generated methane hydrate in undersaturated, synthetic seawater ( S = 35) was investigated in a series of laboratory-based experiments at P-/ T-conditions within the hydrate stability field. A controlled flow field was generated across the smooth hydrate surface to test if, in addition to thermodynamic variables, the dissolution rate is influenced by changing hydrodynamic conditions. The dissolution rate was found to be strongly dependent on the friction velocity, showing that hydrate dissolution in undersaturated seawater is a diffusion-controlled process. The experimental data was used to obtain diffusional mass transfer coefficients k d, which were found to correlate linearly with the friction velocity, u ★. The resulting k d/ u ★-correlation allows predicting the flux of methane from natural gas hydrate exposures at the sediment/seawater interface into the bulk water for a variety of natural P, T and flow conditions. It also is a tool for estimating the rate of hydrate regrowth at locations where natural hydrate outcrops at the seafloor persist in contact with undersaturated seawater.

Controls on methane bubble dissolution inside and outside the hydrate stability field from open ocean field experiments and numerical modeling

The release of methane from the seafloor into the water column within the hydrate stability field (HSF) is a natural and widely observed process. The subsequent bubble dissolution rate determines how far upwards gas is transported and the vertical distribution of this methane source to the water column. Understanding this process is essential to describing natural deep sea seeps, to assess the hazard potential from blowouts in offshore drilling activities for gas and oil, and to refine past and future scenarios of global change involving large-scale destabilization of gas hydrates and free methane gas. We report on in situ experiments on single methane and argon bubbles within and above the HSF for depths from 400 to 1500 m. Single bubbles were injected from the ROV Ventana into an attached, back-illuminated, flow-through imaging box. The ascent of individual bubbles within the imaging box was recorded and analyzed for rise velocity, V B , and radius shrinkage rate, dr/ dt. For all bubbles V B = ~ 30 cms − 1 . Methane bubbles released within the HSF had markedly enhanced lifetimes, attributed to hydrate skin formation. dr/ dt varied from − 7.5 μm s − 1 above the HSF to – 1 μm s − 1 at 1500 m, well within the HSF. Bubble longevity within the HSF increased with distance into the P/T-space from the hydrate phase boundary. There was a delay prior to the slower dissolution, where dr/ dt for methane bubbles was comparable to dr/ dt above the HSF, which was interpreted as a time delay before the onset of hydrate skin formation. Although variable, the onset time generally decreased with distance from the hydrate phase boundary. No delay was observed for the deepest releases. We relate these findings to formal calculations of methane solubility and density as a function of pressure. Pressure-dependent deviations from ideal gas law and Henry's law were implemented in a numerical bubble-propagation model which incorporated the effects of decreased solubility and surface mobility after hydrate formation. Inclusion of these effects greatly improved model prediction of observed methane bubble behavior within the HSF. The effect of these individual depth-related effects on bubble dissolution rate and longevity then was assessed quantitatively, and assumptions of different gas solubility prior and after hydrate nucleation or an effect of hydrates at the interface on the hydrodynamics at the surface were tested. Here, our approach implements our current knowledge on physicochemical and hydrodynamic control and does not seek the best fit for the given data sets, thus it also reveals current uncertainties in methane bubble processes in the HSF.