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  • AGU (American Geophysical Union)  (10)
  • China Geological Survey  (5)
  • ASLO (Association for the Sciences of Limnology and Oceanography)  (2)
  • 1
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    Testing a thermo-chemo-hydro-geomechanical model for gas hydrate-bearing sediments using triaxial compression laboratory experiments (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 18 (9). pp. 3419-3437.
    Details
    Publication Date: 2020-02-06
    Description: Natural gas hydrates are considered a potential resource for gas production on industrial scales. Gas hydrates contribute to the strength and stiffness of the hydrate-bearing sediments. During gas production, the geomechanical stability of the sediment is compromised. Due to the potential geotechnical risks and process management issues, the mechanical behavior of the gas hydrate-bearing sediments needs to be carefully considered. In this study, we describe a coupling concept that simplifies the mathematical description of the complex interactions occurring during gas production by isolating the effects of sediment deformation and hydrate phase changes. Central to this coupling concept is the assumption that the soil grains form the load-bearing solid skeleton, while the gas hydrate enhances the mechanical properties of this skeleton. We focus on testing this coupling concept in capturing the overall impact of geomechanics on gas production behavior though numerical simulation of a high-pressure isotropic compression experiment combined with methane hydrate formation and dissociation. We consider a linear-elastic stress-strain relationship because it is uniquely defined and easy to calibrate. Since, in reality, the geomechanical response of the hydrate-bearing sediment is typically inelastic and is characterized by a significant shear-volumetric coupling, we control the experiment very carefully in order to keep the sample deformations small and well within the assumptions of poroelasticity. The closely coordinated experimental and numerical procedures enable us to validate the proposed simplified geomechanics-to-flow coupling, and set an important precursor toward enhancing our coupled hydro-geomechanical hydrate reservoir simulator with more suitable elastoplastic constitutive models.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2017GC006901
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Linked sediment and water-column methanotrophy at a man-made gas blowout in the North Sea: Implications for methane budgeting in seasonally stratified shallow seas (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (S1). S367-S386.
    Details
    Publication Date: 2019-02-01
    Description: Large quantities of the greenhouse gas methane (CH4) are stored in the seafloor. The flux of CH4 from the sediments into the water column and finally to the atmosphere is mitigated by a series of microbial methanotrophic filter systems of unknown efficiency at highly active CH4-release sites in shallow marine settings. Here, we studied CH4-oxidation and the methanotrophic community at a high-CH4-flux site in the northern North Sea (well 22/4b), where CH4 is continuously released since a blowout in 1990. Vigorous bubble emanation from the seafloor and strongly elevated CH4 concentrations in the water column (up to 42 µM) indicated that a substantial fraction of CH4 bypassed the highly active (up to ∼2920 nmol cm−3 d−1) zone of anaerobic CH4-oxidation in sediments. In the water column, we measured rates of aerobic CH4-oxidation (up to 498 nM d−1) that were among the highest ever measured in a marine environment and, under stratified conditions, have the potential to remove a significant part of the uprising CH4 prior to evasion to the atmosphere. An unusual dominance of the water-column methanotrophs by Type II methane-oxidizing bacteria (MOB) is partially supported by recruitment of sedimentary MOB, which are entrained together with sediment particles in the CH4 bubble plume. Our study thus provides evidence that bubble emission can be an important vector for the transport of sediment-borne microbial inocula, aiding in the rapid colonization of the water column by methanotrophic communities and promoting their persistence close to highly active CH4 point sources.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/lno.10388
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Relating sulfate and methane dynamics to geology: Accretionary prism offshore SW Taiwan (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 14 (7). pp. 2523-2545.
    Details
    Publication Date: 2018-02-28
    Description: Geochemical data (CH4, SO42−, I−, Cl−, particulate organic carbon (POC), δ13C-CH4, and δ13C-CO2) are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest Taiwan. The sampling stations covered three ridges (Tai-Nan, Yung-An, and Good Weather), each characterized by bottom simulating reflectors, acoustic turbidity, and different types of faulting and anticlines. Sulfate and iodide concentrations varied little from seawater-like values in the upper 1–3 m of sediment at all stations; a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments. Below this depth, sulfate was rapidly consumed within 5–10 m by anaerobic oxidation of methane (AOM) at the sulfate-methane transition. Carbon isotopic data imply a mainly biogenic methane source. A numerical transport-reaction model was used to identify the supply pathways of methane and estimate depth-integrated turnover rates at the three ridges. Methane gas ascending from deep layers, facilitated by thrusts and faults, was by far the dominant term in the methane budget at all sites. Differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth-integrated AOM rates. By comparison, methane produced in situ by POC degradation within the modeled sediment column was unimportant. This study demonstrates that the geochemical trends in the continental margins offshore SW Taiwan are closely related to the different geological settings.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/ggge.20168
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Effects of climate change on methane emissions from seafloor sediments in the Arctic Ocean: A review (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (S1). pp. 301-309.
    Details
    Publication Date: 2019-09-24
    Description: Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and seaair exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/lno.10307
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    An abyssal hill fractionates organic and inorganic matter in deep-sea surface sediments (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 (18). pp. 7663-7672.
    Details
    Publication Date: 2017-04-10
    Description: Current estimates suggest that more than 60% of the global seafloor are covered by millions of abyssal hills and mountains. These features introduce spatial fluid-dynamic granularity whose influence on deep-ocean sediment biogeochemistry is unknown. Here we compare biogeochemical surface-sediment properties from a fluid-dynamically well-characterized abyssal hill and upstream plain: (1) In hill sediments, organic-carbon and -nitrogen contents are only about half as high as on the plain while proteinaceous material displays less degradation; (2) on the hill, more coarse-grained sediments (reducing particle surface area) and very variable calcite contents (influencing particle surface charge) are proposed to reduce the extent, and influence compound-specificity, of sorptive organic-matter preservation. Further studies are needed to estimate the representativeness of the results in a global context. Given millions of abyssal hills and mountains, their integrative influence on formation and composition of deep-sea sediments warrants more attention.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2015GL065658
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    A quantitative assessment of methane cycling in Hikurangi Margin sediments (New Zealand) using geophysical imaging and biogeochemical modeling (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 17 (12). pp. 4817-4835.
    Details
    Publication Date: 2019-02-01
    Description: Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2016GC006643
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Linked halokinesis and mud volcanism at the Mercator Mud Volcano, Gulf of Cadiz (2011)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 116 . B05101.
    Details
    Publication Date: 2018-04-27
    Description: Mud volcanoes are seafloor expressions of focused fluid flow that are common in compressional tectonic settings. New high-resolution 3-D seismic data from the Mercator mud volcano (MMV) and an adjacent buried mud volcano (BMV) image the internal structure of the top 800 m of sediment at both mud volcanoes, revealing that both are linked and have been active episodically. The total volumes of extruded mud range between 0.15 and 0.35 km3 and 0.02–0.05 km3 for the MMV and the BMV, respectively. The pore water composition of surface sediment samples suggests that halokinesis has played an important role in the evolution of the mud volcanoes. We propose that erosion of the top of the Vernadsky Ridge that underlies the mud volcanoes activated salt movement, triggering deep migration of fluids, dissolution of salt, and sediment liquefaction and mobilization since the end of the Pliocene. Since beginning of mud volcanism in this area, the mud volcanoes erupted four times while there was only one reactivation of salt tectonics. This implies that there are other mechanisms that trigger mud eruptions. The stratigraphic relationship of mudflows from the MMV and BMV indicates that the BMV was triggered by the MMV eruptions. This may either be caused by loading-induced hydrofracturing within the BMV or due to a common feeder system for both mud volcanoes. This study shows that the mud volcanoes in the El Arraiche mud volcano field are long-lived features that erupt with intervals of several tens of thousands of years.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1029/2010JB008061
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Spatial and temporal modelling of thermal effects during CO2 injection into methane-hydrate reservoirs (2014)
    China Geological Survey
    In:  [Paper] In: 8. International Conference on Gas Hydrates (ICGH8), 28.07.-01.08.2014, Beijing, China . Proceedings of the 8th International Conference on Gas Hydrates (ICGH8-2014), Beijing, China, 28 July - 1 August, 2014 ; T3-56 .
    Details
    Publication Date: 2016-12-21
    Description: Injection of CO2 into CH4-hydrate bearing sediments, and the resulting in-situ replacement of CH4-hydrate by CO2-hydrate, has been proposed as a technique for the emission-free production of natural gas from gas hydrates. While the hydrate conversion is thermodynamically feasible, many studies conclude that the overall process suffers from mass transfer limitations and CH4 production is limited after short time. To improve CH4 production various technical concepts have been considered, including the injection of heated supercritical CO2 combining chemical activation and thermalstimulation. While the feasibility of the concept was demonstrated in high-pressure flow-through experiments and high CH4 production efficiencies were observed, it was evident that overall yields and efficiencies were influenced by a variety of processes which could not be disclosed through bulk mass and volume analysis. Here we present different numerical simulation strategies which were developed and tested as tools to better understand the importance of mass and heat transport relative to reaction and phase transition kinetics for CH4 release and production, or for CO2 retention, respectively. The modeling approaches are discussed with respect to applicability for experimental design, process development or prediction of CH4 production from natural gas hydrate reservoirs on larger scales.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Conference paper
  • 9
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    Experimental investigation of water permeability in quartz sand as function of CH4-hydrate saturation (2014)
    China Geological Survey
    In:  [Paper] In: 8. International Conference on Gas Hydrates (ICGH8), 28.07.-01.08.2014, Beijing, China . Proceedings of the 8th International Conference on Gas Hydrates (ICGH8) Beijing, China, 28 July - 1 August, 2014 ; T1-68 .
    Details
    Publication Date: 2014-11-21
    Description: Water permeability in gas hydrate bearing sediments is a crucial parameter for the prediction of gas production scenarios. So far, the commonly used permeability models are backed by very few experimental data. Furthermore, detailed knowledge of the exact formation mechanism leads to severe uncertainties in the interpretation of the experimental data. We formed CH4 hydrates from a methane saturated water solution and used Magnetic Resonance Imaging (MRI) to measure time resolved maps of the three-dimensional gas hydrate saturation. These maps were used for 3D Finite Elements Method (FEM) simulations. The simulation results enabled us to optimize existing models for permeabilities as function of gas hydrate saturation.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Conference paper
  • 10
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    3-D basin-scale reconstruction of natural gas hydrate system of the Green Canyon, Gulf of Mexico (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 18 (5). pp. 1959-1985.
    Details
    Publication Date: 2020-02-06
    Description: Our study presents a basin-scale 3D modeling solution, quantifying and exploring gas hydrate accumulations in the marine environment around the Green Canyon (GC955) area, Gulf of Mexico. It is the first modeling study that considers the full complexity of gas hydrate formation in a natural geological system. Overall, it comprises a comprehensive basin re-construction, accounting for depositional and transient thermal history of the basin, source rock maturation, petroleum components generation, expulsion and migration, salt tectonics and associated multi-stage fault development. The resulting 3D gas hydrate distribution in the Green Canyon area is consistent with independent borehole observations. An important mechanism identified in this study and leading to high gas hydrate saturation (〉 80 vol. %) at the base of the gas hydrate stability zone (GHSZ), is the recycling of gas hydrate and free gas enhanced by high Neogene sedimentation rates in the region. Our model predicts the rapid development of secondary intra-salt mini-basins situated on top of the allochthonous salt deposits which leads to significant sediment subsidence and an ensuing dislocation of the lower GHSZ boundary. Consequently, large amounts of gas hydrates located in the deepest parts of the basin dissociate and the released free methane gas migrates upwards to recharge the GHSZ. In total, we have predicted the gas hydrate budget for the Green Canyon area that amounts to ∼3,256 Mt of gas hydrate which is equivalent to ∼340 Mt of carbon (∼7 x 1011 m3 of CH4 at STP conditions), and consists mostly of biogenic hydrates.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2017GC006876
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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