107 research outputs found
Magnetic Resonance Imaging of Gas Hydrate Formation and Conversion at Sub-Seafloor Conditions
The production of natural gas from sub-seafloor gas hydrates is one possible strategy to
meet the worldâs growing demand for energy. On the other hand, climate warming scenarios
call for the substitution of fossil energy resources by sustainable energy concepts. Burning
natural gas from gas hydrates could be emission neutral if it was combined with a safe storage
of the emitted CO2. Laboratory experiments, that address corresponding strategies, need to be
performed under high pressures and low temperatures to meet the thermodynamic conditions
of the sub-seafloor environment. In this paper, we present a high-pressure flow-through
sample cell that is suitable for nuclear magnetic resonance (NMR) experiments at realistic
marine environmental conditions, i.e. pressures up to 15 MPa and temperatures from 5 to 20
°C, and we demonstrate its suitability in applied gas hydrate research
Microplastic abundance in beach sediments of the Kiel Fjord, Western Baltic Sea
We assessed the abundance of microplastics (0.2â5 mm) in drift line sediments from three sites in Kiel Fjord, Western Baltic Sea. The first site is intensively used by beach visitors, the second is in close proximity to a sewage plant and the third is polluted with large-sized plastic litter. Samples were split into three grain size classes (0.2â0.5, 0.5â1, 1â5 mm), washed with calcium chloride solution, and filtered at 0.2 mm. Filters were then visually inspected, and a total of 180 fragments was classified as microplastics, of which 39% were analyzed using Raman spectroscopy. At the site that is close to a sewage plant as well as at the site with intense beach use, 1.8 and 4.5 particles (fibers plus fragments) per kg of dry sediment were found, respectively, while particle abundances reached 30.2 per kg of dry sediment at the site with high litter loads. Our data suggest that the fragmentation of large plastic debris at site seems to be a relevant source for microplastics in Western Baltic Sea beach sediments
CO2 injection into submarine sediments: disturbing news for methane-rich hydrates
The production of natural gas via injection of fossil-fuel derived CO2 into submarine gas hydrate reservoirs can be an example of tapping a hydrocarbon energy source in a CO2-neutral manner. However, the industrial application of this method is technically challenging. Thus, prior to feasibility testing in the field, multi-scale laboratory experiments and adapted reaction-modeling are needed. To this end, high-pressure flow-through reactors of 15 and 2000 mL sample volume were constructed and tested. Process parameters (P, T, Q, fluid composition) are defined by a fluid supply and conditioning unit to enable simulation of natural fluid-flow scenarios for a broad range of sedimentary settings. Additional Raman- and NMR-spectroscopy aid in identifying the most efficient pathway for CH4 extraction from hydrates via CO2 injection on both microscopic and macroscopic level. In this study we present experimental set-up and design of the highpressure flow-through reactors as well as CH4 yields from H4-hydrate decomposition experiments using CO2-rich brines and pure liquefied CO2
Technical aspects of gas hydrate conversion and secondary gas hydrate formation during injection of supercritical CO2 into CH4-hydrate-bearing sediments
The injection of CO2 into CH4-hydrate-bearing sediments has the potential to drive natural gas production and simultaneously sequester CO2 by hydrate conversion. Currently, process conditions under which this goal can be achieved efficiently are largely unknown. While the recent Ignik Sikumi field test suggests that a combination of N2/CO2 injection with depressurization yields effective CH4 production, in a previous study (Deusner et al., 2012) we showed that a combination of CO2 injection and thermal stimulation eliminates mass transfer limitations observed at cold reservoir temperatures. These high-pressure flow-through studies revealed that the injection of supercritical CO2 at 95 °C triggers dissociation of CH4-hydrates and counters rapid CO2-hydrate formation in the near-injection region. We also observed a strong effect of reservoir temperature on CH4 production and CO2 retention. The efficiency and yield of CH4 production was highest at a sediment temperature of 8 °C compared to 2 °C and 10 °C. At 2 °C CO2 hydrate formation was rapid and clogged the sediment at the injection spot. Outside the CO2-hydrate stability region, at 10 °C, we observed fast CO2 breakthrough and a comparably low CH4 production. Experiments comparing discontinuous and continuous CO2 injection showed that alternating periods of equilibration and CO2 injection improved the overall CH4 production. We hypothesize that slow formation of secondary CO2-rich hydrate improves the accessibility of the CH4-hydrate distributed in the sediment by locally changing permeability and fluid flow patterns. In situ measurements showed dynamic changes of local p-/T-gradients due to gas hydrate dissociation or dissolution and secondary gas hydrate formation. In addition, continued reconfiguration of guest molecules in transiently formed mixed hydrates maintain elevated gas exchange kinetics. Online effluent fluid analysis under in-situ pressure conditions indicated that CH4 released from CH4-hydrates is largely dissolved in liquid CO2.. It is a current objective of our studies to further elucidate rheological properties and gas exchange efficiencies of CO2-CH4 mixed fluids that approach equilibrium with gas hydrates and to study the effect of in situ CH4-CO2-hydrate conversion and secondary gas hydrate formation on sediment geomechanical parameters
Methane Production from Gas Hydrate Deposits through Injection of Supercritical CO2
The recovery of natural gas from CH4-hydrate deposits in sub-marine and sub-permafrost environments through injection of CO2 is considered a suitable strategy towards emission-neutral energy production. This study shows that the injection of hot, supercritical CO2 is particularly promising. The addition of heat triggers the dissociation of CH4-hydrate while the CO2, once thermally equilibrated, reacts with the pore water and is retained in the reservoir as immobile CO2-hydrate. Furthermore, optimal reservoir conditions of pressure and temperature are constrained. Experiments were conducted in a high-pressure flow-through reactor at different sediment temperatures (2 °C, 8 °C, 10 °C) and hydrostatic pressures (8 MPa, 13 MPa). The efficiency of both, CH4 production and CO2 retention is best at 8 °C, 13 MPa. Here, both CO2- and CH4-hydrate as well as mixed hydrates can form. At 2 °C, the production process was less effective due to congestion of transport pathways through the sediment by rapidly forming CO2-hydrate. In contrast, at 10 °C CH4 production suffered from local increases in permeability and fast breakthrough of the injection fluid, thereby confining the accessibility to the CH4 pool to only the most prominent fluid channels. Mass and volume balancing of the collected gas and fluid stream identified gas mobilization as equally important process parameter in addition to the rates of methane hydrate dissociation and hydrate conversion. Thus, the combination of heat supply and CO2 injection in one supercritical phase helps to overcome the mass transfer limitations usually observed in experiments with cold liquid or gaseous CO2
Experimental investigation of water permeability in quartz sand as function of CH4-hydrate saturation
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
Microscale Processes and Dynamics during CH4âCO2 Guest Molecule Exchange in Gas Hydrates
The exchange of CH4 by CO2 in gas hydrates is of interest for the production of natural gas from methane hydrate with net zero climate gas balance, and for managing risks that are related to sediment destabilization and mobilization after gas-hydrate dissociation. Several experimental studies on the dynamics and efficiency of the process exist, but the results seem to be partly inconsistent. We used confocal Raman spectroscopy to map an area of several tens to hundreds ”m of a CH4 hydrate sample during its exposure to liquid and gaseous CO2. On this scale, we could identify and follow different processes in the sample that occur in parallel. Next to guest-molecule exchange, gas-hydrate dissociation also contributes to the release of CH4. During our examination period, about 50% of the CO2 was bound by exchange for CH4 molecules, while the other half was bound by new formation of CO2 hydrates. We evaluated single gas-hydrate grains with confirmed gas exchange and applied a diffusion equation to quantify the process. Obtained diffusion coefficients are in the range of 10â13â10â18 m2/s. We propose to use this analytical diffusion equation for a simple and robust modeling of CH4 production by guest-molecule exchange and to combine it with an additional term for gas-hydrate dissociation
Persistence of plastic debris and its colonization by bacterial communities after two decades on the abyssal seafloor
The fate of plastic debris entering the oceans is largely unconstrained. Currently, intensified research is devoted to the abiotic and microbial degradation of plastic floating near the ocean surface for an extended period of time. In contrast, the impacts of environmental conditions in the deep sea on polymer properties and rigidity are virtually unknown. Here, we present unique results of plastic items identified to have been introduced into deep-sea sediments at a water depth of 4150âm in the eastern equatorial Pacific Ocean more than two decades ago. The results, including optical, spectroscopic, physical and microbial analyses, clearly demonstrate that the bulk polymer materials show no apparent sign of physical or chemical degradation. Solely the polymer surface layers showed reduced hydrophobicity, presumably caused by microbial colonization. The bacterial community present on the plastic items differed significantly (pâ<â0.1%) from those of the adjacent natural environment by a dominant presence of groups requiring steep redox gradients (Mesorhizobium, Sulfurimonas) and a remarkable decrease in diversity. The establishment of chemical gradients across the polymer surfaces presumably caused these conditions. Our findings suggest that plastic is stable over extended times under deep-sea conditions and that prolonged deposition of polymer items at the seafloor may induce local oxygen depletion at the sediment-water interface
RiverOceanPlastic: Land-ocean transfer of plastic debris in the North Atlantic, Cruise No. AL534/2, 05 March â 26 March 2020, Malaga (Spain) â Kiel (Germany)
Cruise AL534/2 is part of a multi-disciplinary research initiative as part of the JPI Oceans project
HOTMIC and sought to investigate the origin, transport and fate of plastic debris from estuaries to
the oceanic garbage patches. The main focus of the cruise was on the horizontal transfer of plastic
debris from major European rivers into shelf regions and on the processes that mediate this
transport. Stations were originally chosen to target the outflows of major European rivers along the
western Europe coast between Malaga (Spain) and Kiel (Germany), although some modifications
were made in response to inclement weather. In total, 16 stations were sampled along the cruise
track. The sampling scheme was similar for most stations, and included: 1) a CTD cast to collect
water column salinity and temperature profiles, and discrete samples between surface and
seafloor, 2) sediment sampling with Van Veen grab and mini-multi corer (mini-MUC), 3) suspended
particle and plankton sampling using a towed Bongo net and vertical WP3 net, and 4) surface
neusten sampling using a catamaran trawl. At a subset of stations with deep water, suspended
particles were collected using in situ pumps deployed on a cable. During transit between stations,
surface water samples were collected from the shipâs underway seawater supply, and during calm
weather, floating litter was counted by visual survey teams. The samples and data collected on
cruise AL534/2 will be used to determine the: (1) abundance of plastic debris in surface waters, as
well as the composition of polymer types, originating in major European estuaries and transported
through coastal waters, (2) abundance and composition of microplastics (MP) in the water column
at different depths from the sea surface to the seafloor including the sediment, (3) abundance and
composition of plastic debris in pelagic and benthic organisms (invertebrates), (4) abundance and
identity of biofoulers (bacteria, protozoans and metazoans) on the surface of plastic debris from
different water depths, (5) identification of chemical compounds (âadditivesâ) in the plastic debris
and in water samples
Strain RateâDependent HardeningâSoftening Characteristics of Gas HydrateâBearing Sediments
The presence of gas hydrates (GHs) increases the stiffness and strength of marine sediments. In elastoâplastic constitutive models, it is common to consider GH saturation (Sh) as key internal variable for defining the contribution of GHs to composite soil mechanical behavior. However, the stressâstrain behavior of GHâbearing sediments (GHBS) also depends on the microscale distribution of GH and on GHâsediment fabrics. A thorough analysis of GHBS is difficult, because there is no unique relation between Sh and GH morphology. To improve the understanding of stressâstrain behavior of GHBS in terms of established soil models, this study summarizes results from triaxial compression tests with different Sh, pore fluids, effective confining stresses, and strain histories. Our data indicate that the mechanical behavior of GHBS strongly depends on Sh and GH morphology, and also on the strainâinduced alteration of GHâsediment fabrics. Hardeningâsoftening characteristics of GHBS are strain rateâdependent, which suggests that GHâsediment fabrics dynamically rearrange during plastic yielding events. We hypothesize that rearrangement of GHâsediment fabrics, through viscous deformation or transient dissociation and reformation of GHs, results in kinematic hardening, suppressed softening, and secondary strength recovery, which could potentially mitigate or counteract largeâstrain failure events. For constitutive modeling approaches, we suggest that strain rateâdependent micromechanical effects from alterations of the GHâsediment fabrics can be lumped into a nonconstant residual friction parameter. We propose simple empirical evolution functions for the mechanical properties and calibrate the model parameters against the experimental data.
Plain Language Summary
Gas hydrates (GHs) are crystallineâlike solids, which are formed from natural gas molecules and water at high pressure and low temperature. GHs, and particularly methane hydrates, are naturally abundant in marine sediments. It is known that the presence of GH increases the mechanical stiffness and strength of sediments, and there is strong effort in analyzing and quantifying these effects in order to understand potential risks of sediment destabilization or slope failure. Based on our experimental results from highâpressure geotechnical studies, we show that not only the initial amount and distribution of GH are important for the increased strength of GHâbearing sediments but also the dynamic rearrangement of GHâsediment fabrics during deformation characterizes the stressâstrain response and enables strength recovery after failure. We propose that different microstructural mechanisms contribute to this rearrangement and strength recovery of GH sediment. However, we consider these complicated processes in a simplified manner in an improved numerical model, which can be applied for geotechnical risk assessment on larger scales
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