Gas Hydrates 2 E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 550

Veröffentlichungsjahr: 2018

Beschreibung

Gas hydrates in their natural environment and for potential industrial applications (Volume 2).

Das E-Book Gas Hydrates 2 wird angeboten von John Wiley & Sons und wurde mit folgenden Begriffen kategorisiert:
Energie, Energy, Erdgas u. Erdöl, Natural Gas & Petroleum Products

Leseprobe

Table of Contents

Cover

Preface

PART 1: Field study and laboratory experiments of hydrate-bearing sediments

Introduction to Part 1

1 Water Column Acoustics: Remote Detection of Gas Seeps

1.1. Introduction

1.2. Principle of the measurement

1.3 Bibliography

2 Geophysical Approach

2.1. Introduction

2.2. Overview

2.3. Seismic processing

2.4. Example of gas hydrate exploration: the SYSIF instrument

2.5. Bibliography

3 Hydrate Seismic Detection

3.1. Wave velocities of hydrate-bearing sediments

3.2. Bibliography

4 Geomorphology of Gas Hydrate-Bearing Pockmark

4.1. Introduction

4.2. Generalities about pockmarks

4.3. Impact of gas hydrate on seafloor deformation

4.4. Morphological evolution of gas hydrate pockmarks

4.5. Distinction between gas hydrate-bearing and gas hydratefree pockmarks

4.6. Bibliography

5 Geotechnics

5.1. Introduction

5.2. The Penfeld system

5.3. Bibliography

6 Geochemistry

6.1. Introduction

6.2. Sampling geological materials from hydrate-bearing sediment

6.3. Analyses

6.4. Bibliography

7 Benthic Ecosystem Study

7.1. Microbial ecology in hydrate-bearing sediments

7.2. Macrobial ecology studies at cold seeps

7.3. Bibliography

8 Physicochemical Properties of Gas Hydrate-bearing Sediments

8.1. Introduction

8.2. Gas hydrate formation and dissociation

8.3. Fluid transport in gas hydrate-bearing sediments

8.4. Thermal and electrical properties of gas hydrate-bearing sediments

8.5. Distribution and occurrence of gas hydrates in sediments

8.6. Experimental investigation of dynamic processes in gas hydrate-bearing sediments

8.7. Bibliography

9 Small-scale Laboratory Studies of Key Geotechnical Properties which Cannot be Measured from

In Situ

Deployed Technologies

9.1. Introduction

9.2. Influence of gas hydrates on the stiffness and strength properties of sediments

9.3.

Bibliography

PART 2: Modeling of Gas Hydrate-bearing Sediments and Case Studies

10 Geomechanical Aspects

10.1. Introduction

10.2. Geomechanical characteristics

10.3. Constitutive models for continuum mechanics frameworks

10.4. Coupled formulation

10.5. Numerical simulations of the Nankai 2013 gas production test

10.6. Concluding remarks

10.7. Bibliography

11 Geochemical Aspects

11.1. Introduction

11.2. Basic principles

11.3. Model framework

11.4. Model validation and sensitivity tests

11.5. Model application

11.6. Concluding remarks

11.7. Acknowledgments

11.8. Bibliography

PART 3: Geoscience and Industrial Applications

12 Biogeochemical Dynamics of the Giant Pockmark Regab

12.1. Introduction

12.2. Location of the pockmark

12.3. Megafauna distribution on Regab pockmark in relation to fluid chemistry

12.4. General conclusion on the megafauna distribution on the Regab pockmark in relation to fluid chemistry

12.5. Bibliography

13 Roles of Gas Hydrates for CO

2

Geological Storage Purposes

13.1. Introduction

13.2. Hydrate trapping of CO

2

in subsurfaces (onshore, offshore and deep offshore cases)

13.3. CO

2

deep offshore storage capacity in the French and Spanish EEZs

13.4. Summary and prospects

13.5. Bibliography

14 Hydrate-Based Removal of CO

2

from CH

4

+ CO

2

Gas Streams

14.1. Introduction

14.2. Laboratory experiments of gas capture and separation by means of gas hydrates

14.3. Metrics of CO

2

separation

14.4. Results from experiments of CO

2

removal from CO

2

/CH

4

gas mixtures

14.5. Routes to enhance the removal of CO

2

from CO

2

/CH

4

gas mixtures

14.6. Concluding remarks

14.7. Bibliography

15 Use of Hydrates for Cold Storage and Distribution in Refrigeration and Air-Conditioning Applications

15.1. Introduction

15.2. Hydrate systems for cool storage and distribution

15.3. Criteria for use of hydrates in refrigeration

15.4. Hydrate applications in refrigeration and air conditioning

15.5. Conclusion

15.6. Bibliography

List of Authors

Index

End User License Agreement

List of Tables

1 Water Column Acoustics: Remote Detection of Gas Seeps

Table 1.1. Frequency ranges of the main underwater acoustic systems used for target detection in the water column and orders of magnitude of the maximum usable ranges and of the vertical resolution. Frequency ranges of subbottom profilers and seismic sounders are indicatives since they are not usually used for water column targets. F, fisheries; SFM, sea-floor mapping. Modified from [LUR 02]

Table 1.2. Usual working frequencies for seafloor mapping multibeam echosounders according to oceanic domain, swath width, standard survey coverage, horizontal resolution (DTM grid) and working platforms

Table 1.3. Main acquisition, calibration, integration and processing features of the most-used underwater acoustic systems for target detection in the water column

6 Geochemistry

Table 6.1. Long-gravity corer used on the French research vessels. BTBP and MD2 stand for Beautemps–Beaupré and Marion Dufresne 2, respectively

7 Benthic Ecosystem Study

Table 7.1. Total and maximum viable cell counts in hydrate-bearing sediments [PAR 14]

Table 7.2. Isotope signature ranges (δ13C) according to carbon fixation pathways in cold-seep symbiont-bearing invertebrates (references in [CON 94, HÜG 11a]). See text for acronyms

8 Physicochemical Properties of Gas Hydrate-bearing Sediments

Table 8.1. Thermal properties of phases occurring in gas hydrate-bearing sediments and soils

10 Geomechanical Aspects

Table 10.1. MHCS model parameters used for the Nanaki soils

Table 10.2. Thermo–hydro–chemo parameters used for the simulation

11 Geochemical Aspects

Table 11.1. Comparison of the numerical implementation of the three variants

Table 11.2. Fundamental information for the investigated sites

Table 11.3. Constraints for gas hydrate saturation (%) at the investigated sites (NA: Not available)

Table 11.4. Assigned model parameters and model results for the three investigated sites. Numbers in the first column indicate the different runs as shown in Figure 11.4 . The bold values mark the best-fit results

12 Biogeochemical Dynamics of the Giant Pockmark Regab

Table 12.1. Molecular and isotopic composition of the hydrate-bound gases

13 Roles of Gas Hydrates for CO

2

Geological Storage Purposes

Table 13.1. Pure CO

2

(CO2-100) and CO2-96 stream compositions

Table 13.2. Maturity level and perspectives of CO

2

storage technologies

Table 13.3. CO

2

storage capacity in the French EEZ and in the Spanish EEZ

14 Hydrate-Based Removal of CO

2

from CH

4

+ CO

2

Gas Streams

Table 14.1. Overview of published data on gas hydrate formation experiments, in which the gas is a mixture of CO

2

and CH

4

(first column) and the aqueous solution contains additives (second column). See text for the abbreviations. The information on the kinetics is not given here and can be found in the references listed in the rightmost column

15 Use of Hydrates for Cold Storage and Distribution in Refrigeration and Air-Conditioning Applications

Table 15.1. Thermodynamic properties of PCMs suitable for secondary refrigeration; T: melting temperature; ΔH: melting enthalpy; ρ: density; nbh: hydration number.

Table 15.2. Rheological Newtonian and non-Newtonian models and illustrations

Table 15.3. Work on the rheology of hydrate slurry; Italic: Work on refrigeration applications; HC: hydrocarbon; TA: surfactant; AA: anti-agglomerant; ϕ

s

: hydrate volume fraction; R141b: HCFC 141-b refrigerant; ODW: Ostwald-de Waele; S-Thin.:shear thinning; S-Thicken.: shear thickening; HB: Herschel–Bulkley.

Table 15.4. CHTC for various slurries and various experimental apparatus

List of Illustrations

Introduction to Part 1

Figure 1. Conceptual scheme describing the functioning of a gas hydrate deposit on continental margins. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

1 Water Column Acoustics: Remote Detection of Gas Seeps

Figure 1.1. Water column acoustic anomalies of gas escaping bubbles: (a) above mud volcanoes associated with ethane hydrate-bearing sediments in the Sea of Marmara (30 kHz ship-borne multibeam Konsberg EM302 echosounder, MARMESONET expedition 2009), modified from [DUP 15]; (b) above a gas hydrate-bearing large pockmark in deep water Nigeria (24 kHz ship-borne multibeam Reson 7150 echosounder, EGINA expedition 2012), modified from [SUL 14]. Both datasets have been processed, visualized and interpreted using a software platform combining Sonarscope, a Matlab ®-based program and a 3DViewer called Sonarscope3D Viewer [AUG 11]; (c) Estimated distribution of methane volume across the vertical section of an acoustic flare detected above the methane hydrate-bearing Håkon Mosby mud volcano (200 kHz Simrad ER60 single beam echosounder mounted on the Victor ROV, VICKING expedition 2006), modified from [FOU 10]. Methane volume quantification was performed using the MOVIES-B software [WEI 93].

2 Geophysical Approach

Figure 2.1. Schematic view of a 2D marine seismic acquisition

Figure 2.2. Monitoring of the gyration of a marine 3D 8 streamer’s layout. The colored diamonds are the position of the seismic source; the colored dots are the positions of the seismic traces. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 2.3. Example of a common shot gather before and after preprocessing. Raw data (left) display low-frequency noise hiding reflections, and preprocessed data (right) after frequency filtering

Figure 2.4. Example of a common mid-point gather before and after normal move out (NMO) corrections: raw data (left) display differences in time associated with the source-receiver offsets and with variations in velocity. NMO corrected data (right)

Figure 2.5. Stack section (upper) versus migrated section (lower). Note the strong hyperbolae (read arrows) on the stack section compared to the migrated section. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 2.6. Poststack (left) versus prestack (right) seismic processing. In this example, the medium is far too complex to allow the restrictive hypothesis of poststack imaging. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 2.7. Gas hydrate bearing sediment overlying gassy sediments at the Romanian sector of the Black sea (bottom simulating reflector). The transition between the two mediums is marked by a strong amplitude anomaly (red arrows). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

3 Hydrate Seismic Detection

Figure 3.1. Example of BSR offshore of Norway. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

4 Geomorphology of Gas Hydrate-Bearing Pockmark

Figure 4.1. Illustration of the main classes of pockmarks: normal pockmark, regular and asymmetric in shape; elongated pockmark that can also be a composite pockmark; unit pockmarks, with and without “parental” normal pockmark; “eyed” pockmark that contains a relief that could be carbonate or coarser grain size sediment (see [HOV 02] for further descriptions)

Figure 4.2. Seismic sections showing the acoustic characterization of three different chimneys: (A) upward bending reflections corresponding to a velocity pull up artifact (modified from [CAR 07]); (B) deformation of sedimentary layers within a fluid-escape conduit (modified from [RIB 14]); (C) interruption of seismic reflectors due to gas charge (modified from [REI 11])

Figure 4.3. Three-dimensional (3D) view of an area located offshore of Nigeria showing two types of seafloor deformations and the location of two bathymetric profiles. The bathymetric profiles AA’ and BB’ correspond to a pockmark commonly encountered in the literature and an irregular seafloor deformation related to the presence of gas hydrates, respectively (modified from [RIB 16]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 4.4. Pockmark related to gas hydrates studied in [SUL 10, SUL 14]; (A) bathymetric and dip maps of the pockmark, named pockmark A, and showing a subcircular moat delimiting the peripheral. The positive relief observed in the center is due to the presence of gas hydrates close to the seafloor; (B) SYSIF seismic profile Sy01-HR-PR01 through the pockmark showing the geometry of the high-amplitude chaotic seismic facies. The correlations with in situ data demonstrate that this facies is the seismic signature of the presence of gas hydrates within the sedimentary layers. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 4.5. Seafloor maps showing two examples of gas hydrate features discovered in New Zealand (A) and in Gulf of Mexico (B). The bathymetric maps have been modified from [DAV 10] (A) and [SIM 13] (B). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 4.6. The “Rosetta funnel cluster” of the North West Shelf of Australia (modified from [IMB 12]) is interpreted as being a result of decomposition of gas hydrates during the Paleocene-Eocene: (A) the dip map of the yellow reflector indicated by the black arrow in (B) shows a complex geometry. It clearly shows that the cluster is composed of three or four major funnels with additional smaller ones; (B) the architecture of this funnel is detailed in this cross-section and shows a lack of sediment between the black and the yellow reflectors interpreted as the result of gas hydrate processes. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 4.7. Several morphologies of Type-2 pockmarks due to the evolution over time of the sedimentary deformation as a function of gas hydrate dynamics (modified from [SUL 10]): (A) Dip map of the seafloor with bathymetric contours; (B) SYSIF seismic profiles taken from three Type-2 pockmarks at different phases of pockmark evolution during hydrate formation and dissolution; (C) sketch of three major different phases of Type-2 pockmark evolution with time (modified from [SUL 10]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 4.8. Geometrical characteristics of the two types of pockmarks described in the literature. (A): Type-1 pockmarks describe circular depressions associated with a gas chimney and commonly observed on continental margins since 1970; (B) Type-2 pockmarks correspond to irregular and distorted depression characterized in depth by the presence of gas hydrates (modified from [RIB 16]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

5 Geotechnics

Figure 5.1. Illustration of the Penfeld system and of the piezocone and ultrasonic fork. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 5.2. Depth profiles of mass density (ρ), corrected tip resistance (q

t

), sleeve friction (f

s

), and pore pressure (u

2

) from piezocone sounding PM27-A modified from Sultan et al. [SUL 07]. Pore pressure values above the dashed blue line in the right hand graph are in excess of hydrostatic or steady-state pressures. The blue area delineates a hydrate-rich interval. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 5.3. Depth profiles of compressional wave velocity (Vp), attenuation of the compressional wave amplitude, and applied load measured during the acoustic sounding GMPFV03-06 reported by Sultan et al. [SUL 16]. The blue area delineates a hydrate-rich interval. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

6 Geochemistry

Figure 6.1. Schematic description of the Calypso corer (after [BOU 07])

Figure 6.2. Photo showing the location of the sensors on the Calypso corer

Figure 6.3. 3D view of the gas-bubble sampler PEGAZ

Figure 6.4. a) The PEGAZ sampler in operation on the gas vent and b) the gas transfer system

Figure 6.5. Bragg’s law. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.6. Typical scheme of diffractometer Debye–Scherrer. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.7. Typical scheme of diffractometer Bragg–Brentano. This sample is irradiated with X-ray divergence. This X-ray is diffracted by sample, collimated and recorded by a detector. The study of diffraction angles allows to determinate the crystal structure. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.8. Differences between mounting θ−θ (left) and θ−2θ (right). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.9. Diffraction patterns of clays at three different conditions (black: without treatment, blue: ethylene glycol treatment, red: calcination). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.10. Image explaining the Bohr’s model applied to fluorescence. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.11. Operational principle of sequential wavelength dispersive XRF. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.12. Schematic diagram of the CRDS signal with the build-up and ring-down steps. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.13. Example of architecture for the CRDS-analyzer based on the Picarro G2201-i CRDS model (modified after [CRO 08])

Figure 6.14. Spectra obtained from λ2 monolaser for methane. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.15. An example of molecular and isotopic concentration-depth profiles obtained from dissolved CH4 in pore water as obtained with gassy or hydrate-bearing sediment. The low values of δ13C-CH4 indicate a microbial origin for the methane, and the high concentration measured along the core evidence widespread methane occurrence. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 6.16. In this example, cation peaks are perfectly separated. Concentration can be expressed as a function of the integrated peak area. The lithium peak is detected but its low integrated area only provides an estimate of the concentration. This latter will have to be checked by more precise systems such as ICP-MS (see section 6.3.3.2)

7 Benthic Ecosystem Study

Figure 7.1. Habitat diversity (assemblages) at a cold seep site (Regab giant pockmark off Congo). The biogenic habitats (from up left to down right) are created by dense aggregations of Bathymodiolinae mussels (here colonizing a hydrate outcrop covered by carbonates), Vesicomyidae bivalves living in soft sediment, Siboglinidae tubeworms on small concretions, here juvenile bushes, and microbial mats. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 7.2. Morphology, gas emissions features, carbonates and faunal assemblages distribution on a giant pockmark of 800 m diameter. Above: a first image 30 m altitude ROV survey was conducted for microbathymetry (A) and backscatter intensity (B) maps of the whole structure, defining two zones of fluid activity. Below: The second image (8–10 m altitude) and video snapshot (3 m) survey in the most active zone recorded seabed gas emissions (red dots) and hydrate outcrops (blue dots). Dominant faunal assemblages (symbiotic species) were contoured and mapped from photomosaic (8 m photo survey) in the active, central zone. Their distribution is compared to the seabed microtopography and to the backscatter map. High backscatter has been attributed to carbonate distribution and bivalve shells (from [MAR 14b]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 7.3. Temporal evolution of faunal assemblages from a mosaicking video survey [MAR 14b]. Two surveys have been conducted within an interval of 10 years (in 2001 and 2011) in the most active part of the Regab pockmark off Congo. The transects are 200 m long. In 2011, the whole area (200 m × 50 m) was surveyed and mapped, but only the part of the mosaic which overlaps between the 2 years was analyzed (4,600 m

2

). Areas of coverage of the different assemblages (mytilids, vesicomyids, siboglinid tubeworms) were estimated with ArcMap (ArcGIS) and compared between the two visits. The distribution of the assemblages showed very little variation and the changes in coverage per category were lower than 2% of the overlap area (from [MAR 14b]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 7.4. A schematic model of distribution of symbiont-bearing megafauna according to their geochemical drivers (or characteristics of methane seeps), drawn from the different maps produced (Figures 7.2. and 7.3). The main aggregations are distributed in concentric patterns with the mussels in the middle, then the tubeworms on thick concretions, and finally the vesicomyid clams in the surrounding sediments. Mussels which need methane for their methanotrophic symbionts are present in an area of intense fluxes with significant release of methane to the water column, sustained by hydrate stock in the underlying sediments. Siboglinid tubeworms are present on carbonate concretions but they reach the sediments with their roots to take up hydrogen sulfide. The presence of dark, reduced sediments around the concretions indicate that part of the methane and sulfide fluxes are redirected from under the crusts toward more sulfate-rich zones where AOM occurs. Vesicomyid clams colonize the surrounding sulfide-rich sediments. Their patchy distribution suggests that this is controlled by discrete and transient fluxes from below (from [MAR 14b]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 7.5. PEPITO water sampler, location on ROV Victor 6000, and in situ sampling in a mixed assemblage of mussels and tubeworms (Copyright Ifremer, WACS cruise). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 7.6. Deployment of the benthic chamber CALMAR on bare sediment (left) or populated by vesicomyid clams (right). The metallic ring is used to count the bivalve individuals that will be further sampled by blade core (see Figure 7.10)

Figure 7.7. Scheme of the CALMAR chamber

Figure 7.8. Profiles of oxygen and sulfide and pH in typical reduced sediments (modified from [BUR 06])

Figure 7.9. (A) Oxygen measurement on a bench profiler; (B) multiprofiling with in situ RAP

Figure 7.10. Examples of sampling by ROV or manned-submersible tools. From top left to bottom right: macrofauna sampling with blade core in a microbial mat. Meiofauna and microbial sampling with tube cores in a vesicomyid bed. Sampling macromegafauna using the bushmaster Jr. (C. Fisher’s lab) in a tubeworm bush. Macro-megafauna sampling with the mussel pot. The suction sampler was previously used to sample vagile fauna. For all images: © Ifremer, WACS 2011 or Congolobe 2011

Figure 7.11. Top left: tube core slicing (usually each cm) for meiofauna study. The sediment will be fixed in formalin or ethanol in the Petri box (bottom left). Top right: blade core for macrofauna sampling just before slicing. The sediment is then washed on several mesh size sieves before fixing in formalin or ethanol (bottom right)

Figure 7.12. Examples of macrofauna taxa sorted from cold-seep site sediments

Figure 7.13. Expected number of polychaete families (Hurlbert’s calculation) in different chemosynthetic habitats of the Regab pockmark (see Chapter 4). The number of excepted taxa for the same number of sampled individuals is higher in vesicomyid clusters or bare sediment than near a mussel bed or in a bacterial mat. The vesicomyid and sediment habitats are undersampled, as shown by the absence of plateau in the curves (modified from [GUI 17]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

8 Physicochemical Properties of Gas Hydrate-bearing Sediments

Figure 8.1. Gas hydrate stability boundaries of methane (left) and carbon dioxide (right) with pure water (solid line) and seawater (dashed lines, with a salinity of 35 g.kg

−1

). The dashed-dotted line marks the liquid-vapor equilibrium of CO

2

. The following phase combinations are present for the displayed temperature and pressure range: (I) Liquid water + Vapor; (II) Gas hydrate SI + Vapor; (III) Liquid water + Liquid carbon dioxide; (IV) Gas hydrate SI + Liquid carbon dioxide. Plotted from the Matlab

®

toolbox of the German SUGAR project [SPA 96, TIS 05, KOS 13]

Figure 8.2. Solubility of CH

4

and CO

2

in seawater at a pressure of 15 MPa and a salinity of 35. Solid lines are solubilities for gas-hydrate-free systems, and dotted lines show the solubilities in the presence of a gas hydrate phase. The solubility is significantly lower in the presence of gas hydrates

Figure 8.3. Morphology of CH

4

hydrates in a geologic matrix containing sand and clay at variable fractions (modified from [RUF 15]): Pictures 1, 2 and 3 show CH

4

hydrate formation in sands. Small gas hydrate concretions are visible at the end of the core, and within the core, gas hydrates are disseminated. An ignition of the sample highlights the presence of CH

4

(Picture 1). Pictures 4 and 5 show a gas hydrate lens formed within a matrix with 19.80 clay-wt. %. Picture 6 shows the so-called moussy sediment, resulting from the dissociation of gas hydrate in a clay-rich matrix (here 40.3 clay-wt. %)

Figure 8.4. Experimental scheme for studying CH

4

-CO

2

-hydrate conversion and methane production during continuous injection of CO

2

. Gas hydrate-bearing sediments were kept inside a temperature-controlled high-pressure vessel. Fluids were delivered by high-pressure pumps at constant volume flow rates. System pressure was maintained using a back-pressure regulator, and effluent mass flow was measured using a mass flow analyzer after fluid depressurization

Figure 8.5. Pressure–temperature conditions in the center of the sediment sample. Gas hydrate formation was carried out in batch operation during the first 15 days of the experiment, and after day 10 gas hydrate formation was essentially completed. Gas pressure was then increased for another five days. At day 15, methane gas was released and replaced with water. During re-pressurization with water, a relatively high pressure target value was set (23.5 MPa) to increase the re-pressurization rate and minimize gas hydrate dissociation. Continuous CO

2

injection was started on day 16, and maintained over a time interval of 11 days. Controlled depressurization of the sample was carried out at the end of the experiment.

Figure 8.6. Methane production during continuous CO

2

injection: a) pressure, b) intensity of effluent methane signal, c) intensity of effluent CO

2

signal, d) gas hydrate inventory

9 Small-scale Laboratory Studies of Key Geotechnical Properties which Cannot be Measured from In Situ Deployed Technologies

Figure 9.1. Illustration of possible hydrate morphologies in the pore space of coarse-grained sediments (top) or within fine-grained sediments (bottom). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 9.2. Illustration of the influence of hydrate saturation and effective confining stress (σ’

3

) on the stiffness and strength properties of sand as determined from the results of triaxial compression tests. All the samples were prepared with Toyoura sand with initial porosities ranging from 38 to 40%. Hydrates were formed at a temperature of 5 °C and pore fluid pressures of 8 MPa [MIY 11] or 10 MPa [HYO 13a, HYO 14a]. The stiffness is expressed in terms of Young’s modulus at 50% of the stress at failure, E

50

. The stress at failure corresponds to the failure strength, that is the maximum deviator stress (q

max

) recorded during shearing as seen in Figure 9.3 . For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 9.3. Comparison of the stress–strain curves and volumetric strain of Toyoura sand specimens containing methane hydrate (left) and carbon dioxide hydrate (right) (digitized from [HYO 13a] and [HYO 14a]). All the sand specimens had similar initial porosity of 39–40% before the formation of cementing hydrates at a temperature of 5 °C with a pore fluid pressure of 10 MPa. The gray curves have been obtained from testing fully saturated specimens of Toyoura sand with similar initial porosity of 39–40%. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

10 Geomechanical Aspects

Figure 10.1. Illustration of typical responses of gas hydrate-bearing sediment to triaxial testing

Figure 10.2. Stress relaxation due to hydrate dissociation

Figure 10.3. Enlargement of yield surface due to hydrates ( captures the cohesive effect of the hydrate and captures the increase in dilatancy)

Figure 10.4. CT image showing a thin water layer between gas hydrate (white) and soil grains; after [CHA 15]

Figure 10.5. Volumetric behavior in the MHCS model

Figure 10.6. Triaxial test data by (a and b) Miyazaki et al. [MIZ 11], (c) Winters et al. [WIN 07], (d) Ghiassian and Grozic [GHI 13], (e) Winters et al. [WIN 02] and (f) Yoneda et al. [YON 15] and the MHCS model fitting

Figure 10.7. Chart for evaluating p’cd (modified from Uchida et al. [UCH 16b])

Figure 10.8. DEM cylindrical sample (hydrate saturation of 25%)

Figure 10.9. Results from computerized triaxial tests. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 10.10. Calculation cycle of the coupled process

Figure 10.11. Schematic diagram of the production well (modified fromYamamoto et al. [YAM 15])

Figure 10.12. a) The Nankai site condition and b) the adopted model geometry

Figure 10.13. Profiles of a) hydrate saturation and b) permeability with and without gas hydrate based on field measurements (2013 Nankai offshore test). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 10.14. Hydrate phase boundary adopted for the Nankai simulation. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 10.15. Deviatoric and volumetric responses of a) the Nankai sand and b) the Nankai clay

Figure 10.16. Recorded bottom hole pressure during the 2013 Nankai production test

Figure 10.17. Production history during the 2013 Nankai gas production test

Figure 10.18. Relative permeability relation adopted for the simulation

Figure 10.19. Thermo–hydro responses of the Nankai hydrate-bearing sediments during the 2013 Nankai gas production test. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 10.20. Mechanical response of the Nankai hydrate-bearing sediments during the 2013 Nankai gas production test. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

11 Geochemical Aspects

Figure 11.1. The nonlinear behavior among chloride concentration, methane saturation and gas hydrate (GH) abundance

Figure 11.2. The three numerical schemes implemented in the model [PES 16]. The figure is modified after [PES 6]. See text for discussion. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 11.3. I) Time progression model results (IODP Exp311 U1328) for the model scenario “contrasting GH formation modes”. Formation of gas hydrate from 250,200 years (A) to 200 years (B) is slow while the latest hydrate formation, from 200 years (B) to present (C), is contrastingly fast. II) Time progression model results (IODP Exp311 U1328) for the model scenario “prolonged Cl relaxation mode”. Formation of gas hydrate is equally fast for all incidents. The two episodes of hydrate formation (E and G) are separated by an inactive period (F) during which the positive chloride anomalies disappear due to diffusion. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 11.4. Model results by applying the “contrasting GH formation” mode. Different combinations of methane supply and model time were assigned (the combinations were marked by numbers indicated on the Cl profiles, see Table 11.4 for the assigned parameters) to constrain the longest time (and smallest methane supply) required for the most recent massive hydrate formation events. We show only one model-estimated hydrate saturation and methane concentration from one combination for each site as the results for different cases are very similar. Close-up chloride profiles were shown by the inset figures for U1328 and UBGH2–3. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

12 Biogeochemical Dynamics of the Giant Pockmark Regab

Figure 12.1. Location of the Regab pockmark. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.2. Bathymetric map of the Regab pockmark. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.3. a) Diagram of δ

13

C-CH4 versus δD-CH

4

for the hydrate-bound gases (modified after [SCH 83, WHI 99]); b) diagram of δ

13

C-CH

4

versus C1/(C1+C2) for the hydrate-bound gases (modified after [BER 78]; c) diagram of δ

13

C-CH

4

versus δ

13

C-C

2

H

6

for the hydrate-bound gases (modified after [BER 78]). The red star and the black square correspond to the hydrates of KZR-42 and WACS-02 cores, respectively. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.4. Bathymetric map of the Regab pockmark obtained during the WACS cruises in 2011. Megafauna distribution on the Regab pockmark (modified from [OLU 07] and [MAR 14b]). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.5. Localization of the gas bubble streams, detected hydrates and core recovered during Zaiango, Guineco and WACS cruises and dominant megafauna distribution [MAR 14b, OLU 07, OND 05]. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.6. Mytilids colonize area close to hydrate deposits outcrop and seabed gas emission observed by ROV during WACS cruise in a mytilid and siboglinid areas (Copyright IFREMER). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.7. General view of a mytilid cluster living in a soft sediment area, located in the middle part of the Regab pockmark (left) and sampling by tube core close to the mytilids (and siboglinids) cluster (right) (Copyright IFREMER). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.8. Chloride and sulfate profiles from push core 429-CT5 collected close to the mytilids. See Figure 12.4 for core location. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.9. General view of a bacterial mat and sampling by tube core (Copyright IFREMER). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.10. Chloride, sulfate, methane, δ

13

C

CH4

and sulfide profiles from push core 427-CT1 within the bacterial mat. See Figure 12.4 for core location. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.11. Photo of the vesicomyid bivalve cluster sampled at the southwestern part of the Regab pockmark (Copyright IFREMER). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.12. Chloride and sulfate profiles from core 425-CT1 collected close to the dead/living vesicomyid bivalves. See Figure 12.4 for core location. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.13. Picture of the vesicomyid bivalve habitats located in the middle part of the Regab pockmark (Copyright IFREMER). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.14. Picture of the core 481-CT4 recovered in the vesicomyid bivalve habitats located in the central part of the Regab pockmark (Copyright IFREMER). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.15. Methane and ΣH

2

S profiles from core 481-CT4 collected close to the dead/living vesicomyid bivalves (unpublished data). See Figure 12.4 for core location. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.16. Upper: picture of the seabed gas emission observed by ROV during WACS cruise in a area populated by sparse mytilids and siboglinids, as well as vesicomyids (mainly empty shell). Lower: vesicomyids cemetery in the seabed gas emission zone (Copyright IFREMER). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 12.17. Chloride, sulfate and ΣH

2

S profiles from calypso cores WACS-01 and 03 collected within and outside the pockmark, respectively. See Figure 12.4 for core location. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

13 Roles of Gas Hydrates for CO

2

Geological Storage Purposes

Figure 13.1. Comparison between CO

2

-hydrate experiments in pure water with impurities (N

2

, O

2

, Ar, CH

4

) [CHAPOY 14] and the hydrates equilibrium values of the pure CO

2

case calculated by CSMGem code Version 1.10 (January 1, 2007) [SLO 08]. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 13.2. Mixed gas hydrates equilibrium values of CO

2

with 10%, 25% and 50% of CH

4

in pure water compared to pure CO

2

and CH

4

cases calculated by CSMGem code Version 1.10 (January 1, 2007) [SLO 08]. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 13.3. Variation with depth of the liquid density of pure CO

2

, CO2-96 calculated by GERG-2008 [KUN 12] and comparison with the seawater density assuming a mean bottom water temperature of 2.5°C. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 13.4. Influence of the seafloor depth (below 4,000 mbsl) on the thickness of NBZ and GHSZ with pure CO

2

or CO2-96 (assuming a bottom water temperature of 2.5°C and a local heat flow of 46.5 mW/m

2

). The black arrow indicates the vertical direction of the buoyancy-driven migration after a deep injection of CO

2

-rich liquid at about 450 m. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 13.5. Location map of the area of the Bay of Biscay and the Galicia Plateau showing surface heat flux observations by circles and DSDP/ODP boreholes by white triangles (L48 400 in French EEZ, L12 118-119 and L103 638 in Spanish EEZ). The red lines are the isocontours of 4,000-mbsl using EMODnet data (http://www.emodnet-bathymetry.eu). Also shown are the main onshore CO

2

sources in 2010 by white squares (Source: EC-JRC/PBL. EDGAR version 4.0. http://edgar. jrc.ec.europa.eu). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 13.6. Sediment thickness (color legend = isopachs in meter) in the area of the Bay of Biscay and the Galicia Plateau (Source: NCEI’s global ocean sediment thickness grid version 2 http://www.ngdc.noaa.gov/mgg/sedthick). Also shown are the isopachs of 800 m (see text). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 13.7. Map of the area of the Bay of Biscay and Galicia Plateau showing locations of both CO

2

storage zones in the French EEZ (in green) and in the Spanish EEZ (in pink) after applying the three criteria (see text). Also shown are some physiographic features of interest in the studied area: (1) Trevelyan Escarpment, (2) Gascogne Knoll and (3) Jovellanos Seamount. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

14 Hydrate-Based Removal of CO

2

from CH

4

+ CO

2

Gas Streams

Figure 14.1. Pure component hydrate dissociation boundaries in the temperature and pressure plane. These boundaries separate regions of hydrate stability (on the left) from regions where liquid water coexists with the guest phase (on the right). The H

2

hydrate boundary would require lower temperatures and higher pressures on the axis to be apparent. The operational zone of interest for CO

2

capture is shown in this diagram

Figure 14.2. Isothermal phase diagrams of the CH

4

+ CO

2

hydrate at 274 and 278 K (from [LEE 12]). Triangles: hydrate phase compositions. Squares: gas phase compositions. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 14.3. Example of a batch experiment [RIC 12, RIC 13]. (a) Following the solubilization at 293 K of a 75 mol% CO

2

/25 mol% CH

4

gas mixture in an aqueous solution (4 wt% THF, 3,000 ppm SDS), temperature is decreased (A). Hydrate nucleation takes place (B and then C) prior to reaching the target temperature of 2 °C (there is no induction period). Once the pressure reaches the three-phase equilibrium pressure (F), temperature is raised to the initial temperature and the hydrate is dissociated. (b) The upper curve is the composition of the gas phase, which witnesses the preferential removal of CO

2

; the lower curve, inferred from the pressure curve in (a) by means of an equation of state, is the quantity of gas removed

Figure 14.4. (a) Path followed in the temperature–pressure plane for the experiment reported in Figure 14.3 . From D to F to G, the path is that of the three-phase (L

w

-H-V) equilibrium. (b) Photographs at various times in the course of the experiment: the THF hydrate phase appears at B (the associated exothermicity is shown in the inset), followed at C by the CO

2

–CH

4

hydrate (from [RIC 12, RIC 13])

15 Use of Hydrates for Cold Storage and Distribution in Refrigeration and Air-Conditioning Applications

Figure 15.1. Illustration on a microscopic scale of different kinds of slurry: a) paraffin slurry [XU 05]; b) ice slurry [STA 05]; c) hydrate slurry [CLA 15]

Figure 15.2. Lw–H–V equilibrium conditions of water–CO

2

, water–CO

2

–salt (with salt: TBMAC, TBACl, TBANO

3

, TBPB or TBAB). For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 15.3. Dissociation enthalpies of mixed hydrates of CO

2

+ additive (with additive: TBMAC, TBANO

3

, TBPB, TBACl, TBAB, THF) in kJ·kg

water

−1

. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

Figure 15.4. Air-conditioning system by ice slurry for CAPCOM building in Osaka [KUR 01]

Figure 15.5. CFC-12 hydrate-based cool storage system

Figure 15.6. Air-conditioning system by TBAB hydrate slurry for NKK Corporation offices in Japan [TAK 01]

Figure 15.7. Refrigerant hydrate-based refrigeration system

Figure 15.8. CO

2

hydrate-based refrigeration loop [JER 10b]

Figure 15.9. TBAB hydrate-based air-conditioning system [DOU 13]

Guide

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Series Editor

Alain Dollet

Gas Hydrates 2

Geoscience Issues and Potential Industrial Applications

Edited by

Livio Ruffine

Daniel Broseta

Arnaud Desmedt

First published 2018 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd

27-37 St George’s Road

London SW19 4EU

UK

www.iste.co.uk

John Wiley & Sons, Inc.

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USA

www.wiley.com

© ISTE Ltd 2018

The rights of Livio Ruffine, Daniel Broseta and Arnaud Desmedt to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2018932786

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN 978-1-78630-221-2

Preface

Clathrate hydrates are crystalline inclusion compounds resulting from the hydrogen bonding of water (host) molecules enclosing relatively small (guest) molecules, such as hydrogen, noble gases, carbon dioxide, hydrogen sulfide, methane and other low-molecular-weight hydrocarbons. They form and remain stable under low temperatures – often well below the ambient – and high pressures – ranging from a few bar to hundreds of bar, depending on the guest molecule. Long considered as either an academic curiosity or a nuisance for the oil and gas producer facing pipeline blockage, they are now being investigated for applications as diverse as hydrogen or methane storage, gas separation, cold storage and transport, water treatment, etc. The ubiquitous presence of natural gas hydrates not only in the permafrost, but also in deep marine sediments, has been identified, and their role in past and present environmental changes and geohazards, as well as their potential as an energy source, are under intense scrutiny.

These perspectives are motivating an ever-increasing research effort in the area of gas hydrates, which addresses both fundamental issues and applications. Gas hydrates exhibit fascinating yet poorly understood phenomena. Perhaps the most fascinating feature exhibited by gas hydrates is self-preservation, or the existence of long-lived metastable states in some conditions far from stable thermodynamic equilibrium. Strong departures from equilibrium are also noted in gas hydrate compositions, depending on their formation and kinetic pathways. A proper understanding of these two effects could serve in developing gas storage and selective molecular-capture processes. The memory effect, or the ability of gas hydrates to reform rapidly in an aqueous solution where gas hydrates have been freshly melted, is another puzzling phenomenon. Gas hydrates are likely to be soon exploited for storing gas (guest) molecules or for separating or capturing some of them selectively; yet, the occupancy rates of the different hydrate crystal cavities by the various guest molecules are not fully understood. Very little is known as well on hydrate formation and stability in the extreme conditions (e.g. low or high pressures) met on extraterrestrial bodies such as comets and planets. How hydrates interact with substrates is a topic of prime interest for understanding not only the behavior of hydrates in sediments, but also why some mesoporous particles act as hydrate promoters. Nucleation and growth processes are still unsettled issues, together with the mechanisms by which additives (co-guest molecules, surfactants, polymers, particles, etc.) promote or inhibit hydrate formation. Depending on the application, these additives are needed to either accelerate or slow down the crystallization process; but their selection is still carried out on a very empirical basis. This book series gathers contributions from scientists who actively work in complementary areas of gas hydrate research. They have been meeting and exchanging views regularly over the past few years at a national (French) level, and recently at a European level, within the COST Action MIGRATE (Marine gas hydrate - an indigenous resource of natural gas for Europe). Most of them are involved in the CNRS research cluster “Hydrates de gaz”. The proposed book series is the written expression of those meetings and exchanges. It is divided in two volumes: the first volume, published in 2017, addresses the “Fundamentals, Characterization and Modeling of Gas Hydrates” in the absence of sedimentary material. It deals with physico-chemistry investigations of fundamental properties (structure and dynamics from the molecular to the microscopic scale thanks to the contributions of neutron scattering, vibrational spectroscopy and optical microscopy), calorimetric characterization and phase thermodynamic-modeling, and thermodynamic-kinetic coupling approaches of non-equilibrium effects met during hydrate formation.

This volume addresses geoscience issues and potential industrial applications. The first part is devoted to field study and laboratory experiments of hydrate-bearing sediments. Marine gas-hydrate deposits are very complex geological structures, which often host rich and diverse ecosystems. They can be studied via multiple approaches, which all entail three major steps: an exploratory step to locate the deposit, a sampling and in situ measurement step and further onshore analyses. Thus, this part is meant to provide the reader with a general overview of the tools and techniques commonly used during the three aforementioned steps. It ends with a detailed description of the physicochemical properties of hydrate-bearing sediments with new results obtained from high-pressure flow-through experiments to investigate hydrate dynamics. The second part presents modeling approaches of the geochemical and geomechanical behavior of hydrate-bearing sediments, with applications to the Nankai gas production test and other settings. Finally, the last part presents a field case study for a giant hydrate-bearing pockmark and potential industrial applications: the volume ends with state-of-the-art reviews on the promises and challenges of using clathrate hydrates in technologically important areas - geological storage of CO2 in sub-marine sediments, the capture of CO2 from gaseous methane-rich streams, and cold storage and distribution.

Livio RUFFINEIFREMER

Daniel BROSETAUniversity of Pau and Pays de l’Adour

Arnaud DESMEDTCNRS – University of Bordeaux

February 2018

PART 1Field study and laboratory experiments of hydrate-bearing sediments

Introduction to Part 1

Natural-gas hydrate deposits concentrated a huge amount of hydrocarbons stored beneath the seafloor [BUR 11]. It represents the largest methane reservoir on earth [BUR 11, KRE 15, KVE 88, MIL 04, WAL 12], and is of central relevance in the carbon cycle on continental margins. Gaining in-depth knowledge of its contribution to this cycle would lead to a better estimation of the methane budget of the ocean and the lithosphere, with implications in climate evolution, geohazards and energy resources [BOS 11, COL 10, KEN 03, MAS 10, MAX 06, MCC 12]. Likewise, a fundamental understanding on how natural gas hydrates affect the development and distribution of chemosynthetic communities on the seafloor is needed [FOU 09, KNI 05].

Figure 1 represents a conceptual scheme of the functioning of a natural-gas hydrate system. On continental margins, hydrates are formed within a sedimentary interval characterized by high-pressure and low-temperature conditions. Such conditions are met in the few hundreds of meters of the upper sedimentary column, at water depth of more than 500 m on average. Natural gas hydrates are the product of a crystallization reaction from a mixture of gas molecules, primarily methane, and interstitial water. The gas molecules can be either of thermogenic or microbial origin [MIL 05]. Thermogenic gases imply a long-distance upwards migration from deep-seated reservoirs, whereas in the case of microbial sources the gases can be either generated within the sedimentary interval where the hydrates crystallize or migrate upwards from shallow reservoirs. The majority of natural gas hydrate deposits already discovered contain primarily microbial methane generated from particular organic matter degradation [BUR 11, PIN 13, WAL 12].

Despite the apparent simplicity of its chemical composition, our knowledge of natural gas hydrates is far from perfect, and the reason is threefold:

–

in situ

measurements of key properties and parameters of natural gas hydrates are limited due to the highly expensive cost of drilling expeditions;

– attempts to recover well-preserved samples at

in situ

conditions frequently fail due to the unstable behavior of the hydrates upon pressure decrease, and the development of easy-to-use pressure corers is currently at its early stage;

– the inception and lifetime of hydrate deposits strongly depends on the gas availability in the area. In fact, during its ascent within the sedimentary column, part of the gases can bypass the hydrate formation process and be released at the seafloor, forming plumes into the water column (Figure 1). Another part of the ascent gases is oxidized by the so-called anaerobic oxidation of methane (AOM), and methane from natural gas hydrate deposits can also meet the same fate [DE 15]. This reaction takes place at a specific sedimentary horizon called the sulfate–methane transition zone. It is mediated by a consortium of microbes including bacteria and archae [BOE 00, HON 13, JOY 04, NIE 98, REE 76] and allows the mitigation of methane release to the seafloor. It is coupled with the reduction of sulfate into sulfide, and this coupled redox reaction is the cornerstone of a variety of secondary geochemical processes like the dissolution of barite and the precipitation of authigenic carbonates. Indeed, carbonate precipitation in hydrate-bearing sedimentary environments or at any other submarine methane seep setting is closely related to the AOM. In such environments, methane is oxidized whenever upward migrating gas-rich fluids encounter downward diffusing seawater sulfate. This biogeochemical process is driven by a consortium of microbes [BOE 00], which releases bicarbonate (HCO

3

−

) and sulfide (HS

−

) into the surrounding pore waters. At cold seeps, the AOM often proceeds in the near seafloor environment, typically in the upper first meters below the sediment–water interface. Thus, a significant portion of the dissolved bicarbonate (HCO

3

−

) produced through AOM can precipitate as authigenic carbonates [LUF 03]. Since their first discovery on the Cascadia margin, numerous deposits of authigenic carbonate crusts and nodules have been documented at ocean margins [SUE 14]. In gas hydrate-bearing sediments, authigenic carbonates often occur as millimeter- to centimeter-size nodules of carbonate-cemented mudclast breccias or nodules [BAY 07, BOH 98, GRE 01, NAE 00]. Such carbonates represent suitable “fossilized” indicators of the presence of gas hydrates in marine sediments [BAY 07, NAE 00, PIE 00]. Absolute dating of authigenic carbonate breccias or nodules recovered within hydrate-bearing sediments can hence provide unique constraints on the evolution of gas hydrate reservoirs in marine sediments through time and their relationship with past climate change [BAY 15, BER 14, CRÉ 16, RUF 13, WAT 08]. This set of reactions supports the development of chemosynthetic communities at the [BOE 13, OLU 09].

Figure 1.Conceptual scheme describing the functioning of a gas hydrate deposit on continental margins. For a color version of this figure, see www.iste.co.uk/broseta/hydrates2.zip

From Figure 1 and what has been exposed above, it becomes clear that investigating on the natural gas hydrate dynamics is of particular concern to biogeochemists as it deals with the processes related to their formation, distribution and destabilization within the sedimentary column. Additionally, the observation of densely concentrated chemosynthetic communities at hydrate deposits has motivated the need for developing multidisciplinary-based approaches combining geochemistry, microbiology and fauna-related biology to investigate the interplays between the hydrate dynamics and the development of such ecosystems [OLU 09, OND 05, RAB 16, SIB 09].

Chapters 1 to 7 seek to offer insight into the multidisciplinary approach used to improve our understanding on the dynamics of natural gas hydrate systems. We purposely put forward the approach applied at Ifremer, and it may differ from the combination of analytical tools, technics and methods implemented by other research institutions or research groups. It offers an overview of the roles of geophysics, geology, geochemistry and (micro-)biology in the investigation of natural gas hydrate deposits. Chapters 8 and 9 present laboratory experiments of key properties of hydrate-bearing sediments which are either very difficult or impossible to measure from field studies.

Bibliography

[BAY 07] BAYON G. et al., “Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments”, MarineGeology, vol. 241, nos 1–4, pp. 93–109, 2007.

[BAY 15] BAYON G. et al., “U-Th isotope constraints on gas hydrate and pockmark dynamics at the Niger delta margin”, Marine Geology, vol. 370, pp. 87–98, 2015.

[BER 14] BERNDT C. et al., “Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard”, Science, vol. 343, no. 6168, pp. 284–287, 2014.

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