Gas Hydrates 2: Geoscience Issues and Potential Industrial Applications

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 RUFFINE

IFREMER

Daniel BROSETA

University of Pau and Pays de l’Adour

Arnaud DESMEDT

CNRS – University of Bordeaux

February 2018

PART 1

Field 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 (HCO3−) 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 (HCO3−) 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.

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Chapter written by Livio RUFFINE.

1

Water Column Acoustics: Remote Detection of Gas Seeps

1.1. Introduction

Gas bubbles in the oceans can be detected by acoustic remote devices exploring the water column between the water–atmosphere interface and the seafloor [MER 85]. It is admitted that natural seepage can be observed in most of sedimentary basins and continental shelves around the world [ETI 15, HOV 93, JUD 07]. However, under certain temperature and pressure conditions, only a fraction of gas seepage areas can be directly associated with the presence of gas hydrate-bearing sediments. Hence, the relationship between gas hydrates and the acoustic detection of gas bubbles in the water column may be mostly divided into four main categories:

– bubbles of gases associated with gassy sediments located outside the hydrate stability field, as reported by Dupré et al. [DUP 14] for the seepage field recently discovered at the Aquitaine shelf (Bay of Biscay, France);

– bubbles of gases escaping from the seafloor and coated in oil [LEI 03] and a hydrate skin if located within the hydrate stability field [SAU 06]. The existence of this hydrate skin on the bubble surface prevents them from rapid dissolution [MAK 05]. The decrease in the dissolution rate of methane bubbles within the hydrate stability field provides a mechanism by which methane gas released from the seafloor can be efficiently transported above the hydrate stability boundary [REH 02]. However, the occurrence of hydrate-coated bubbles in the water is not necessarily in relation with hydrate-bearing sediments, and conversely gas escaping from a hydrate deposit is not systematically coated with a hydrate skin;

– bubbles of gases related to the process of gas hydrate dissociation within the underlying sedimentary column. Dissociation of hydrates in response to changes in environmental parameters (e.g. pressure, temperature, salinity) has the potential to produce a release of gas that was stored as hydrates over a long period of time [KEN 03]. This process is well documented in areas such as the West Spitsbergen where hundreds of methane bubble seeps were acoustically detected and associated with gas hydrate occurrence in the sediments [WES 09];

– bubbles of gases in the water column are also related to the presence of free gas in the sediments in surrounding cohabitation with hydrate-bearing sediments as reported by Dupré et al. [DUP 15] from mud volcanoes associated with thermogenic hydrates in the Sea of Marmara (Figure 1.1(a)), by Sultan et al. [SUL 14] in studying the formation and evolution of large active pockmarks in the Gulf of Guinea (Figure 1.1(b)) and by Foucher et al. [FOU 10] when investigating the dynamics of one of the largest submarine mud volcanoes, the Håkon Mosby mud volcano located in the Barents Sea (Figure 1.1(c)).

Acoustic remote sensing has been used for almost a century for the detection of biological and non-biological targets in the oceans. Thus, the first echogram illustrating backscattered signals of both the seafloor and a cod school in the water column along the vessel track was published by Sund [SUN 35]. Since the First World War, underwater acoustic technology has been continuously improved toward different applications such as detection, tracking, imaging, communication and measurement of a large variety of objects within the water column, and is now essential for mastering the oceans for scientific, military and industrial purposes [LUR 02]. Indeed, spatial and temporal scales of acoustic observations are more appropriate for the aforementioned purposes than electromagnetic waves since acoustic waves are much less attenuated [LUR 02].

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 hydratebearing 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].

1.2. Principle of the measurement

Active acoustic systems transmit sound that propagates toward the target and is backscattered to the system. The acoustic signals are backscattered when the acoustic material properties of the target are different from those of the surrounding medium. This produces an acoustic impedance contrast, which is function of both the sound–speed and the density contrasts between the target (e.g. gas bubbles discharged in the water) and the ambient seawater. Depending on the frequency of the acoustic wave and the nature of the target, the range of detection varies from meters to tens of kilometers in the ocean.

1.2.1. Instrumentations

Different acoustic sensors are able to collect water column data with a range of frequencies and subsequent resolution: single-beam echosounders (e.g. EA600 (38 kHz), ER60 (120 kHz)), multibeam echosounders (e.g. ME70 (70–120 kHz), Kongsberg EM302 (30 kHz), EM122 (12 kHz), EM2040 (nominal frequency of 300 kHz), Reson 7150 (12 and 24 kHz) and 7111 (100 kHz)), side-scan sonars (e.g. IFREMER SAR (180 kHz), EDGETECH DTS1 (75 and 410 kHz)), acoustic cameras (e.g. Echoscope 3D, ARIS 2D) and scanning sonars (e.g. ALDS, Simrad, Furuno). A higher frequency provides a better resolution in range, whereas a lower frequency offers wider insonification coverage (Table 1.1).

1.2.2. Qualitative and quantitative measurements

Water column acoustics, rather known as fishery acoustics, has been used for several decades as a non-intrusive method with which to establish fishery independent assessments of marine resources [MAC 92] and for ecosystem studies, including plankton [MAC 96]. Calibrated single and multibeam (mainly ME-70 [TRE 08]) echosounders are used to provide accurate estimates of target abundance [TRE 09].

Marine geosciences studies dedicated to seepage, initiated a few decades ago, essentially benefited from deep-towed side-scan sonar (e.g. mounted on a fish or an autonomous underwater vehicle [AUV]) and ship-borne single beam echosounder surveys (e.g. [DUP 10, DUP 14, FOU 10, MER 85]). These surveys are relatively time-consuming and only provide limited information with regard to the insonified volume and the accuracy of the fluid source location on the seafloor, respectively, for the single beam and side-scan sonars.

Calibrated acoustic systems for amplitude associated with knowledge about the acoustic wave propagation environment, enable quantitative measurements of the backscattered signals. Calibrated measurements can then be used to provide abundance estimates of plankton or fish populations based on the echo-integration technique [DRA 65], which relies on the proportionality of the backscattered signal amplitude with the number of targets in the insonified area or volume [FOO 83]. The amplitude of the backscatter signal for an individual target, derived from numerical modeling or in situ measurements, is then used to compute the number of targets, for instance fishes or free gas bubbles in the water [GRE 04, LEB 14].

Beyond vessel uses, this well-known fishery acoustic technology and the processing approach have been integrated in different platforms like AUV [SCA 09], remotely operated vehicles (ROV) [FOU 10, SIM 07] and sea-floor observatories [SCA 05] with unexpected applications for other scientific and industrial fields studying the presence of non-biological targets in the water column, such as natural gas/oil/water seeps from the seafloor and gas/oil leaks from offshore installations. It has been used for the exploration of natural seeping sites [DUP 14], for continuous monitoring of gas seeps from the seafloor [BAY 14, LEB 14] and accident-monitoring of oil spills [WEB