Experimental investigation of THF-clathrates using microfocus x-ray computed tomography

Master’s Thesis Submitted in Fulfillment of the Degree

Master of Science Energy Technology and Economics

University of Applied Sciences Vorarlberg

Submitted to Stefan Arzbacher

Handed in by Christian Kopf

Dornbirn, 21.01.2016

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Abstract

Experimental investigation of THF-clathrates using microfocus x-ray computed tomography

A clathrate sample formed of water and tetrahydrofuran was investigated using

microfocus x-ray computed tomography, with the aim of assessing whether the technique

is suited to corroborate the ice-rinding hypothesis, and so to explain the phenomenon of

anomalous self-preservation well-known from some gas hydrates. A quantitative analysis

based on x-ray absorption physics was carried out to assess the maximum degree of

contrast that could be expected on an x-ray image between THF-clathrate and H2O, the

results of which suggest that the contrast would be insufficient to enable visual evidence

of ice-rinding on the surface of a gas hydrate clump. Initial imaging confirmed this result.

With the addition of barium chloride as a contrast agent, a time series of scans was

carried out to investigate the growth of THF hydrate. Although a clear-cut distinction

between the hydrate and solution components was not possible on the basis of their gray

value histograms, a linear regression of volumetric analysis data suggests an initial,

steady state hydrate growth rate of ~0.05 mm³/hour. An unexpected result was hydrate

growth continuing, at elevated rates, at ambient temperatures.

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Kurzreferat

Experimentelle Untersuchung von THF-Klathraten mittels Mikrotomographie

Ein Klathrat bestehend aus Wasser und Tetrahydrofuran wurde mittels Mikrotomographie

untersucht, um festzustellen, ob die Methode geeignet ist, eine Eisschicht auf der

Oberfäche einer Gas-Hydrat-Probe identifizieren zu können, welche als Erklärung für das

―self-preservation‖ Phänomen von Forschern vorgeschlagen wurde. Eine quantitative

Analyse, basierend auf der Physik der Röntgenabsorption wurde zunächst ausgeführt,

welche ergab, dass der maximal zu erwartende Kontrast zwischen THF-Klathrat und H2O

auf einem Röntgenbild unzureichend für diesen Zweck ist. Erste Aufnahmen bestätigte

dies. Mit Zugabe von Bariumchlorid als Kontrastmittel wurde eine weitere Probe für eine

Zeitreiheanalyse herangezogen, um das Wachstum von THF Klathrate zu untersuchen.

Obwohl eine vollständige Trennung zwischen Klathrat- und Lösungs-Komponente nicht

möglich war, wurde eine initiale lineare Klathrat-Wachstumsrate von ~0,05 mm³/Stunde

ermittelt. Ein unerwartetes Ergebnis stellt das anhaltende, erhöhte Klathrat-Wachstum bei

Umgebungstemperatur (T=293 K) dar.

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Table of Contents

List of Figures VI

List of Abbreviations/Symbols IX

1. INTRODUCTION 1

1.1 What are clathrates? 1

1.2 The history of their discovery 1

1.3 Phase diagram and geographic distribution 1

1.4 How large are global gas hydrate reserves? 3

1.5 Energy relevant aspects of natural gas hydrates 5

1.6 Gas hydrates as a geohazard 6

1.6.1 Submarine mass movement events 6

1.6.2 The Storegga Slide 7

1.6.3 Difficulties 8

1.7 Methane hydrates and climate change 8

1.7.1 Methane hydrates and paleoclimatological evidence 9

1.7.2 Evidence against the ―clathrate gun hypothesis‖ 10

1.8 Objectives 11

2. THEORY 12

2.1 Clathrate structure 12

2.2 Anomalous preservation 14

3. METHODOLOGY 18

3.1 Tomography overview 18

3.2 Basic principles of x-ray computed tomography 21

3.3 Literature review of hydrate CT studies 24

3.4 Experimental design 26

3.4.1 Overview 26

3.4.2 Equipment 26

3.4.3 Study I 29

3.4.4 Study II 34

4. RESULTS AND DISCUSSION 36

4.1 Study I: Contrast 36

4.1.1 Quantitative analysis 36

4.1.2 Imagery 36

4.2 Study II: THF hydrate growth 37

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5. CONCLUSION 48

6. LITERATURE 50

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List of Figures

Figure 1: Phase diagram of methane hydrate with respect to temperature and pressure,

and temperature and methane concentration. ........................................ 2

Figure 2: World map of sites where natural gas hydrates have been discovered ............... 3

Figure 3: Estimates of the global volume of hydrate-bound gas in marine sediments,

plotted against the year in which the estimate was made ....................... 4

Figure 4: Pie charts showing the distribution of organic carbon (in Gt C) at the Earth’s

surface ................................................................................................... 5

Figure 5: The typical structure of gas hydrate .................................................................. 12

Figure 6: Schematic presentation of the three different, most common, gas hydrate

structures. ............................................................................................ 13

Figure 7: The anomalous preservation regime between the temperatures of 242 – 271 K

for methane hydrate at standard atmospheric pressure (0.1 MPa) ....... 15

Figure 8: The ice rinding hypothesis to explain the phenomenon of anomalous self

preservation of gas hydrates ................................................................ 16

Figure 9: Schematic of a basic microfocus computed tomography setup......................... 19

Figure 10: Schematic drawing of a microfocus transmission x-ray tube ........................... 21

Figure 11: The interaction of x-rays with matter ............................................................... 23

Figure 12: Two images showing the hardware used to conduct this investigation. .......... 27

Figure 13: A CAD drawing of the container made from PTFE used in the first part of this

inverstigation. ....................................................................................... 28

Figure 14: Graph showing the calibration results of the independently variable stage

temperature ......................................................................................... 29

Figure 15: Illustration of the take-off angle θ for inclined and perpendicular anodes, as

used in the SpekCalc software. ............................................................ 30

Figure 16: Emission spectrum modelled using SpekCalc software .................................. 31

Figure 17: Attenuation coefficients for THF and H2O + THF, as well as H2O, dry air,

aluminium and barium for the energy ranges 1 keV to 20 MeV. ........... 33

Figure 18: Temperature setting (solid blue line) on the self-made Peltier cooling stage

over the course of the time series experiment. ..................................... 35

Figure 19: Vertical 2D slice of THF clathrate and a THF + H2O solution inside a

homemade PFTE sample cell, showing the low degree of contrast

between the individual components. .................................................... 37

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Figure 20: A vertical 2D slice showing THF clathrates, THF + H2O solution and a matrix of

glass beads; as well as the histogram of the image’s gray value

distribution ........................................................................................... 38

Figure 21: Exemplary images and histogram of gray value distribution for the class of

analysis: ―conservative‖, an underestimation of the true volume of THF

clathrate in the sample ......................................................................... 39

Figure 22: Time series images of a horizontal section (at constant height) through the

cylindrical sample.. ............................................................................... 43

Figure 23: 3D images of the THF hydrate region, according to the ―best estimate‖

threshold criterion, at the end of the time series investigation. ............. 44

Figure 24: Plot of the ―conservative‖ and ―best estimate‖ data obtained from volumetric

analysis, showing the growth of THF clathrate over a 240 hour period. 46

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List of Tables

Table I: Technical specifications of the GE Systems Phoenix Nanotom m used in the

investigation. ........................................................................................ 27

Table II: Results of the various volumetric analyses carried out on the reconstructed data

from the time series investigation to quantify the THF hydrate growth

rate. ..................................................................................................... 45

Table III: Results of the linear regression analyses of the growth rate data, seperated

according to temperature regimes: "Zero" for temperatures near the

freezing point of water and "Anomalous" for temperatures at ambient. 45

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List of Abbreviations/Symbols

Ångström Å

Apperture area Ap

Bottom simulating reflector BSM

Computed tomography CT

Chemical vapor deposition CVD

Energy E

Focal spot size F

Fachhochschule Vorarlberg/University FHV

of Applied Science

General Electric GE

Gas hydrate stability zone GHSZ

Gas hydrate occurance zone GHOZ

Gigatons of carbon Gt C

Global warming potential GWP

Planck constant h

Hour hr

Intensity I

Cubic ice Ic

Hexagonal ice Ih

Thousand years ka

Liquefied natural gas LNG

Million years Ma

Magnetic resonance MR

Nuclear magnetic resonance NMR

Pee Dee Belemnite PDB

Paleocene-Eocene thermal maximum PETM

Polytetrafluoroethylene PTFE

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Coefficient of determination R²

Hexagonal structure H sH

Cubic structure I sI

Cubic structure II sII

Tetrahydrofuran (CH2)4O THF

Percentage by vol vol%

Percentage by weight wt%

Atomic number Z

Delta carbon-13 (isotope signature) δ13C

Delta deuterium (isotope signature) δD

Anode take-off angle θ

Wavelength λ

Michelson’s contrast κ

Mass attenuation coefficient µ

Microfocus x-ray computed tomography µCT

Microgray µGy

Frequency ν

Mass density ρ

Exposure time τ

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1. INTRODUCTION

1.1 What are clathrates?

Clathrate hydrates, also called gas hydrates, are nonstoichiometric inclusion compounds,

in which a network of H2O molecules held together by hydrogen bonds forms the host

molecule, inside which guest molecules are contained. Most gases of a low molecular

weight—including molecular oxygen (O2), molecular hydrogen (H2), molecular nitrogen

(N2), carbon dioxide (CO2), methane (CH4) and hydrogen sulphide (H2S)—as well as

some higher hydrocarbons such as ethane (C2H6) and propane (C3H8) form gas hydrates

within defined ranges of comparatively low temperatures and high pressures (compared to

average ambient conditions at the Earth’s surface).

1.2 The history of their discovery

Gas hydrates were known form laboratory investigations as early as 1810, when they

were described by Sir Humphrey Davy, to whom their discovery is credited. At this time,

they were a mere scientific curiosity: the transformation of gas and water into a solid

substance. They garnered increasing attention from the 1930s onwards, when pipeline

operators for the natural gas industry started noticing that gas hydrates formed plugs

which obstructed pipeline flow. During the second half of the 20th century, vast reserves of

methane hydrates—the most common naturally occurring gas hydrate—were discovered

in oceanic sediments along continental shelves as well as in permafrost soils of the Arctic

(to date, no estimates have been made of Antarctic reservoirs; Maslin et al., 2010). Since

the guest molecules mentioned above are also components of natural gas—with methane

again being the most common component—hydrates formed with them are of great

interest as a potential primary energy carrier. As a consequence, there has since been not

only a large scientific interest in these inclusion compounds, but also a substantial

economic interest.

1.3 Phase diagram and geographic distribution

A phase diagram (cf. Fig. 1) provides an easy-to-read graphical illustration of the

combination of temperature and pressure (shown as depth below ground surface and sea

level respectively) for which methane hydrates are stable.

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Figure 1: Phase diagram of methane hydrate with respect to temperature and pressure (a), and

temperature and methane concentration (b). (Source: Buffett, 2010)

As may be seen, the gas hydrate stability zone (GHSZ) for permafrost soils are at a much

lower depth as compared to those marine sediments, due to lower temperatures. A world

map of sites where natural gas hydrates have been discovered (cf. Figure 2: World map of

sites where natural gas hydrates have been discovered (through bottom simulating

reflectors and core drilling, solid yellow and red circles respectively) and three production

sites (squares). (Source: Makogon et al., 2007)) corresponds to such a phase diagram,

with the large majority of sites situated in submarine continental shelves. Another factor

accounting fort he geographic distribution shown is the large input into the ocean of

organic matter from the continents. This material, containing organic carbon, is buried in

marine sediements and is subsequently metabolized by bacterial communities in the

process of methanogenesis. Once methane concentrations increase to a given point (cf.

Fig 1), methane hydrates may form and/or remain in a stable state, depending on the

combination temperature and pressure.

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Figure 2: World map of sites where natural gas hydrates have been discovered (through bottom

simulating reflectors and core drilling, solid yellow and red circles respectively) and

three production sites (squares). (Source: Makogon et al., 2007)

1.4 How large are global gas hydrate reserves?

Since it became known that clathrate hydrates naturally form (biogenically and

thermogenically) and remain stable within particular depths of marine sediments and

permafrost soils, estimates of the total global reserves of these compounds and the

amounts of natural gas contained within them have been made and periodically revised

(cf. Fig 3).

The most widely cited ―consensus value‖ is 21 x 1015 m³ of methane at STP (standard

temperature and pressure, 273.15 K and 1 kPa), which is based on a number of

independent estimates, including those of Kvenvolden (1988) and MacDonald (1990).

This is equivalent to approximately 10,000 Gt of methane carbon. Based on these

estimates, the carbon contained in hydrate-bound methane amount to more than double

the amount of carbon stored in all other fossil fuels (4200 Gt C; Kump et al., 2000). A

reservoir of this size would also constitute a major component of the global (organic)

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carbon cycle, the importance of which for past and future climate change is not well

understood to date (Dickens, 2003).

The basic approach of the various estimation methods are similar: the spatial extent of the

gas hydrate occurance zone (GHOZ) and/or the GHSZ is estimated; volumes are arrived

at by multiplying the estimated area of the Earth’s surface at which gas hydrates occur

and/or are stable, by the vertical thickness for each location across which such an

occurance and/or stability is assumed to be given. This volumetric estimate (of hydrate

bearing sediment) is then multiplied by the estimated average porosity (%) of a given

sediment and a yield factor: the estimated average gas yield of hydrate bearing sediments

(m³/m³, the volume of hydrate-bound gas, at STP, per cubic meter of hydrate-bearing

sediment) (Milkov, 2004).

Figure 3: Estimates of the global volume of hydrate-bound gas in marine sediments, plotted against

the year in which the estimate was made and/or published. These estimates only

represent a small fraction of all published estimates, namely those to which a higher

degree of justification has been ascribed. Dots indicate average or best-estimate

values, bars indicate the range of estimates reported in individual publications.

(Source: Milkov, 2004)

Despite the similarity of the approaches taken, estimates of the global reserves of gas

hydrates varied widely over the course of the last three decades of the 20 th century, with

roughly one order of magnitude reduction per decade between estimates (Milkov, 2004;

cf. Fig. 3). This continual reduction in the estimated amount of hydrate-bound gas can be

explained by the growing knowledge, over the same period, of gas hydrate distribution

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and concentration within marine sediments. Whereas estimates up to the early 1980s, of

both the spatial extent of hydrate-bearing sediments as well as yield factors, were arrived

at without mapping or drilling data, these parameters were continually constrained by data

from such investigations, especially direct measurements from drilling operations, as they

became available over the course of the next two decades. As a result, the estimated

global reserves of organic carbon as well as the proportion thereof that methane hydrates

contribute have become progressively smaller (cf. Fig. 4).

Figure 4: Pie charts showing the distribution of organic carbon (in Gt C) at the Earth’s surface, with

all reservoirs except gas hydrates kept constant. The three charts, from (a) through

(b) to (c) show various estimates of gas hydrates, in chronological order. The figures

provide clear visual evidence of how gas hydrates make up an ever smaller slice

(coloured yellow) of a diminishing pie of organic carbon reserves, as their total

estimated reserves are updated on the basis of new knowledge. (Adapted from

Milkov, 2004)

1.5 Energy relevant aspects of natural gas hydrates

As mentioned in the previous section, the discovery of vast global reserves of methane

hydrates in the second half of the 20th century has spurred hopes of a significant new

fossil fuel resource to help meet long-term energy needs (Boswell, 2005).

However, as outlined in the previous section, initially very high estimates of the total

amount of naturally occurring gas hydrates have continually been revised downwards as

our knowledge of in-situ conditions has increased. Although the latest estimate of the

global inventory—between 500 and 2500 Gt C—is smaller than the approximately 5000

Gt C estimated for all other fossil fuel inventories, it is still much larger than the roughly

230 Gt C estimated for other natural gas sources (Maslin et al., 2010), such as coalbed

methane, shale gas and so-called tight gas.

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Furthermore, the low concentrations of gas hydrates registered at most sites surveyed

suggest that only a small fraction of deposits may constitute an economically viable gas

hydrate resource (Ibid.). Further research and development, experience gleaned from

test-sites, advances in exploration technology, as well as the future development of

energy prices may of course alter this current, cautious assessment concerning the role of

natural gas from gas hydrates in the question of overall energy supply and demand. On a

smaller scale, countries such as Japan, which possess no significant domestic deposits of

fossil fuels, have a clear interest in the economic production of natural gas from gas

hydrates and have implemented sizeable programmes toward that aim; note for example,

the Nankai production site in the Nankai Trough located off the southeast coast of Japan

in Fig. 2.

Other positive applications of gas hydrates include natural gas storage and transportation,

which is made economically feasible through the anomalous preservation effect. This

effect (described in Section 2.2), allows methane to be stored and transported at higher

temperatures and lower levels of pressure than those required for the liquefaction and

compression processes used in the production of liquefied natural gas (LNG) (Chatti et al.,

2005).

1.6 Gas hydrates as a geohazard

Methane hydrates also pose a number of risks, all of which are related to its clathrate

structure dissociating and the previously trapped natural gas being released. There are a

variety of hazards associated with such a release, depending on the rate at which the

release occurs. So-called catastrophic releases involves the sudden release of large

amount of methane and it is this high rate of release that is associated with geohazards or

geomorphological processes such as underwater landslides, which can in turn trigger

tsunamis and so have an impact far beyond the local site of the gas blowout event.

On a smaller scale, local gas blowouts or plumes clearly pose a risk to both shipping and

human infrastructure related to marine oil or gas production (such as drilling rigs and

ocean platforms), not least through the ability of such a gas plumes to produce negative

buoyancy in the water column (Maslin et al., 2010).

1.6.1 Submarine mass movement events

Various studies have proposed a correlation between the frequency of submarine mass

movements and the rate of climate change, noting that these occur more frequently during

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periods of rapid climate change (Lee, 2009; Maslin et al., 2004, 2005). The scientific

consensus is that this is predominantly caused by changes in the sedimentary regime (for

more details, see section 1.3.2), rather than the dissociation of gas hydrates (Masson et

al., 2010). This does not, however, rule out the catastrophic release of methane from

hydrate deposits as a result of submarine mass movements, nor does it preclude gas

hydrates from playing a role in slope stability. Slope failures occur when the shear stress

brought to bear on a layer of sediment exceeds the shear strength. Gas hydrates which

occupy sedimentary pore space may act like a cement (Groznic, 2010) and it stands to

reason that a continued dissociation of such gas hydrates deposits would lower the shear

strength of the host sedimentary layer.

1.6.2 The Storegga Slide

A large and well known submarine slide is the Storegga Slide along the continental shelf

of Norway (Brown et al., 2006). The slide, with a surface area of around 104 km², occurred

approximately 8000 years (8 ka) ago, following deglaciation, excavated an average depth

of 250 m of sediment and generated a tsunami that impacted Norway, the Faeroes,

Scotland and northern England (Bondevik et al., 2005). The slide area is known to contain

methane hydrate deposits through seismic investigations which have revealed a bottom-

simulating reflector that corresponds to the base of the hydrate stability zone at 200-300

m (Zillmer et al., 2005). The presence of pockmarks, which indicate gas expulsion from

the sediment (Hovland et al., 2005), offers further support for the previous line of

evidence.

However, other factors are thought to have led up to and ultimately caused the slide, and

as such, the precise role of methane hydrate in this slide is unclear. Although the slide is

presumed to have been triggered by an earthquake, the hazard disposition is thought to

have been increased through a destabilization of the sediment column (Archer, 2007). As

previously mentioned, the general consensus is that such a destabilization is primarily

caused by changes in the sedimentary regime; in this case, by a rapid accumulation of

glacial sediment associated with the Fennoscandian ice sheet (Bryn et al., 2005). The

comparatively high sediment loading that occurs on the continental margin seabed due to

glacial erosion processes results in pore water being trapped in sediments faster than it

can be expelled by the increasing weight of the overlying sediment column. This

mechanism also offers an explanation for comparable landslides along the Norwegian

margin, the return period of which, at approximately every 105 years, is approximately

synchronous with Quaternary glacial cycles (Solheim et al., 2005). Whereas such rapid

sediment loading may be considered a preconditioning factor for the increased instability

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of submarine slopes, the isostatic rebound that follows deglaciation may provide a likely

trigger mechanism for submarine slope failures or slumps (Lowe and Walker, 1997;

Maslin et al., 2010).

1.6.3 Difficulties

Although a review of the literature does hint at a correlation between North Atlantic

submarine mass movements and atmospheric methane concentrations, the data are far

from conclusive, owing mostly to the difficulty of accurately dating the known mass

movement events, as well as to the small sample number (23 events in the last 45 ka),

which does not support statistical analysis (Maslin et al., 2010).

Nevertheless, the risk of submarine mass movements and resulting tsunamis should be

seriously considered in any professional assessment of potential gas hydrate exploitation.

That certain geohazards only occur relatively infrequently often entails the additional

danger that awareness thereof is lacking or that the hazard posed by it is not included in

risk analysis and management activities. However, the frequency of a given type of

hazardous event is often inversely related to the magnitude thereof (a good example

thereof are the magnitudes and frequencies of earthquakes), and hence also with the

potential damage that it can cause.

1.7 Methane hydrates and climate change

Methane stored in naturally occurring hydrate clathrates pose a potentially even greater

threat to the Earth’s climate system, since methane has a global warming potential (GWP)

of 86 over a time horizon of 20 years and 34 over 100 years (compared to the baseline

GWP value of 1 for carbon dioxide) (Myhre et al, 2013). The GWP is a ratio of the time-

integrated radiative forcing from the instantaneous release of 1 kg of a trace substance

relative to that of 1 kg of a reference gas, usually carbon dioxide, and is calculated as

follows (Ramaswamy et al., 2001):

𝐺𝑊𝑃 𝑥 = 𝑎𝑥 ∙ 𝑥(𝑡) 𝑑𝑡𝑇

0

𝑎𝑟 ∙ 𝑟(𝑡) 𝑑𝑡𝑇𝑂

(1)

where x refers to the trace substance in question and r to the reference gas; ax is the

radiative forcing (in W/m2·kg) that can be ascribed to the unit increase in the atmospheric

abundance of a substance and [x(t)] is the time-dependent decay in abundance of that

substance following an instantaneous release at t = 0; T refers to the time horizon under

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consideration. The denominator contains the corresponding calculation for the reference

gas, CO2.

1.7.1 Methane hydrates and paleoclimatological evidence

Large-scale releases of methane gas from dissociating methane hydrates are also

postulated to be linked with a number of paleoclimatic events (associated with elevated

global average temperatures), notably the carbon isotope excursion at the Cretaceous-

Tertiary boundary about 65 million years (Ma) ago, and the Paleocene-Eocene thermal

maximum (PETM, approximately 55.5 Ma) (see for example Maslin et al., 2010). Two

lines of evidence are primarily used in these postulates: One is the identification and

dating of seabed geomorphologic features associated with hydrate dissociation

processes, such as pockmarks and seabed slumps (Haflidason et al, 2004). The second

line of evidence involves changes in regional and/or global records of different carbon

isotopes, primarily from ice cores and deep-sea sediment cores.

Of three naturally occurring carbon isotopes, 12C and 13C are by far the most abundant,

accounting for 98.9 % and 1.1 % respectively, while 14C accounts for 1 part in 1012.

However, many natural processes result in a fractionation of this ratio. For example, 14C is

more readily absorbed by ocean waters than the other carbon isotopes are, while

photosynthesis leads to a relative enrichment of 12C (Lowe and Walker, 1997). Not only

does this knowledge, along with known decay rates of radioactive isotopes, allow for

dating techniques, but the various fractionations allow inferences to be made about the

origin of materials containing carbon. Carbon isotope ratios of 13C to 12C are compared to

a standard, namely PDB limestone: belemnite carbonate from the Cretaceous Peedee

Formation of South Carolina, with the values of a sample under investigation being

expressed as a deviation from this standard, as follows (Lowe and Walker, 1997):

𝛿 𝐶 13 =

𝐶

13

𝐶 12

sample

−

𝐶

13

𝐶 12

standard

𝐶

13

𝐶 12

standard

(2)

The results are often expressed on a per mille (‰) basis.

Most methane is produced biogenically, in other words, it is the product of organic matter

degradation, with an associated δ13C range from -60 to -110 ‰. With narrower ranges of

δ13C, as well as different ranges of deviations from standard hydrogen to deuterium ratios

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(δD), which constitute an additional criterion, it may be distinguished whether a methane

sample was formed in fresh water or in salt water (Archer, 2007). The difference is caused

by seawater containing higher concentrations of SO42- than freshwater, which acts as a

final electron acceptor, after O2, NO3-, Mg2+ and Fe2+ have successively been depleted,

before methanogenesis may commence.

A number of events characterized by extreme environmental change have occurred

sporadically throughout Earth’s history. One line of evidence used to identify some of

these events is carbonate and organic matter, ideally with a wide geographic distribution,

which display prominent drops (greater than 2 ‰) in δ13C that are indicative of a large

perturbation in the global carbon cycle. Such perturbations have proven difficult to account

for with the various known reserves of and fluxes between the various components of the

global carbon cycle. Dissociating methane hydrate reserves were consequently proposed

as a possible explanation, a hypothesis popularly known as the ―clathrate gun

hypothesis‖. This hypothesis also suggests a positive feedback loop, with global climate

warming leading to the release of large amounts of methane, the radiative forcing of

which leads to further warming.

In addition to the two paleoclimatic events mentioned above (the Cretaceous-Tertiary

boundary at about 65 Ma and the Paleocene-Eocene thermal maximum, or PETM, of

approximately 55.5 Ma), a role for methane has also been hypothesized for the much

higher frequency glacial-interglacial cycles of the Quaternary (roughly the previous 2 Ma)

(Maslin et al., 2010). Moreover, a link between methane hydrates and even older

paleoclimatic events has been proposed: at the Permian-Triassic boundary, of

approximately 252 Ma (Krull et al., 2000) as well as Jurassic episodes of approximately

183 and 157 Ma (Hesselbo et al., 2000; Padden et al., 2001). Of these, the PETM is the

most significant, as it is the only paleoclimatic event for which stratigraphic evidence

unequivocally indicates a δ13C excursion that was both global and rapid (the input of 13C-

depleted carbon indicated to have occurred in less than 20 ka) (Dickens, 2003).

1.7.2 Evidence against the “clathrate gun hypothesis”

A serious problem to the proposed ―clathrate gun hypothesis‖ is the size of the global

inventory of gas hydrates, which current best estimates peg as three to four orders of

magnitude smaller than the first estimates posited (see Section 1.1.3.). During the PETM,

the GHSZ (as a volume) is estimated to have been roughly 43% of the present one

(Dickens, 2001), which in turn suggests that the likely maximum amount of methane

released from gas hydrates during the PETM was an order of magnitude lower than that

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which would have been necessary to account for the observed isotopic excursion. This

line of argument assumes that the hydrate-bearing marine sediments of the Paleocene

had the same average gas yield that present ones do. Current best estimates, constrained

by measurements from the Blake Ridge and the Hydrate Ridge, place this value at

between 1.4 – 2.4 m³/m³ (Milkov, 2004). In this context, it is not irrelevant that the

estimated value of this parameter decreased by more than an order of magnitude over the

past few decades (Milkov, 2004) – which is directly linked to the orders of magnitude

reduction in the estimated global reserves of gas hydrates over the same period, as

explained in Section 1.1.3.

That these reservations apply to the PETM, which is the most unequivocal example

refered to in regards tot he clathrate gun hypothesis, suggests that the proposed link is to

date still very speculative indeed.

1.8 Objectives

For all of the questions described above and to which gas hydrates are relevant, it is

important to have better understanding of the conditions under which these compounds

are stable or unstable, including their in-situ growth and dissociation. It was towards these

ends that the present investigations were carried out with the available equipment and

resources.

The aim of this thesis was to investigate tetrahydrofuran (THF) hydrate using microfocus

x-ray computed tomography (µCT). More specifically, the work aimed to identify which

avenues of investigation may successfully be pursued with the available technology, first

and foremost with regards to the anomolous preservation phenomenon. THF was chosen

as a guest molecule for the reason that it is well suited to experiments in a basic

laboratory setting and because of its ready availability.

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2. THEORY

Theoretical aspects of clathrate hydrates that are relevant to this investigation are breifly

considered below.

2.1 Clathrate structure

Although more than 130 compounds are known to form clathrates with water molecules,

the most commonly occurring natural gas hydrates form one of the three following

repeating crystal structures: cubic structure I (sI), cubic structure II (sII), or hexagonal

structure H (sH) (Sloan and Koh, 2007). In each case, water molecules form the outer

cages, within which different guest molecules of gas can be encaged (cf. Fig. 5). The

individual structures can be packed like polyhedral cages to form the overall gas hydrate

structure.

Figure 5: The typical structure of gas hydrate with H2O molecules hydrogen bonded to form cages

that trap a gas molecules as a so-called guest molecule inside, with the individual

structures packed together so as to occupy a minimum of space. (Source: Maslin et

al., 2010)

Structure I, which is the most common, can enclose molecules which have diameters

between 4.2 and 6 Å, such as methane (CH4), ethane (C2H6), carbon dioxide (CO2) and

hydrogen sulphide (H2S), which are commonly occurring biogenic gases found in marine

- 13 -

sediments along continental shelves (Sloan and Koh, 2007). The unit cell of structure II

consists of 24 cages in total, namely 16 small cages and 8 large ones, so that nitrogen,

hydrogen and small molecules of d < 4.2 Å can form single guests in this structure, as can

larger molecules (between 6 and 7 Å) such as propane (C3H8) or isobutane (C4H10) (Ibid.);

this structure therefore entraps a mixture of natural gases and is most prevalent in marine

sediments where thermogenic formation of gas occurs (Maslin et al., 2010). Molecules

larger still (typically between 7 and 9 Å), such as isopentane (C5H12) or neohexane (2,2-

dimethylbutane, C6H14) may form structure H when they are accompanied by smaller

molecules like methane to occupy the smaller cages of structure H (Sloan and Koh,

2007). Fig. 6 shows the three different cage structures and the type of polyhedra which

they contain. Different polyhedra result in different cavity sizes and hence in the different

size classes of guest molecules they can contain. The nomenclature used to describe

these cages are of the form nm, with n denoting the number of edges of a polygonal face

type, and m being the number of faces, per cage, with n edges. For example, a

pentagonal dodecahedron, labelled 512, has 12 pentagonal faces, and is shown in Fig. 5.

Figure 6: Schematic presentation of the three different, most common, gas hydrate structures;

various polyhedra, with different numbers of different polygonal faces, results in

different cavity sizes and hence the different size classes of guest molecules that

can be enclosed in the different structure types, as listed in the text. For an

explanation of the polyhedral nomenclature, see text. (Source: Maslin et al., 2010).

- 14 -

2.2 Anomalous preservation

The anomalous preservation phenomenon is one that is of practical interest, as it has

potential applications for the storage of natural gas, as well as the transport of natural gas

at much reduced pressures and high temperatures (close to the melting point of ice) as

compared to the case of liquefied natural gas (LNG).

A number of researchers (for example Davidson et al., 1986; Yakushev and Istomin,

1992; Stern et al., 2001) have described a phenomenon of gas hydrates exhibiting long-

term stability outside of their thermodynamic field of stability. Russian pipeline operators

attempted to explain anomalous gas hydrate behaviour (delayed decomposition) of gas

hydrates at negative Celsius temperatures as early as 1969 in a pipeline running from

Ust-Vilyuisk gas/condensate field in eastern Siberia (Istomin et al, 2006). They isolated a

sample of gas hydrate at atmospheric pressure (0.1 MPa) and 253 K, which showed no

outwardly visible changes for a period of two weeks, whereas the equilibrium temperature

under atmospheric pressure was calculated to be approximately 223 K.

In laboratory tests, Stern et al. (2001) carried out numerous temperature-ramping and

rapid depressurization tests on pure, polycrystalline methane hydrate at standard

atmospheric pressure (0.1 MPa) and a range of constant test temperatures, directly

measuring decomposition rates over a period of days for each run (cf. Fig. 7). The authors

provide a clear description of the various pressure-temperature-time (p-T-t) paths taken

for the various experimental runs as well as the time-dependant dissociation rates

recorded for each such run. In so doing, they were able to sharply delineate a low

temperature region—between 193 and 240 K—within which dissociation rates increased

monotonically with increasing temperature, from a warmer region—between 242 and 271

K, the anomalous preservation regime—in which dissociation rates deviated markedly

(being a number of orders of magnitude lower) from an extrapolated trend of the previous,

monotonically increasing regime (see dotted line, Fig. 7). Within the temperature range of

the anomalous preservation regime, following a rapid depressurization to 0.1 MPa, a short

rapid dissociation phase was registered during which 5-20 vol% of the total methane in

the hydrate samples was released, after which the samples remained anomalously stable

for a period of two to three weeks, which amounts to a average dissociation rate which is

a few orders of magnitude slower than that expected based on an extrapolation of

measurements for the range T < 242 K.

- 15 -

Figure 7: The anomalous preservation regime between the temperatures of 242 – 271 K for

methane hydrate at standard atmospheric pressure (0.1 MPa), as opposed to the

continuous, monotonic dissociation regimes below and above this temperature

range. These data were recorded in rapid depressurization experiments at the

respectively shown constant external temperatures. The equilibrium dissociation

temperature T = 193 K for methane hydrates at p = 0.1 MPa is shown on the left and

the melting point of ice at 273 K on the right. As dissociation rates varied with time

for each given temperature, the results shown here constitute an average rate: the

inverse of the total time required for a sample to dissociate to 50% (a corresponding

time scale is shown on the right axis) (solid circles). The open circles plot

extrapolated rates for tests in which anomalous preservation was so marked, that

the sample did not attain 50% dissociation within the allotted 6.5 hours per run.

(Source: Stern et al., 2001)

The phenomenon has become a well-defined and reproducible one. Nevertheless, the

mechanisms underlying the self-preservation of gas hydrates is poorly understood to date.

A number of processes and contributing factors have been suggested to explain this

phenomenon. Canadian researchers were the first to propose the idea that thin-film ice

formed on the surface of a hydrate sample during hydrate decomposition is responsible

for the self-preservation effect (Davidson et al., 1986). The proposed mechanism involves

the formation of a thin film of supercooled water on the surface of hydrate clump during

the initial hydrate dissociation process, which later freezes to form a layer of ice that acts

as a mechanical barrier which prevents or significantly retards further dissociation by

maintaining the necessary partial pressure for the hydrate to remain metastable (cf. Fig.

8). Japanese researchers who studied this phenomenon using methane hydrates and x-

ray diffraction techniques likewise proposed (Takeya et al., 2002) a two-stage

- 16 -

decomposition, with the first, rapid phase followed by the formation of a thin ice film which

dramatically slows the rate of decomposition.

Figure 8: The ice rinding hypothesis to explain the phenomenon of anomalous self preservation of

gas hydrates following depressurization (p < peq) and with 238 K < T < 273 K.

(Source: Istomin et al., 2006)

Kuhs et al. (2004) describe the results of in situ neutron diffraction experiments, which

suggest a role for the structural crystalline changes that occur in such an ice layer in

explaining the onset of anomalous preservation and its development across the

temperature range previously delineated (Stern et al., 2001). They propose the idea that

the low-temperature onset of the anomalous preservation effect at 240 K coincides with

the annealing of stacking faults of the ice initially formed around a hydrate sample at lower

temperatures (T < 240 K). The first, rapid phase of hydrate decomposition was found to

coincide with the formation of cubic ice, Ic, which is associated with deformation stacking

faults. These stacking faults are ascribed to the mobility and concentration of Bjerrum

faults (specific irregularlities in the arrangement of oxygen and hydrogen atoms in an ice

lattice), which in turn allow for the high diffusion rates required during this initial phase of

rapid hydrate decomposition. Diffraction data indicate the transition of cubic ice, Ic, to

hexagonal ice, Ih, at T ≈ 240 K with appreciable annealing of stacking faults as well as

grain growth of the ice crystallites, the result of which is that gas molecules can only

escape by solid-state diffusion, which is said to account for the depression, by many

orders of magnitude, of the dissociation rate and hence to explain the anomalous

preservation effect.

Stern et al. (2001 and 2003) give credence to the hypothesis that ice-shielding causes the

self-preserving behaviour observed in low-temperature, rapid-depressurization tests as

- 17 -

well as temperature-ramping tests, although they point out that this only applies to the

self-preservation of residual gas hydrate (< 8 %) and suggest that it is not sufficient to

explain self-preservation of methane hydrate, owning particularly to its sII hydrate

structure. In a survey of available literautre, Istomin et al. (2006) furthermore point out a

number of discrepancies between the results of various research teams, suggesting that

decomposition rates vary widely, depending on temperature, pressure, process

initialisation conditions and gas composition. The use of scanning electron microscopy

has furthermore not confirmed the presence of a visible thin ice film on the surface of

hydrate particles under the necessary temperature and pressure conditions.

- 18 -

3. METHODOLOGY

Before describing the experimental setup of the present investigation, tomographic

methods and the basic principles underlying them will be briefly be described below.

3.1 Tomography overview

Tomographic methods are used to produce two- or three-dimensional images through

sectioning of a sample of interest with the use of one of the following physical

phenomena: electromagnetic radiation of wavelengths ranging from radio waves to

gamma rays (cf. Fig. 9); oscillating pressure waves (ultrasonic imaging); the magnetic field

(nuclear magnetic resonance imaging); electric field (for electrical impedance or

capacitance tomography) (Baruchel et al., 2000). In most cases, numerous sections or

slices of the object of interest are made and depending upon the given geometric

arrangement or distribution of these sections, they are combined into a coherent three

dimensional image using various mathematical procedures called tomographic

reconstruction.

Although the technology is best known from medicine, having first been used to

differentiate between various human tissue types (of different density), industrial

computed tomography (CT) scanners have since been developed that are capable of

capturing high resolution images of materials with greater densities than those of human

tissues. Tomographic methods have been successfully employed in a great number of

academic fields ranging from archaeology, soil science and sediment core analyses,

oceanography, seismology and materials science (see for example van Kaick and

Delorme, 2005; Orsi et al., 1994; Munk and Wunsch, 1979; Bording et al., 1986; Baruchel

et al., 2000).

- 19 -

Figure 9: Schematic of a basic microfocus computed tomography setup, showing how an individual

section image of a sample is obtained through the different attenuation of x-ray

beams within the sample depending upon the material. Many such slices, taken from

different angles as the sample is rotated, are then recombined to non-destructively

obtain high resolution 2D and 3D images of the sample. (Source: Egan et al., 2015)

The use of microfocus x-ray sources constitute an important advance in the technology,

which allowed for more accurate images to be captured by reducing penumbral blurring.

The improvement is based on achieving a smaller focal spot size of the x-ray source, by

focusing the beam of impingent electrons onto the cathode with the aid of magnetic

lenses. Microfocus x-ray tubes are able to achieve focal spot sizes in the order of a few

micrometers (Landis and Keane, 2010). So-called nanofocus computed tomography (CT)

systems were developed as a further improvement, decreasing the focal spot sizes by

another order of magnitude to F = 600 nm (0.6 µm).

Another method typically used in medical imaging applications, but which has also been

used successfully for other purposes (e.g. see Gupta et al., 2006) is that of nuclear

magnetic resonance (NMR). This technique relies on the interaction of matter with

electromagnetic fields. Specifically, protons (hydrogen nuclei) and certain other nuclei

have non-zero spin and as a result, a magnetic moment. If these particles are placed in an

external magnetic field (produced by the powerful magnets of MR scanners), they realign

themselves accordingly. Known radio pulse frequencies are then applied, causing the

short-lived deviation of these alignments. These deviations cause changes of magnetic

flux, which can be recorded and from which tomographic reconstructions can be made.

Phase contrast tomography is a technique that appears to have a good potential for

investigating gas hydrates (Takeya, et al., 2007). This method relies on phase contrast as

opposed to the attenuation contrast. An x-ray beam’s phase is shifted as it passes through

- 20 -

a material due to interactions with the electrons of that material. Imaging reliant on this

physical basis may be used qualitatively, especially for edge-detection purposes, or

quantitatively by recording multiple images of a sample at various distances from the

sample and reconstructing an image from the data thus recorded. What makes the

method particularly powerful for the applications under consideration here, is that it is

suited to investigating light materials with similar x-ray attenuation coefficients but with

different electron densities (Baruchel et al., 2000).

Another noteworthy development in CT technology was the use of synchrotron radiation

as an x-ray source. The radiation is emitted when electrons are accelerated along a

curved path, which is produced in industrial CT scanner by bending a high-energy

electron beam with the aid of a magnetic field. This form of radiation constitutes the

brightest artificially created x-rays, being many orders of magnitude brighter than x-rays

emitted by conventional sources. The key implication for the purposes of industrial

tomography is that the much higher flux enables the resolution of comparatively much

subtler variations in absorptivity and hence, of a sample’s internal structure (Landis and

Keane, 2010). This much higher flux also allows for shorter exposure times, which not

only reduces total scan times but also offer greater capabilities for studying time-

dependant processes. A further advantage of synchrotron radiation is the ability to set the

x-ray energy to a comparatively narrow energy band. Used in conjunction with a

monochromator, an x-ray beam may thus be produced that is very intense, parallel and

monochromatic (Baruchel et al., 2000). A potential drawback of this technology is that it

can only be applied advantageously to relatively small sample sizes, in the order of 5 to

10 mm, which does however not exclude many materials sciences investigations or the

investigation of gas hydrates.

One of the key advantages of tomographic methods is that they are non-destructive. This

allows for investigations to be made on samples that are undergoing a transformation,

given that discrete stadia of the transformation in question can be scanned within the

typically long scan time (typically one hour to several hours per 3D scan). This capability

is of great importance in materials research, allowing for microstructural changes to be

recorded in samples undergoing loading (Landis and Keane, 2010). In the case of gas

hydrates, the hope is that hydrate formation and dissociation processes may be

investigated using a suitable computed tomography technique, in order to obtain data that

would further our understanding of these processes and their controlling mechanisms.

- 21 -

3.2 Basic principles of x-ray computed tomography

In this work µCT was used in this investigation, which relies on x-ray absorption physics

on the one hand and on tomographic reconstruction mathematics on the other. The basic

principles dealt with here concern x-ray absorption physics.

X-rays are produced in medical or industrial scanners by an x-ray tube, a vacuum tube in

which electrons emitted by a cathode filament are accelerated by an electric field before

colliding with a target (the anode, usually tungsten, molybdenum or copper) (cf. Fig. 10).

This process produces mainly Bremsstrahlung radiation. Bremsstrahlung is

electromagnetic radiation produced when a charged particle is decelerated as it

approaches and is deflected by another charged particle (typically, an electron

approaching the nucleus of an atom). The kinetic energy associated with its motion is thus

reduced, resulting in the emission of a photon and so satisfying the law of conservation of

energy. In fact, only a very small proportion, approximately 1 %, of the energy is

converted to Bremsstrahlung radiation; the rest is released as heat (Buzug, 2004). Early

x-ray tubes for this reason employed a rotating anode, in order to distribute the thermal

stress and prevent it from melting. The radiation is emitted as a continuous spectrum of

wavelengths (cf. Fig. 15, for example), along with distinct peaks or spikes associated with

the characteristic K and L lines of the target material.

Figure 10: Schematic drawing of a microfocus transmission x-ray tube, showing how an electron

beam (coloured blue) is focused onto a very small focal spot of a target material with

- 22 -

the use of magnetic lenses. (Source: www.phoenix-xray.com; retrieved September

2015)

Although x-rays (roughly categorized as electromagnetic radiation within the wavelength

range 10-13 m to 10-8 m, between the longer wavelength ultraviolet light and the shorter

wavelength of gamma-rays) have a high penetrating ability, which makes them useful to

non-destructively image the inside of objects, they are nevertheless attenuated—suffering

a loss of intensity—through their interaction with matter. This attenuation is largely brought

about by absorption and scattering as the x-rays pass through a material, in particular

through the mechanisms of Rayleigh scattering, the photo effect, Compton scattering and

pair production.

Rayleigh scattering or the elastic scattering of electromagnetic radiation occurs when the

diameter of a particle is small relative to the wavelength of the incident radiation. The

kinetic energy and therefore the wavelength of the incident photon remain constant during

Rayleigh scattering, only the direction of its propagation is changed. Although this effect is

therefore not expected to be of significance in x-ray computed tomography, owning to the

short wavelengths of x-rays, it should not be neglected as the scattering probability is

proportional to the fourth power of the radiation wavelength.

The other three mechanisms—the photoelectric effect, Compton scattering and pair

production—cause the absorption that occurs during the interaction of x-rays and matter

(cf. Fig. 11). The Compton effect or Compton scattering by contrast is an inelastic

scattering of electromagnetic radiation following its interaction with matter. In other words,

an incident photon is scattered in such a way that its kinetic energy and therefore its

wavelength decreases, whereby the conservation of energy is fulfilled through the ejection

of an electron. The photoelectric effect, first described mathematically by Albert Einstein,

occurs when the total energy of an incident x-ray radiation is absorbed by an atom,

causing an electron to be ejected from, for x-ray energies, typically the K or L shell, thus

ionizing the atom. For the energy ranges typically employed in medical and industrial x-ray

computed tomography, these last two effects are the most significant (Buzug, 2004). At

quantum energy > 1.022 MeV, the interaction of x-ray radiation and matter may lead to the

production of an electron positron pair. The positron may shortly thereafter collide with an

electron, producing radiation of a longer wavelength than that originally incident.

- 23 -

Figure 11: The interaction of x-rays with matter: (a) the photoelectric effect, (b) Compton scattering,

and (c) pair production. (Source: Buzug, 2004)

The absorption of an electromagnetic wave as it passes through a given material is a

logarithmic function of the absorptivity of the material and the distance traversed by the

wave through the material. This relationship is expressed by the Beer-Lambert Law, which

can be written as follows:

𝐼(𝜆) = 𝐼0(𝜆)𝑒−𝜇 (𝜆)∙𝑥 (1)

The intensity of the x-rays emitted by the x-ray tube is given by I0(λ). I(λ) represents the

intensity of the x-rays that arrive at the x-ray detector, after they have traversed a distance

x of a homogenous material with a mass attenuation coefficient µ. The attenuation

coefficient is proportional to a function of wavelength, energy, and the atomic number of

the material:

𝜇 ∝ 𝑓 𝜆,𝐸,𝑍 (2)

The x-ray beams are said to be attenuated after traversing different object regions, with

the attenuation dependant on material properties as well as the amount of the material

traversed. The absorptivity of a material depends on the type of atoms it consists of as

well as the number of atoms (their density) along the path of the x-ray beam. For a given

photon energy, the absorptivity of elements with a lower atomic number, and therefore

with fewer electrons, is lower than that of elements with high atomic numbers. So-called

x-ray absorption ―edges‖ constitute an exception to this general rule. For the range of

photon energies under consideration here (10-3 to 101 MeV), distinct jumps occur in the

- 24 -

attenuation coefficients of heavier elements (cf. Fig. 17, barium and aluminum), at the

precise photon energy level that is necessary to eject an electron from their atoms.

3.3 Literature review of hydrate CT studies

Hydrates formed with THF have been used in various studies, including studies of its

crystal growth and kinetic inhibition (Makogon et al., 1997; Storr et al., 2004). Takeya et

al. (2007) investigated THF clathrate hydrates grown from a 19 wt% THF solution, using

phase-contrast x-ray computed tomography and a monochromatic synchrotron x-ray

source at 35 keV. They report being able to carry out density mapping of single hydrate

crystals with a density resolution of several mg/cm³. A density difference of up to 30

mg/cm³ was found for THF hydrate within a single crystal.

The authors rule out the possibility of the difference being caused by previously dissolved

air being trapped in pore spaces between THF hydrate grains, as such a large density

range within a sample (the density of air being two orders of magnitude smaller than that

of THF hydrate) would cause the phase shift to exceed 2π. Instead, they hypothesize the

density difference to be caused by different cage occupancies of THF molecules in 16-

hedral cages, pointing out that difference of 10% in cage occupancy can account for a

THF hydrate density difference of 20 mg/cm³. Although this hypothesis cannot be tested

with this technique, the authors have shown that sufficiently high density resolution

measurements can be made using phase-contrast x-ray CT systems in order to study the

inhomogeneous growth of THF hydrate crystals.

Kerkar et al. (2009) used x-ray CT with a synchrotron light source between 24 and 26 keV

to visualise the growth of THF hydrates from a 60 wt% THF solution in porous media,

which they simulated using a glass bead bed with uniform diameter of 0.5 mm. A 25 wt%

BaCl2 solution was used to enhance the density contrast the THF solution and THF

hydrate. The primary interest of the authors was the microstructure of sediment-hydrate

interaction, which directly affects the mechanical strength of wellbores.

The resulting images, which constitute a time series over 79 hours, have sufficient

attenuation contrast to clearly distinguish the said components, which was furthermore

used to determine the sample’s porosity; the result (34.7 %) was in good agreement with

the theoretical porosity of a randomly packed bed of uniform spheres (38 %). A

comparison of individual glass beads across the time series show that the growth of

hydrates displaces glass beads in the unconsolidated bed. The manner in which hydrates

grew in the available pore spaces is said to conform with a known pore-filling model

described by Dvorkin et al. (1999). The converse of such growth, namely that hydrates

- 25 -

retract from pore walls as they shrink within the pore spaces upon dissociation, is

interpreted by the authors as implying ―a progressive but significant reduction of

mechanical strength of the sediment‖.

Two recent studies by a group of Japanese researchers using microfocus x-ray computed

tomography to perform structural analyses of artificial methane hydrate sediments (Jin et

al., 2004; Jin et al., 2006) have shown the increased difficulty of experimenting with

methane hydrates (as opposed to THF hydrates) in a laboratory setting on the one hand,

and of the insufficient attenuation contrast between free CH4 gas, CH4 hydrate and H2O

on the other hand. The former point is readily illustrated by the need, in the 2004 study, to

keep the sample of methane hydrate near 173 K in order to prevent its decomposition (at

standard atmospheric pressure), which was achieved by blowing cold, dry nitrogen gas

onto the sample from above. In the 2006 study, a temperature of roughly 278 K and a

pressure of 5.1 MPa was maintained, for which a high-pressure vessel was specially

constructed. Both studies used actual hydrate sediment (sand) in order to study the

spatial distribution of grains and gas hydrates in sedimentary layers. In particular, the

porosity of natural, hydrate-bearing sediment is said to be closely related to its methane

content. The hope is that a better understanding of such relationships would ultimately

enable more accurate surveying through the improved interpretation of seismic seafloor

data.

The histograms of grey scale values—which are a direct translation of the intensities of

the attenuated x-ray beams after having passed through the sample—did not allow a

clear-cut distinction between free gas, hydrate and water especially. Sand could be clearly

distinguished from free gas, with an increasing overlap between the—presumed—

Gaussian distribution curves of sand and hydrate, and sand and water. Nevertheless,

porosity values determined from a summation of the area ratios of pore spaces to the total

area from successive 2D slices was in good agreement with porosity values determined

through direct mass and volume measurements from samples that were allowed to melt:

Three samples were measured with both techniques and the maximal difference of the x-

ray CT estimation as opposed to the direct measurement was an overestimation of 2.4 %

and an underestimation of 3.6 %. The overlap in the grey scale histograms from these

studies however rule out any investigation that relies upon a relatively clear-cut

attenuation contrast between methane hydrate and an H2O component, as would for

example be required when trying to visualise µm-scale water or ice layers on the surface

of methane hydrate regions.

- 26 -

3.4 Experimental design

3.4.1 Overview

For this investigation of gas hydrates using µCT, THF — chemical formula (CH2)4O —

was used as it forms hydrates with water under comparatively high temperatures and low

pressures, namely at temperatures T < 278 K at atmospheric pressure (Gough and

Davidson, 1971). As such, it is particularly suited to experimental investigations, in

addition to which, this commonly used solvent is readily available.

Two seperate studies were carried out. For both, THF hydrates were The first, Study I,

investigated the contrast that could be achieved between THF hydrate and the solution

from which it formed, without the aid of a contrast agent. For this first study, a quantitative

analysis was also carried out to assess the maximal contrast that could be expected

between THF and H2O (liquid), based on x-ray absorption physics.

The second part, Study II, aimed to investigate and quantify the growth of THF hydrate,

for which purpose BaCl2 was used as a contrast agent.

3.4.2 Equipment

The x-ray computed tomography was carried out at the Fachhochschule Vorarlberg (FHV;

eng: University of Applied Science) in Dornbirn on a GE Systems Phoenix Nanotom m (cf.

Fig. 12), with an open type x-ray tube with internal cooling and nanofocus capabilities. The

target consists of tungsten on a CVD diamond. An overview of the technical specifications

are provided in Table 1.

- 27 -

Figure 12: Two images showing the hardware used to conduct this investigation: the GE Systems

Phoenix Nanotom m (left); a view of the inside (right), showing the housing of the x-

ray tube in yellow, the Peltier cooling stage, on top of which the homemade sample

cell is placed.

Table I: Technical specifications of the GE Systems Phoenix Nanotom m used in the investigation.

(Source: GE Technische Dokumentation)

High voltage Min 20 kV

Max 180 kV

Tube power Max 48.4 W

Tube power at max voltage Max 26.1 W

Target power Max 20 W

Target power at max voltage Max 15 W

Dose rate at 1 m Max 43 µGy/s @ 180 kV

Tube current Min 5 µA

Max 880 µA

Target current Max 400 µA

Target current at max voltage Max 83 µA

Detail detectablility Min 0.2 µm

Cone beam Typically 170°

Target properties Type Transmission

Material Tungsten

Cooling Air

X-ray window Material CVD

Thickness 0.3 mm

Focus object distance Min 0.3 mm

Filament Type Tungsten-Hairpin, PlugIn

Sample containers were designed and constructed at the FHV. For the contrast study, the

container was constructed from a Polytetrafluoroethylene (PTFE) rod (cf. Fig. 13). For the

- 28 -

crystal growth study, a similar container was constructed from aluminium, with a height of

36 mm, an outer diameter of 14 mm and an inner diameter of 11.4 mm .

Figure 13: A CAD drawing of the container made from PTFE used in the first part of this

inverstigation, the contrast study. All measurements in mm.

In order to have full temperature control and since no commercial solution could be found

that fit the needs, a Peltier cooling stage was designed and constructed in-house.

Temperature could be controlled to within a fraction of a K and to a minimum of

approximately 233 K.

Calibration of the temperature of the cooling stage with that which existed inside the cell

was carried out with K-type thermocouples and the results shown in Fig. 14.

- 29 -

Figure 14: Graph showing the calibration results of the independently variable stage temperature

and the temperature deviation of the dependently varying cell temperature from the

stage temperature. The cell temperatures were measured with a K-type

thermocouple. Also shown, a linear regression best fit.

The regression analysis yielded the following linear relationship:

𝑇𝑐𝑒𝑙𝑙 = 0.988 ∙ 𝑇𝑠𝑡𝑎𝑔𝑒 + 3.963 (3)

3.4.3 Study I

For the contrast study, a 19 wt% THF solution, which corresponds to a stoichiometric ratio

for THF:H2O of 1:17 was prepared. The cooling stage temperature was set to 263 K. The

x-ray tube settings were 40 kV and 300 µA. A spatial resolution of 10 µm was used,

resulting in a voxel size of 1000 µm³. Total scan time was 2 hours and 14 minutes.

3.4.3.1 Quantitative analysis

For this investigation, a purely theoretical quantitative analysis was carried out to

determine the maximum possible difference which could be expected between the

- 30 -

attenuation that an x-ray beam is subject to as it passes through a sample of liquid water

as compared to the attenuation that it would be subject to as it passes through (an equal

distance) of a sample of THF. The motivation is that the contrast between THF clathrate

and a THF + H2O solution would be smaller still than this calculated theoretical maximum,

due to both the molecular composition or arrangement, as well as tot he inhomogenous

spatial arrangement of all of these components.

The initial intensity I0(λ) of the emission spectrum produced by an x-ray tube is related to

the attenuated emission spectrum that arrives at the detector (after having traversed an

homogenous material) according to the Beer-Lambert Law as stated in Eq. (1).

The initial intensity I0(λ) of the x-ray tube’s emission spectrum was approximated with

SpekCalc, a program developed first and foremost for applications of medical physics, but

which may also be of use for industrial CT applications (Poludniowski et al., 2009).

Although the spectra produced by this model are in satisfactory agreement with those

produced by the more sophisticated Monte Carlo method and by some experimentally

measured spectra, can model a wide range of tube potentials (40-300 kVp), can

incorporate filtration effects for seven materials (air, water, Be, Al, Cu, Sn and W) and is

free to download and use, its notable drawback for this investigation is that its predictions

are not suitable for modelling of transmission targets, as used here.

SpekCalc was nevertheless used for lack of alternatives. The theoretical discrepancy

between emission spectra from inclined anode and transmission targets is presumed to

be least for anode take-off angles of θ = 90° (cf. Fig. 15). The reasoning is that for acute

(small) anode take-off angles, a larger distance of anode material must be traversed than

is the case for take-off angles approaching the perpendicular, resulting in lower intensities

(fewer emitted photons) toward the lower end of the emission spectra; this can be

confirmed with the software by specifying a set of parameters and varying the take-off

angle θ.

Figure 15: Illustration of the take-off angle θ for inclined anodes (left) and perpendicular anodes

(right), as used in the SpekCalc software. (Source: Poludniowski et al., 2009)

- 31 -

An emission spectrum was modelled for a photon energy range from 13 keV to 130 keV,

and is shown in Fig. 16. This particular photon energy range was chosen to correspond to

the x-ray tube settings used in the investigations here. For the quantitative analysis

carried out for this investigation, no filtration was used apart from 10 mm of air, for the

scenario of placing the sample cell very close to the transmission target.

Figure 16: Emission spectrum modelled using SpekCalc software for a 130 kVp x-ray tube and

tungsten anode target, with 10 mm air filtration and anode take-off angle θ = 90°.

The wavelength range corresponds to a photon energy range of 13 - 130 keV. The

continuous part of the spectrum is due to Bremsstrahlung radiation, while the local

peaks are characteristic K-lines and L-lines for tungsten.

Mass attenuation coefficients of elements with atomic number Z = 1 to Z = 92 (hydrogen

to uranium) which have been determined experimentally as a function of photon energy

(for x-ray, gamma ray, bremsstrahlung), ranging from 1 keV to 20 MeV, are available

online (Hubbell and Seltzer, 2015 May). These values are the quotient of an element or

molecule’s attenuation coefficient to its mass density (µ/ρ).

The attenuation coefficients for THF was calculated over the energy range of 1 keV to 20

MeV with the method used by El-Kateb and Abdul-Hamid (1991), according to which the

mass attenuation coefficient of an HCO-material (compound or mixture) is equal to the

- 32 -

sum of the products of the mass attenuation coefficients and the weight fractions of the

hydrogen, carbon and oxygen constituents (cf. Eq. 4). The sum of the weight fractions of

H, C and O are equal to 1. In the case of THF, (CH2)4O, the weight fractions are fH =

0.1118, fC = 0.6663 and fO = 0.2219. The investigation carried out by El-Kateb and Abdul-

Hamid (1991) was for the much narrower energy range of 54 keV to 1.333 MeV. For the

case of THF:

𝜇

𝜌

THF = 𝑓H

𝜇

𝜌

H+ 𝑓C

𝜇

𝜌

C+ 𝑓O

𝜇

𝜌

O (4)

with 𝑓H + 𝑓C + 𝑓O = 1

The attenuation coefficients for THF and H2O are shown in Fig. 16, along with the

corresponding values for a mixture of THF (19 wt%) and H2O, dry air near sea level,

barium and aluminium for comparison. The graph clearly illustrates that the attenuation

coefficients for THF and H2O, the guest and host compounds of the clathrate structure

under investigation here, differ only marginally from one another, and that both only differ

marginally from a 19 wt% THF + H2O mixture. This suggests that the achievable contrast

between these components on a microfocus x-ray computed tomography image would be

insufficient to resolve them from one another, as would be necessary for investigating the

hypothesis of ice-shielding described above. An air bubble trapped in a sample cell, or a

sample container constructed of aluminium, would have much greater contrast to both

THF and to water than these two components would have between each other.

- 33 -

Figure 17: Attenuation coefficients for THF and H2O + THF (19 %wt; blue line) (calculated), as well

as H2O (red line), dry air, aluminium and barium (from Hubbell and Seltzer, 2015

May) for the energy ranges 1 keV to 20 MeV.

The attenuation coefficients for elemental barium (cf. Fig 17) show a marked difference to

that of H2O, THF and the specified mixture thereof, especially over the photon energy

range of roughly 10-2 to 10-1 MeV which is typically emitted in industrial x-ray tomography

machines, for which reason barium chloride, BaCl2, is often used as a contrast agent in

studies investigating THF clathrates with this technique (for example, Kerkar et al., 2009).

Using the attenuation coefficients for THF and liquid water, the attenuated spectrum I(λ)

was calculated from the emission spectrum I0(λ) emitted by the source. This was done

once each for THF and liquid water, for an equal distance of x.

Subsequently, the energy E of the electromagnetic radiation impingent upon an area 𝐴𝑝 of

the x-ray detector over the duration of an exposure time 𝜏, after having passed through a

sample of material, was calculated, again, once each for THF and liquid water, as follows:

𝐸 = 𝐴𝑝 ∙ 𝜏 ∙ 𝐼 𝜆 𝑑𝜆∞

0 (5)

- 34 -

Finally, the contrast between the attenuated x-ray emission spectra as they pass through

liquid water as opposed to THF was evaluated using Michelson’s Contrast as follows:

𝜅 = EA − EB

EA + EB (6)

E refers to the total energy that arrives at the x-ray detector as calculated in Eq. 5 while

the subscripts here refer to the attenuation caused either by the passage through a

sample of water or THF respectively. The constants outside of the intergral in Eq. 5 may

be neglected as they cancel out in the nominator and denominator. A very small value of κ

corresponds to two materials that result in very similar attenuation of x-ray beams passing

through them, and therefore, show up very little contrast in the resulting imagery.

3.4.4 Study II

For the second part of the study, a time series of CT scans was carried out over a 240

hour period in order to investigate the growth of THF hydrate regions. A THF + H2O

solution was prepared as in the first part of the study, however, BaCl2 was also added to

the THF solution, constituting 5 wt% of the THF-H2O solution. This was done in order to

enhance the achievable contrast on the reconstructed images. The x-ray tube settings

were 130 kV and 200 µA. A spatial resolution of 6.67 µm was attained, corresponding to a

voxel size of 296.7 µm³. A bed of glass beads (of 1.5 mm diameter) was also added to

mimic a sediment bed within which crystal growth of the hydrate could occur. Images were

recorded with an exposure time of 750 ms in 0.2° angular increments from 0° to 360°.

Total scan time constituted 1 hour and 27 minutes per scan.

The container was not sufficiently air tight, allowing all of the THF to evaporate and

escape the container at room temperature prior to cooling. The use of an O-ring in the

screw-on lid in subsequent scans provided a sufficiently air tight environment and

prevented the highly volatile THF from escaping the container.

For this part of the investigation, a temperature profile as shown in Fig. 18 was set for the

cooling stage and a series of 14 scans made over the entire 240 h period. From the

reconstructed 3D datasets, volumetric analysis was carried out in the attempt to quantify

the crystal growth rate of THF clathrates in the sample over the course of the time series

investigation.

- 35 -

Figure 18: Temperature setting (solid blue line) on the self-made Peltier cooling stage over the

course of the time series experiment. The dotted line is the freezing point of water,

273.15 K.

270

275

280

285

290

295

300

0 24 48 72 96 120 144 168 192 216 240

Tem

pe

ratu

re /

K

Time / h

- 36 -

4. RESULTS AND DISCUSSION

4.1 Study I: Contrast

4.1.1 Quantitative analysis

The integral of the emission spectrum (cf. Eq. 5) was approximated numerically with 0.125

keV increments. The resulting Michelson’s Contrast between pure liquid water and THF

was calculated as κ = 7.4271 x 10-2. For comparison, the corresponding value for the

contrast between aluminium and dry air is κ = 6.6638 x 10-1, an order of magnitude higher.

As such, the quantitative analysis strongly suggests that the nanofocus µCT method is not

a suitable method for investigating anomalous preservation through ice rinding, as the

contrast between a gas hydrate and a proposed ice shield would be insufficient to allow a

clear distinction to be made between them. To repeat: the value of κ calculated represents

a theoretical maximum: the contrast achievable on an actual sample containing hydrates

is expected to be even poorer, due to the heterogenous nature of any such sample.

4.1.2 Imagery

The imagery captured and reconstructed of THF hydrates, without use of a contrast agent,

confirmed what was shown through the quantitative analysis above: The differences in x-

ray attenuation caused by the various components (THF solution and THF hydrate) in the

sample cell are too slight to enable a clear-cut distinction between them. In the resulting

image (cf. Fig. 19), this insufficient difference in x-ray attenuation is translated as an

insufficient degree of contrast between the various components.

Certainly, the laboratory set-up used is not suitable for investigating the proposed

mechanisms of self-preservation. This is due first and foremost to the insufficient degree

of contrast attainable between the various components. THF hydrate produces an x-ray

attenuation very similar to liquid water and hexogonal ice, an das such, this would make a

visual identification of a layer of supercooled water or ice around a hydrate particle difficult

at best. Furthermore, the spatial resolution attained in this study (10 µm, or a voxel of 10³

µm³) is insufficient to resolve the presence of an ice- or supercooled water shield

surrounding hydrate clumps, assuming such a layer to be on the order of 1 µm.

- 37 -

Figure 19: Vertical 2D slice of THF clathrate and a THF + H2O solution inside a homemade PFTE

sample cell, showing the low degree of contrast between the individual components.

Temperature control was a problem during the initial trial runs, with the actual temperature

either so far above or below what the cooling stage was set to, that the result was either

the growth of a THF hydrate region that completely filled the ―pore spaces‖ between the

glass beads, or, no formation of hydrate taking place at all.

Another problem was posed by the presence of air bubbles trapped in the glass bead

matrix, as the contrast between air and all the other components were so great as to

render even worse the already insufficient contrast between those components (THF

hydrate and THF solution). It was found that first adding the solution to the sample cell

and then the glass beads—rather than the other way around—minimized the probability o

fair bubbles being present.

4.2 Study II: THF hydrate growth

Use of barium chloride (BaCl2) as a contrast agent significantly improved the contrast

between the THF solution and the glass beads, as well as between these components

and the THF hydrates (cf. Fig. 22). Nevertheless, an absolutely clear-cut separation

between THF hydrate and the THF + H2O solution was not possible, as is illustrated in

Fig. 18.

Owning to the inherent error caused by the overlapping gray value histograms of THF

clathrate and THF solution, a set of three volumetric analyses was carried out for each

reconstructed dataset, i.e. for each scan within the time series investigation. With the

resulting data, a range of volumes could be provided for each scan, within which the

actual volume of clathrate formed must be situated. The three classes of analyses carried

out were as follows:

- 38 -

Conservative: For this class, the upper threshold was set so that no voxels of what

is most likely THF solution were included in the gray value range classified as

THF clathrate (rendered yellow in the images); this analysis came at the cost of

also excluding a number of voxels of what could clearly be identified as being part

of one or more THF clathrate regions in the images. It is therefore an

underestimation of the true volume. (For an exemplary image and histogram, see

Fig. 21.)

Best estimate: For this class, a judgement based on visual inspection was made,

consisting of a trade-off between excluding voxels of THF clathrate on the one

hand and including voxels of THF solution on the other. As such, this class is

considered a best estimate, the most likely intermediate between the

underestimation of the ―conservative‖ and the overestimation of the ―complete fill‖

classes. However, although the delineation of all three classes of analyses is

based on a limited visual inspection, and is therefore subject to human error, the

delineation criterion itself is not as precise for this class as it is for the other two.

Complete fill: For this class, the upper threshold was set so that as many voxels

as possible of what could clearly be identified as THF clathrate regions were

included in the chosen gray value range; this analysis came at the cost of also

including a (potentially great) number of voxels of what could clearly be identified

as THF solution. It is therefore an overestimation of the true volume. (For an

exemplary image and histogram, see Fig. 20.)

Figure 20: A vertical 2D slice (left) showing THF clathrates (solid yellow), THF + H2O solution (dark

grey) and a matrix of glass beads (light grey); as well as the histogram (right) of the

image’s gray value distribution. The image (and its gray value histogram) shown

here is exemplary of one of three classes of results: ―complete fill‖, which constitutes

a potentially significant overestimation of the volume of THF clathrate in a sample

(note the grainy yellow rendering of the THF solution).

- 39 -

Figure 21: Exemplary images and histogram of gray value distribution for the class of analysis:

―conservative‖, an underestimation of the true volume of THF clathrate in the

sample. The horizontal blue line in the upper left image shows the location of the

horizontal slice shown on the right.

Horizontal slices taken from the same height in the sample over the course of the time

series investigation (figure 20) clearly shows the initial formation and growth of small (<

500 µm), round clumps of hydrate in the center of the sample cell. Over the course of 89

hours from scan number 6 (t = 88 hr, T = 275 K) onwards, as the temperature approaches

and sinks immediately below the freezing point of water, larger regions of hydrate form

and successively fill a relatively large and continuous vacant space between two rows of

glass beads at the wall of the sample cell; this vacant space was also present from the

beginning of the experiment (cf. Fig. 22, yellow elipse).

The images also show that some glass beads were moved about, presumably by the

growth of THF hydrate; an example of this may be seen towards the right of the previously

mentioned vacant space in Fig. 22 where, for all of the images except the second-to-last,

a small circle representing either the top or the bottom of a glass bead, which was moved

either downwards or upwards respectively, changing in size before being pushed out of

view of the horizontal slice from the second-to-last scan (t = 187 h) and again reappearing

in the last one (t = 240 h).

The movement of individual glass beads and the resultant space between them and the

region of THF hydrate rules out the possibility that these regions are in fact air bubbles.

- 40 -

- 41 -

- 42 -

- 43 -

Figure 22: Time series images of a horizontal section (at constant height) through the cylindrical

sample. Each slice (right) is accompanied by a copy of the cooling stage

temperature profile (left) originally shown in Fig. 18, indicating (red arrow) the time

(x-axis, in hours) and temperature (y-axis, in K) at which the scan was made. Glass

beads have the lightest shading of grey and a diameter of approximately 1.5 mm.

THF hydrate has the darkest shade of grey. Diameter of the cell: 11.4 mm. Yellow

elipse indicating a continuous space between beads progressively filled by THF

hydrate. Note the anomolous growth shown by the last two scans at ambient

temperature.

Three dimensional images of the maximal extent reached by the THF clathrate region are

shown in Fig. 23. Smaller, individual clumps of hydrate can be seen at a position of higher

elevation in the cell, above an extended, continuous body of hydrate which forms the bulk

of the clathrate region, filling up the pore spaces between glass beads and thus clearly

recognizable by the concave surfaces. These (visual) results correspond well with the

results of Kerkar et al. (2009).

- 44 -

Figure 23: 3D images of the THF hydrate region, according to the ―best estimate‖ threshold

criterion, at the end of the time series investigation (t = 240 h). The solution and

glass beads have not been rendered. The box in the background in each case

measures 11.7 x 11.7 x 8.2 mm (x, y and z dimensions) or 1.12 cm³.

The results of the volumetric analyses are listed in Table II. The error inherent in the

overlapping gray value distributions of the various components on the one hand, and the

subjectivity involved in making a visual assessment on the other, is especially apparent in

the ―best estimate‖ and ―complete fill‖ results, which do not show a continuous growth

process throughout the time series, even though individual 2D slices at various heights

throughout the sample do show growth throughout (in terms of surface area), and there is

no reason to expect growth to have been temporarily halted or gone into reverse. The

range of values between the ―conservative‖ underestimation and the overestimation of the

―complete fill‖ class are also indicative of the inherent error and uncertainty of the data.

Throughout, the ―best estimate‖ volume lies much closer to the ―conservative‖ estimate

than to the overestimation of the ―complete fill‖ estimate.

- 45 -

Table II: Results of the various volumetric analyses carried out on the reconstructed data from the

time series investigation to quantify the THF hydrate growth rate.

Time (hr) Cell temp

(K) Scan

Number

Volume (mm³)

Conservative Best

estimate Complete

Fill

0 296.27 18 275.72 0 0 0 0

36 275.72 1 1.4 2.7 22.6

42 275.72 2 2.2 5.4 26.4

58 275.72 3 3.5 12.2 57.7

67 275.72 4 3.3 14.9 66.9

83 275.72 5 4.2 8.5 64.4

88 275.23 6 5.1 11.6 61.4

105 274.74 7 5.2 18.8 111.6

111 273.76 8 5.5 13.9 47.4

134 273.76 9 6.9 12.6 48.6

154 273.76 10 8.1 13.2 57.4

177 272.78 11 9 15.8 22.9

187 293.33 12 15.4 19.1 65.5

240 293.33 13 31.1 35.3 107.4

Table III: Results of the linear regression analyses of the growth rate data, seperated according to

temperature regimes: "Zero" for temperatures near the freezing point of water and

"Anomalous" for temperatures at ambient.

Data class

Zero Anomalous

N R² Rate

[mm³/hr] N R²

Rate [mm³/hr]

Conservative 12 0.98 0.054

3 0.981 0.332

Best estimate 12 0.538 0.083

3 0.999 0.308

A graphical plot of the ―conservative‖ and ―best estimate‖ data is shown in Fig. 24, along

with lines of best fit. The results of the regression analyses are listed in Table III. It may be

clearly seen that the methodology behind the conservative class of volume analyses

delivered the highest degree of correlation. The increase in THF hydrate growth rate

between the period of near freezing temperatures (for water, 273.15 K) and those found at

ambient is dramatic: registering nearly four times higher for the best estimate data and

over six times higher for the conservative data.

- 46 -

Figure 24: Plot of the ―conservative‖ and ―best estimate‖ data obtained from volumetric analysis (cf.

Table II), showing the growth of THF clathrate over a 240 hour period. Also shown

are two sets of lines of best fit (linear regression) for the two data sets: "best fit zero"

is a regression for the data points for which temperature was close to or at the

freezing point of water; "best fit anomalous" is a separate regression only

considering the data points associated with the temperature increase to ambient.

The growth rate of a hydrate crystal is not a linear process (a logarithmic one would be

expected), but depends instead upon the concentration gradient between the bulk solution

and a boundary layer on an individual crystal, the crystal’s surface area, as well as

diffusion and reaction coefficients, which in turn depend upon other parameters such as

temperature (Sloan and Koh, 2007). However, the THF hydrate which grew over the

course of this time series investigation represents a very small fraction (approximately 35

mm³ out of 2 cm³, or 1.7 %) of the components available in the bulk solution, which is not

- 47 -

expected to amount to a significant modification of the initially prevalent concentration

gradient. For this reason, it is assumed that the hydrate crystal growth process may over

the initial phase investigated here, at least be approximated as a steady state process.

By far the most interesting and unexpected result from the time series investigation

reported upon here, is the fact that the THF clathrate region was not only still present

during the last two scans, but had furthermore grown and done so at a rate much higher

than throughout the rest of the investigation (cf. Table III and Fig. 24). This result was

entirely unexpected, because the last two scans had been carried out at an ambient of

293 K. THF hydrate is reported to have a melting point between 277.53 and 277.76 K,

depending on the stoichiometry (Gough and Davidson, 1971).

A hypothesis to explain the anomaly of at least the first scan, which was started

immediately after the cooling stage temperature increase (cf. Fig. 22)—namely that the

temperature increase was delayed within the sample cell, through a much retarded heat

transfer—seems unlikely considering the scan time of 1 hour and 27 minutes, and merely

shifts the burden of explanation to finding a mechanism that provided such a level of

thermal insulation, one which could moreover not be discerned over the entire preceding

investigation. The hypothesis becomes even less credible when considering the last scan,

which was started 53 hours after the temperature increase, and which once again

revealed elevated growth rates as compared to the rest of the investigation.

Further investigation is necessary in order to clarify this result. Avenues which may be

explored include the possibility that a reaction occurred between either THF or BaCl2 on

the one hand and the glass beads or the aluminium container on the other, or that

photolysis occurred, which led to gas formation. The former idea may be investigated by

using different materials; for the latter, a methodology must be developed to test for and

ideally quantify the presence of gas. If further initial experiments support the idea of

photolysis having taken place, a row of similar experiments but with variable x-ray tube

settings could be carried out, in order to test the hypothesis that a dependently variable

gas production takes place.

- 48 -

5. CONCLUSION

A very great relevance has been attributed to gas hydrates in academic literature over the

past few decades. Vast global reserves thereof have been proposed as a potential new

source of natural gas, one that, according to early estimates, surpassed the amount of

organic carbon stored in all other fossil fuel reserves, which would be of great interest in

meeting future energy needs. On the other hand, great caution has been advised in

moving from research to commercial exploitation of such gas hydrate reserves, owning to

the heightened radiative forcing that a large release of methane gas into the atmosphere

would entail. Such catastrophic releases from gas hydrates have in this respect also been

hypothesized to have played a major role in prominent negative carbon isotope

excursions (i.e. periods of marked warming) in the remote past.

Over the same time period, estimates of how large global reserves of gas hydrates are

have fallen by orders of magnitude. The much reduced youngest estimates refute many of

the extravagant propositions that have historically been made. It is of course not unknown

in the field of scientific endeavour that new objects of study are subject to a manner of

promotion work by the researchers involved, in other words, that the relevance and

importance of the object in question is trumpeted in claims and propositions that may in

the future, as more knowledge is accrued, turn out to have been exaggerated. This

appears to have also been the case following the inference of very large gas hydrate

stability zones in marine sediments as well as in permafrost soils.

This is by no means meant to negate the need for further research on gas hydrates, but

rather, to discount to inflated hopes and fears attached to the proposition of commercially

exploiting gas hydrate reserves to meet future energy needs.

The present investigation has shown that microfocus computed tomography methods are

not suitable for investigating the self-preservation effect, through ice rinding, of gas

hydrates, due to insufficient contrast between hydrates and solid or liquid H2O. Of course,

this tool cannot be completely ruled out for this type of investigation. A better distinction

could for example be made possible if a more suitable contrast agent was found, one that

dissolves exclusively with the molecule to form a clathrate, so that a much greater degree

of contrast could be achieved between the various components which need to be

distinguished in order to confirm or disconfirm the presence of an ice rind on the surface

of a gas hydrate clump.

The quantitative analysis utilized here is a useful tool in this regard, to provide a first

assessment prior to undertaking x-ray computer tomographic investigation. It is however

- 49 -

not known whether comparable methods exist which extend the methodology of El-Kateb

and Abdul-Hamid (1991) for calculating the photon attenuation coefficients beyond

relatively light compounds of carbon, hydrogen and oxygen, as would be necessary to

evaluate the likely efficacy of novel contrast agents. A better degree of contrast would also

benefit volumetric analyses or studies assessing crystal growth rates, although further

improvements should also be possible with the level of contrast prevalent in this

investigation, for example, with a random walker algorithm.

Of primary importance however, is to further investigate the anomalous result obtained in

the final two scans of the time series investigation, which registered above average

clathrate growth at ostensibly ambient temperatures.

- 50 -

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Statement of Affirmation

I hereby declare that this master’s thesis was in all parts exclusively prepared on my own,

without using other resources than those stated.The thoughts taken directly or indirectly

from external sources are properly marked as such. This thesis or parts of it were not

previously submitted to another academic institution and have also not yet been

published.

Dornbirn, am 21.01.2016 Christian Kopf