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Instrumental analysis of gas hydrates properties.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2010; 5: 310–323
Published online 7 July 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.293
Review Article
Instrumental analysis of gas hydrates properties
Y. Rojas and X. Lou*
Department of Chemical Engineering, Curtin University of Technology, Bentley WA 6102, Australia
Received 21 August 2008; Revised 13 February 2009; Accepted 15 February 2009
ABSTRACT: Gas hydrates attracted intense research interest when it was first recognised some 70 years ago that
they were responsible for the blockage of flow lines, valves and well heads, thereby causing great loss of production
and other severe safety hazards to the oil and gas industry. After many decades, these compounds are still the topic
of research activities in various multi-disciplinary fields, including chemical and petroleum engineering, earth and
geophysics, chemistry and environmental sciences. This is not only due to the great impact that these compounds
have on the oil and gas industry, but also to the potential applications they have in many evolving areas, including,
but not only, natural gas storage and transportation, carbon dioxide sequestration, and sea-water desalination. It is
generally accepted that gas hydrates represent the largest source of hydrocarbons on earth, something which has not
been appreciated until only recently.
Management, either prevention or application or both, of gas hydrates requires a complete knowledge and
understanding of the formation, decomposition and inhibition mechanisms of gas hydrates, which in turn demands
advanced experimental methods and instrumental techniques for gas hydrate characterisation. This paper reviews a
broad range of techniques that have been used for natural gas hydrate characterisation. It includes the basic physical
science principles of each method and the gas hydrate properties that each method is capable of detecting, including
some modern instrumental analyses that enable direct determination of gas hydrate phases and possible measurement
of molecular interactions within the fluid phases.  2009 Curtin University of Technology and John Wiley & Sons,
Ltd.
KEYWORDS: gas hydrates; gas hydrates properties; instrumental analysis; LDHIs
INTRODUCTION
Natural gas hydrates are crystalline solid compounds
each consisting of a host lattice formed by hydrogenbonded water molecules that enclose a large variety of
small guest molecules. They form at low temperatures
and elevated pressures and are found in a vast volume of
sediments on the ocean floor and polar regions.[1] Over
a long period following their discovery by Sir Humphry
Davy in 1811, interest in clathrate hydrates was purely
academic.[2] Extensive researches on gas hydrates were
promoted, some 70 years later, when Hammerschmidt
found that the compounds were responsible for the
blockage of flow lines, valves and well heads where
the operation conditions, i.e. low temperature and high
pressure, are ideal for clathrate hydrates to form.[3]
Figure 1 shows a gas-hydrate plug taken from an
offshore production line.
Gas hydrate plugging can lead to over pressuring and
sometimes an eventual shutting down of the operation
*Correspondence to: X. Lou, Department of Chemical Engineering,
Curtin University of Technology, Bentley WA 6102, Australia.
E-mail: X.Lou@curtin.edu.au
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
facilities, causing loss of production and serious safety
problems.[5] Removal of hydrate plugs from subsea production/transmission systems can be time consuming.
In some cases, the loss in drill time was as long as
70 days.[6] The economic loss is unquestionably significant. Various strategies have been investigated in order
to combat hydrate plugs and to ensure regular flow
in oil and gas operations. These include mechanical,
thermal, hydraulic and chemical methods.[2,7 – 16] The
chemical method involves injection of thermodynamic
inhibitors such as alcohols, glycols, aqueous electrolytes
or a combination of these and others. The injections
of these chemicals shift the equilibrium temperature
and thus prevent gas hydrate crystallisation/formation at
the pipeline working conditions. Although the method
has proven to be effective in preventing gas hydrate
formation and is currently the most commonly used
in the oil and gas industry, the economic drawbacks
are significant. Large volumes of the inhibitors are
required, generally between 20 and 60% by weight.
The cost associated with the operation and recovery of
the inhibitors in such volumes is very high. Worldwide
annual expense for the most commonly used thermodynamic inhibitor, methanol alone, was estimated at
Asia-Pacific Journal of Chemical Engineering
CHARACTERISATION OF GAS HYDRATES
20 years. These research activities have been extensively reviewed by Kelland in a recent report.[17]
On the other hand, many recent studies have shown
that gas hydrates could potentially be used in various areas. It was reported that gas hydrates represent
the largest source of hydrocarbons on earth[1,7] and
can be utilised as a possible source of energy.[16,18 – 20]
They can also be used as novel technologies in separation processes, gas storage and transportation,[21] carbon
dioxide sequestration,[22] sea-water desalination,[2,7] and
in cool storage or air-conditioning applications.[23,24]
These positive observations about gas hydrates have
motivated increasing research and development activities in the areas of chemical and petroleum engineering,
earth and geophysics, chemistry and environmental sciences. Figure 2 reveals the increasing number of publications in the past 10 years (Data collected from Engineering Village II data base). Over 350% increase is
shown in the total publications. The increase in characterisation of gas hydrate properties is even higher (data
not shown). This further demonstrates the significant
importance in gas hydrate studies and management.
Management of gas hydrates, for both prevention or
utilisation, requires insightful understanding of hydrate
Figure 1. A gas hydrate plug taken from an offshore
production line[4] . This figure is available in colour online at
www.apjChemEng.com.
US$220 million in 2003.[11] The high demand for more
cost-effective and environmental friendly inhibitors has
led to enormous research activities in the development
of various low-dosage gas hydrate inhibitors including kinetic inhibitors and anti-agglomerants in the past
800
747
700
654
664
Number of Publications
600
498
500
431
400
349
286
300
250
228
212
200
100
0
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
Year
Figure 2. Number of publications on gas hydrates between 1998 and 2007
(Engineering Village II data base, Elsevier Inc.). This figure is available in colour
online at www.apjChemEng.com.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
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Y. ROJAS AND X. LOU
properties, the hydrate formation and dissociation processes, and the factors that might affect these properties and processes, which in turn requires meticulously
designed experimental methods and state-of-the-art
equipment to measure and characterise hydrates properties. Traditional studies on both theoretical prediction
(molecular thermodynamic simulation) and experimental characterisation of properties, including phase equilibria, structures and occupancy, are generally related
to the measurement of parameters such as pressure,
temperature and fluid-phase compositions, and involve
mainly macroscopic and mesoscopic instruments such
as high-pressure visual cells, rheometers, calorimeters, flow loops, flow wheels, and X-ray computerised tomography.[8,9,25] In the most recent one to two
decades, the development and advancement of more
powerful and selective instruments have allowed one
to obtain a more accurate estimation of hydrate properties as well as a more precise information about the
characteristics of gas hydrates.[1] Many of the recent
research activities have been shifted to thermal analysis, crystallographic analysis, topographic analysis and
more significantly, spectroscopic analysis at the molecular or atomic level.
A review on hydrate characterisation methods has
been provided by Malone in early 1990s,[26] which
focused mainly on geophysical analysis. Some techniques and selected case studies that are associated with
the application of these methods and the phase equilibria
and thermal property measurement have been described
by Sloan and his co-authors[1,9,25] and have become
useful references for many researchers. Recent works
by Tulk and Susilo and their co-workers respectively
have indicated the importance and dissimilarity of a
few spectroscopic methods that are increasingly used
in hydrates studies in recent years.[27,28] There has not
been a handy reference systematically covering a large
spectrum of instrumental methods that are useful in the
characterisation of gas hydrates properties.
This paper provides an overview on all key instrumental analysis methods that have been employed in
the gas hydrateR&D activities in the past 15 years with
the focus on the gas hydrates property measurements in
various phases and through complementary experimental methods which have become increasingly attractive
for the ultimate understanding of their formation, dissociation and inhibition. These include thermal analysis,
crystallographic analysis, topographic analysis, size and
size distribution analysis, spectroscopic analysis, interfacial tension and intermolecular particle force analysis,
and methods involving gas hydrates inhibition. Complementing methods and confirmative methods might have
been cross referenced when necessary. The paper is
intended for researchers from various disciplinary fields
who are knowledgeable of gas hydrates, but may or may
not be fully aware of instrumental analysis. We will
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
first give a brief introduction on the gas hydrates structures and promoters. This will be followed by a short
summary of reactors, cells and apparatus which are
commonly used for gas hydrates formation and analysis. Instrumental analysis of gas hydrates properties will
then be given in details which include a brief description of the physical scientific principles of the methods
and hydrates properties that the methods are capable
of examining. Comments on current hurdles and future
challenges are given at the conclusion of the paper.
GAS HYDRATES STRUCTURES AND
PROMOTERS
A great number of gas molecules are known to form
hydrates at high pressure and low temperature conditions. The three most commonly appearing structures
in natural gas hydrates, namely cubic I, cubic II and
hexagonal H structures, are displayed in Fig. 3. The
formation of a particular structure is largely dependent
on the size of the trapped molecules.[29,30] In this paper,
we use sI, sII and sH representing the three structures
respectively.
Hydrates systems based on gas molecules
Most natural gas molecules such as methane, ethane,
hydrogen sulfide or carbon dioxide are small (0.4–
0.55 nm) and form structure sI.[11] Larger molecules
(0.6–0.7 nm) such as propane, iso-butane form sII
hydrates.[11] Even larger molecules (0.8–0.9 nm) such
as iso-pentane, 2,2-dimetylbutane, methylcyclohexane
and tert-butyl methyl ether form sH hydrates in the
presence of small molecules such as methane.[11]
Interestingly, molecules smaller than 0.4 nm including
argon, krypton, xenon, oxygen, hydrogen and nitrogen also form sII hydrates.[8,11,28,31] Binary, ternary
and multi-component gas systems have also been found
in gas hydrates, exhibiting transitions between different structures.[11] Even though different gases can
form hydrates, the focus of this paper is on natural
gas hydrates, including the hydrocarbons and some
other organic molecules involved in the oil and gas
industries.
Hydrate systems based on liquid molecules
Substances that are in the liquid form at room temperature and form hydrates at low temperature and
atmospheric pressure, are of particular interests to many
researchers as they form similar types of hydrate structures, as some gas molecules do, and can be used to
Asia-Pac. J. Chem. Eng. 2010; 5: 310–323
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CHARACTERISATION OF GAS HYDRATES
Figure 3. Three common hydrate unit crystal structures.[11] 512 64 indicates a water
cage composed of 12 pentagonal and four hexagonal faces. The numbers over arrows
indicate the number of cages of a particular type in the structure. For example, the
structure I unit crystal is composed of two 512 cages, six 512 62 cages. This figure is
available in colour online at www.apjChemEng.com.
study the latter without the requirement of high pressures. For example, ethylene oxide forms the sI structure
type and tetrahydrofuran (THF) forms the sII structure
type of hydrates[14] at low temperature and atmospheric
pressure. THF hydrate crystals form in the presence
of water or sea-water at 277.4 K and at atmospheric
pressure at a molar ratio of 1 : 17 (THF to water).
They form sII type structures that are usually found
in natural gas hydrates and have been widely used
for screening natural gas hydrate inhibitors.[32] Other
less commonly investigated liquid hydrates promoters include tetrahydropyran,[31] chloride fluorocarbon
compounds,[33 – 36] hydrotrope molecules[37,38] and some
alcohols.[39 – 42]
REACTORS, CELLS AND APPARATUS FOR
HYDRATES FORMATION
A large number of experimental apparatuses have
been developed to form and to measure the equilibrium properties (phase equilibria) of gas hydrates
in bulk fluid phases. These include multiphase flow
loops and miniloops, flow wheels, high-pressure stirred
cells, autoclave and mini-autoclaves with visual hydrate
observation, different types of reactors and highpressure circulating vessels, sea-floor process simulators, static mixers, cylindrical glass columns, spray
chambers and bubble towers. More details of these
equipments can be found in Sloan and Koh.[25] Lately,
cool storage apparatuses have been produced in order
to preserve gas hydrates samples and to characterise
gas hydrate formation in the direct-contact cool storage
process.[34,43]
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
INSTRUMENTAL ANALYSIS OF GAS
HYDRATES
Thermal analysis of gas hydrates
Thermal analysis profiles the physical property changes
of a substance as a function of temperature while
the substance is subjected to a controlled temperature
programme.[44] Differential scanning calorimetry (DSC)
is one of the most commonly used thermal analysis
methods, which has been utilised to obtain detailed
equilibrium property data of gas hydrates during their
solid–liquid phase transitions at a high pressure.[45,46]
DSC determines the difference in the amount of heat
required to increase the temperature of a sample and a
reference when both materials are subjected to identical
temperature regimes in an environment cooled or heated
at a controlled rate. It can be used to investigate the
hydrate dissociation in various aqueous media including highly concentrated salt solutions, water/oil emulsions, and actual drilling fluids, at a high pressure ranging between 5 and 12 MPa.[47 – 50] The same technique
has also been used for probing the effect of inhibitors
on natural gas hydrate formation and dissolution at
low dosage levels.[14] Time and/or temperature transformation profiles have been constructed for isothermal
DSC data, yielding valuable information about complex
hydrate nucleation and growth mechanisms. Heats of
fusion and crystallisation involving ice and hydrates
respectively have been identified by DSC studies.[51]
DSC was also used to quantify polymeric inhibitor and
water interactions and the dependence of the interaction on the chemical structure of the polymer.[52,53] The
kinetics, thermodynamics, nucleation period and mode
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Y. ROJAS AND X. LOU
of action of a range of inhibitors on THF and trichlorofluoromethane hydrates have been determined using
DSC.[54 – 57]
MicroDSC and thermal modulated differential scanning calorimetric (TMDSC) have also been used for
hydrates analyses at a greater range of working temperatures and pressures, and for the detection of processes that occur simultaneously, such as melting, lattice
destruction and decomposition.[58,59]
Crystallographic analysis of gas hydrates
Crystallographic methods are often used for the determination of the atomic and/or magnetic structure of materials. The analysis is dependent on the diffraction patterns
emerging from a sample that is targeted by a beam
of X-rays [X-ray diffraction (XRD)], neutrons (neutron
diffraction), or electrons (electron diffraction).[60] The
three types of radiation/particles interact with a specimen in different ways. X-rays interact with the spatial
distribution of the valence electrons which are affected
by the total charge distribution of both the atomic nuclei
and the surrounding electrons.[61] Neutrons are scattered
through atomic nuclei by the strong nuclear forces. It
can also be contributed by the magnetic field if the
magnetic moment of neutrons is non-zero. Electrons
are charged particles. Their interactions with matter are
influenced by both the positively charged atomic nuclei
and the surrounding negatively charged electrons.
X-ray diffraction (XRD)
The application of X-ray diffraction (XRD) is focused
on revealing and identifying the structures of gas
hydrates, including the positions of the oxygen atoms of
the water crystal lattice.[8,29,30,62 – 65] Thermal expansion
in various types of gas hydrate structures has also been
examined using XRD.[66,67] Studies on gas hydrate formation, decomposition and inhibition at low temperature and atmospheric pressure[68] or at low-temperature,
high-pressure conditions have been conducted by several research teams using XRD.[69 – 75] Studies have
shown that single crystal XRD is a more powerful
tool for the analysis of the structure and composition
of different hydrates systems including absolute cage
occupancies and lattice parameters.[31,76]
Other diffraction technologies that have been utilised
in gas hydrate research in the past several years
include powder X-ray diffraction (PXRD), X-ray computerised tomography (CT), neutron diffraction and
neutron powder diffraction (NPD), as well as inelastic
neutron scattering (INS) and small-angle neutron scattering (SANS).
Powder x-ray diffraction (PXRD)
PXRD is used to characterise the crystallographic structure, crystallite size, and the preferred orientation of
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
polycrystalline or powdered solid samples. It may also
be used to characterise heterogeneous solid mixtures
to determine the relative abundance of crystalline compounds and, when coupled with lattice refinement techniques, can provide structural information on unknown
materials.[44] Structural and other property analysis of
different hydrate samples from mixed gases and different hydrate promoters have been reported using this
technique.[28,77 – 82] A complete conversion of melting
ice grains to hydrate at pressurised conditions has been
reported using PXRD analysis.[83] The same analysis
has also been employed to study solid–liquid phasetransition properties under high pressure.[46]
X-ray computerised tomography (CT)
This method is used for projecting and photographing an image of crystals or the defect of crystals on
a real space unit. It has been proved useful for determining the location of methane hydrates in sediments
and the relationship of the gas hydrate to the host sediments, without significantly disturbing the samples at
the grain scale. The method has also been used to
determine local density changes, 3-D morphology, porespace pathways, porosity, and permeability values during the hydrate formation and dissociation on porous
media samples containing trapped methane hydrates, to
quantify pure methane hydrate dissociation, and to generate model parameters that predict the behaviour of
the methane hydrate bearing sediments at meso- and
macro-scales.[84 – 86] Formation of natural gas hydrates
has been observed visually at the drill site and in core
samples preserved in pressurised storage vessels using
the X-ray CT scanner technique.[87 – 89]
Neutron diffraction
Neutron diffraction has been used to study the dynamics
of the molecules involved in gas hydrates, and to obtain
the characteristics of vibrational, rotational and translational motion of molecules in gas hydrates.[8] This
sophisticated and expensive technique has made it possible to determine the proton placement in the hydrate
lattice and caging occupancy,[1,90] and the guest and
host molecule positions.[91,92] Crystal formation, growth
and decomposition processes have been examined for
methane, carbon dioxide, and other gas hydrates using
this technique[64,93 – 95] and an in situ neutron diffraction
with isotopic substitutions.[96] Time-dependent kinetic
studies of hydrate formation and dissociation processes
using neutron diffraction have been reported by several
researchers.[75,97 – 101] A review on the applications of
neutron diffraction to the understanding of transformation processes between ices and gas hydrates is
available.[102]
Neutron powder diffraction
Both X-ray and neutron powder diffractions have
proved to be powerful for thermodynamic and structural analysis of natural gas hydrates.[69,83,103 – 107] The
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structure, composition, and the growing kinetics of natural gas hydrates have been investigated using this
method.[108,109] The effects of surfactants on the rate
of formation of carbon dioxide and methane hydrates
from polycrystalline mixtures of deuterated ice and
methanol have been studied using in situ neutron powder diffraction.[110] The same study has shown evidence
that methanol, in certain concentrations, tremendously
accelerates the formation of hydrates from deuterated ice.
Inelastic neutron scattering (INS)
The scattering caused by colliding neutrons with a substance which does not change the kinetic and internal energies of the neutron, even after the collision,
is called elastic scattering, while that changes those
energies of the incident neutrons by letting the substance absorb a part of those energies or release neutron energy is called inelastic neutron scattering.[111]
INS is commonly used in condensed matter research
to study atomic and molecular motion as well as
magnetic and crystal field excitations. The vibrational
dynamics of natural methane hydrates and fully deuterated methane hydrates has been investigated using INS
methods.[112,113] Neutron incoherent inelastic scattering
(IINS) experiments have been performed to demonstrate
unambiguously the coupling of guest and water framework vibrations.[114] Results indicated that the coupling
is the consequence of symmetry forbidden crossings of
the localised guest motions with the acoustic branches
of the lattice vibrations. This phonon interaction leads
to an energy exchange mechanism that explains the
anomalous thermal behaviour. They also pointed out
the importance of the repulsive part of the host–guest
interaction, which is responsible for the stability of gas
hydrate structures.
Small-angle neutron scattering (SANS)
SANS is particularly useful because of the dramatic
increase in forward-scattering that occurs at phase transitions, known as critical opalescence, and because
many materials, substances, and biological systems possess interesting and complex features in their structure, which match the useful length scale ranges
that these techniques probe.[115] It has been used
to characterise molecular conformations and adsorptions of several polymeric gas hydrate inhibitors
including poly(N -vinyl-2-pyrrolidone) (PVP), poly(N vinyl-2-caprolactam) (PVCap) and poly(N -vinylacetamide/N -vinyl-2-caprolactam) (pVIMA/VCap) in a
THF/water mixture.[116,117] Results showed that in the
presence of a hydrate-crystal/liquid slurry, the inhibitor
polymers significantly change their conformation which
can be a signal of an adsorbed polymer layer on the
hydrate crystal surface. It was also suggested that an
association between the hydrate-forming components
and the polymer might act to alter the nucleation
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
CHARACTERISATION OF GAS HYDRATES
behaviour. Using a small-angle neutron diffractometer, the structure of water molecules around dissolved
methane molecules during methane hydrate formation
and decomposition was investigated.[106,118]
Topographic analysis of gas hydrates
Topographic analysis of gas hydrates can be carried out
using optical microscopy (OM) and scanning electron
microscopy (SEM), or atomic force microscopy (AFM).
These microscopic technologies can produce a highresolution scan of a sample surface, allowing magnified
images and measuring and manipulating small samples
at atomic scale. They have proved to be highly useful
for studies of crystal-growth behaviour and morphology, as well as the distribution of gas hydrates phases
in both synthetic and natural gas hydrates.
Optical microscopy
Optical studies provide an opportunity to examine several macroscopic properties of gas hydrate crystals.
These include crystal-growth behaviour, equilibrium
crystal morphologies of different hydrate structures, and
the relative growth rates of specific crystallographic
planes or directions.[52,119] Equilibrium data of several sH type promoters have been measured using this
method.[119] Clathrate hydrate film growth has been
investigated through the measuring of the film thickness and the propagation rate across the hydrocarbon/water interface for gas hydrate systems, using video
microscopy.[120 – 121] The variation in propagation rate of
a methane hydrate film when changing the sub-cooling
conditions has been investigated, finding a linear relationship between the growth rate and the bulk temperature of the system for each equilibrium temperature.[122]
The measurement of the film thickness with different superficial velocity of the moving water phase was
possible using OM.[123] Observations of the formation
and growth of clathrate hydrate crystals in the presence of kinetic inhibitors using an OM have also been
reported.[124]
Scanning electron microscopy (SEM)
The SEM takes images of the surface of samples by
focusing a high-energy beam of electrons onto the sample. The electrons interact with the atoms that make
up the sample producing signals that give information
about the sample.[111] SEM offers yet another powerful technique for providing additional information on
hydrate growth processes due to its high resolution,
large depth of focus, and versatility in detection capabilities. It has been used to investigate grain texture
and pore structure development associated with gas
hydrate formation from a hydrate-forming gas or liquid
in the presence of melting ice under static and constantvolume conditions. Various compositions of pure sI and
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sII gas hydrates either synthesised in the laboratory
or retrieved from marine and permafrost settings have
been analysed and compared.[125] The application of the
cryogenic scanning electron microscopy (cryo-SEM)
imaging technique has allowed one to examine the
phase distribution, grain contacts, and microstructural
development within hydrate-bearing sediment assemblages, and to identify grain structures in CH4 , CH4 -N2 ,
and CO2 hydrates prepared from ice. The results have
revealed the remarkable development of highly mesoporous gas hydrates formed from ice at temperatures
close to the ice point.[64,126,127] SEM has been useful in the investigations of pure and partially dissociated methane hydrates,[128] compacted and deformed
methane hydrates,[129] and porous CO2 hydrates[109]
which are of both laboratory and natural origins. The
technique has also been used to compare natural samples with samples made using Stern’s method.[83,125]
Size and size distribution analysis of gas
hydrates
Particle size and size distribution analyser is a useful
tool to monitor the nucleation/formation and growth of
gas hydrates,[130 – 132] and to determine the size and size
distribution of methane hydrates during their formation
in a pressurised reactor.[133] It has also been used to
monitor gas hydrate nucleation and growth processes in
the presence of positively and negatively charged latex
particles and kinetic inhibitors.[134,135]
Spectroscopic analysis of gas hydrates
Spectroscopic methods covering a full range of the
electromagnetic spectrum have been widely used for
gas hydrate investigations. These include techniques
based on the measurement of absorption of electromagnetic radiation in the radio-frequency where nuclei of
atoms possessing spin or protons [nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI)]
and electrons [electron paramagnetic resonance (EPR)]
developing energy states that can be transformed into
an image by computer techniques.[136] Techniques that
involve the spectroscopy of photons in the ultra violet -visible region (UV/VIS spectroscopy); infrared (IR)
region (IR spectroscopy); and that rely on inelastic scattering of monochromatic light, usually from a laser in
the visible and near IR, or near UV range (Raman spectroscopy) are also included in the category.[137] In these
ranges of electromagnetic spectrum, molecules undergo
electronic transitions that offer the possibility to measure different types of inter-atomic bond vibrations,
rotation at different frequencies.[138] In many cases,
these methods provide information about the structure
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
type, cage occupancies, and guest composition of solid
hydrate phase that can be used to improve existing prediction models.[9,139,140]
Nuclear magnetic resonance (NMR)
Since the first reports on the spectra of methane, ethane,
propane, and isobutene hydrates by Davidson and Garg
and their co-workers,[141,142] NMR has been frequently
used to determine the structure identification, chemical composition, cage occupancy, and water mobility
in gas hydrates, and more recently the magnitude of
spin–spin and spin–lattice interactions of gas hydrates,
the character of relaxation processes, and their dependence on temperature and pressure. It has also been used
to examine various defects, thermal fluctuations, translation, and rotation of molecules in gas hydrates, and
to identify the enclathrated guest molecules and their
concentrations.[8]
Using 13 C and 129 Xe NMR spectroscopy, the occupation of cages by methane (with propane) in the small
and large cages of sI and sII was first solved.[143] 13 C
NMR spectroscopy was used to obtain the guest composition of the hydrate phase on a water-free basis at
various vapour compositions near the lower transition
point.[140] NMR has also been used to study the storage
and content of gas molecules, showing the preferential
occupancy of various molecules.[28,76,144]
Dynamic studies of methane hydrate formation and
decomposition, and identifying changes in the hydrate
structure and number of guest-occupied cages during
hydrate formation are possible using NMR.[14,28,145 – 148]
The introduction of 1 H cross-polarisation techniques
has allowed measuring relaxation times compatible
with hydrate kinetics measurements.[149,150] The kinetics
of transition between sI, sII, and sH hydrates was
studied using 129 Xe NMR.[151] The first kinetic NMR
measurements of binary natural gas hydrate formation
from ice have been reported.[152] With time-resolved
spectroscopy, it is also possible to monitor the real-time
kinetic transition from methane in the aqueous phase to
methane in the hydrate phase.[1]
Water mobility, molecular reorientation and diffusion in various hydrate structures, and dynamic properties of both guest and water molecules have been
investigated using 129 Xe, 13 C, or 1 H NMR by several researchers.[105,150] Results demonstrated that the
water molecule reorientation process was dependent
on the nature of the guest molecule which could
involved in establishing a transient hydrogen bond with
the water molecules. At least two dynamically distinguishable types of water molecules have been identified, both undergoing a four-site tetrahedral jump
motion but with different jump rates. A solid-state
2
H NMR spectroscopic study on THF hydrate by
Bach-Verges et al ., indicated that THF molecules reorient more rapidly compared with water molecules.[153]
It was also proposed that there is a distribution in
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terms of anisotropic characteristics associated with the
re-orientational motions of different guest molecules,
reflecting that the dynamics of the water molecules is
substantially slower than the guest dynamics, such that
different guest molecules experience a distribution of
local water cage structures.[14]
Based on the same principle as NMR, MRI has been
proved useful in studies of gas hydrate formation and
growth in porous sandstone, sediment, and in single or
multiple liquid droplets;[154 – 156] the interaction of gas
with ice surfaces;[157,158] and the methane hydrate production rate and distribution in a mixed gas system.[159]
Raman spectroscopy
Raman spectroscopy has been proved extremely useful
and convenient in characterising gas hydrates. It can
directly and non-destructively measure the vibrational
energies of the interstitial gas molecules. Ripmeester
and co-workers’ report on structural phase equilibria
data of sI and sII hydrates involved the employment
of both NMR and Raman spectroscopy.[150] Dissociation point data was collected from Raman study
for structure prediction.[160] The existence of structural phase-transition in several mixed gas hydrate systems was confirmed using Raman spectroscopy.[140,161]
Structure and structure stability information were readily obtained using Raman.[28,162,163] The formation
and decomposition processes, the molecular dynamics, the composition, and cage occupancies of several hydrate systems have also been investigated using
Raman analysis. These include THF sII hydrate, sI
methane hydrate, sII methane + deuterated THF (THFd8 ) hydrate, carbon dioxide hydrate, and various fluorocarbon hydrates.[164 – 169] Using Raman, the effect of
the inhibitors on gas hydrate formation kinetics was
also studied.[9,57,170] The results indicated that upon the
addition of a polymer inhibitor, the ratio of occupancy
of the large and small cavities of methane hydrates
was reduced, the rate of hydrate cavity formation was
changed, but the hydrate host crystal structure was not
affected.[14]
Recent research using Raman has focused on the
vibrational spectra of the guest molecules.[27,171] Vibrational modes of a molecule shift in frequency as a
function of a molecule’s local environment and can
be measured by Raman spectroscopy, allowing identification of a molecule in the gas phase or/and that
in a discrete hydrate cage.[97,172] Since a solid–water
phase does not necessarily indicate a hydrate structure
(it can be ice), guest molecules are typically used to
identify the presence of solid hydrate phase in this type
of study.[172] On the other hand, profile and frequency
of the Raman signals including the O–H stretching and
the H–O–H bending of water molecules are hydrates
structure dependent and therefore can be used to identify the type of structure of the gas hydrate systems.[171]
The hydration number and relative cage occupation of
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
CHARACTERISATION OF GAS HYDRATES
pure guest molecules in three natural gas hydrate structures have been measured and calculated based on their
Raman spectra.[140,172] In a multi-molecule system, the
linear relationship of Raman intensities and the concentration no longer exists and, additionally, the scattering cross-sections do not change equivalently;[93,140]
therefore, Raman analysis becomes only qualitative.
Assuming that the Raman scattering at the cross-section
of the guest species is the same as that within the
hydrate host structure,[173] Raman spectra can also be
used to identify both interfacial and dendrite–hydrate
cage occupancies in various multiple phase hydrate
systems.[1,71,174]
More recently, Raman study has been extended to the
exploration of in situ ocean clathrate hydrates[175,176]
and to the investigation of the methane hydrate stability
in relation to the deep sea conditions.[177] Raman spectroscopy can also identify the preferential enclathration
of various guest molecules into the hydrate structure.[82]
Interfacial tension and intermolecular particle
forces
Interfacial phenomena are the ones that occur at the
limit between two immiscible phases, the so-called
surface or interface, and take into account attractive
forces acting on molecules at the surface/interface that
keep them tight together. The greater the force of
attraction between the molecules, the greater is the
surface tension. Since the nucleation of the hydrate is
in part an interfacial phenomenon, interfacial properties
such as interfacial tension have been used to explain the
memory effects and the membrane force working in the
hydrate films.[35] Recently, interfacial and/or superficial
phenomena analysis has been employed to study gas
hydrates formation systems in the presence of different
surfactants. The effect of micelles and critical micellar
concentration (CMC) under hydrate-forming conditions
has also been investigated. The studies have indicated
that the presence of surfactants at certain concentrations
can promote hydrate formation and improve hydrate
storage capacity.[178 – 181]
Studies by the authors and colleagues on two wellknown kinetic inhibitors, Gaffix VC-713, a terpolymer of N -vinylcaprolactam/N -vinylpyrrolidone/N ,N dimethylaminoethyl methacrylate, and Luvicap , a
homopolymer of N -vinylcaprolactam have shown an
apparent link between the critical concentration and
inhibition performance of the inhibitors in a THF-NaCl
system (manuscript in preparation). Interfacial adsorption kinetics of these polymers is under investigation
using surface tension measurements in order to further
explore the inhibition mechanisms of these inhibitors
and to develop novel low-dosage gas hydrates inhibitors
(manuscript in preparation).
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Y. ROJAS AND X. LOU
Asia-Pacific Journal of Chemical Engineering
Other studies have focused on the interactions
between hydrate particles in the presence of antiagglomerants. It was shown that effective antiagglomerants decrease the interfacial tension and
increase contact angle through the water phase, thereby
reducing the particle size of the hydrates.[182] A separate study on adhesion forces between THF hydrate
particles in n-decane has shown that the adhesion
force of hydrates is directly proportional to the contact time, contact force, and interfacial energy of the
surrounding medium with water and it increases with
temperature.[183]
Other gas hydrate characterisation methods
IR spectroscopy, dielectric spectroscopy (DS), EPR
spectroscopy, acoustic resonance spectroscopy (ARS),
mass spectrometry (MS), laser imaging (LI) and quartz
crystal microbalance (QCM) have also been used
in some of the gas hydrate studies as confirmatory
tools. The research associated with the application of
these instruments is summarised in Table 1. In addition, a short summary of each of the aforementioned
instrumental analysis methods and its relevance to
the gas hydrate properties is given in Table 1 and
Fig. 4.
Mechanical strength including elastic constants and
acoustic velocities of gas hydrates has been investigated by many researchers using various instrumental
methods.[8] Molecular thermodynamic simulation plays
an equally important and useful role as experimental
measurement in the prediction of hydrate properties,
formation/dissociation processes and kinetics.[186] However, overview of these methods is beyond the scope of
this study.
Methods involving gas hydrate inhibition
As mentioned before, prevention of gas hydrates formation has been a central focus for the oil and gas industry
due to its significant impact on gas production and
transportation. There have been numerous R&D activities in developing and studying various gas hydrate
inhibitors in the past decades.[17] While most of aforementioned instrumental methods are useful and have
been utilised in such investigations, screening of various gas hydrates inhibitors often require simple laboratory equipments and an easy-forming gas hydrates
system, such as THF hydrate, which allow effective
measurement and representative data collections. The
most commonly used apparatus for studying gas hydrate
inhibition and multiple screening tests is the ball-stop
Methods
(Equipments)
Thermal
Topographic
(DSC, MicroDSC, MDSC)
Properties
Thermodynamic and equilibrium properties
Formation
(OM, SEM)
Decomposition
Structure identification, lattice parameters,
and thermal expansion
Crystallographic
(XRD, PXRD, CT, ND,
NPD, INS, SANS)
Phase transition and composition
Molecular dynamics and molecular
interactions
Growth rate, hydrate film thickness
and morphology
Size and size distribution of hydrate
particles
Size and size
distribution
Spectroscopic
(NMR/MRI, Raman)
Intermolecular
forces
(Interfacial tension)
Inhibition
Processes
Figure 4. A diagram showing gas hydrate properties and associated characterisation methods and
instruments. This figure is available in colour online at www.apjChemEng.com.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 310–323
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
CHARACTERISATION OF GAS HYDRATES
Table 1. Instrumental analysis used for gas hydrate characterisation.
Instrument
Thermal
DSC, MicroDSC,
TMDSC
Crystallographic
XRD
PXRD
CT
Neutron Diffraction
NPD
INS
SANS
Topographic
OM
SEM
Size and size
distribution
Spectroscopic
NMR/MRI
Raman
Interfacial tension and
intermolecular
particle forces
Minor methods
IR
DS
ARS
EPR
MS
LI
QCM
Properties of gas hydrates
Thermodynamic and equilibrium properties (formation and dissociation enthalpies, thermal
conductivity, and heat capacities); solid–liquid phase transitions and compositions under high
pressure; slow dynamics; nucleation and melting, lattice destruction and decomposition; kinetics,
thermodynamics, polymer–water interactions, mode of action of gas hydrate inhibitors;
emulsion stability.
Structure identification; lattice parameters; guest occupancy and composition during formation;
decomposition; thermal expansion; growth rate.
Size and structure of crystallite; preferential orientation in polycrystalline or powdered solid
samples; phase identification; decomposition temperatures; changes in lattice parameters vs
temperature; thermal expansion; phase transitions properties under high pressure.
Formation and dissociation of core samples; location and identification of gas hydrates in
sediments; 3-D morphology; pore-space pathways; thermal conductivity, diffusivity and
permeability.
Molecular dynamics; vibrational characteristics; rotational and translational motion; guest/host
placement; caging occupancy; hydration structure around guest molecules; kinetics of formation,
dissociation and transformation processes.
Thermodynamic/structural studies; composition and kinetic behaviour; structural changes and
transitions during hydrate formation.
Guest/host molecular interactions in the hydrate lattice; thermal conductivity of crystalline
clathrates; vibrational dynamics.
Structure of water molecules around dissolved methane molecules during methane hydrate
formation.
Crystal-growth behaviour; equilibrium morphologies; relative growth rates of specific
crystallographic planes; equilibrium data; hydrate film thickness and growth rate in a water
droplet.
Growth processes and morphology; phase distribution, crystal contacts and structures;
microstructural development in hydrate-bearing sediment assemblages.
Hydrate nucleation/formation and growth, hydrates particle formation in water/gas hydrate
interface, size and size distribution of hydrate particles, induction time.
Structure identification; chemical composition; cage occupancy and water mobility; the magnitude
of spin–spin and spin–lattice interactions; relaxation processes and their dependence on
temperature and pressure; defects, thermal fluctuations, translation and rotation of molecules in
hydrate; enclathrated guest molecules and their concentrations.
Vibrational energies of the interstitial gas molecules, microstructural features of natural samples,
structure and stability, formation and decomposition processes, molecular dynamics,
composition and cage occupancies.
Forces between the molecules and interactions between hydrate particles, memory effects and the
membrane force working in the hydrate films, effect of the presence of different surfactants and
polymers, effect of micelles and CMC under hydrate-forming conditions.
Molecular interactions related to the vibrational, rotational, and translational motion of the
molecules.[8]
Internal dynamics related to electrical properties of gas/water molecules.[8]
Measurement of hydrate equilibration temperatures; hysteresis of growth, decomposition and
kinetics.[51]
Hydrates formation in water droplets; study of radicals produced in ethane hydrate through
irradiation with γ -rays.[8,184]
Molecular clustering structure; mass spectra of the clusters of water–methanol and water–propanol
solutions.[46]
Rapid formation and dissociation processes in slow motion.[51]
Screening low-dosage hydrate inhibitors (LDHIs); structural changes of macromolecules upon
adsorption to the surface; dissociation temperatures.[185]
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
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Y. ROJAS AND X. LOU
rig or rocker rig.[17,187,188] Several other types of apparatuses have been designed and used for gas hydrates
inhibition studies. These can be found in Refs [52, 106,
189, 190].
As for the instrumental analysis of gas hydrates in the
presence of inhibitors, rheological studies, apart from
the methods described in the previous session, are frequently used.[101,131,191,192,193] Most of the time these
studies have been conducted using anti-agglomerants in
hydrate slurry observing the phase change from water
to hydrates, which results in the formation of the suspension. Variables such as concentration, particle size
distribution, particle density, and from a flow assurance
point of view, apparent viscosity are considered in such
analysis.[101]
CONCLUSIONS
The growing interests in gas hydrates from various
disciplinary fields have demanded extensive studies
on gas hydrates properties and characteristics that
are associated with their formation, decomposition,
and inhibition. The insightful understanding of these
properties will help in planning long-term effective
and sustainable strategies for the management and
application of gas hydrates. Traditional macroscopic
and mesoscopic analyses have been successful in gas
hydrates phase equilibrium studies. The use of modern
advanced instrumental techniques in recent years has
provided further insights about gas hydrate processes,
which could not be obtained using traditional methods.
These modern microscopic measurement tools enable
direct determination of gas hydrates phase and possible
measurement of molecular interactions within the fluid
phases, which can only be estimated by theoretical
methods otherwise.
Although each of these instrumental techniques provides useful and valuable information about hydrate
properties, no single technique can, on its own, reveal
the complex gas hydrate systems.[194] There are challenges ahead. For instance, in situ studies are important for the direct analysis of hydrate systems which
require meticulously designed reactors and/or cells that
facilitate hydrate formation at high pressure and low
temperature.[25] Some properties and kinetics of gas
hydrates are apparatus-dependent[28] ; accurate correlation and interpretation of experimental data from different methods require a feasible approach that allows
simultaneous measurement on the same hydrate systems
using different analytical tools. For the same reason,
translation from laboratory to field work, a natural gas
pipeline for example, is also a challenge.[9] Distinguishing hydrates from ice is another challenge for the implementation of most of the aforementioned techniques.[59]
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Apart from these, extensive data management, excellent computational models, and efficient communication between experimental engineers, scientists, and the
molecular simulation engineers are also critical.
Overall, studies of gas hydrate systems require not
only extensive technology support but also integrated
knowledge base and expertise from engineers, physical
and chemical scientists, experimentalists, instrumental
and information technologists, computational scientists,
and industrialists. The collaborative work among them
is probably more vital than any other aspects for
insight, understanding, and adequate management of gas
hydrates. The sharing and exchange of acquired and
accumulated knowledge and expertise are fundamental
for more rapid progress to occur, which can probably be
facilitated by a consortium of industrial, academic and
government laboratories/researchers working together
to solve major problems in the field for the mutual
benefit of all concerned.
Acknowledgements
The authors are grateful to Dr Karl J. Jalkanen of Bremen Center for Computational Material Science at the
University of Bremen Germany for his comments and
help in editing the manuscript. Yenny Rojas also wishes
to acknowledge the financial support of the Curtin International Research Tuition Scholarships (CIRTS) and a
PhD scholarship from Commonwealth Scientific and
Industrial Research Organization (CSIRO) Petroleum
Resources.
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