07 jpse mahajan hydrates

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Editorial

An introduction to natural gas hydrate/clathrate:

The major organic carbon reserve of the Earth

1. Existence of hydrates

Gas hydrates belong to a general class of inclusion

compounds commonly known as clathrates. A clathrate

is a compound of molecular cage structure made of host

molecules encapsulating guest molecules. It is also

considered a chemical substance consisting of a lattice of

one type of molecule trapping and containing a second

type of molecule (Sloan, 1998; Taylor et al., 2004).

Natural gas clathrates owe their existence to the ability of

H2O molecules to assemble via hydrogen bonding and

form polyhedral cavities as shown in Figs. 1 and 2.

Natural gas hydrate is a naturally occurring ice-like

solid (see Fig. 3), which is made of water molecules as the

cage forming host and other molecules (mostly methane)

as the guest. The guest molecules, like methane or carbondioxide, are of an appropriate size such that they fit within

cavities formed by the host material. Common clathrate

compounds of interest are those formed from CO2/H2O

and CH4/H2O mixtures: the former for application in

carbon sequestration and the latter for methane extraction.

The physical appearance of the natural gas hydrate is

like other crystalline substances. At standard pressure

and temperature, a methane hydrate molecule contains

approximately 160 volumes of methane for each volume

of water.

Until recently, methane hydrates, known to scientistsfor almost 200 years, have remained a scientific

curiosity. It was not until the 1930's that it was realized

that methane hydrate was responsible for plugging

natural gas pipelines, particularly those located in cold

environments. For the next 40 years, a small body of

researchers investigated the physics of various clath-

rates, including the construction of the first predictive

models of their formation. A prime focus of this work

was (and continues to be) the development of chemical

additives and other methods to inhibit hydrate formation.

The United States Geological Survey (USGS) esti-

mates that there are more organic carbon reserves around

the earth as methane hydrate than all other forms of fossil

fuels combined (Booth et al., 1996). The Department

of Energy (DOE) considers about 1% recovery of me-

thane from the known methane hydrate reserves within

the U.S. enough (over 2000 TCF) to satisfy the U.S.

consumption for the next eight decades.

As it is demonstrated in the pie-chart, Fig. 4, the

amount of organic carbon contained in natural gas hydrate

reserves around the globe is estimated to be twice the

amount contained in all fossil fuels on Earth. By fossil

fuels it is meant coal, oil and conventional natural gas

reserves all around the world.

According to the U.S. Geological Survey, the

estimated global natural gas hydrate reserves are in therange from 100,000 to about 300,000,000 trillion cubic

feet. This estimate when compared with the 13,000 tril-

lion cubic feet of conventional natural gas reserves

demonstrates the vastly more abundant natural gas

hydrates around the globe.

Interest in methane hydrate as an energy resource

was initially ignited in 1960s' by Russian scientists who

claimed contribution from hydrates during conventional

gas drilling in the Messoyakha field, Siberia. The U.S.

was a leader in the 1970s' in this area. In mid-1990's,

two countries, with a large energy demand but limitedresources (Japan and India), began to explore the

possibility of extracting methane from hydrates. The

U.S. research effort got a big push in Year 2000 with the

passage of the Methane Hydrate Research and Devel-

opment Act. Under the Act, the U.S. DOE coordinated a

five-year effort by the Federal Agencies to promote the

research, identification, assessment, exploration, and

development of methane hydrate resources. Advances

in the basic understanding of hydrates have occurred

during these 5 years, including several hydrate-specific

Journal of Petroleum Science and Engineering 56 (2007) 18

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expeditions in the polar region and in the deep water on

the continental shelves around the world (see Fig. 5).

The Methane Hydrate Research and Development Act

(H.R. 1753, 2000) was extended by an amendment

through 2010 as part of the Energy Policy Act of 2005.

Methane hydrates have been discovered in the

subsurface in permafrost regions, but most occur in

oceanic sediments hundreds of meters below the sea

floor where water depths are greater than about 500 m.

As it is shown in Fig. 5 natural gas hydrates have beendiscovered at numerous locations along continental

margins as well as in the Arctic.

Many issues are under investigation by hydrate

researchers around the world. These include implica-

tions on the global carbon cycle, long-term climate-

change effects, seafloor stability, future energy source,

hydrate formation and dissociation properties, physical

and chemical properties, and global distribution of

hydrate. We have attempted to represent papers report-

ing on the latest research in these areas. As research is

progressing, it may lead to an environmentally-benignmethane extraction method in a not too distant future.

2. About this special issue

This special issue is primarily dedicated to natural

gas hydrate, an unconventional energy source that has

the potential to supplant the world's energy supply. A

thematic symposium on Gas hydrates and clathrates

was organized during the 2005 Spring National Meeting

of the American Chemical Society in San Diego, CA.

The two-day symposium consisted of 28 invited speak-

ers from industry, government, and academia discussing

the latest research issues and advances in the field of gas

hydrates and clathrates. Of those presented, below are

the titles and authors, followed by brief summaries of

the 20 papers that are included herein:

1. Resource potential of methane hydrate coming intofocus by Ray Boswell

2. Natural Gas-Hydrates A potential energy source

for the 21st century by Y.F. Makogon, S.A.

Holditch, and T.Y. Makogon

3. Methane hydrate exploration on the mid-Chilean

coast: A geochemical and geophysical survey by

Richard Coffin, John Pohlman, Joan Gardner, Ross

Downer, Warren Wood, Leila Hamdan, Shelby Walker,

Rebecca Plummer, Joseph Gettrust, and Juan Diaz

4. Growth kinetics of ethane hydrate from a seawater

solution at an ethane gas interface

by John P.Osegovic, Shelli R. Tatro, Sarah A. Holman, Audra

L. Ames, Michael D. Max

5. Effects of gas hydrates on the chemical and physical

properties of seawater by Chung-Chieng A. Lai

6. Effect of pressure vessel size on the formation of gas

hydrates by Scott D. McCallum, David E. Riesten-

berg, Olga Y. Zatsepina, Tommy J. Phelps

7. Raman spectroscopy of a hydrated CO2/water

composite by Monsuru O. Gborigi, David A.

Riestenberg, Michael J. Lance, Scott D. McCallum,

Yousef Atallah, Costas Tsouris

8. Formation of HFC-134a hydrate by static mixingby Hideo Tajima, Toru Nagata, Akihiro Yamasaki,

Fumio Kiyono, Tadashi Masuyama

9. Investigations into surfactant/gashydrate relation-

ship by Rudy Rogers, Guochang Zhang, Jennifer

Dearman, Charles Woods

Fig. 1. The schematic drawing of one type of natural gas clathrate

structure in which a methane molecule is encaged by a lattice of water

molecules. (Courtesy of NETL).

Fig. 2. Methane clathrate dual structure.

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10. Effect of surfactant carbon chain length on hydrate

formation kinetics Takamichi Daimaru, Akihiro

Yamasaki

, by Yukio Yanagisawa11. Effect of bubble size and density on methane

conversion to hydrate by Jonathan Lekse, Charles

E. Taylor, and Edward P. Ladner

12. A novel high-pressure apparatus to study hydrate

sediment interactions Michael Eaton, Devinder

Mahajan, and Roger Flood

13. Methane hydrate formation and dissociation in a

partially saturated core-scale sand Sample" by

Timothy J. Kneafsey, Liviu Tomutsa, George J.

Moridis, Yongkoo Seol, Barry M. Freifeld, Charles

E. Taylor, and Arvind Gupta

14. Methane gas hydrate effect on sediment acousticand strength properties by W.J. Winters, W.F.

Waite, D.H. Mason, L.Y. Gilbert, and I.A. Pecher

15. Characterization of methane hydrate host sedi-

ments using synchrotron-computed microtomogra-

phy (CMT) by Keith W. Jones, Huan Feng,

Stanmire Tomov, William J. Winters, Maa Proda-

novi, and Devinder Mahajan

16. Predicting gas generation by depressurization of

gas hydrates where the sharp-interface assumption

is not valid by Shahab Gerami and Mehran

PooladiDarvish

17. Parametric study of methane hydrate dissociation

in oceanic sediments driven by thermal stimulation

by Ioannis N. Tsimpanogiannis, and Peter C.

Lichtner

18. Down-hole combustion method for gas production

from methane hydrates by Marco J. Castaldi, Yue

Zhou, and Tuncel M. Yegulalp

19. Methane hydrate research at NETL, research tomake methane production from hydrates a reality"

by Charles E. Taylor, Dirk D. Link, and Niall

Eglish

20. Gas hydrates and clathrates: flow assurance, environ-

mental, economic perspectives, and the Nigerian

Liquified Natural Gas Project by B. C. Gbaruko,

J. C. Igwe, P. N. Gbaruko, and R. C. Nwokeoma

The first three papers discuss the resource potential of

methane hydrate as an energy source. In the 1st invited

paper, Boswell (2007) (NETL/U.S. DOE) discussesincreasing recognition of the potentially vast global

occurrence of methane hydrate that has raised a number

of critical public interest questions. Chief among these is

the potential for methane hydrate to serve as a significant

new resource to help meet long-term energy demands. To

address this and other questions, the Methane Hydrate

R & D Act of 2000 was enacted promoting un-

precedented collaboration between six federal agencies

and enabling 5 years of government-industryacademia

R & D partnerships. The paper discusses significant

developments over the past 5 years that have sharpened

the focus of R and D into methane hydrate's resource

Fig. 3. Photographs of methane hydrate made in a 1 L cell. A: pure methane hydrate; B: methane hydrate in sea sand; C: methane hydrate in F-110

sand (Taylor et al., 2004).

Fig. 4. Distribution of organic carbon in Earth reservoirs (excluding

dispersed carbon in rocks and sediments, which equals nearly 1000

times this amount). More than half of the carbon is locked up in gashydrates (Source: www.usgs.gov).

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potential. The 2nd paper by Makogon et al. (2007) (Texas

A & M) presents various energy scenarios. He contends

that the rate of modern civilization growth in the future

will depend on numerous factors, but the quality and

quantity of energy used will be among the most

important factors. The data presented in this paper

reflects the distribution and changes of energy sources

over time with oil and natural gas as the primary present

fuels. According to British Petroleum (BP), as of 1January 2005, the proven hydrocarbon reserves are

161.9 billion tons of oil and 179.5 trillion m3 of natural

gas. At the current level of consumption, the reserves are

sufficient for 41 years for oil and 66 years for natural gas.

A new source of fossil fuel is needed and gas hydrates

may be this source. In the 3rd paper, Coffin et al. (2007)

(Naval Research Laboratory) summarize data from a

methane hydrate exploration that was conducted off the

Mid-Chilean margin (west of Concepcin) during

October 2004. The goal of the expedition was to survey

regions of possible hydrate accumulations by integratingdata from seismic surveys, geochemical analysis of

porewater samples from piston cores, and heat flow

probing. Piston cores were collected at 13 sites along a

NESW transect. This sample pattern was based on a

previous deep towed acoustics geophysics system

(DTAGS) seismic survey. In the study region, measure-

ment of the shallow SMI and vertical sulfate diffusion,

coupled with the seismic survey and heat flow provides a

more thorough indication of the presence of deep

sediment methane gas pockets. This work is representa-

tive of worldwide interest in developing hydrate to

supplant conventional natural gas reserves.

Papers 4 through 8 describe laboratory studies that

are underway to enhance our understanding of hydrates

of CH4, CO2, and other gases. In paper 4, Osegovic et al.

(2007) (MDS, LLC) reports the growth rate and

morphology of gas hydrate at an interface of seawater

and ethane gas. Ethane hydrate crystals that have the

macroscopic appearance of concentric rings were

nucleated and grown from a seawater solution under a

range of supersaturations. The nucleation rate wasdetermined by plotting the increase in the number of

observed particles with time and approached a maxi-

mum rate implying a diffusion limited process. The disk

crystals eventually agglomerated to completely cover

the interface. An understanding of the formation of

hydrate at gas/liquid interfaces is of interest for its

potential role in clogging gas pipelines. In the 5th paper,

Lai (2007) (Los Alamos National Laboratory) presents

analysis of individual profiles on the seafloor and

continental margins that imply dissociation of gas

hydrates occurs according to temperature and pressure(depth) conditions at the hydrate phase boundary.

According to Lai, the oceanic hydrates provide a huge

biochemical fuel source that is continually generating

internal heat in the world oceans. This may explain the

observed ocean warming at the intermediate depth

during last several decades. Analysis of the relationship

among seawater temperature, apparent oxygen utiliza-

tion (AOU) and the concentration of CO2, based on

WOCE ocean observations, reveal some characteristics

that are not part of a stoichiometric relation. The role of

microbes in altering the chemical and physical proper-

ties of seawater is also discussed. In paper 6, McCallum

Fig. 5. Distribution of natural gas hydrates around the globe ( Max et al., 1997).

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et al. (2007) (Oak Ridge National Laboratory) describe

the effect of pressure vessel size on gas hydrate

formation. A Seafloor Process Simulator (SPS) consist-

ing of a 72 L vessel was used for mesoscale experiments

investigating the nature of hydrate nucleation and

dissociation at pressures and temperatures required forstability of hydrates of CH4 and CO2. The same

experiments were duplicated in a smaller (0.45 L) Parr

vessel. It was found that experiments in the SPS resulted

in hydrates consistently forming at lower overpressures

and in shorter induction times than equivalent experi-

ments in the smaller vessel. The variability of pressure

and/or induction time for hydrate formation was not

eliminated by using the SPS, but it appeared to be less

dramatic (small coefficients of variation) when com-

pared with a 450 mL Parr vessel. The observed

differences were attributed to increased bubble surfacearea, gas concentration, lifetime of bubbles, total

volume of the SPS, or a combination of the above.

The authors conclude that mesoscale experiments such

as those in the SPS, may perhaps be more representative

of hydrate accumulation in the natural environment. In

the 7th paper, Gborigi et al. (2007) (Oak Ridge National

Laboratory) investigated formation of a sinking carbon

dioxide (CO2) hydrate composite as an alternative to

direct liquid CO2 injection and pure CO2 hydrate

formation for ocean carbon sequestration. Raman

spectroscopy was used as a tool to understand the

formation and spectroscopy of a semi-solid sinking CO2hydrate composite formed using a coflow injector in

the 72 L and 0.45 L pressurized vessels at pressure and

temperature conditions equivalent to approximately

1.3 km depth in the ocean. The Raman shifts cor-

responding to CO2 and water molecules as well as shifts

in peak positions due to different CO2 phases were

obtained. The Raman spectra of the composite showed

that both liquid and hydrate phases of CO2 were present.

The dissolution rate of CO2 hydrate composite in water

was also studied. An attempt was made to calculate the

hydration number for the CO2 hydrate composite andalso the percentage of liquid CO2 and water loss during

formation.

In recent years, the applications of gas hydrate research

are expanding into fields of energy and environmental

management. In the 8th paper by Tajima et al. (2007)

(National Institute of Advanced Industrial Science and

Technology, Japan) the authors propose a more efficient

continuous gas hydrate formation method using a static

mixer. Flowing HFC-134a gas was mixed with water in a

static mixer, and the resulting hydrate was observed as

plug-like hydrate and hydrate slurry. Plug-like hydrate was

agglomerated HFC-134a bubbles covered with hydrate,

while the hydrate slurry consisted of small hydrate particles

dispersed in water. The hydrate formation patterns

depended on the rate of hydrate formation per pressure

and temperature conditions. From the experimental results,

the pressure and temperature conditions feasible for

continuously forming HFC-134a hydrate were determined.Papers 9 and 10 describe the effect of surfactants on

hydrate formation. The 9th paper by Rogers and his

group (2007) (Mississippi State U.) presents results

showing how anionic synthetic surfactants helped

develop an industrial gas-hydrate storage process for

natural gas and how naturally-occurring in situ anionic

biosurfactants influence the formation and placement of

gas hydrates in ocean sediments. The catalytic effects,

mechanisms, and surface specificities imparted by

synthetic surfactants in the gas storage process and

imparted by biosurfactants in porous media are dis-cussed. The Bacillus subtilis bacterium that is indige-

nous to gas hydrate mounds in the Gulf of Mexico was

cultured in the laboratory. Its biosurfactant was separated

and found to catalyze gas hydrates in porous media. The

experiments indicate that seafloor biosurfactants can be

produced rapidly in situ to achieve threshold concentra-

tions whereby hydrates are promoted. The biosurfactants

accumulate and promote hydrate formation on specific

mineral surfaces such as sodium montmorillonite.

Paper 10 by Daimaru et al. (2007) (University of

Tokyo) describes an experimental study to establish the

effect of surfactants on hydrate formation kinetics. Aseries of surfactants with sodium sulfonic acid groups in

common but of different carbon chain lengths (C4, C12,

and C18) were tested, and the effects of carbon chain

length and concentration were systematically investigat-

ed. Hydrate formation rates were measured by a batch-

type method with a high-pressure vessel made of

stainless steel, with 100 mL inner volume, and the

hydrate formation rate determined by the rate of pressure

decrease caused by hydrate formation. Energy saving

potential was estimated when the C4 surfactant was

applied to a natural gas transportation scenario in theform of hydrates; it was estimated that power consump-

tion could be reduced by about 40% with the use of the

C4 surfactant in the hydrate formation process.

In paper 11, Lekse et al. (2007) (NETL/U.S. DOE)

describe research to economically store methane as

hydrates that may open many commercial opportunities

such as transport of stranded gas, off-peak storage of

line gas, etc. The authors observed during investigation

that the ability to convert methane to hydrate was

enhanced by foaming of the methane-water solution

using a surfactant. The density of the foam, along with

bubble size, was important in this conversion reaction.

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Papers 1215 focus on the study of sediment (host)/

hydrate (guest) interactions. In order to develop or

improve understanding of hydrates under natural

environments that may lead to a methane recovery

method, it is important to be able to mimic natural

conditions in a laboratory and study dynamics ofmethane hydrates in host sediments. To date, a large

data set from laboratory studies are available for pure

methane hydrates for which kinetic models have been

proposed but reproducible data collection in the

presence of sediments has proved challenging. In

paper 12 by Eaton et al. (2007) (Brookhaven National

Laboratory/Stony Brook University), a new experimen-

tal apparatus namely FISH (Flexible Integrated Study of

Hydrates) is described. The unit was designed to confine

artificial and natural sediments in a pressure vessel and

mimic oceanic conditions to study various aspects ofhydrate research. The authors studied kinetics of

methane hydrate formation/decomposition in depleted

sediments from a well-studied hydrate site. The

availability of accurate data on the formation/decompo-

sition cycle and acoustic properties of hydrates will aid

in developing a much sought after economical method

to extract methane from this vast resource. The

following paper 13 by Kneafsey et al. (2007) (Lawrence

Berkeley National Laboratory) focused on a series of

experiments to collect data for validating numerical

models of gas hydrate behavior in porous media.

Methane hydrate was formed and dissociated undervarious conditions in a large X-ray transparent pressure

vessel, while pressure and temperature were monitored.

In addition, X-ray computed tomography (CT) was used

to determine local density changes during the experi-

ment. In a series of experiments, thermal perturbations

on the sand/water/gas system were performed to form

methane hydrate and studied their reverse behavior.

Winters et al. (2007) (USGS) report in paper 14 the

interaction of methane gas hydrate with host sediment.

The authors studied: (1) the effects of gas hydrate and

ice on acoustic velocity in different sediment types;(2) effect of different hydrate formation mechanisms on

measured acoustic properties; (3) dependence of shear

strength on pore space contents; and (4) pore-pressure

effects during undrained shear. A wide range in acoustic

p-wave velocities (Vp) were measured in coarse-grained

sediment for different pore space occupants. Acoustic

models based on measured Vp indicate that hydrate,

which formed in high gas flux environments, can

cement coarse-grained sediment, whereas hydrate

formed from methane dissolved in the pore fluid may

not. In paper 15, Jones et al. (2007) (Brookhaven

National Laboratory) studied the hydratesediment

interaction, an important aspect of gas hydrate studies

that needs further examination. The study describes the

applicability of the computed microtomography (CMT)

technique that utilizes an intense X-ray synchrotron

source to characterize sediment samples, two at various

depths from the Blake Ridge area (a well-knownhydrate-prone region) and one from Georges Bank,

that once contained methane trapped as hydrates.

Detailed results of the tomographic analysis performed

on the deepest sample (667 m) from Blake Ridge are

presented as 2D and 3D images which show several

mineral constituents, the internal grain/pore microstruc-

ture, and, following segmentation into pore and grain

space, a visualization of the connecting pathways

through the pore space of the sediment. Various

parameters obtained from analysis of the CMT data

are presented for all three sediment samples.The last four papers (1620) present studies to

understand hydrate behavior during gas production. A

number of analytical models have been reported for

predicting gas production from gas hydrates. The

analytical models assume that decomposition happens

at a sharp-interface that divides the medium into two

regions; the hydrate zone and the dissociated zone.

However, several detailed studies have shown that in the

presence of a mobile (gas or water) phase in the hydrate

cap, pressure reduction propagates from the interface

into the hydrate zone, leading to decomposition of the

hydrate throughout the hydrate zone. Under theseconditions, the sharp-interface assumption is not valid

and much needs to be learned in the production area.

Papers 16 by Gerami and PooladiDarvish (2007)

(U. Calgary) and 17 by Tsimpanogiannis and Lichtner

(2007) (Los Alamos National Laboratory) examine

methane hydrate dissociation in porous media caused by

thermal stimulation of the system. They consider a semi-

analytical model of the problem based on local

equilibrium and build upon previous studies to under-

stand the effect of various parameters on methane

production. While previous studies focused on porousmedia with relevance to permafrost regions, the in-

creasing significance of hydrates in oceanic sediments

warrants further studies on this aspect. Sediment pro-

perties such as pore-size distributions, permeabilities,

porosities, thermodynamics, and transport, etc. of

oceanic versus permafrost origins could be significantly

different and impact hydrate behavior. These data could

help delineate possible range of parameters where

methane production could be economically viable. The

18th paper by Castaldi et al. (2007) (Columbia U.)

considers natural gas as an excellent fuel for transition to

a hydrogen economy. The best fuel for a transition to

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hydrogen economy will be natural gas. With its high H/

C ratio, natural gas will remain major source of

hydrogen for next generation power systems. Most

proposed techniques for recovering natural gas from

hydrates require extensive energy input, raise safety

concerns or can only recover a fraction of the gas. Thepaper discusses the technical feasibility of a novel gas

production method in hydrate reservoirs. The method

assumes an in situ process to create a point heat source

in hydrate formations. Calculations and simulations

using FEMLAB show that the method offers an

energy efficient and environmental friendly way to

produce natural gas from hydrate reservoirs.

The 19th paper by Taylor et al. (2007) (NETL/U.S.

DOE) describes laboratory experiments and computa-

tional modeling to address several key areas in hydrate

research. The laboratory results are used in the compu-tational models and the results from the computational

modeling is used to help direct future laboratory research.

Laboratory research is accomplished in one of the

numerous high-pressure hydrate cells: thermal conduc-

tivity of hydrates (synthetic and natural) at temperature

and pressure, computational modeling studies are inves-

tigating the kinetics of hydrate formation and dissociation,

modeling methane hydrate reservoirs, molecular dynam-

ics simulations of hydrate formation, dissociation, and

thermal properties, and Monte Carlo simulations of

hydrate formation and dissociation

The 20th paper by Gbaruko et al. (2007) (Abia StateUniversity, Nigeria) is an overview of natural gas, a

plentiful natural resource in Nigeria. The authors high-

light gas hydrates, asphaltenes and waxes as three major

threats to flow assurance that must be well assessed by

design team uptime. Gas hydrates are also being looked

upon as a future energy source in Nigeria. The paper

discusses chemistry and mechanism of gas hydrate

formation, the problems they pose, especially to flow

assurance, their system implications, their environmen-

tal and economic perspectives with respect to their

prospects as storage and transport alternative to theliquefied natural gas (LNG) technology.

In closing, energy supply has become an urgent issue

as consumers, especially in the U.S., are hit with multi-

fold increase in a short span of few years. Methane

hydrate has the potential to change the global energy

equation. Collectively, the 20 contributions included

in this volume cut across several hydrate R & D areas

to give the reader appreciation of the system complexity,

an understanding of which is needed to develop a

commercially viable extraction method in the near

future.

Acknowledgements

The editors thank all the contributors who made it

possible to compile this special volume. The editors sin-

cerely acknowledge the technical assistance of numerous

experts who gave their time and effort by participating inthe review process of manuscripts contained within this

issue. The editors commend the editorial staff of Elsevier

Science for their assistance in publishing this timely issue

on gas hydrates and clathrates.

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Devinder Mahajan*b Department of Materials Science and Engineering,

Stony Brook University, Stony Brook, NY 11794 USA

E-mail address: [email protected].

Charles E. Taylor*c U.S. Department of Energy,

National Energy Technology Laboratory,

P.O. Box 10940, Pittsburgh,

PA 15236-0940, USA

E-mail address: [email protected] authors. Mahajan is to be contacted at

Energy Sciences and Technology Department,

Brookhaven National Laboratory, Upton,

NY 11973-5000 USA. Tel.: +1 631 344 4985;

Taylor, Tel.: +1 412 386 6058.

G. Ali Mansoorid

dDepartments of Bio and Chemical Engineering,

University of Illinois at Chicago,

851 S. Morgan Street (M/C 063),

Chicago, IL 60607-7052

E-mail address: [email protected].

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