Ứng dụng hữu ích của Gas hydrates trong ngành công nghiệp dầu khí. Cung cấp các thông tin cần thiết cho các kỹ sư dầu khí nhằm đáp ứng cho nhu cầu năng lượng không chỉ của riêng nước ta mà còn trên thế giới.
Trang 1Until mankind learns how to economically
gener-ate hydrogen for fuel cells, natural gas will be the premium fuel for this century for two reasons
First, gas burns cleanly, causes few pollution problems and, relative to oil or coal, produces less carbon dioxide And second, liquid fuels are better used as feedstocks (raw material) for generation of petrochemicals
Two examples herald this coming change: many power plants are being converted from coal to natural gas, and fleets
of cars have been converted from petrol to natural gas fuel
As we deplete the readily accessible reserves, we will need
to obtain natural gas from conditions that are both more severe and more remote We will need to explore deep ocean environments with higher pressures, and permafrost envi-ronments with lower temperatures than we presently do
And gases that were previously thought to be uneconomical, such as those containing non-combustible components of nitrogen, carbon dioxide and hydrogen sulphide, will also
be explored Such unusual conditions also promote the for-mation of a solid compound of gas and water — namely clathrates of natural gas — commonly called gas hydrates
Here, I indicate the motivation for hydrate science and engineering; that is, the applications where technical workers find a use for the physics, chemistry and biology that are associated with science This is not to indicate that hydrate engineering is simply an applied science As indicated recently in a defining book1on differences in technical phi-losophy, engineering frequently cannot afford the luxury of
a thorough scientific foundation, and must proceed at risk
As only one example, the past decade’s development of the new ‘low-dosage’ pipeline hydrate inhibitors proceeded in
an Edisonian research mode (a process of intelligent guess-work, intuitive reasoning and testing), and scientific progress is currently being made to refine trial and error research gaps in inhibitor developments
Below, I describe five major applications of hydrate research: flow assurance, safety, energy recovery, gas stor-age/transportation and climate change Before the applica-tions are addressed, an introduction to hydrate structures and their overall properties is presented I conclude this review with an outline of future challenges For readers who want a more detailed understanding, several hydrate reviews2–8are available
Hydrate structures
Clathrate hydrates typically form when small (0.9 nm)
‘guest’ molecules such as methane or carbon dioxide contact
water at ambient temperatures (typically less than 300 K) and moderate pressures (typically more than 0.6 MPa) On a molecular scale, single small guest molecules are encaged (enclathrated) by hydrogen-bonded water cavities in these non-stoichiometric hydrates Guest-molecule repulsions prop open different sizes of water cages, which combine to form the three well-defined unit crystals shown in Fig 1, and structural details and references have been provided in a recent book2
Cubic structure I predominates in the Earth’s natural environments, and contains small (0.4–0.55 nm) guests; cubic structure II generally occurs with larger (0.6–0.7 nm) guests in mostly man-made environments; and hexagonal structure H may occur in either environment, but only with mixtures of both small and large (0.8–0.9 nm) molecules The smallest hydrated molecules (Ar, Kr, O2and N2), with diameters less than 0.4 nm, form structure II Most hydrate science, and thus most applications, concentrates on struc-ture I and strucstruc-ture II, with strucstruc-ture H in anecdotal occur-rence Although this review will emphasize structure I and structure II, many analogues occur for structure H hydrates Figure 1b lists the properties of the three common unit crystals Water molecules form hydrogen bonds in a basic building block for both structure I and structure II, called the 512(pentagonal dodecahedra) because there are 12 faces
of pentagonally bonded water molecules in that cavity Within the cavity, small guest molecules are enclathrated, with limited translation motion but substantially more rotation and vibration ability The 512building blocks are joined to other 512s either through the vertices (structure I)
or through the 512faces (structure II)
Yet all structures need to fill space within their cavities to prevent hydrogen-bond strain and breakage The 512basic building blocks cannot fill space without bond breakage, and so interstices between the 512cages are filled with other cavities that relieve the strain by incorporating hexagonal faces — two in the 51262cavity of structure I, and four hexag-onal faces in the 51264 of structure II, in addition to the
12 pentagonal faces in each cavity Thus, the cages can con-tain larger guest molecules.The cages form basic repeating unit crystals with ratios of 2•512+6•51262in structure I and
16•512+8•51264in structure II, indicating sixteen 512s and eight 51264s in the structure II unit crystal Although both large and small cages are present in each crystal structure, sometimes single guests are too large for the smaller cage, which must go empty so that only the larger cage is filled However, smaller molecules can fill both cages
Fundamental principles and applications of natural gas hydrates
E Dendy Sloan Jr
Center for Hydrate Research, Colorado School of Mines, Golden, Colorado 80401, USA (e-mail: esloan@.mines.edu)
Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water They form when the constituents come into contact at low temperature and high pressure The physical properties of these compounds, most notably that they are non-flowing crystalline solids that are denser than typical fluid hydrocarbons and that the gas molecules they contain are effectively compressed, give rise to numerous applications in the broad areas of energy and climate effects In particular, they have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and they could play a significant role in past and future climate change.
Trang 2In all three structures, typically there is only one guest molecule
within each cage However, at unusual conditions such as very high
pressure, it is possible to have multiple-cage occupancy with
unusu-ally small guests, such as hydrogen or noble gases For example, it was
recently shown9that hydrogen can form hydrates at very high
pres-sure with as many as two occupants in the small cage and four in the
large cage of hydrate structure II However, very small guests and
multiple occupancies are considered an aberration
Remarkably, when all hydrate cavities are filled, the three crystal
types have similar concentrations of components: 85 mol% water
and 15 mol% guest(s) Hydrate formation is most likely to take place
at the interface between the bulk guest and aqueous phases, because
the hydrate component concentrations greatly exceed the mutual
fluid solubilities The solid hydrate film at the interface acts as a barrier
to prevent further contact of the bulk-fluid phases, and fluid surface
renewal is required for continued clathrate formation
Guest to cage ratio
Examination of the size ratios of the guest molecules to the cages they
occupy can aid the understanding of hydrates This heuristic controls
not only the occupancy but also the thermodynamic properties of
these structures, to a first approximation
Simple hydrate guests
A number of researchers have commented on the fit of the guest
mol-ecule within the hydrate cage, beginning with von Stackelberg10,
whose modified original diagram is shown in Fig 2 Table 1 lists the
size ratios of guests of natural gas in the four common cavities of structure I and structure II The discriminating size ratio is not absolute for each cage, instead it occurs over a molecular size range, which has a number of important implications The implication is that clathrate hydrates have no fixed size ratio of guest to host, as shown by the ranges in Fig 2 It is particularly interesting to note the resulting behaviour of molecules at cage size borders
Take, for example, the most common natural gas compound, methane, in the phase diagram for methane and water in Fig 3 (ref 11) The non-stoichiometry of the hydrate causes the hydrate composi-tion area (vertical axis parabola) shown in green in Fig 3 rather than the vertical stoichiometric line originally proposed12for methane hydrate Assuming equilibrium, the implication of this hydrate phase area is that laboratory-made hydrates (from methane-rich systems)
may differ slightly in composition from in situ hydrates, which can
form in methane-lean systems This methane hydrate composition difference, although small (4%), when multiplied by the entire hydrate reserve is sufficient to supply the USA for 600 years at the cur-rent energy usage
Binary hydrate guests
For binary systems, Holder and Manganiello13indicated that an opti-mized fit of guests in the cages was sufficient for hydrate azeotropes, for which the vapour composition is equal to that of the hydrate This
is particularly unusual because azeotropy in vapour–liquid equilib-ria is only possible for a non-ideal solution (with an activity coeffi-cient), whereas hydrates typically form ideal solid solutions
5 12
4 3 5 6 6 3
5 12 6 2
5 12 6 4
5 12 6 8
2
Cavity types Hydrate structure
Structure I
Structure II
Structure H
‘Guest molecules’
Methane, ethane, carbon dioxide and so on
3 16
H
II
Number of cavities per unit cell
Average cavity radius (Å)
Coordination number*
Number of waters per unit cell
*Number of oxygens at the periphery of each cavity.
† Estimates of structure H cavities from geometric models.
8
34 H2O
iso-butane and so on
Methane + neohexane, methane + cycloheptane, and so on
a
b
5 12 5 12 6 2 5 12 5 12 6 4 5 12 4 3 5 6 6 3 5 12 6 8
Description
Figure 1The three common hydrate unit
crystal structures Nomenclature: 51264
indicates a water cage composed of 12 pentagonal and four hexagonal faces The numbers in squares indicate the number of cage types For example, the structure I unit crystal is composed of two 512
cages, six
512
62
cages and 46 water molecules
Trang 3Table 1 Ratios of molecular diameters* to cavity diameters† for some molecules including natural gas-hydrate formers
Molecule Guest diameter Structure I Structure II
(Å)
5 12 5 12 6 2 5 12 5 12 6 4
N 2 4.1 0.804 0.700 0.817 F 0.616 F
CH 4 4.36 0.855 F
0.744 F
0.868 0.652
H 2 S 4.58 0.898 F 0.782 F 0.912 0.687
CO 2 5.12 1.00 0.834 1.02 0.769
C 2 H 6 5.5 1.08 0.939 F
1.10 0.826
C 3 H 8 6.28 1.23 1.07 1.25 0.943 F
i-C 4 H 10 6.5 1.27 1.11 1.29 0.976 F
n-C 4 H 10 7.1 1.39 1.21 1.41 1.07
F indicates the cavity occupied by the single hydrate guest.
*Molecular diameters obtained from von Stakelberg 12
†
Cavity radii from Table 2-1 minus 1.4 Å water radii.
The earlier measurements of von Stackelberg10showed that some
single guest molecules that are structure I formers will form structure II
in binary guest mixtures — a somewhat counter-intuitive notion
This idea was later predicted14and measured15over a wide
composi-tion range for methane and ethane mixtures However, other
struc-ture I simple guest combinations such as methane and carbon
diox-ide will not form stable structure II as binary mixtures16 The
ques-tion arises as to why this occurs for methane and ethane mixtures but
not for methane and carbon dioxide mixtures
The reason for such structural transitions was addressed initially
by Ripmeester3and subsequently by Hester and Sloan17 The structure
II transition occurs when two molecules are at each extreme of the structure I molecular sizes shown in Fig 2 That is, small structure I formers in the 512cage (which are almost small enough to form struc-ture II) and large strucstruc-ture I formers in the 51262cage (which are almost large enough to form structure II) will, when mixed, cause structure II to form from two structure I formers
Two other examples serve to indicate that hydrate structure research has been a minefield for the complacent who think that everything structural is known First, for almost 40 years, until the prediction of structure II formation13by the smallest guests was
vali-dated by Davidson et al.18, it was erroneously thought that they formed structure I as simple hydrates Second, in 1987, structure H was dis-covered19, but on close inspection structure H crystals had been mea-sured much earlier in the diffraction data of von Stackelberg10 Again for four decades, the hydrate community paradigm forced the data into structure I and structure II categories, counting structure H as an aberration
Physical properties and implications
Several key physical properties of hydrates determine the roles that they play (or might play in the future) in both industry and the envi-ronment They are solids with densities greater than those of typical fluid hydrocarbons, and this has practical implications for flow assurance in pipelines and the safety thereof Furthermore, the fact that, in effect, hydrates concentrate their guest molecules results in
three potential applications: that energy can be recovered from in situ
hydrates; that hydrates can be used to transport stranded gas; and that hydrates may be a factor in climate change Each of these implica-tions and applicaimplica-tions is briefly discussed
Flow assurance
First, and perhaps most importantly, when hydrates form, they are solid, non-flowing crystalline structures This leads to the most urgent consideration — that of flow assurance in oil and gas pipelines Oil and gas wells always produce undesired water along with hydrocarbons that are in the hydrate guest size range As the flowing mixed phases cool, hydrates form and plug transmission lines, causing costly production stoppages, sometimes for as long as months, in large pipelines, while the hydrates are dissociated
The petroleum industry would like to maintain their processes outside the hydrate stability range Fortunately, the hydrate stability temperature and pressure range is predictable to within experimental accuracy using modern thermodynamic programs usually based upon a Gibbs energy extension20of the van der Waals and Platteeuw21
method Unfortunately, however, low temperatures (such as the deep-sea floor temperature of 277 K) and the mandates of high pressure for economic energy densities place many pipelines well within the hydrate-formation region High pressures and low temperature require hydrate-inhibition methods During the past decade this phe-nomenon has initiated a new type of engineer — the flow-assurance engineer — whose major objective is to prevent pipeline blockages,
former
Cavities occupied
n-C4H10
Ar
Kr
N2
O2
CH4
Xe H2S
CO2
C2H 6
C3H8
iso-C4H10
C-C3H6
(CH2)3 O
3
4
5
6
7
No hydrates
No SI or SII hydrates
Structure II
5 12 + 5 12 6 4
Structure II
5 12 6 4
Structure I
5 12 + 5 12 6 2
Structure I
5 12 6 2
5 2 / 3 H2O
7 2 /3 H2O
5 3 / 4 H2O
17 H2O
Figure 2 Hydrate guests versus hydrate cavity size ranges Along the line are the
size of the guest molecules in hydrates The broad shaded areas and the numbers on
the right discriminate the number of water molecules in hydrates occupied by single
guest occupants shown on the left For example, methane has a typical hydration
number of 53⁄4and occupies both the 512
and 512
62
cavities of structure I However, propane is so large, it can only fit into the largest structure II hydrate cavity (512
64
)
Adapted from ref 43 Copyright Geological Society, London
Trang 4primarily of hydrates, but also of waxes and other solids.
To provide flow assurance, the energy industry injects
hydrogen-bonding fluids (for example, alcohols or glycols) into the flowing
stream at the wellhead to compete with solid hydrates for the
avail-able water Worldwide methanol costs for hydrate inhibition are
esti-mated at US$220 million annually (P K Notz, personal
communica-tion) In addition, severe financial penalties are paid for large
methanol storage capacity on offshore platforms and for greater than
50 p.p.m methanol contaminations in refinery feedstocks During
the production process from hydrocarbon reservoirs, any amount of
water, which generally increases over the life of the well, must be
hydrate inhibited Although the aqueous phase is the primary place
where hydrate inhibition occurs, a small concentration (typically less
than 0.1 mol%) of methanol partitions into the hydrocarbon vapour
and liquid phases
Yet the bulk of methanol is lost to the hydrocarbon phases
because, even with low methanol concentrations, the hydrocarbon
phase amount greatly exceeds that of the aqueous phase Methanol is
a difficult molecule to model partitioning into both hydrocarbon
liq-uid and vapour phases, particularly with high accuracy, at low
con-centrations If we apply Hildebrand’s solubility maxim, ‘like dissolves
like’, we conclude that the methyl group makes methanol soluble in
hydrocarbons, whereas the hydroxyl group increases its solubility in
water Thus any equation-of-state that models methanol
partition-ing must address the challenge of modellpartition-ing hydrocarbon and
aque-ous phases equally well
In addition to hydrogen-bonding inhibitors, which require high
concentrations, new low-dosage hydrate inhibitors — both kinetic
inhibitors and anti-agglomerants — have been developed in the past
decade to prevent hydrate crystal growth and agglomeration,
respec-tively These new chemicals are rapidly being adopted in the field22
and provide a fertile research area for molecular modelling
Safety
Secondly, the hydrate solid specific gravity is typically 0.9 compared
with typical fluid hydrocarbon specific gravities of 0.8 or less This
higher density leads to the problem of ensuring hydrate safety and
preventing the annual loss of property and lives When hydrate
blockages dissociate in pipelines, they detach first at the pipe wall;
therefore, any pressure gradient across the high-density hydrate plug
will cause the hydrate to travel rapidly (measured at 300 km hr–1)
down the pipeline This effect will compress the downstream gas,
either causing pipeline blowouts or causing the plug to erupt though
pipeline bends Case studies of hydrate safety problems are given in a
monograph on this subject4
A second safety concern arises when hydrate plugs are locally
heated (for example, using a blowtorch outside a pipeline) to
dissoci-ate them Frequently, the evolving gas from the hydrdissoci-ate is contained
by the ends of the plug until the pipeline bursts owing to the pressure
being too high This safety concern is a result of the next hydrate
property — the ability of hydrates to concentrate high levels of gas in
a hydrated form
Energy recovery
When small (0.9 nm) hydrocarbon guest molecules are encaged in
hydrates, with typically one molecule per cavity, the guests are
sepa-rated approximately 0.5 nm by the water cages This means that the
energy density in hydrates is approximately the same as that of a
com-pressed gas, that is, less than the energy density of liquefied natural
gas (LNG) For example, if every hydrate cavity were filled with a
guest molecule, one volume of hydrate would dissociate to 180
vol-umes (STP) of gas The gas concentration in clathrates is comparable
to that of a highly compressed gas (that is, methane gas at 273 K and
18 MPa)
A large fraction of the Earth’s fossil fuels is stored in clathrate
hydrates Even the most conservative current estimates23suggest that
the amount of energy in hydrates is equivalent to twice that of all
other fossil fuels combined Most of the natural-gas-containing hydrates are in the ocean bottom, and although production of gas from such deep-lying hydrates is now too expensive, it is likely that within the next two decades we will tap that fuel source to meet grow-ing energy demands Table 2 compares hydrated methane to that in conventional reserves for 11 arbitrary divisions of the world Most of the natural hydrates around the world are biogenic — the guest gas comes from bio-degraded plant and animal matter that have been buried in the sea floor at low temperature over long peri-ods Substantial but anecdotal evidence exists for thermogenic hydrates from deeper gas sources in places like the Gulf of Mexico24
and the Caspian Sea25 Most of the estimates of gas hydrates have come from indirect seismic evidence using a bottom simulating reflector (BSR), which indicates reflections from gas at the base of the hydrate (see Fig 4)
BSR indications are not totally reliable, and other more accurate methods are needed There are a significant number of cases in which hydrates occurred without bottom simulating reflections, or when the BSR did not indicate the presence of hydrates Notwithstanding this problem, the resource numbers are so large that they warrant energy-recovery studies, even if they are in error by as much as two orders of magnitude
Much of the available public funding of hydrate research has been channelled toward industrial field experiments, aimed at the produc-tion of energy Although results from industrial experiments (for example, in Alaska and in Japan) may not be publicly available, results from two recent drilling expeditions are soon to be published These detailed field experiments will probably serve as design bases for the foreseeable future, owing to their thorough documentation
Pilot drilling, characterization and production testing of hydrates have begun in permafrost regions, which have higher concentrations
of hydrates (for example, 30 vol.% in the 1998 Mallik 2L-38 well in Canada), to learn how to approach the more dispersed, but much greater, ocean resource in the future The Mallik 5L international field experiment was concluded in March 2002 on Richards island in the MacKenzie delta of Canada at a cost of US$17 million This per-mafrost experiment provided the first direct evidence that hydrates
L W
H 2 O CH 4 ·?H 2 O CH 4
L W –I
LM–H LM–H H–I
H–V
V–LW
V
V Vapour
L W Liquid water
H Hydrate
M Solid methane
LM Liquid methane
I Ice
H H–LW
Previous hydrate line
by Kobayashi and Katz
Figure 3 The isobaric methane and water phase diagram Compare the vertical
parabolic hydrate area (green) with the previous vertical stoichiometric hydrate line of Kobayashi and Katz12
The parabolic region is a result of incomplete filling of the small cages (512) in structure I Variation in hydrate cage filling and the resulting hydrate parabola is a function of temperature, overall methane composition and pressure (not shown here)
Trang 5could be economically recovered at high concentrations The
infor-mation provided by this experiment, when it becomes available in a
Geological Survey of Canada report early in 2004 (ref 26), will be a
landmark upon which the industry will base designs for energy
recovery
For ocean hydrates, the benchmark will be Leg 204 of the Ocean
Drilling Program (ODP), completed in September 2002 on Hydrate
ridge of the Cascadia Margin off Oregon This is one of the first ocean
drillings for hydrates to recover large, pervasive hydrates, other than
anecdotal evidence Much of the information from Leg 204 was
pre-sented in preliminary findings at the combined American Geological
Union, European Geophysical Society, European Union of
Geo-sciences meeting in Nice (April 7–10, 2003) This drilling, the result
of which should appear in print in the final quarter of 2003, provides
the first evidence that ocean hydrates may be present in sufficient
concentration to be economically producible
Until the publication of the benchmark results from the Mallik 5L
and Leg 204 wells, the best literature for natural hydrates can be
found in summaries and volumes about the Mallik 2L well27, the
Blake Ridge ODP28Leg 164 and the Gulf of Mexico24
It should be noted that the amount of energy in ocean hydrates is
several orders of magnitude greater than that in permafrost hydrates
Put another way, the error in the ocean hydrated energy estimate is
greater than the entire permafrost hydrated energy estimate Until
ODP Leg 204, however, it was thought that the most concentrated
hydrates were in the permafrost, which provided more accessible
recovery
Methods for the economic recovery of methane from natural
hydrates are uncertain, and substantial creativity has gone into
devis-ing new recovery methods, as well as into applydevis-ing existdevis-ing oil and gas
technology to hydrate recovery All recovery methods apply one or
more of the following three principles: (1), reduction of the pressure
below that of hydrate stability; (2), addition of enough energy to
dis-rupt the water hydrogen bonds; and (3), addition of strong
hydro-gen-bonding chemicals (such as alcohol or glycol) to disrupt the
hydrate structure at reservoir conditions
Finite difference reservoir recovery models29 indicate that gas
production is only economical at rates larger than 500,000 standard
cubic metres (0.1 MPa and 289 K) per day This will require both
depressurization and thermal/inhibitor stimulation The most
pro-ducible of the permafrost hydrate deposits are those lying adjacent to
a gas reservoir, because free gas production will dissociate hydrates by decreasing reservoir pressures below hydrate stability Heat from the Earth allows hydrate decomposition to slowly replenish the gas reser-voir Makogon5indicated that a Siberian permafrost reservoir was produced in this manner during the 1970s
Gas production from hydrates close to conventional permafrost reservoirs will begin in the West during the next decade at incremen-tal costs over normal gas production Production from stand-alone hydrates in the permafrost or in the ocean will be much more costly, but is technically feasible Both Japanese and American programmes forecast that stand-alone hydrated energy recovery will begin by 2015
Storage and transportation
It is estimated that about 70% of the total gas reserve is either too far from an existing pipeline or too small to justify a liquefaction facility Gudmundsson and Borrehaug30suggested that it is economically fea-sible to transport stranded gas in hydrated form In the fourth inter-national hydrate conference, workers from Mitsui Shipbuilding31
showed that work in conjunction with the Japan Maritime Research Institute32 provides a basis for extending the basic concept of
Gudmundsson et al.33to transport stranded gas
Climate change
A recent publication34thoroughly documents evidence for Late Qua-ternary climate change caused by hydrates, commonly called ‘the hydrate gun hypothesis’ The concept is that, as little as 15,000 yr ago, methane from hydrates caused significant global warming
The hydrate gun hypothesis seems analogous to another,
some-what less controversial, hypothesis, proposed by Dickens et al.35–37 They suggested that an ancient (55.5 Myr ago), massive ocean methane hydrate dissociation might explain a 4–8 °C temperature rise over a brief geological time interval (103years) called the Late Palaeocene Thermal Maximum (LPTM) This is documented in deep ocean drilling samples as a prominent negative carbon isotope (13C = –2.5‰) in ocean sediments, in fossil tooth enamel, and in carbonates and organic sediments in terrestrial sequences This 13C reduction in the ocean and the recovery over the ensuing 0.2 million years (see Fig 5a) is consistent with pronounced dissolution of
calci-um carbonate in the deep sea sediment deposited during the LPTM,
as shown in Fig 5b
3.5
4.0
4.5
5.0
NE SW
VE =10.0
Line 8
NE SW
3.5
4.0
4.5
5.0
10 km
VE =10.0
BSR
Figure 4 A seafloor slump in the Blake-Bahama
ridge shown in both seismic (top) and cartoon (bottom) relief40
Note the bottom simulating reflector (BSR) parallel to the ocean bottom, except in the middle section (dotted line) where it appears that a seafloor eruption has occurred Reproduced with permission from ref 40 Copyright Geological Society, London
Trang 6Table 2 Conventional and hydrated gas resources in trillion cubic metres
(TCM) (TCM)
TCM, trillion cubic metres
3.0
2.0 1.0 0.0 –1.0
–2.0
Shallow Atlantic
Time after methane release (10 3 yr)
a
3.0
1.0 0.0 –1.0
–2.0
ODP site 1001:
Caribbean bulk carbonate
Time after the LPTM (10 3 yr)
b
c
d
Time after the LPTM (10 3 yr)
2.0 1.5 1.0 0.5 0
Average flux: 1.12 × 10 14 g yr
1,000
1,200
1,400
1,600
1,800
2,000
12 CH4 (+6H2O)
12 CH4 6H2O
∆T = +4°C
15
Temperature (°C)
Original geothermNew geotherm
Water Sediment
CH4-hydrate-pore water equilibrium curve A
D
B
C
Figure 5 Hypothesized causes of the Late Paleocene Thermal Maximum (LPTM) a,
Carbon isotope reduction and recovery during LPTM b, Dissolution of calcium
carbonate during LPTM c, Evolution of methane from hydrates d, Initial shift in
ocean hydrate equilibrium curve to cause the methane release In sequence: in d,
the geotherm shifted by 4 °C, causing release of a large quantity of methane from
hydrates, shown in c The result was a 13
C isotope reduction and recovery, as
shown in a and b through conversion of methane, first to carbon dioxide and then to
calcium carbonate Adapted from ref 44 Copyright Société géoglogique de France
In the LPTM hypothesis, the evolution of a large amount of methane from hydrates (1.121018g of methane) is the only plausi-ble explanation that has been offered to explain this environmental perturbation The abnormal 13C isotope indicates that the source was external to the normal ocean–atmospheric–biomass carbon pool Figure 5c shows a rapid evolution of methane from hydrates;
methane is hypothesized to be oxidized to carbon dioxide that is greatly enriched in 12C
Figure 5d shows the hydrate equilibrium curve as a function of depth and temperature in the ocean Hydrates are only stable between the equilibrium line and the original geotherm to the left of the curved line, at depths below the sediment surface, shown by the small vertical rectangle at A In the LPTM, if the ocean was warmed by
4 °C, the hydrates between the original geotherm and the equilibrium curve would melt, as the new geotherm was established The warm-ing from the original to the new geotherm would result in methane expulsion to the environment, where it would be oxidized to carbon dioxide, resulting in significant further warming It was hypothe-sized that the resulting carbon dioxide was re-absorbed by the ocean over the ensuing 0.2 Myr
The importance of the LPTM perturbation is that it is the first well documented instance of an explanation for how the global carbon cycle and other systems relate to a rapid, massive input of fossil fuel, such as may occur in modern industrial times The data and
summa-ry in the publication by Kennett34are the most thorough source of information in support of extending the theory to more modern times (the Late Quaternary), about 15,000 yr ago However, there is a considerable controversy concerning the validity of the hypothesis,
as suggested below
In the recent meeting, it was suggested38that ‘the hydrate gun is firing blanks’, and that the atmospheric methane spike was due to emissions from wetlands and peat bogs This new theory requires a glacial–interglacial vegetation time shift of 1,000 Gt C, which the proposers of the theory, Maslin and Thomas, admit is difficult
However, even this counter hypothesis requires some hydrate-derived methane for a mass balance, along with a shift in time for the wetlands
In a review of the hydrate gun monograph34, Dickens39generally concurs with the theory, but criticizes it on the grounds that it “per-petuates the common misconception that present-day methane hydrates are stable These systems may be in a steady state, but they must be viewed as dynamic, with large carbon fluxes to and from the ocean, even at [the] present day”
In closing the discussion on hydrate-related climate change, it should be noted that seafloor hydrate dissociation is also directly related to slumping of sediments on the sea floor Significant
hydrat-ed shydrat-ediment slumps in the ocean can jeopardize the foundation of sub-sea structures, such as platforms, manifolds and pipelines The single incident off the Carolina coast shown in Fig 4 took place about 15,000 years ago40 and increased the extant Earth’s atmospheric methane by as much as 4% The effect of subsidence on sub-sea struc-tures and foundations represents the initial meeting point for the two
Trang 7energy communities — the first is concerned with hydrates in the
Earth, and the second with concerns for hydrates in man-made
pro-duction systems The interested reader is referred to the recent
monograph41on this topic
Future challenges
Currently, it appears that hydrate research has acceptably addressed
the thermodynamic challenge for most conditions
Time-indepen-dent hydrate quantification, courtesy of extensions20of the van der
Waals and Platteeuw21model, is at the bounds of experimental
accu-racy, and is the most common industrial exemplar of statistical
ther-modynamics use
The most accurate thermodynamics have been provided
through modern spectroscopy for hydrate phase measurements, by
Raman, NMR and diffraction spectroscopy, through the bridge of
statistical thermodynamics to the macroscopic domain The
incor-poration of these three spectroscopic methods have enabled more
accurate descriptions of hydrate mixtures, so that mixture
thermo-dynamic predictions no longer depend solely on single hydrate
guest measurements An overview of such hydrate spectroscopic
methods and results is provided in a recent review6
Although the central concerns of hydrate thermodynamics have
been addressed, challenges remain at the periphery — for example,
at very high pressures (1,000 bar), in unusual fluids such as black
oils, hydrate–sediment mixtures, and the methanol-partitioning
challenge indicated earlier As an example of one such challenge,
hydrate– sediment mixtures have an unexplained thermal
diffusiv-ity maximum when plotted against sediment concentration As we
begin to examine hydrates in nature, such challenges for
time-inde-pendent properties will require decades to resolve
However, the largest challenge is to describe the kinetics of
hydrates42 The fact that hydrates are solid compounds makes their
slow, solid-phase kinetics particularly challenging to researchers An
additional challenge arises from the fact that hydrate solids form
interfacial barriers between the liquid and vapour phases that
typical-ly compose them Hydrate research is most accurate when studying a
time-independent target Typically, time-dependent (kinetic)
research is much more difficult and at least an order of magnitude of
accuracy is lost, relative to time-independent (thermodynamic)
research
The use of kinetic-model results to predict data from other
labo-ratories is problematic Molecular dynamic simulations of hydrate
kinetics have been hindered by stochastic nucleation and the large
number of molecules and time required for growth processes More
hydrate phase measurements are required to provide a needed
breakthrough — a unified hydrate kinetics model
Conclusions and outlook
Wherever small molecules contact water, the potential for a hydrate
phase should be considered The size ratio (guest to cavity)
deter-mines hydrate structural stability to a first-order approximation
Other simple hydrate properties such as solid behaviour, density and
concentration of guest molecules affect the major applications of
hydrate safety, flow assurance, energy production and storage and
climate change
During the next decade, gas production will begin from
per-mafrost hydrates associated with conventional gas reservoirs
How-ever, efficient production of ocean hydrates is problematic and
requires an engineering breakthrough to be economically feasible
Yet, the potential to tap the Earth’s largest hydrocarbon energy
resource cannot be ignored
Although hydrate thermodynamics are understood to an
accept-able degree for most engineering applications, the kinetics arena will
represent the largest challenge for advancing the information on
hydrates Although we know quite a lot about what hydrates are, the
question of how hydrates form is still very much unanswered
Find-ing the answers to such questions provides the intrinsic motivation
doi:10.1038/nature02135
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