a large potential methane source--natural gas hydrates

Energy Sources, Part A, 29:217–229, 2007Copyright © Taylor & Francis Group, LLCISSN: 1556-7036 print/1556-7230 onlineDOI: 10.1080/009083190948676

A Large Potential Methane Source—Natural Gas Hydrates

R. A. DAWES. THOMAS

Department of Chemical EngineeringThe University of the West IndiesSt. Augustine, Trinidad

Abstract Natural gas, essentially methane, can be obtained from natural gas hydrate(NGH). NGH reserves are difficult to pinpoint in the subsurface, but large sourceshave been identified by seismic reflection. This is particularly so below the sea floornear continental shelf plates in the oceans deeper than 300 m as NGH is stable at4◦C and 50 bar pressure. When extracted, 1 m3 of NGH can contain 160 sm3 ofgas. Currently, estimates of this gas resource are very uncertain, but recent estimatessuggest perhaps 2,500 trillion sm3, but how much gas can actually be produced fromthese accumulations is totally unclear at present. NGH could possibly solve much ofthe energy needs after 2020, but safe ways of extraction still have to be designed.Possible methods include the injection of hot water or inhibitor or reduction of reser-voir pressure, but none have yet been commercially tested. Great caution will beneeded because catastrophic environmental damage is likely if the methane is care-lessly released from the sediments. This article reviews the ‘state of the art’ of NGH.

Keywords energy, gas resources, methane, natural gas, natural gas hydrate

1. Introduction

Natural gas is taking on a greater role in energy requirements (currently 20%), partic-ularly for power generation (Cranmore and Stanton, 2000), largely due to increasingenvironmental pressure for clean fuels, energy efficiency and the relatively low capi-tal costs of building new natural gas–fired power equipment. Current reserves of fossilhydrocarbons should be sufficient to satisfy the World’s energy needs until 2020 (BPStatistics, 2004). However, Japan, China, India, and the USA are all energy deficient andhave to import energy. These countries are therefore spending increasing percentages oftheir economy on energy imports, which is putting pressure on the price of oil. Othermajor world economies (e.g., Germany, France, UK, etc.) will be following soon.

Energy needs after 2020 may be unsatisfied unless new processes to extract energyfrom the sun or new indigenous sources are found and developed. Unfortunately, the ex-penditure by governments on renewable energy research is small, which suggests that thesituation is not yet seen as urgent. However, a ‘new’ potential source of methane exists—natural gas hydrate (NGH). This article reviews some of the major factors about NGH.

Address correspondence to Richard A. Dawe, Dept. of Chemical Engineering, The Universityof the West Indies, St. Augustine, Trinidad. E-mail: [email protected]; [email protected]

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218 R. A. Dawe and S. Thomas

2. Natural Gas Hydrates (NGH)

Natural gas hydrates (NGH) are crystalline solids of natural gas and water (Katz and Lee,1990; McCain, 1990; Englezos, 1993; Makogon, 1997; Sloan, 1997; Collett, 2000, 2002;Kromah et al., 2003; Taylor et al., 2003; Pooladi-Darvish, 2004). Such compounds aretermed clathrates, and are defined as molecules of one kind included within a cavity inthe crystal lattice of another. With NGH, molecules of natural gas are trapped within thethree-dimensional water lattice creating a solid ice-like substance with a white snow-likeappearance. The water, the host, forms a hydrogen bond lattice, giving a three-dimensionalcage-like structure, and the gas molecules are guest molecules (Figure 1). As the guestenters the lattice, the internal alignment of the electrons in the guest molecule interactswith the hydrogen bonds of the host altering and subsequently lowering the energy andstabilizes the system. The stable lattice can then precipitate as a crystalline solid, NGH,under the appropriate conditions described later. Each cage contains at most one guestmolecule, which is held within the cage by dispersion van der Waals forces. The guestmolecule must be of size that it fits in the voids created by the host lattice.

NGH are non-stoichiometric (CH4.nH2O where n ≥ 5.75), and can be formed withnatural gas (methane, ethane, propane, and i-butane), but not n-butane (this moleculebeing too large) (Katz and Lee, 1990; Makogon, 1997; Sloan, 1997) and liquid water, atthe right conditions.

There are three host structures, types sI, sII, and sH (Sloan, 1997). Type sI—where48 water molecules per 8 gas molecules form polyhedral cages to hold the smaller gasmolecules, such as methane, ethane, and carbon dioxide, and is the most important forNGH. The most common NGH is methane hydrate as a type sI hydrate (Figure 1). TypesII with 136 water molecules per 24 gas molecules, contains larger molecules such aspropane, isobutane and nitrogen. Type sH is sometimes found in oil pipelines (Sloan,1997; Mehta and Sloan, 1999).

3. Occurrence of NGH

In 1964 methane hydrates were found in Messoyakha, a northern Siberian gas field(Katz and Lee, 1990; Englezos, 1993; Makogon, 1997; Collett and Ginsberg, 1997; Max,2000; Collett et al., 2003). They exist in areas where low temperatures and high pressurespersist, i.e., temperatures ranging between −10 to +10◦C and pressures between 1–500bar. Growing interest occurred after NGH were discovered in offshore USA and Japanin water depths around 500–1000 m (Max, 2000).

Figure 1. A type sI hydrate molecule cage with methane inside as guest.

Natural Gas Hydrates 219

Today NGH have been detected in the oceans below the sea-bed near almost everycontinental margins, some distance, perhaps 200 km, from the coastline, which is withinmany territorial waters. Also, NGH has been found in permafrost regions below theEarth’s continental surface (Makogon, 1997; Kvenvolden, 2000). Over sixty large fieldshave been discovered thus far in oceanic sediment in the North Sea, the Arctic Oceans,and deep water (900–3000 m) Gulf of Mexico, and eight fields have been detected onland in the permafrost regions along the continental plates of Alaska, Canada, and Siberia(Makogon, 1997; Max, 2000) (Figure 2).

Marine hydrate is found in the detritus in marine sediments from the seafloor down tosome depth determined by rising temperature (Wellsbury and Parkes, 2000). The methanein most hydrate deposits is generated by anaerobic bacteria living beneath the sea floor bya process termed methanogenesis—i.e., biogenic generation (Norville and Dawe, 2007),although some thermogenic NGH has been found (thermogenic gas is produced by ther-mal decomposition at temperatures greater than 150◦C and depths perhaps greater than4000 m so is upward migrating gas). Bacteria that produce CH4 are called methanogensand are anaerobic. As the bacteria consumes plant and animal remains in the sediment,they excrete mainly methane, with minor by-products including carbon dioxide, hydro-gen sulphide, ethane and propane. The bacteria are killed by even traces of oxygen. Theexact mechanisms of methane production are part of the complex bacterial activity withinthe decay of organic matter in marine sediments at the ocean floor, and are under activeresearch.

Elsewhere, the oil/gas industry has known about hydrates for many years, mainly asa pipeline nuisance and safety hazard (Katz and Lee, 1990; Makogon, 1997; McCain,1990; Kromah et al., 2003). NGH occur in high pressure pipelines at pressures >50bar and temperatures <15◦C, which is common for gas transport pipelines, particularlyin colder climates, or subsea where the temperature often can be around 4◦C, i.e., atconditions to the left of the phase equilibrium curve (Figure 3). In pipelines, NGH canform (gas plus liquid water [water dropout in the line] under appropriate conditions),and block them, particularly at spots where turbulence occurs, e.g., valves. Millions of

Figure 2. Map of the world showing potential land and offshore hydrate sediments. (http://www.eduplace.com/ss/maps/pdf/world_cont.pdf Copyright © Houghton Mifflin Company. Reprinted bypermission of Houghton Mifflin Company. All rights reserved.)


dollars are spent annually in the prevention of NGH by ensuring that the gas is fullydried, or adding large volumes of antifreeze, often methanol (McCain, 1990). Drillinghazards when drilling into NGH, or where bottom hole drilling conditions are such thatNGH are formed.

On the other hand, there is also possible use of hydrates for gas transportation(Kromah et al., 2003; Taylor, 2003), and pilot plant facilities have been tested. Hydratesmay also be an alternative route to desalinate water (Singh et al., 2007).

4. Hydrate Detection

Seismic-acoustic imaging is the major method that has been used to identify NGH sub-surface (Collet et al., 2000; Collett, 2002). The base of the gas hydrate layer can beobserved in seismic reflection profiles where echoes from the base hydrate zone (BHZ),known as bottom stimulating reflectors (BSR), show up very strongly. This is because gashydrate cementation significantly increases the sound velocity within the sediments, andthe drop in velocity below the BSR generates a strong seismic reflection. BSR reflectionshave been observed widely in continental margin sediments when there is sufficient gasto form NGH. However, deeper reflections are difficult to identify because of the unfa-vorable refection coefficients due to the slower seismic velocities below the BSR (Miles,2000; Peecher and Holbrook, 2000), which can cloud interpretation.

Drilling can confirm the presence of NGH. Sometimes core samples of sedimentare also taken (Goldberg et al., 2000; Ohara and Dallimore, 2000; Peltzer and Brewer,2000; Stern et al., 2000). Unfortunately, the sampling and transporting of NGH fromnatural habitat is difficult and the material often decomposes spontaneously and rapidlyevolving methane gas and leaving water and the sand–sediment (Goldberg et al., 2000).This is because ambient conditions of the laboratory (temperature rise and pressuredrop) take the P/T conditions to the right of the phase diagram (Figure 3) (Tomutsaet al., 2002). Core samples are therefore difficult to study. Some information con-cerning the physical chemistry of the pure hydrates is reasonably well known fromlaboratory prepared samples (Makogon, 1997; Peltzer and Brewer, 2000; Stern et al.,2000), but natural NGH may not be totally similar. The required in situ NGH propertieswill eventually be obtained by new downhole measurement techniques (Goldberg et al.,2000).

Figure 3. The thermodynamic diagram of hydrate, pressure with temperature; of particular noteis the hydrate phase boundary where hydrate is stable/unstable (McCain, 1990; Sloan, 1997).


5. The Potential of Natural Gas Hydrates—Quantities

Quantitative estimations of the volume of methane held within NGH are difficult andsubject to great uncertainty, ranging over 3 orders of magnitude, or perhaps more. This isbecause NGH can be widely scattered in small concentrations within marine sediments, orin more concentrated consolidated form, making volumetric estimations of gas content pervolume of sediment speculative. This uncertainty is magnified when making worldwideestimates of hydrate gas resource so they are very speculative, with a very large range,with perhaps as much as 1017 sm3 (some estimates have been even higher, althoughrecent reviews are greatly downgrading the estimates), but it is probably lower but morethan 1015 sm3 (1 sm3 = 35 scf) (Katz and Lee, 1990; Englezos, 1993; Makogon, 1997;Collett, 2000; Kvenvolden, 2000; Milkov and Sassen, 2002; Milkov, 2004).

Compared to the world’s currently known conventional gas reserves, which are ap-proximately 155 trillion sm3 (0.155×1015 sm3) (BP Statistics, 2004), there may be 6–600times more gas in NGH. This huge potential is the reason for the excitement concerningNGH. It could create energy security for many nations for many decades.

The majority (perhaps >95%) of these hydrate reserves are inferred by seismicreflection. They have been found in the marine sediments off-shore such as the deepwater (900–3000 m) Gulf of Mexico, the west coast from California to Washington, theeast coast, including the Blake Ridge off-shore the Carolinas, the North Sea, Japan, Indiaand the Artic Oceans (Figure 3) (Max, 2000). There is possibly more elsewhere but havenot been explored for, as yet.

After a NGH resource has been identified, the knowledge of quantity and placementis clearly essential before planning economic, feasible, and safe extraction processes canbe contemplated, but knowledge is needed on how stable the sediment is, particularly ifthe sea-floor is sloping.

6. The Thermodynamics of NGH

The thermodynamic equilibrium curves of NGH (PT diagram, Figure 3) indicate that thecrystalline solid is stable at temperatures between +34 to −5◦C and pressures of 100-1bars (1500-15 psia) (Katz and Lee, 1990; McCain, 1990; Englezos, 1993; Makogon, 1997;Sloan, 1997; Collett, 2000; Kromah et al., 2003; Taylor et al., 2003; Pooladi-Darvish,2004). NGH are stable at temperature conditions above those where solid ice normallyexists. Figure 4 shows the same phase diagram and boundary conditions for methane

Figure 4. Hydrate stability with depth. The hydrate phase boundary defines the stability of hydrate.


hydrate stability but with depth (pressure = water depth ∗ water density ∗ accelerationdue to gravity, and water density and acceleration due to gravity are constants). We shalluse Figure 4 as the basis for our discussion of the occurrence and stability of NGH inthe sub-sea sediment and permafrost.

The hydrate phase boundary defines the stability of hydrate. From Figure 4, hydratecan only exist when the P/T conditions are to the left of the hydrate phase boundary. Thehydrate stability zone (HSZ) is shown in Figure 5. The top is governed by the depth ofthe sea (pressure) and its temperature. Thus, hydrate cannot exist at shallow sea depths.The base of the HSZ depends on the temperature, which increases with depth createdby the geothermal gradient by some 2–4◦C per 100 m (with an average of 3◦C) insidethe sediment. It means that at 1000 m depth below the sea-bed, the temperature is some20–40◦C higher than at the sea-bed. The pressure in the oceans exerted by the waterwithin the marine sediment (the hydrostatic gradient) creates approximately 1 bar extrapressure for every 10 m, so will stabilize the hydrate until the geothermal gradient actsagainst hydrate stability (Figure 4) (Dillon and Max, 2000). The base of the hydratezone (BHZ) is the temperature and pressure at which the conditions again cross the PTstability line, circled areas in Figure 5.

Therefore, the BHZ is a phase boundary of hydrate formation—no hydrate will formbelow this point; its exact position is subject to the local controls of temperature andpressure, but usually not at depths greater than 1500 m.

Vast quantities of sediment impregnated with hydrate are therefore in the earth inthe HSZ, a relatively narrow zone (Katz and Lee, 1990; Makogon, 1997; Max, 2000),both in the oceans and onland in the permafrost regions. The HSZ is some 500–1000 m

Figure 5. Conditions for NGH stability in sediments. Hydrate can exist when the P/T conditionsare to the left of the hydrate phase boundary. The hydrate phase boundary defines the stability ofhydrate. The geothermal gradient defines the temperature. The hydrostatic gradient determines thepressure. Where they cross (circles) is the hydrate stability zone, HSZ. The lower crossover is thebase of hydrate zone, BHZ. Hydrate within the sediment is trapped. Hydrate will form in the seawhen the P/T conditions are to the left of the phase diagram, but as hydrate is lighter than water,it will rise and dissociate when the conditions cross the upper boundary. Hydrate cannot exist atshallow sea depths nor at depths below usually 1500 m.


thick and the hydrate is most stable at the top (coldest part) of the HSZ. The depth levelsfor finding NGH are 150–500 m below sea-bed, depending on the temperature (smallerdepths for the artic regions) down to about 1000 m near the equator, e.g., near Trinidad(although in some areas perhaps even to depths of 2500 m). In permafrost terrain, theyhave been found near the surface because of the cold, down to depths where the thermalgradient creates temperature conditions where hydrate is no longer stable.

Hydrate within the sediment is trapped and does not move once formed. Hydratewill also form in the sea when the P/T conditions are to the left of the phase diagram, butas hydrate is lighter than water it will not be trapped but will rise and dissociate whenit crosses the boundary of the sea temperature and pressure conditions evolving gas asshown in Figure 5. It maybe the reason for the strange behavior within the Bermudatriangle, where ships have “disappeared” possibly due to the sea becoming a fluidizedbed due to the evolving gas, and not supporting the weight of the ship (Corfield, 2002).

7. Production of Natural Gas from Gas Hydrates

The formation of hydrate is exothermic, and conversely gas release from NGH is en-dothermic, so that heat is needed to decompose NGH back into gas and water. A singleunit of hydrate when heated and depressurized can release around 160 times its volumein gas if it fully saturated, i.e., 1 m3 of hydrate can contain 160 m3 of gas measuredunder standard conditions, which is a considerable concentration of gas (Makogon, 1997;Max, 2000; Taylor et al., 2003).

The major gas extraction methods being considered are decomposition of NGH in situby thermal stimulation, depressurization or inhibitor injection (Figure 6) (Makogon, 1997;Max, 2000; Moridis, 2002; Sawyer et al., 2000; Pooladi-Darvish, 2004).

7.1. Thermal Stimulation

Thermal stimulation uses a source of energy (e.g., steam, hot brine) to raise the localreservoir temperature to outside the hydrate region (Figure 3). The hydrate will decom-pose by breaking the host hydrogen bonds of the water, thus releasing the gas. Computermodels suggest that gas can be produced from NGH at sufficient rates to make gas hy-drates a technically recoverable resource (Ceyhan and Parlaktuna, 2001; Sawyer et al.,2000; Swinkels and Drenth, 2000).

Figure 6. The major methods of extraction of methane from hydrate sediments.


7.2. Inhibitor Injection

Inhibitor injection moves the hydrate equilibrium conditions to outside the conditionsfor hydrate formation through injection of some inhibitor (antifreeze) in the liquid phaseadjacent to the hydrate. The antifreeze (methanol or perhaps hot brine) modifies the phaseequilibrium diagram (Figure 3) and lowers the dissociation temperature to outside thehydrate forming area, in addition to providing heat for dissociation (Sung et al., 2002).The process is analogous to putting salt on icy pavements. This is the method commonlyused in inhibitor prevention in pipelines by the gas industry (McCain, 1990).

7.3. Depressurization

Depressurization involves reducing the hydrate pressure to below the equilibrium valueto initiate decomposition. As the reservoir pressure drops below the equilibrium value,the hydrate dissociates (an endothermic process) absorbing energy from the surroundingsand decreasing the local reservoir temperature (Moridis, 2002; Sung and Kang, 2003;Pooladi-Darvish, 2004). Hydrate will dissociate until sufficient gas has evolved to achievethe new equilibrium pressure at the lower temperature. Heat flows by conduction to thedissociating hydrate interface due to the thermal gradients. It is believed that often freenatural gas underlie many sediments containing NGH, in which case it might be possibleto drill down to below the NGH into the free gas to release it.

7.4. Other Schemes

Other schemes postulated to get the energy into the formation required to dissociate NGHinclude heat injection using horizontal and multilateral wells; fire flooding; injectionof non-hydrating gases, such as air at suitable conditions of temperature and pressure;downhole electromagnetic heating; and even burial of nuclear waste. One idea could beto put an impermeable sheet over the sea-bed area but ensuring that it is fully attachedto guarantee that no gas escapes from the sediments, and then drill suitable boreholes toallow production. However, much testing would have to be carried out before confidentapplication.

The fact that NGH may not be in a concentrated form in the sediment and theproblems of ecological dangers and safety as discussed below, means that none of themethods have yet been tried commercially.

8. Ecological Dangers

Though NGH represent a tantalizing energy source, there are significant environmentaland safety issues that must be considered before exploitation is developed. Any exploita-tion of NGH must not create instability of the sediment as methane is twenty times morepotent in affecting greenhouse effects than carbon dioxide.

A fear is that the NGH in hydrate–bearing sediment, if melted, could be converted toa gassy water-rich slurry, and if the gas evolves it might trigger seafloor subsidence andcatastrophic landslides (Max, 2000). Such destabilization conditions could be createdeither by natural processes or human interventions if the phase line in Figures 3–5is crossed. NGH will only remain stable within the range of temperature and pressureindicated. Reducing the pressure by less than a few psi (10 kPa) or raising the temperatureby a couple of degrees will cause NGH to decompose spontaneously. Such decompositioncould release vast quantities of methane in an uncontrolled manner (Corfield, 2002).


There is geological evidence that in the past massive naturally occurring releases ofmethane contributed to abrupt changes in the earth’s climate. For instance, greenhousewarming could raise the temperature of the atmosphere, which in turn could warm thesea and hence the seafloor. If the sea depth changes, the temperature and pressure condi-tions change so that the position on the NGH phase diagram (Figures 3–5) moves to theunstable side of the line. Massive quantities of methane could be released, further desta-bilizing the sediment, perhaps creating landslides or create huge potholes in the oceanfloor, leading to hugely destructive sediment undersea spills, as well as towering tsunamiwaves (tidal waves), which would probably wreak havoc on coastlines. Methane releasedas a result could then enter the atmosphere so magnifying the greenhouse warming. Withsuch global warming, the polar icecaps would melt, resulting in sea level rise and muchcoastal flooding.

Evidence implicating gas hydrates in triggering seafloor landslides have been foundalong the Atlantic Ocean margin of the United States (Collett et al., 2003). The mech-anism controlling slope stability of the ocean floor, seafloor subsidence and landslidesare not well known. A repeat of this occurrence now could be a worldwide catastrophe.Alternatively global warming could cause massive evaporation of the sea so that the sea-levels fall, as well as creating worldwide desertification (Collett et al., 2003). Massivemethane release from sea-bed NGH is speculated as one of a sequence of causes forthe abrupt climatic events on the geological record. For example, the Cretaceous-Tertiarychange (∼65 million years ago), the Permian–Triassic boundary (∼254 years ago), andthe Palaeocene–Eocene boundary (55 million years ago) (Corfield, 2002).

In summary, the dangers of NGH instability are:

• change in atmospheric composition,• change in the thermal regime of the earth,• global warming, causing desertification and/or coastal flooding,• alteration in the dynamics of ocean levels and Arctic shores,• sea-floor instabilities and landslides perhaps resulting in tsunami waves.

This instability could be triggered by a methane release caused by an inept extractionprocedure.

9. Safety Problems in Drilling and Production

NGH are a safety problem during drilling and production particularly in artic or deep-seaconditions. NGH can be formed when deep-sea drilling (>300 m), which can lead todangerous conditions (Englezos, 1993; Max, 2000; Ebeltoff et al., 2001; Petersen, et al.,2001), where hydrate forming conditions occur, or in subsea flowlines when gas andliquid water are present. Deep sea exploration is becoming more common so the dangersare becoming more apparent. For instance, when drilling through hydrate impregnatedsediment, the warmth of circulating drilling fluids within the drillpipe and heat fromsetting cement can move the conditions to the right of the phase equilibrium so that gasmay then be released.

Gas influx into the wellbores can create gas-kicks or even blowouts, or the gasescapes to surface. Alternatively the NGH can melt in the formation creating slush thatwould not be strong enough to hold the well tubing in place. Such drilling could triggertubing collapse or sea floor instability and possible seafloor slope failures. Clearly onemust drill through NGH when drilling in the deep sea (>300 m) but the dangers are notyet sufficiently understood to ensure that this drilling is always fully safe. Additionally,


the gas could further nucleate hydrates within the well tubing which then move aroundthe drill string to a place where they can block, creating dangerous situations, or betransported to surface where it decomposes at the blowout preventor and/or valves andcreate very dangerous gas well control situations.

If there is gas ingress into the wellbore, hydrates can form and create gas kicks orother dangerous situations. This could be especially so when using water-based drillingfluids. Maybe the solution will be to control the well temperature so that the conditionsdo not create hydrates, at least inside the drill string.

10. Discussion

The development of safe and environmentally friendly exploration and extraction tech-niques of gas from NGH to commercial gain will be costly. Clearly governments ratherthan commercial enterprises will drive the NGH research, and this will probably remainso until economic returns can be seen to be close. Then private companies will invest.In 2001, the bill Methane Hydrate Research and Development Act was signed into lawby President Clinton (106th US Congress, 2000). It states that a US program of methanehydrate research should be carried out to identify, explore, assess, and develop methanehydrate as a source of energy as well as to mitigate environmental impacts of hydratedegassing both natural ad that associated with commercial development, and to developtechnologies to minimize risks of drilling through methane hydrate. Some £11/12 USmillion per annum for years 2003–2005 have been budgeted. This research is on-going(Collet, 2004).

Most commentators predict that commercial applications will only happen later(2030+) rather than sooner, as there are probably sufficient supplies of gas and oil tosatisfy needs until 2030 (Max, 2000). For the United States, if only 1% of the methanehydrate source believed to be in US waters could be made technically and economicallyrecoverable, NGH might be able to supply their energy needs for over 200 years at thecurrent rate of use. There is urgency for Japan to have its own energy supply for its se-curity, which could mean that Japan may develop commercial techniques earlier, perhapseven by 2015 (Max, 2000; Ohara and Dallimore, 2000).

10.1. Mineral Rights

It must be remembered that the law currently states that whoever owns the sea-floorhas the mineral and exploration rights. This could mean energy security for countrieswho have territorial waters at the continental margins and sea depths >500 m, whichincludes Trinidad. But who has the mineral and exploitation rights and owns the gasis an uncertain and intriguing question, needing to be finalized by the United Nations.Discussions have been dragging on for years.

Conclusions

• NGH could become a large source of natural gas, particularly methane.• Currently, the volume of resource base is uncertain but is believed to be greater

(possibly much greater) than conventional gas reserves. The quantity of gas thatcan be recovered is totally speculative.

• Seismic methods are being improved to enhance estimation of quantities and cer-tainty of position of resources.


• Because of the instability of NGH at normal laboratory conditions, obtaining infor-mation and data on NGH for research for commercial exploitation is problematicaland riddled with uncertainty.

• Exploitation with production of methane from NGH, is technically feasible, butnot yet safely. Also, no process has yet been reported in the open literature ashaving been tested commercially.

• Gas hydrates have the potential for creating hazards that could create catastropheson a global scale, particularly as methane is some twenty times more potentthan carbon dioxide as far as greenhouse effects are concerned. NGH is a safetyproblem during drilling and production in artic and deep subsea conditions. Ifthere is gas ingress into the wellbore, hydrates can form and create gas kicks orother dangerous situations.

• The role NGH will play in contributing to the worlds’ energy needs depends onthe ability and the cost of extracting the methane from the sub-sea sediment.Should gas from NGH come on stream, energy security for those counties wouldbe assured, at least, for some decades. Japan maybe the first country to developNGH exploitation because their need is greatest. If/when they do, the whole energypolitical scene would rapidly change.

• A fuller understanding of exploration and extraction techniques of the NGH ac-cumulations and the effect of hydrate extraction mismanagement on regional eco-logical situations and global changes must be developed.

Acknowledgments

We thank the UWI Research and Publication Committee and CDB for support, and J.-A.Babwah for help with an early draft of this paper.

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a large potential methane source--natural gas hydrates

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