42 Oilfield Review

Growing Interest in Gas Hydrates

Timothy S. CollettUnited States Geological SurveyDenver, Colorado, USA

Rick LewisGreenwood Village, Colorado

Takashi UchidaJapan Petroleum Exploration Company, Ltd.Chiba, Japan

For help in preparation of this article, thanks to GerardDaccord, Clamart, France; William Dillon, US GeologicalSurvey, Woods Hole, Massachusetts, USA; YoshiKawamura, Nagaoka, Japan; and Robert Kleinberg,Ridgefield, Connecticut, USA.AIT (Array Induction Imager Tool), DSI (Dipole Shear Sonic Imager), FMI (Fullbore Formation MicroImager) and Platform Express are marks of Schlumberger.

To some people in the energy industry, gas hydrates are known for the trouble

they cause by blocking pipelines and production tubing. But for others, hydrates

are a potential hydrocarbon resource, far surpassing the potential of conventional

natural gas resources. For both groups, learning more about hydrates is essential.

Summer 2000 43

Hydrocarbons are special combinations of thecommon elements hydrogen and carbon. Forthousands of years, these naturally occurringcompounds have been harvested, first from rareseeps—for use as adhesives in road and buildingconstruction, waterproofing for ships and bas-kets, for weaponry, paint, mosaic artwork, fireworship, medicinal purposes, and cooking andlighting fuel—and then from wells. During thelast two centuries, oil and gas production hasreached a global scale and now powers most ofthe world’s activities.

Someday, perhaps in the 21st century, theEarth’s store of conventional hydrocarbons willno longer supply adequate energy to its growingeconomies and population. By then, the unfamiliarbut kindred hydrocarbons called hydrates may beready to take their place as significant sources ofenergy. Hydrates, too, are a special combinationof two common substances, water and naturalgas. If these meet under conditions in whichpressure is high and temperature low, they join toform a solid, ice-like substance. Vast volumes ofsediments in the ocean bottoms and polarregions are conducive to hydrate formation.

This article describes how and wherehydrates exist, how they might be evaluated asresources, and other questions and challengesassociated with their exploitation.

Compacted EnergyThe basic hydrate unit is a hollow crystal ofwater molecules with a single gas moleculefloating inside. The crystals fit together in a tightlatticework. Hydrates, also known as gashydrates, methane hydrates, or clathrates, (fromthe Greek and Latin words for “cagework”) havea structure similar to ice, except the gasmolecules are located within crystals instead ofbetween them. Hydrates also look like ice—thefew times they have been seen. But they don’tact like ice: when lit with a match they burn.

Chemists have known about hydrates foralmost 200 years, but until recently, these sub-stances were treated as laboratory curiosities.1

The petroleum industry began to take an interestin hydrates in the 1930s when gas-hydrate formation was found to be the cause of trouble-some pipeline blockage in Kazakhstan.2 Sincethen, most industry efforts related to hydrateshave been directed toward avoiding them or hindering their accumulation (see “HydrateHazards,” page 55 ).

In the 1960s, a Russian drilling crew discov-ered hydrates occurring naturally in a Siberiangas field. Then, in the 1970s, scientists on deep-sea drilling expeditions discovered that hydratesoccur naturally not only in polar continentalregions but also in deepwater sediments at outercontinental margins. 1. Sloan ED Jr: Clathrate Hydrates of Natural Gas, 2nd ed.

New York, New York, USA: Marcel Dekker, Inc., 1998.2. Bagirov E and Lerche I: “Hydrates Represent Gas

Source, Drilling Hazard,” Oil & Gas Journal 95, no. 48(December 1, 1997): 99-101, 104.

Many studies show that the gas that goesinto naturally occurring hydrates is generatedwhen anaerobic bacteria break down organicmatter under the seafloor, producing methaneand other gaseous by-products including carbondioxide, hydrogen sulfide, ethane and propane.All of these can be incorporated as the guestmolecules in hydrates, but methane predomi-nates.3 Some evidence exists to support the argu-ment that in a limited number of settings,methane in hydrates also comes from thermo-genic sources deeper within the earth.4

The compact nature of the hydrate structuremakes for highly effective packing of methane. Acubic volume of hydrate contains gas that willexpand to somewhere between 150 and 180 cubicvolumes at standard pressure and temperature.

Most marine hydrates seem to be confined toedges of continents where water is about 1500 ft[about 500 m] deep and where nutrient-richwaters unload organic detritus for bacteria to con-vert to methane (above). Gas hydrates have beenfound at the seafloor, but their usual range is 325to 1600 ft [100 to 500 m] beneath it. In permafrost

regions, they can occur at shallower depthsbecause surface temperatures are lower. Largeaccumulations have been identified offshoreJapan, in the Blake Ridge off the US easternseaboard, on the Cascade continental margin offVancouver, British Columbia, Canada, and offshoreNew Zealand.5 Only a small proportion of the evi-dence for hydrate accumulations around the worldcomes from direct sampling, however; most isinferred from other sources, such as seismicreflections, well logs, drilling data and pore-water-salinity measurements.

44 Oilfield Review

Hydrate location> Known and inferred occurrences of gas hydrates.

3. Krajick K: “The Crystal Fuel,” Natural History 106, no. 4(May 1997): 26-31.

4. Collett TS: “Natural Gas Hydrates of the Prudhoe Bayand Kuparuk River Area, North Slope, Alaska,” AAPGBulletin 77, no. 5 (May 1993): 793-812.MacDonald IR, Guinasso N, Sassen R, Brooks JM, Lee L and Scott KT: “Gas Hydrate That Breaches theSeafloor on the Continental Slope of the Gulf of Mexico,”Geology 22, no. 8 (August 1994): 4539-4555.

Suess E, Torres ME, Bohrmann G, Collier RW, Greinert J,Linke P, Rehder G, Trehu A, Wallman K, Winckler G andZuleger E: “Gas Hydrate Destabilization: EnhancedDewatering, Benthic Material Turnover and LargeMethane Plumes at the Cascadia Convergent Margin,”Earth and Planetary Science Letters 170, no. 1-2 (June 1999): 1-15.

5. Collett TS and Kuuskraa VA: “Hydrates Contain VastStore of World Gas Resources,” Oil & Gas Journal 96, no. 19 (May 11, 1998): 90-95.

6. Krajick, reference 3.7. Kvenvolden K: “Gas Hydrates—Geological Perspective

and Global Change,” Reviews of Geophysics 31, no. 2(May 1993): 173-187.

8. Makogan YF: Hydrates of Hydrocarbons. Tulsa,Oklahoma, USA: PennWell Books, 1997.

Summer 2000 45

Laboratory experiments show how the stabilityof the solid methane-hydrate phase depends onpressure and temperature (left). Theoretically,these stability requirements are met over a highpercentage of continental slope seafloor. Theearliest encounters with naturally occurringoceanic gas hydrates corroborated these stabilityconditions. Deep-sea research programs drilledand cored hydrate-rich sediments and attemptedto retrieve samples for shipboard and laboratorystudy.6 When the first cores were broughtonboard, however, they depressurized and self-destructed. Few naturally occurring hydrateshave survived long enough to be studied.

Hydrates have been found, or suspected, insufficient quantities to allow estimates of theirtotal volume to be computed. There is a roughconsensus that about 20,000 trillion cubic meters[about 700,000 Tcf] of methane are locked up inhydrates.7 About 99% of these are in marine sed-iments offshore.8 The total is about two orders ofmagnitude greater than the amount of conven-tional recoverable methane, which is estimatedto be 250 trillion m3 [about 8800 Tcf]. Statedanother way, hydrates may hold 10 trillion tons ofcarbon, more than twice as much as the world’scoal, oil and conventional gas reserves combined(above). These vast estimates of this potential

> Pressure-temperature dependence of methane-hydrate stability. The methane-water combination is a solid at low temperatures andhigh pressures (shaded). Adding sodium chloride to the water shiftsthe pink curve to the left, while adding carbon dioxide, hydrogensulfide and other hydrocarbons moves the curve to the right.

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> Carbon content of the world’s known hydro-carbon resources, with gas hydrates accountingfor more than half. (This figure excludes dispersedorganic carbon such as kerogen and bitumen.)(Adapted from Kvenvolden, reference 7.)

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hydrocarbon resource are leading several coun-tries to initiate research and exploration pro-grams to understand hydrate behavior, identifyaccumulations and develop recovery methods.Japan, India, USA, Canada, Norway and Russiaare among the countries with ongoing gas-hydrate investigations.

An Early Serendipitous Find—MessoyakhaThe only known example of gas productionattributed to hydrates occurs in the SiberianMessoyakha gas field (above). The Messoyakhafield, discovered in 1968, was the first producingfield in the northern West Siberian basin. By themid-1980s more than 60 other gas fields had

been discovered in the basin, together containingabout 777 Tcf [22 trillion m3], or one-third of theworld’s gas reserves. Before production, theMessoyakha field was estimated to contain2.8 Tcf [79 million m3] gas, one-third of which iscontained in hydrates that overlie the interval offree gas in the field (next page, top).

46 Oilfield Review

> The Messoyakha gas field, Russia, discovered in 1968. Much of the gas production is attributedto dissociation of methane hydrates.

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Production began in 1969 from the free-gaszone of the reservoir, and for a couple of yearspressures declined as expected (left). Then,higher-than-expected pressures and productionwere measured in 1971. These were attributed toproduction of gas originating in the hydrate layer:as the pressure in the free-gas layer decreased,the hydrate layer depressurized and liberated gasfrom the dissociating hydrates. About 36%, or183 Bcf [5 billion m3], of the gas produced fromMessoyakha has been ascribed to dissociation ofgas hydrates.9

This method of depressurization to producegas from hydrates works when there is free gasassociated with the hydrate accumulation.Therefore, it may also work for the Prudhoe Bay-Kuparuk River fields in Alaska, USA.

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> Gas hydrates overlying free gas in the Messoyakha field.

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> Messoyakha production history showing predicted (dashedblue) and measured (solid blue) reservoir pressures and gas volumes (black curves). Five periods of production have beenidentified: I. Production of free gas; II. Production of gas fromfree-gas zone and hydrate deposit; III. Production of gas fromhydrate alone; IV. Shut in; V. Small amount of gas production from hydrate. The produced volumes under the dashed blacklines are attributed to hydrate dissociation.

9. For a counterargument: Collett TS and Ginsberg GD:“Gas Hydrates in the Messoyakha Gas Field of the WestSiberian Basin—A Re-Examination of the GeologicEvidence,” International Journal of Offshore and PolarEngineering 8, no. 1 (March 1998): 22-29.

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> Hydrate occurrences and calculatedthickness of the hydrate-stability zone in the Prudhoe Bay region, North Slope ofAlaska. Contour intervals in the middle figure are in meters.

Prudhoe BayAfter the gas hydrates in the Russian Messoyakhafield, those in the Prudhoe Bay-Kuparuk Riverarea in Alaska are the next most studied hydrateaccumulations in the world. In 1972, gas hydrateswere recovered in pressurized core barrels at theARCO and Exxon 2 Northwest Eileen wildcat wellon the North Slope of Alaska (below).10 From tem-perature and pressure gradients in the region,the thickness of the zone of gas-hydrate stabilityfor the Prudhoe Bay-Kuparuk area can be calcu-lated. Potentially, hydrates are stable between210 and 950 m [690 and 3120 ft].

Examination of well-log data from 445 addi-tional wells in the North Slope revealed 50 wellswith hydrates in six laterally continuous sand-stones at the east end of the Kuparuk River production unit and the west end of the PrudhoeBay production unit. The analysis tied logs fromthese wells to logs from the wildcat well inwhich the hydrate core had been recovered.These and other studies indicate that it is difficultto distinguish hydrates on the basis of singlelogs; multiple types of logs seemed to work better.Acoustic logs record values similar to thosefound in ice. Resistivity logs measure high values

indicative of hydrocarbon. The increase in back-ground gas measured by the drilling-mud log wasthe best indicator of hydrates, but its responsewas not very different from a log through a free-gas zone (next page, top).

These early discoveries of hydrates associatedwith conventional hydrocarbon accumulationsshowed how borehole logging tools could identifyhydrate zones in arctic settings. Drilling holes tolook for hydrates in the marine environment iscostly. However, another tool is available—the seismic section.

Finding Hydrates at SeaTo quantify the likely volume of hydrates andtheir potential as a resource, it is important tounderstand both the distribution of hydrates insediments and the mechanical properties ofhydrate-bearing formations. Retrieved samplesshow individual particles of hydrate dissemi-nated in the sedimentary section, but hydratesalso occur as intergranular cement, nodules, lam-inae, veins and massive layers (next page, bottom).In both marine and terrestrial permafrost deposits,hydrate-bearing sections typically vary in thickness

48 Oilfield Review

Summer 2000 49

from a few centimeters to 30 m [1 in. to 100 ft].Once, a 3- to 4-m [10-to 13-ft] thick layer of solidhydrate was sampled.

Hydrate that acts as cement will stiffen thesediment matrix. It may also occur in the porespace without significantly affecting sedimentrigidity. When gas hydrates form in the intersti-tial pore spaces of consolidating sediment, solidhydrate rather than liquid water occupies thepore spaces, and the diagenetic processes ofconsolidation and mineral cementation aregreatly inhibited.

The velocity of sound in pure hydrate isbelieved to be similar to that of ice, but the exactvalue has not been agreed upon, and probablydepends on hydrate chemistry. Acoustic velocityin a hydrate-cemented layer is also high—higherthan in fluid-filled sediment. As a result, the con-tact between a hydrate-rich layer and a gas-filledlayer can act as a prominent seismic reflector.These reflectors, which occur at the base of thehydrate zone, are known as bottom-simulatingreflectors (BSR). Their shape tracks the shape ofthe seabottom, and the polarity of their seismicpulse is reversed. Depth of the BSR below seabot-tom depends on the temperatures and pressures

required for hydrate stability. Offshore, BSR havebeen mapped at depths below the seafloor rang-ing from 100 to 500 m [330 to 1640 ft].11

The occurrence of a BSR in seismic reflectiondata is the most important indicator of hydratesin marine sediments. However, hydrates canexist without creating a BSR if there is no under-lying free gas or if the hydrate does not appre-ciably stiffen the sediment matrix. Researchers inCanada have reported success in characterizingmarine hydrate zones using electrical remotesounding in areas where no BSR is visible.12

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> Logs through the ARCO and Exxon 2 Northwest Eileen well. Sonic velocity (Track 2)increases through the hydrate zone, as it would through an ice layer. Resistivity (Track 1)increases due to methane. The mud log (Track 4) shows an increase in background gas,similar to the response seen while drilling through a free-gas zone.

Disseminated cement Nodules Veins Massive layers

> How hydrate is distributed in sediments. A formation can contain (left toright) hydrate as disseminated cement, nodules, veins and massive layers.

10. Collett, reference 4.Collett TS: “Well Log Characterization of SedimentPorosities in Gas-Hydrate-Bearing Reservoir,” paper SPE 49298, presented at the SPE Annual TechnicalConference and Exhibition, New Orleans, Louisiana,USA, September 27-30, 1998.

11. Collett T: “Methane Hydrate: An Unlimited EnergyResource?” Proceedings of the International Symposiumon Methane Hydrates Resources in the Near Future?,Chiba City, Japan, October 20-22, 1998: 1-12.

12. Yuan J and Edwards RN: “The Assessment of MarineGas Hydrates Through Electrical Remote Sounding:Hydrate Without a BSR?” Geophysical Research Letters 27, no.16 (August 15, 2000): 2397-2400.

Bottom-simulating reflectors have beenobserved in many parts of the world. One of thebetter studied regions is the Blake Ridge, off-shore North Carolina, USA (left).13 Here, scien-tists from the United States Geological Survey(USGS) have conducted two-dimensional (2D)marine surface seismic surveys and boreholeseismic surveys in conjunction with researchdrilling for experiments on the chemical and iso-topic properties of hydrates.14

While standard marine multichannel seismicsurveys detect the high contrast in acousticimpedance between a gas-hydrate layer and anunderlying layer of free gas, BSR are not as evidenton higher frequency surveys, which sample atgreater vertical resolution.15 A high-resolutionsurvey off the coast of Vancouver, BritishColumbia, Canada, recorded signals in the 250- to 650-Hz range, but recorded only weakreflections in an area where the BSR is strong onlower frequency, multichannel data. This indi-cates that the velocity contrast at the interfacebetween the hydrate and free-gas zones is grada-tional and occurs over a few meters. Velocitiesinferred from the high-resolution survey are con-sistent with those obtained from multichannelsurveys and from Ocean Drilling Program (ODP)logging (left).

The contrast in seismic compressional andshear velocities at the BSR may also give rise toan amplitude variation with offset (AVO) signa-ture that can help determine whether the hydrateis acting as a cement or is filling pores withoutcementing sediment grains.16 Workers at ChevronPetroleum Technology Company, La Habra, and atStanford University, both in California, USA, haveanalyzed the AVO response at the BSR along aportion of the Blake Outer Ridge offshore Floridaand Georgia, USA, and have concluded that thehydrate in that setting is noncementing.17

In addition to marine seismic surveys, whichcover large areas, pore-water-salinity measure-ments can detect the existence, or recent pres-ence, of hydrates near the seafloor. Hydratescontain only pure water and exclude salts andother compounds that may be present in sea-water. Immediately following hydrate-moleculecrystallization, the surrounding seawaterbecomes enriched in the expelled components,such as chloride. Salinity measurements taken atthis time show high chloride concentrations.Soon after crystallization, mixing causes thechloride anomaly to disappear. Conversely, disso-ciation of hydrate freshens water by reducingpore-water salinity. Detection of fresh water duringdrilling can indicate dissociation of hydrate andtherefore its recent presence.

50 Oilfield Review

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> Reflection-strength seismic section across the Blake Ridge, US Atlantic coast, showing abottom-simulating reflector (BSR) tracking the base of the gas-hydrate layer. The BSR is elevatedin the center of the profile, where a salt diapir has intruded. High-strength reflections pointto free gas trapped beneath the hydrate layer. [From Taylor MH, Dillon WP and Pecher IA:"Trapping and Migration of Methane Associated with the Gas Hydrate Stability Zone at theBlake Ridge Diapir: New Insights from Seismic Data," Marine Geology 164 (2000): 79-89, courtesyof William P. Dillon.]

> Acoustic velocities at the contact between a hydrate-bearinglayer and an underlying free-gas zone offshore Vancouver, BritishColumbia, Canada. Velocities from an Ocean Drilling Program (ODP)sonic log, multichannel seismic (MCS) surveys and high-frequencyseismic (DTAGS, for deep-towed acoustic/geophysics system)soundings show consistency. The higher-frequency results imply agradational contact (solid black curve). (Adapted from Gettrust et al,reference 15.)

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Exploring for Hydrates TodayA gas-hydrate reservoir in the Mackenzie Delta,Northwest Territories, Canada, has been investi-gated through a project comprising membersfrom the Geological Survey of Canada (GSC),Japan National Oil Corporation (JNOC), JapanPetroleum Exploration Company (JAPEX), USGS,US Department of Energy, and several companies,

including Schlumberger.18 Drilled in 1998 nearthe site of an earlier Imperial Oil Ltd. well that hadencountered hydrates, the new research well,Mallik 2L-38, was designed to evaluate in-situ prop-erties of hydrates and assess the ability of wirelinelogging tools to characterize them (above).

Like other wells in the Arctic, the Mallik 2L-38well was drilled and cased through the permafrost

interval, which reached a depth of 640 m [2100 ft].Before casing was set, the permafrost zone waslogged with several Schlumberger tools. Theseincluded the AIT Array Induction Imager Tool, DSIDipole Shear Sonic Imager and Platform Expresstool strings. Below the permafrost, the well wasdrilled and cored to 1150 m [3770 ft]. This subper-mafrost section was logged with the same tools,

Research well, Mallik 2L-38, designedto evaluate in-situ properties and assessthe ability of wireline logging tools tocharacterize hydrates.

13. http://abacus.er.usgs.gov/hydrates/index.html14. http://obs.er.usgs.gov/BlakeRidge95.html15. Gettrust J, Wood W, Lindwall D, Chapman R, Walia R,

Hannay D, Spence G, Louden K, MacDonald R andHyndman RD: “New Seismic Study of Deep Sea GasHydrates Results in Greatly Improved Resolution,” EOS Transactions of the American Geophysical Union 80,no. 38 (September 21, 1999): 439-440.

16. The variation of reflection amplitude with offset betweenseismic source and receiver is indicative of the contrastin Poisson’s ratio across the reflector.

17. Ecker C, Dvorkin J and Nur A: “Sediments with GasHydrates: Internal Structure from Seismic AVO,”Geophysics 63, no. 5 (September-October 1998): 1659-1669.

18. Collett TS, Lewis RE, Dallimore SR, Lee MW, Mroz THand Uchida T: “Detailed Evaluation of Gas HydrateReservoir Properties Using JAPEX/JNOC/GCS Mallik 2L-38 Gas Hydrate Research Well Downhole Well-LogDisplays,” in Dallimore SR, Uchida T and Collett TS (eds):Scientific Results from JAPEX/JNOC/GSC Mallik 2l-38Gas Hydrate Research Well, Mackenzie Delta,Northwest Territories, Canada. Ottawa, Ontario, Canada:Geological Survey of Canada, Bulletin 544 (1999): 295-311.

as well as with the FMI Fullbore FormationMicroImager tool (above).

Log and hole quality were excellent in thehydrate-bearing section, which extended from897.5 to 1109.5 m [2945 to 3640 ft]. A mud chillerwas employed that greatly increased hydrate stability. The downhole electrical resistivity andacoustic velocity logs confirmed the presence ofgas hydrate in an interval more than 200 m [656 ft]

thick. Deep electrical resistivity values range from10 ohm-m to 100 ohm-m. Compressional-wavespeeds (Vp) range from 2.5 to 3.6 km/sec, andshear velocities (Vs) range from 1.1 to 2.0 km/sec.The low value of compressional to shear velocityratio, Vp / Vs, at the base of the hydrate zone indi-cates a thin layer of free gas. A small amount offree gas in other parts of the wellbore can be

interpreted from the slight density-neutroncrossover, but this may be from hydrates that weredisturbed by the drilling process. Measurementson the core agree well with log-derived values ofpore-water resistivity, porosity, and bulk and graindensity (next page). Wellbore images and coreindicate that the reservoir is high-quality sand-stone with hydrates filling the pores. Porosity

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Summer 2000 53

ranges from 20% to 40%. Hydrates are not foundin the neighboring shale layers.

Gas-hydrate saturations were computedbased on the “standard” Archie equation.19

Saturations were also estimated from the acous-tic log data, but the acoustic-derived saturationswere lower than those measured from the recov-ered cores. The resistivity-based calculations

indicate the presence of some zones withhydrate saturations that exceeded 90%. The vol-ume of hydrates inferred from log and core datais equivalent to 3 to 4 x 109 m3 of gas in the 1-km2 [0.36-sq. mile] area surrounding the well.

The experience gained in the Mallik researchwell helped clarify the characteristics of natural-

gas hydrates and encouraged JAPEX and theassociated groups to undertake the next hydrate-drilling project in the Nankai trough, offshore

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> Comparison of logs with core measurements in Mallik 2L-38. Pore-water resistivities measured in cores correlate well withlog-derived values in Track 2. Core porosities in Track 3 match density and neutron porosities. Bulk densities are shown inTrack 4 and grain densities in Track 5.

19. Collett TS: “Well Log Evaluation of Gas Hydrate Satura-tions,” Transactions of the 39th SPWLA Annual LoggingSymposium, Keystone, Colorado, USA, May 26-29, 1998,paper MM.

Japan. About a dozen areas have been identifiedby BSR as potential hydrate reservoirs (below).

As a first step, an exploratory well was drilledin late 1999 and early 2000 in 945 m [3100 ft] ofwater.20 Two pilot holes and a main hole weredrilled to 1600 m [5250 ft] and 3300 m [10,830 ft],respectively. Cores were acquired as well asmeasurements including chlorine anomalies; logging-while-drilling (LWD) density, neutron,dual induction and bit resistivity; and openholewireline dipole shear and compressional veloci-ties, laterolog and nuclear magnetic resonance(NMR).21 Maximum gas-hydrate saturation wasestimated at about 80% of total porosity in thereservoir sandstones.

New Ways to Monitor Hydrate CreationIn the laboratory, gas hydrates have been manu-factured from gas and water, but only with diffi-culty.22 Hydrates form slowly in pressure vessels,even at temperatures and pressures well withinthe thermodynamic phase boundaries. The pro-cess is also self-limiting: as pressure rises andtemperature drops, a layer of solid hydrate formsat the gas-water interface. Left undisturbed, thislayer effectively stops further hydrate production.The hydrate barrier can be broken by vigorousagitation, and many investigators have resortedto installing a grinding apparatus within theirpressure vessels to hasten crystallization. Eventhen, it can take several days to fill a small pres-sure vessel with hydrate.

Early in 1996, a group led by Peter Brewer ofthe Monterey Bay Aquarium Research Institute(MBARI), California, devised a new way to study hydrate formation. These researchers rec-ognized that the seafloor provides not only appro-priate temperature and pressure conditions forhydrate manufacturing, but also a setting inwhich the dynamics of natural hydrate formationcan be replicated.23

Clear plastic tubes loaded with seawater ormixtures of sediment and seawater were trans-ported to the ocean bottom by a small remotelyoperated submarine. At the appropriate depth,methane from a tank was allowed to bubble upfrom the bottom of each cylinder. Worried thatthe reaction might not occur within the three tofour hours available, the investigators were sur-prised when a translucent mass of hydrateformed within minutes.

The research submarines used in these inves-tigations were instrumented with thermometers,pressure gauges, electrical conductivity sensorsand navigation instruments. The prime instru-ment used to observe hydrate formation was thevideo camera.24 The camera provided astonishinggraphic images, but no quantitative data. Futureexperiments are being designed to help under-stand the spatial and textural distribution ofhydrates in sediments.

54 Oilfield Review

> Regions with bottom-simulating seismic reflections offshore Japan.

20. Uchida T, Hailong L, Tomaru H, Dallimore S, Matsumoto R,Oda H, Delwiche M and Okada S: “Japan’s Efforts toExplore Marine Gas Hydrates off Tokai at the NankaiTrough and Their Occurrences: Geological Overview,”Supplement to EOS, Transactions, American GeophysicalUnion 81, no. 22 (May 30, 2000): WP59.

21. Kazuhiko T, Uchida T and Akihisa K: “Well Log Evaluationof Gas Hydrate Saturation in the MITI Nankai TroughWell Drilled Offshore Tokai, Japan, Supplement to EOS,Transactions, American Geophysical Union 81, no. 22(May 30, 2000): WP60.

22. Sloan, reference 1. 23. Brewer PG, Orr FM Jr, Friedrich G, Kvenvolden KA,

Orange DL, McFarlane J and Kirkwood W: “Deep OceanField Test of Methane Hydrate Formation from a RemotelyOperated Vehicle,” Geology 25, no. 5 (May 1997): 407-410.

24. To download a video of the experiment:http://www.mbari.org/~brpe/gas_hydrates.html

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Gas hydrates are a concern to oil and gas com-panies wherever water and natural gas contacteach other. Hydrates present constraints to oiland gas flow, cause drilling and subsea comple-tion hazards and induce risks to offshore plat-form stability.

In the 1930s, gas hydrates were identified asresponsible for blocking surface pipelines inthe former Soviet Union. When crude oil or gasis piped through pressurized pipelines in coldclimates, there may be enough water andmethane in the mix to form solid hydrates,which can plug the pipeline.

Dislodging a hydrate plug can be dangerous. A depressurizing hydrate plug can travel at ballistic speeds, injuring workers and causingpipelines to rupture. One way to avoid pipelineblockage is to heat the pipeline, but excludingthe water before compressing the hydrocarbonsmay be a more cost-effective treatment. A combi-nation of techniques may work to prevent hydrateformation in pipelines: remove water to lowerthe dewpoint, keep the temperature above thehydrate-formation point, keep pressure belowthat point, and use inhibitors to prevent themixture from solidifying.

For operators drilling in deep water, encoun-tering naturally occurring solid gas hydratesduring drilling can pose a well-control problem,if large amounts enter the borehole and depres-surize. Furthermore, circulation of warm fluidwithin the wellbore can reduce the temperaturein the surrounding hydrate-rich sediments, lead-ing to hydrate melting and destabilization of the

sediments holding up the well. Heat releasedduring cement setting may also destabilize thehydrate-bearing formation. Special cement sys-tems designed to minimize released heat canhelp prevent hydrate dissociation.

Developing solid hydrates in the wellbore as a result of fluid mixing is another significantwell-control problem in deepwater situations. If gas enters the wellbore, the high hydrostaticpressure and low seafloor temperature cancause hydrates to form in water-base fluids aswell as in the brine phase of oil-base and syn-thetic muds.1 Repercussions of hydrate forma-tion are plugging of choke lines, kill lines andblowout preventers (BOP), difficulties in moni-toring well pressure, restriction of drillstringmovement and deterioration in mud propertiesdue to dehydration.2 Choke and kill lines andBOP stacks are particularly susceptible sincethey are located where temperature is lowest,and they cool quickly when circulation stops.

One practical way to stop hydrate formation is by adding salts, glycol or other chemicalinhibitors to reduce the amount of free water.Maintaining high wellhead temperature and circulating drilling fluid may help for waterdepths down to 1000 ft [305 m], but at greaterdepths, it is harder to transmit sufficient heatby circulating mud. Preheating fluids also maybe useful, as can keeping mud weight as low assafely possible, since lower pressures reducehydrate stability.

Finding the right solution for treating hydratehazards starts with being able to foresee the

risk of encountering them. Several organizationshave developed computer programs for predict-ing hydrate formation—under environmentalconditions input by the user—for some idealizedmud chemistries, and for determining appropriatelevels and types of inhibitor additives.3 However,the diversity of composition of real drillingmuds limits the applicability of these programs.

Subsea operations also are affected byhydrates.4 Hydrate formation during subseacompletion and intervention can be avoided byinjecting methanol into the injection lines atthe subsea tree. Subsea flowlines are also subjectto blockage by hydrates, especially when longtieback distances between subsea tree andplatform subject fluids to low temperaturesand high hydrostatic pressures for extendedperiods of time.

Certain oil and gas operating areas areprone to naturally occurring hydrate problems.The Caspian Sea has been characterized as auniquely hazardous basin, with a huge amountof sediment and a high rate of sedimentation.5

Large overpressures lead to mud diapirism andvolcanism. Mud diapirs are buoyant masses ofunconsolidated rock, water and gas that riseand penetrate sedimentary formations. Mud volcanoes vent mud, water and gas from thediapirs. During a mud-volcano eruption, tens of millions of cubic meters of methane can beejected. The low water temperatures and highpressures are conducive to hydrate formation.Direct gravity coring of three Caspian mud volcanoes has sampled many gas hydrates.

Hydrate Hazards

1. Brandt W, Dang AS, Magne E, Crowley D, Houston K,Rennie A, Hodder M, Stringer R, Juiniti R, Ohara S andRushton S: “Deepening the Search for Offshore Hydro-carbons,” Oilfield Review 10, no. 1 (Spring 1998): 2-21.

2. Barker JW and Gomez RD: “Formation of HydratesDuring Deepwater Drilling Operations,” paper SPE 16130,presented at the SPE/IADC Drilling Conference, NewOrleans, Louisiana, USA, March 15-18, 1987.

4. For more on subsea activities: Christie A, Kishino A,Cromb J, Hensley R, Kent E, McBeath C, Stewart H, Vidal A and Koot L: “Subsea Solutions,” Oilfield Review 11, no. 4 (Winter 1999/2000): 2-19.

5. Bagirov and Lerche, main text, reference 2.

3. Tohidi B, Danesh A, Burgass RW and Todd AC: “Effect of Heavy Hydrate Formers on the Hydrate Free Zone ofReal Reservoir Fluids,” paper SPE 35568, presented at the SPE European Production Operations Conference and Exhibition, Stavanger, Norway, April 16-17, 1996.

Hydrate ChallengesInterest in hydrates is growing, and several of the technologies proven effective for conven-tional hydrocarbon exploration and formationevaluation are being applied to the hydrate-characterization problem. However, enormouschallenges remain. Specialists disagree over theamount of hydrates present in the accessible portions of the subsurface.

Many in the industry believe the widely citedestimates of methane in gas hydrates to be over-stated.25 Also, even if the estimates prove true, ifthe hydrate is sparsely distributed in the sedi-ment rather than concentrated, it may not berecoverable easily, economically, or without dangerto the environment.

Exploitation—Gas hydrates are grouped withother unconventional hydrocarbon resources—coal-bed methane, tight sands and black shales.With the exception of hydrates, some part of the

total world volume of these unconventionalsources is being produced commercially today. Inmost cases, evolution from a nonproducibleunconventional gas resource to a producible onehas relied on significant capital investment andtechnology development.26

The gas industry has been slow to developmethodologies to recover methane from hydrates.Three principal methods are under consideration:depressurization, thermal injection and inhibitorinjection (above). In depressurization, the pressureof the gas hydrate is decreased sufficiently tocause dissociation. This method is feasible onlywhen associated free gas can be produced todecrease hydrate reservoir pressure, as has beenreported in the Messoyakha field.

In the absence of a free-gas zone beneath thehydrates, thermal injection, or stimulation, may bea viable solution. Heat is added to the gas hydrate-bearing strata to increase the temperature enough

to cause the hydrate to dissociate. An example ofthis is injection of relatively warm seawater intoan offshore gas-hydrate layer.27

Injection of inhibitors such as methanol shiftsthe pressure-temperature equilibrium so that thehydrates are no longer stable at their normal con-ditions and methane is released.

Of the three methods, dissociation by warm-water injection may be most practical. However,gas hydrates will become a potential resourceonly when it can be shown that the energy recov-ered is significantly greater than the energyrequired to release methane gas.

Seafloor stability—Dissociation of hydratescan cause instability in seafloor sediments on thecontinental slopes. The base of the gas-hydratezone may represent a discontinuity in the strengthof the sediment column. The presence of hydratesmay inhibit normal sediment consolidation andcompaction, and free gas trapped below the

56 Oilfield Review

> Three main methods under consideration for hydrate exploitation: depressurization, thermal injection and inhibitor injection.

Gas out

Impermeable rock

Impermeable rock

Steam orhot water

Hydrate Dissociatedhydrate

Thermal Injection

Gas out Methanol

Dissociatedhydrate

Impermeable rock

Impermeable rock

Hydrate

Inhibitor InjectionDepressurization

Gas out

Hydrate cap

Dissociatedhydrate zone

Free-gasreservoir

25. Haq BU: “Gas Hydrates: Greenhouse Nightmare? Energy Panacea or Pipe Dream?” GSA Today 8, no. 11(November 1998): 1-6.Hovland M and Lysne D: “Is the Fear and Promise ofGas Hydrates in Deep Water Overstated?” Proceedings,International Conference on Oceanology, vol. 3.Brighton, England (1998): 263-271.

26. Collett, reference 11.27. Okuda Y: “Introduction to Exploration Research on Gas

Hydrates in Japan,” Bulletin of the Geological Survey ofJapan 49, no. 10 (1998): 494-500.

28. Kvenvolden KA: “Potential Effects of Gas Hydrate onHuman Welfare,” Proceedings of the National Academyof Sciences 96 (March 1999): 3420-3426.

29. Licking E: “The World’s Next Power Surge,” BusinessWeek (December 14, 1998): 79-80.

30. Kvenvolden, reference 7.31. Haq, reference 25.

32. Gudmundsson JS, Andersson V and Levik OI: “HydrateConcept for Capturing Associated Gas,” paper SPE 50598,presented at the SPE European Petroleum Conference,The Hague, The Netherlands, October 20-22, 1998.

33. Gudmundsson JS, Andersson V and Levik OI: “GasStorage and Transport Using Hydrates,” Proceedings ofthe Offshore Mediterranean Conference [OMC 97] vol. 2.Ravenna, Italy (March 19-21, 1997): 1075-1083.

Summer 2000 57

hydrate zone may be overpressured. Any techniqueproposed for hydrate exploitation would have tosucceed without causing additional instability.

An example of the problems that arise whenhydrates dissociate can be found off the USAtlantic margin. There, the seafloor slope isabout 5° and as such should be stable. However,many submarine landslide scars have beenobserved. The depth of the scars is near the shal-low limit of the hydrate-stability zone. The BSRare weaker in the areas that have experiencedlandslides, indicating perhaps that hydrates arenot present currently and may have escaped.Scientists theorize that if pressure on thehydrates decreased, as would happen with a fallin sea level during a glacial period, then hydratescould dissociate at depth and cause the gas-saturated sediments to slide (right).28

Such zones have been detected near thecoast of South Carolina, USA. In the region of ahuge submarine landslide 40 miles [66 km] wide,a seismic section indicates a massive hydrateformation on either side of the landslide, but nohydrates directly below the slide.

Offshore platforms and pipelines are alsosubject to hydrate-related marine landslides. Oiland gas exploration and production companiesthat operate in deepwater areas are interested infinding ways to detect areas of the seafloor proneto instability, to avoid placing structures onunsteady ground.29

Greenhouse effect—Worldwide, hydratescontain methane in amounts several orders ofmagnitude greater than are currently found in theatmosphere. Methane increases the greenhouseeffect about 20 times more aggressively than anequivalent weight of carbon dioxide [CO2].30

Climate scientists suggest that dissociation ofhydrates during a glacial period of lower sealevel would release methane into the atmo-sphere and warm the earth, possibly acting tostabilize the climate.31 For example, during thelast glaciation, a 120-m [390-ft] sea-level drop isestimated to have taken place. This would haveraised the base of the hydrate-stability zone by 20 m [66 ft], destabilizing sediments, causing

slumping and release of methane into the atmo-sphere—in turn causing greenhouse warming.The warming would have melted the glaciers,terminating the Pleistocene glacial period.

On the other hand, methane released frompermafrost layers in the Arctic during a period ofglobal warming could further warm the atmo-sphere, exacerbating climatic warming anddestabilizing the climate. Scientists are investi-gating which geological processes could mostaffect the stability of hydrates in sediments andso control the possible release of methane intothe atmosphere.

Converting to HydratesWhether or not naturally occurring hydratesbecome the world’s next fuel source, other usesmay be found for the knowledge gained abouthydrate formation. Researchers at theNorwegian University of Science and Technology(NTNU) in Trondheim are investigating the possi-bility of storing and transporting natural gas in itshydrate state at atmospheric pressure.32

Experiments at NTNU show that once formed,gas hydrate at atmospheric pressure will not dis-sociate if it is kept at or below –15°C [5°F].Potential applications of this technology abound: • Associated gas produced in oil fields could be

converted to solid gas hydrate and transportedin shuttle tankers, or mixed with refrigeratedcrude oil and transported as slurry in shuttletankers or pipelines.

• Like liquified natural gas (LNG), frozen hydratecan be transported long distances whenpipelines are not available.

• When storage of gas is required, natural gascan be converted to hydrates and stored refrig-erated at atmospheric pressure.

• Nitrogen, carbon dioxide and hydrogen sulfidecan be separated from methane throughhydrate formation.

• Salts and biological materials can be separatedfrom water by the hydrate-formation process.

• Carbon dioxide can be removed from the atmo-sphere and stored in solid hydrate form to betransported into deep water for disposal.33

With more countries restricting flaring andsome producers unwilling to build pipelines, con-verting gas to hydrate form may afford conve-nient disposal and transport alternatives.

Much of the uncertainty in the viability of gashydrates as a resource stems from a lack ofunderstanding of the nature of hydrate accumu-lations. The similar physics and chemistry of thenatural and manufactured processes are encour-aging hydrate proponents and opponents to contribute to one another’s understanding.Opportunities exist for researchers and fieldspecialists to focus additional expertise on theproblem and continue advancing our state ofknowledge of gas hydrates. —LS

Gasplume

Debrisflow

Originalslope surface

Hydrated zone

Large block of hydratedsediment breaking off andsliding down slope

Dissociatedgas hydrate

Lower boundary of hydrateat high sea level

Lower boundary of hydrateat low sea level

> Dissociation of hydrates responsible for a submarine landslide. A decreasein pressure on the hydrate zone would allow hydrates at depth to dissociate,and cause the unconsolidated sediments above them to slide. [Adaptedfrom Kvenvolden KA: "Potential Effects of Gas Hydrate on Human Welfare,"Proceedings of the National Academy of Sciences 96 (March 1999): 3420-3426.]