Gas Hydrates Final

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GAS HYDRATES – a future fuel 2011

1. INTRODUCTION

Hydrates of natural gas were discovered by Sir Humphrey davy in 1810 but remain somewhat of

scientific curiosity until hammerschmidt reported in 1934 that they could form in natural gas

pipelines leading to blockages and reduced or zero gas flow. Hydrate is a crystalline solid

consisting of gas molecules, usually methane, each surrounded by a cage of water molecules

Each volume of hydrate contains up to 160 volumes of methane (natural gas).

For decades, gas hydrates have been discussed as a potential resource, particularly for countries

with limited access to conventional hydrocarbons or a strategic interest in establishing

alternative, unconventional gas reserves. Methane has never been produced from gas hydrates at

a commercial scale and, barring major changes in the economics of natural gas supply and

demand, commercial production at a large scale is considered unlikely to commence within the

next 15 years. Given the overall uncertainty still associated with gas hydrates as a potential

resource, they have not been included in the EPPA model in MITEI’s Future of Natural Gas

report. Still, gas hydrates remain a potentially large methane resource and must necessarily be

included in any consideration of the natural gas supply beyond two decades from now.

Gas hydrates are crystalline substances composed of water and gas, in which a solid water-lattice

accommodates gas molecules in a cage-like structure, or clathrate. Gas hydrates are widespread

in permafrost regions and beneath the sea in sediment of outer continental margins. While

methane, propane, and other gases can be included in the clathrate structure, methane hydrates

appear to be the most common in nature (Kvenvolden, 1988). The amount of methane

sequestered in gas hydrates is probably enormous, but estimates of the amounts are speculative

and range over three orders-ofmagnitude, from about 100,000 to 270,000,000 trillion cubic feet

(modified from Kvenvolden, 1993). The estimated amount of gas in the hydrate reservoirs of the

world greatly exceeds the volume of known conventional gas reserves. The production history of

the Russian Messoyakha gas hydrate field demonstrates that gas hydrates are an immediate

source of natural gas that can be produced by conventional methods (Makogon, 1981; Collett,

1993b).

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2. CHEMICAL COMPOSITION:

In an effort to determine whether a significant connection exists between the

process of hydrate formation and gas fractionation, gas samples were obtained from two

known hydrate deposits, Eel River Basin and Hydrate Ridge. The composition of the gas

was analyzed using an HP 5890 Gas Chromatograph, for C1-C6 hydrocarbons, N2 and

O2 combination peak, and CO2. The samples collected from the Eel River site consisted

of ~98% methane with trace amounts of ethane, propane, N2 + O2, and CO2. Whereas,

the Hydrate Ridge site samples contained 88-98% methane and detectable amounts of

C1-C4 hydrocarbons, i-pentane, N2, O2, and CO2. The N2 + O2 peak, in the Hydrate

Ridge samples, was more concentrated then expected. The concentrated N2 + O2 peak

and the presence of i-pentane suggests that there is a possibility of a connection between

the process of hydrate formation and gas composition.

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3. GAS HYDRATES RESOURCES

There are several challenges to commercially exploiting gas hydrates. How much and where gas

hydrate occurs in commercially viable concentrations are not well known, and how the resource

can be extracted safely and economically is a current research focus. Estimates of global gas

hydrate resources, which range from at least 100,000 TCF to possibly much more, may greatly

overestimate how much gas can be extracted economically. Reports of vast gas hydrate resources

can be misleading unless those estimates are qualified by the use of such terms as in-place

resources, technically recoverable resources, and proved reserves:

• The term in-place is used to describe an estimate of gas hydrate resources without regard for

technical or economical recoverability. Generally these are the largest estimates.

• Undiscovered technically recoverable resources are producible using current technology, but

this does not take into account economic viability.

• Proved reserves are estimated quantities that can be recovered under existing economic and

operating conditions.

For example, the U.S. Department of Energy’s Energy Information Agency (EIA) estimates that

total undiscovered technically recoverable conventional natural gas resources in the United

Statesare approximately 1,300 TCF, but proved reserves are only 200 TCF.4 This is an important

distinction because there are no proved reserves for gas hydrates at this time. Gas hydrates have

no confirmed past or current commercial production. Until recently, the Department of the

Interior’s U.S. Geological Survey (USGS) and Minerals Management Service (MMS) have

reported only in-place estimates of U.S. gas hydrate resources. However, a November 12, 2008,

USGS estimate of undiscovered technically recoverable gas hydrates in northern Alaska

probably represents the most robust effort to identify gas hydrates that may be commercially

viable sources of energy.5 Despite a lack of a production history, the USGS report cites a

growing body of evidence indicating that some gas hydrate resources, such as those in northern

Alaska, might be produced with existing technology despite only limited field testing.

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Gas Hydrates on the North Slope, Alaska

The USGS assessment indicates that the North Slope of Alaska may host about 85 TCF of

undiscovered technically recoverable gas hydrate resources (Figure 1). According to the report,

technically recoverable gas hydrate resources could range from a low of 25 TCF to as much as

158 TCF on the North Slope. Total U.S. consumption of natural gas in 2007 was slightly more

than 23 TCF. Of the mean estimate of 85 TCF of technically recoverable gas hydrates on the

North Slope, 56% is located on federally managed lands, 39% on lands and offshore waters

managed by the state of Alaska, and the remainder on Native lands.6 The total area comprised by

the USGS assessment is 55,894 square miles, and extends from the National Petroleum Reserve

in the west to the Arctic National Wildlife Refuge (ANWR) in the east (Figure 1). The area

extends north from the Brooks Range to the state-federal offshore boundary three miles north of

the Alaska coastline.

Gas hydrates might also be found outside the assessment area; the USGS reports that the gas

hydrate stability zone—where favorable conditions of temperature and pressure coexist for gas

hydrate formation—extends beyond the study boundaries into federal waters beyond the

threemile boundary (Figure 1).

Figure 1. Gas Hydrate Assessment Area, North Slope, Alaska

Source: USGS Fact Sheet 2008-3073, Assessment of Gas Hydrate Resources on the North

Slope, Alaska, 2008, at http://pubs.usgs.gov/fs/2008/3073/.

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Note: TPS refers to total petroleum system, which refers to geologic elements that control

petroleum generation, migration, and entrapment.

Gas Hydrates in the Gulf of Mexico

On February 1, 2008, the MMS released an assessment of gas hydrate resources for the Gulf of

Mexico.7 The report gives a statistical probability of the volume of undiscovered in-place gas

hydrate resources, with a mean estimate of over 21,000 TCF. The MMS report estimates how

much gas hydrate may occur in sandstone and shale reservoirs, using a combination of data and

modeling, but does not indicate how much is recoverable with current technology. The report

notes that porous and permeable sandstone reservoirs have the greatest potential for actually

producing gas from hydrates, and gives a mean estimate of over 6,700 TCF of sandstone-hosted

gas hydrates, about 30% of the total mean estimate for the Gulf of Mexico.8 Even for sandstone

reservoirs, however, the in-place estimates for gas hydrates in the Gulf of Mexico likely far

exceed what may be commercially recoverable with current technology. The MMS is planning

similar in-place gas hydrate assessments for other portions of the U.S. Outer Continental Shelf

(OCS), including Alaska.

Gas Hydrates Along Continental Margins

Globally, the amount of gas hydrate to be found offshore along continental margins probably

exceeds the amount found onshore in permafrost regions by two orders of magnitude, according

to one estimate.9 With the exception of the assessments discussed above, none of the global gas

hydrate estimates is well defined, and all are speculative to some extent.10 One way to depict the

potential size and producibility of global gas hydrate resources is by using a resource pyramid

(Figure 2).11 The apex of the pyramid shows the smallest but most promising gas

hydratereservoir—arctic and marine sandstones—which may host tens to hundreds of TCF. The

bottom of the pyramid shows the largest but most technically challenging reservoir—marine

shales. Sandstones are considered superior reservoirs because they have much higher

permeability—they allow more gas to flow—than shales, which can be nearly impermeable. The

marine shale gas hydrate reservoir may host hundreds of thousands of TCF, but most or all of

that resource may never be economically recoverable. It is likely that continued research and

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development efforts in the United States and other countries will focus on producing gas

hydrates from arctic and marine sandstone reservoirs.

Figure 2. Gas Hydrate Reservoir Pyramid

Source: Roy Boswell and Timothy S. Collett, “The Gas Hydrate Resource Pyramid,” Fire in the

Ice, Methane Hydrate R&D Program Newsletter, Fall 2006.

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4. Gas Hydrate Hazards

Gas hydrates are a significant hazard for drilling and production operations.12 Gas hydrate

production is hazardous in itself, as well as for conventional oil and gas activities that place wells

and pipelines into permafrost or marine sediments. For activities in permafrost, two general

categories of problems have been identified: (1) uncontrolled gas releases during drilling; and (2)

damage to well casing during and after installation of a well. Similar problems could occur

during offshore drilling into gas hydrate-bearing marine sediments. Offshore drilling operations

that disturb gas hydrate-bearing sediments could fracture or disrupt the bottom sediments and

compromise the wellbore, pipelines, rig supports, and other equipment involved in oil and gas

production from the seafloor.13 Problems may differ somewhat between onshore and offshore

operations, but they stem from the same characteristic of gas hydrates: decreases in pressure

and/or increases in temperature can cause the gas hydrate to dissociate and rapidly release large

amounts of gas into the well bore during a drilling operation.

Oil and gas wells drilled through permafrost or offshore to reach conventional oil and gas

deposits may encounter gas hydrates, which companies generally try to avoid because of a lack

of detailed understanding of the mechanical and thermal properties of gas hydrate-bearing

sediments.14 However, to mitigate the potential hazard in these instances, the wells are cased—

typically using a steel pipe that lines the wall of the borehole—to separate and protect the well

from the gas hydrates in the shallower zones as drilling continues deeper. Unless precautions are

taken, continued drilling may heat up the sediments surrounding the wellbore, causing gas from

the dissociated hydrates to leak and bubble up around the casing. Once oil production begins, hot

fluids flowing through the well could also warm hydrate-bearing sediments and cause

dissociation. The released gas may pool and build up pressure against the well casing, possibly

causing damage.15 Some observers suggest that exploiting the gas hydrate resources by

intentionally heating or by depressurization poses the same risks—requiring mitigation—as

drilling through gas hydrates to reach deeper conventional oil and gas deposits.

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5. Gas Hydrate Research and Development

The Mallik Project (Goho)

• Is located in the McKenzie River Delta in Canada’s Northwest Territories

• This research began in the fall of 2001 with a large team of international scientists.

• This research demonstrated that natural gas could be obtained from gas hydrates.

The Nankai Trough

• Off of Japan’s Eastern Coast (Goho)

• Drilling began in 2004 with decent success in finding that hydrates were abundant.

(O’Driscoll)

• The results showed that 60 to 70 percent of the sediments’ pores were filled with

hydrates. (Goho)

• Starting in 2007, the Japanese team plans to begin production tests to assess the economic

potential of the trough. If all goes well, the site could be producing methane by 2016.

(Goho)

Alaska’s North Slope (Bradner)

• BP Exploration is working with the U.S. Department of Energy (DOE) on a joint research

program to test the North Slope for confirmation of the presence of hydrates around

current oil fields.

• The drilling would be stratigraphic, in order to gather geologic information about future

drilling.

• No decision about the actual drilling has been made yet.

• One of the problems related to this area is that removal of the hydrates may cause the

surface to sink and pose risks for nearby oil and gas facilities.

• A federal energy bill was passed in August 2004 which contained two provisions that

encouraged hydrate development.

A goal of the DOE methane hydrate research and development (R&D) program is to develop

knowledge and technology to allow commercial production of methane from gas hydrates by

2015.17 Since the Methane Hydrate Research and Development Act of 2000 (P.L. 106-193) was

enacted, DOE has spent $102.3 million through FY2009, or approximately 67% of the $152.5

million authorized by law. The Omnibus Appropriations Act, 2009 (P.L. 111-8), provided $20

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million in FY2009 for natural gas technologies R&D, which included $15 million for gas

hydrates R&D. The Obama Administration requested $25 million for the natural gas

technologies program in FY2010, or half of the $50 million authorized for methane hydrates

R&D by the Energy Policy Act of 2005 (P.L. 109-58). Congress appropriated $17.8 million for

natural gas technologies in FY2010, giving DOE direction to fund research into unconventional

gas production from basins containing tight gas sands, shale gas, and coal bed methane, as well

as for gas hydrates.18 The gas hydrate R&D program is authorized through FY2010 under

current law.

The DOE program completed a Gulf of Mexico offshore expedition in May 2009 and an Alaska

production test in the summer of 2009. The Gulf of Mexico program was aimed at validating

techniques for locating and assessing commercially viable gas hydrate deposits.19 In Alaska, a

two-year production test is expected to provide critical information about methane flow rates and

sediment stability during gas hydrate dissociation.20 Results from the two-year test in Alaska

may be crucial to companies interested in producing gas hydrates commercially. Both projects

have international and industry partners.

Researchers identify a need to better understand how geology in the permafrost regions and on

continental margins controls the occurrence and formation of methane hydrates.21 They

underscore the need to understand fundamental aspects—porosity, permeability, reservoir

temperatures—of the geologic framework that hosts the gas hydrate resource to improve

assessment and exploration, to mitigate the hazard, and to enhance gas recovery. Together with

advances in R&D, economic viability will depend on the relative cost of conventional fuels, as

well as other factors such as pipelines and other infrastructure needed to deliver gas hydrate

methane to market. Additionally, price volatility will likely affect the level of private sector

investment in commercial production of gas hydrates.

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6. Utilization of Gas hydrates

Hydrates decompose releasing hydrocarbons as a gas when removed from low temp/high

pressure environment

High costs of long pipelines across unstable continental slopes

Pipelines in deep cold water become plugged with hydrates during transport

Damage to sensitive chemosynthetic communities

7. Advantages of Gas Hydrates

Denser source of hydrocarbons than conventional sources

Amount of conventional fossil fuels will decline in next century

Redirect/dispose of greenhouse methane away from the atmosphere

Cleaner fuel source than oil, coal, and oil shale

Abundant supplies in deep sea and permafrost

8. GAS HYDRATES – A FUTURE FUEL FOR INDIA

Gas-hydrates are crystalline materials (Fig. 3) of water and light hydrocarbons (mainly methane),

and are found in the permafrost and outer continental margins of the world where the methane

concentration exceeds their solubility limit (Sloan, 1990; Kvenvolden, 1998). These are formed

at high pressure (8-30 MPa) and low temperature (10 to 20°C) in shallow sediments, and are

stable up to a few hundred metres below the seafloor which defined as the gas-hydrates stability

zone (Fig. 4). One volume of gashydrates releases about 164 volume of methane and 0.8 volume

of fresh water at standard temperature and pressure (STP). Gas-hydrates have attracted the global

attention due to their widespread occurrences, potential as future major energy resource, role in

climate change and submarine geohazards. Dissociation of gas-hydrates by destabilization

releases methane (a greenhouse gas) to the atmosphere accentuating global warming and

reducing the sediment strength that producs the slope failure or seafloor instability.

Methane stored within and trapped as ‘free-gas’ below the hydrates-bearing sediments is huge.

The world-wide carbon in gas-hydrates is estimated to be 10,000 X1015 g, which is double the

carbon content in total fossil fuel (crude oil, natural gas and coal) reserves of the world

(Kvenvolden,1998). Among all renewable (solar, wind, wave, hydro, nuclear, hydrogen,

bioprocess, geothermal, etc.) and nonconventional (gas shale, tight gas shale, coal bed methane,

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basin centred gas, gas-hydrates, etc) energy resources, gashydrates are regarded as one of the

most suitable candidates for cleaner energy resources in this century.

Fig. 3: The schematic drawing of a type of gas-hydrates structure in which methane molecules

are caged in hydrogen-bonded water molecules (after Mahajan et al., 2006)

Fig. 4: Stability thickness map in the outer continental margin (after Kvenvolden and Barnard,

1982)

Fig. 5 shows the global locations where gas-hydrates have been established from geophysical,

geochemical and geological methods and deep sea/ocean drilling programme sampling. Many

countries have formed national programmes for the research and production of natural gas from

gas-hydrates. Drilling at the Blake Ridge (Paull et al.,1996) of the US Atlantic margin; Hydrate

Ridge at the offshore Oregon (Tréhu et al., 2004) and the Nankai Trough of the Japan margin

(Henriet & Mienert, 1998; Ginsburg & Soloviev, 1998) and more recently in the eastern offshore

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of India by the National Gas Hydrate Programme (NGHP)-Expedition 01 (Collett et al., 2008)

has recovered gas-hydrates samples. Samples of gas-hydrates have also been recovered from the

permafrost in the Messoyakha gas field of the western Siberia (Makogon, 1981), Prudhoe Bay oil

fields of Alaska (Collet et al., 1988) and the Mackenzie delta of Canada (Dallimore et al., 1999).

Other countries which are actively involved in gas-hydrates research are Norway, Germany, UK,

France, Belgium, Bulgeria, South Korea, China, Taiwan, Australia, Nigeria, Turkey, Egypt,

Italy, New Zealand, Greece, etc.

Gas-hydrates are mostly identified by mapping a botton simulating reflector (BSR) on seismic

section. The BSR is recognized based on its characteristic features such as mimicking the shape

of seafloor (Fig. 4), cutting across underlying dipping strata and exhibiting large amplitude

but opposite polarity event with respect to the seafloor reflections. Presence of gas-hydrates

reduces the permeability and hence trap ‘free-gas’ underneath. Thus the BSR is an interface

between the gas-hydrates-bearing sediments above and free-gas-saturated sediments below, and

is often associated with the base of gas-hydrate stability field.

India is the 4th largest oil consumer in the Asia-Pacific region and follows Japan, China and

South Korea. The energy requirement of India will be more than double in the next 20 years, The

country has low per capita (346 kg oil equivalent) energy consumption against the world average

per capita (1599 kg oil equivalent) value. To meet this requirement, import of 70% of oil per

year equivalent to Rs. 1,60,000 crores equivalent to 40 billion US $ at the rate of US $74 per

barrel. However, as the global price of oil has already reached US $147 per barrel the import bill

will be increased tremendously. To keep the pace of growth, we earnestly look for an alternate

form of energy, as the new discoveries of oil & gas fields has remained almost static.

Gas-hydrates are regarded as one of the most feasible sources for future energy, as bathymetry

(pressure) seafloor temperature, organic carbon content, sedimentary thickness, rate of

sedimentation etc. indicate favourable environment for the occurrences of gas-hydrates in both

offshore of India (Malik and Varadarajan, 2002). A total volume of 1894 TCM of gas has been

predicted from gas-hydrates reserves within the Indian exclusive economic zone EEZ (http://

www.dghindia.org), which is 1,900 times the country’s current gas reserve. Tapping only 1%

would meet the energy requirement for a decade or so. Thus, it was felt necessary to look into

available geological, geophysical, microbiological, geochemical data for the investigation of gas-

hydrates along the Indian margins.

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The Ministry of Petroleum & Natural Gas and the Ministry of Earth Sciences, Government of

India formed the Indian gas-hydrates programme and entrusted activities to various organizations

like Oil and Natural Gas Commission

Fig. 5: Global occurrences of gas-hydrates in the permafrost and outer continental margins

(after Kvenvolden and Lorenson, 2000)

(ONGC), Oil India Limited (OIL), Gas Authority of India Limited (GAIL), Directorate General

of Hydrocarbons, (DGH), Reliance and national institutes like National Geophysical Research

Institute (NGRI), National Institute of Oceanography (NIO) and others, to review the existing

datasets for the exploration of gas-hydrates followed by technology development for producing

gas from gashydrates. Using available seafloor temperature and geothermal gradient data, NGRI

has prepared the gashydrates stability thickness map of India (Fig. 4) that serves as a depth

window within which features of BSR can be looked into for detecting gas-hydrates. This map is

similar to the maps prepared by NIO and ONGC independently. Analysis of available seismic

data shows BSRs in the Saurashtra and Kerala-Konkan (K-K) Basins in the western offshore,

and the the Krishna-Godavari (K-G) K-G and Mahanadi Basins and the Andaman region in the

eastern offshore.

The drilling and coring using the “JOIDES Resolution” drill-ship by the NGHP – Expedition 01

(Collett et al., 2008) have confirmed the presence of gas-hydrates in the K-G and Mahanadi

offshore basins and the Andaman region. The main results are the discovery of the (i) richest

gas-hydrates deposit (~130 m thick with ~70% saturation in 64% porous fractured shale) in the

K-G basin and (ii) thickest (260 to 600 mbsf) and deepest BSR in the Andaman Sea. Sandstone

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and siltstone dominated gas-hydrates reservoir have also been established. It has become all the

more important now to demarcate the lateral/areal extent of gas-hydrates and free-gas-laden

sediments and assess their true potential.

Fig. 6: Locations (shown by black ovals) in the eastern offshore, superimposed on gas-hydrates

stability thickness map (after Hanumantha Rao et al., 1998) along the Indian margins from

which gas-hydrates samples have been recovered by recent drilling in 2006 (NGHP Expedition

01)

For this, expertise in processing, modelling and inversion of seismic data will be needed using

tools such as travel time tomography; waveform inversion; Amplitude variation with offset

(AVO) inversion; AVO attributes; rock physics modeling; prestack depth migration; seismic

attributes and attenuation etc. (Fig.6).

Gas-hydrates have been reported at many places in the world without BSR (Westbrook et al.,

1994; Wood and Ruppel, 2000; Ashi et al., 2002). They are also not found by sampling even if

BSR has been identified on seismic data in the Bering Sea. Therefore, other proxies are to be

looked into for the detection of gas-hydrates without BSR and/or to ascertain whether a BSR is

related to gas-hydrates. Sometimes double BSRs (Posewang and Mienert, 1996 a; b) are also

observed on the seismic section and cause enigma to the hydrates stability field. We discuss

some approaches for the identification of gas-hydrates in absence of BSR and to ascertain if a

BSR is related to gas-hydrates. As seismic velocity of gas-hydrates is high compared to the host

sediments, presence of gas-hydrates increases the velocity depending on saturation and nature of

distribution, and even a small amount of underlying freegas reduces the velocity considerably.

Thus estimating seismic velocities using AVO and/or waveform inversion followed by rock

physics and/or effective medium modelling has been an effective strategy for quantifying gas-

hydrates and free-gas. This study may require log data for calibration. Additional tools that can

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be used for the identification of gas-hydrates are geological, geochemical and microbiological

studies. The geological evidences include sediment properties, grain size, stratigraphic

relationships, gas-migration pathways, and most importantly the actual recovery and description

of gas-hydrates samples. Porefluid chemistry and gas compositions (molecular and isotopic) are

important aspects of gas-hydrates geochemistry. Higher values of total organic carbon content

help in identification of probable locales of gas-hydrates occurrences from the microbiological

study.

EXPLORATION STRATEGY

So far most of the multi-channel seismic MCS data were acquired for the exploration of

conventional hydrocarbons and are not adequate to understand the nature of distribution and

accurately determine the velocities of gas-hydrates and free-gas bearing sediments due to their

limited offset of recording. Major portions of the deep-water regions within the Indian EEZ are

yet to be explored. To fill the gap,concerned ministries of the Govt. of India have launched

research oriented gas-hydrates program with more comprehensive scientific study for exploration

and technology development for extracting gas from gas hydrates. Recovery of gas-hydrates

samples by the NGHP Expedition-01 (Collett et al., 2008) has boosted the Indian exploration

scientists and technologists to proceed with a concerted effort. Currently, they are in the process

of acquiring large-offset MCS and ocean bottom seismic (OBS) data from the prospective zones

in the eastern offshore. The travel time tomography of large-offset MCS data would be used in

delineating the lateral and areal extent of gashydrates/ ‘free-gas’ saturated sediments by imaging

the velocity ‘build-up’ and ‘drop’ in the velocity tomogram. Independent P- and S-wave seismic

velocities from the OBS data would provide the nature of distribution of gas-hydrates within the

pore spaces of sediments, and help in quantitative assessment of gas-hydrates. Pre-stack depth

migration of large-offset MCS and OBS data using the derived velocity tomogram would

provide an improved structural image of BSR and the base of free-gas saturated sediments. This

may delineate faults or fractures, acting as migration path for fluid flow to understand the genesis

of gas-hydrates. Though, a large thickness of gas-hydrates from the K-G offshore basin at one

site (10D) has seen sampled, gashydrates couldn’t be recovered by drilling from nearby sites.

This implies that the processes which govern the formation and migration of gas to form gas-

hydrates are yet to be established. An intensive research in that direction is going on throughout

the world. From drilling at one location in the K-K basin in the western margin, which couldn’t

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sample any gas-hydrates, we cannot rule out the possibility of gas-hydrates occurrences in the

entire western Indian margin. In fact, surface seismic, geological and other data sets indicate

presence of gas-hydrates in both margins of India (Malik and Varadarajan, 2002). The BSR

identified on the seismic data in the western margin has later been related to the geological

feature. To ascertain whether a BSR is related to gas-hydrates, we need to further analyse the

seismic data and look for other proxies or attributes. Identification of BSR as such would not

inform about gas-hydrates apart from indicating their mere existence.

There is a need therefore to extend the study to delineating the lateral/areal and vertical extent of

the gas-hydrates bearing sediments, and quantifying the amount from surface seismic data for

evaluating the resource potential. This will foster the exploitation technology for producing gas

from gas-hydrates. By employing various seismic tools (Fig.5), available at NGRI to the large-

offset and/or high-resolution seismic data, provide a quantitative assessment of gashydrates

along with demarcating the zone of gas-hydrates and free-gas -bearing sediments by seismic

tomography. Information on the global status of gas-hydrates research and development are

reviewed by Gupta (2003) and Sain (2004).

Recently, Ojha and Sain (2008a) have shown that the seismic attributes (Fig. 6) such as

reflection strength, blanking or reduction in seismic amplitudes and instantaneous frequency can

be used for ascertaining whether a BSR is related to the gas-hydrates. Blanking phenomena can

be used in quantifying gas-hydrates, provided we know the effect of homogenization of lithology

on amplitude reduction. Study of these attributes is also important in identifying gas-hydrates in

absence of BSR or when the BSR becomes a suspect in absence of crosscutting phenomena.

Presence of gas-hydrates stiffens the sediment matrix and thus allows the seismic waves to pass

through the hydratesladen sediments easily as compared to the host sediments i.e. sediments

without hydrates. Using the log spectral ratio method, Sain et al. (2008) have demonstrated

through a field example that the region with a hydrates-related BSR exhibits low attenuation or

high-quality factor compared to the region without the BSR. This implies that the study of

attenuation or quality factor can also be used as an indicator for identifying gas-hydrates without

a BSR or to ascertain whether a BSR is caused by gas-hydrates. For quantitative assessment of

gas-hydrates, Ojha and Sain (2007) have proposed a scheme of constrained AVO modelling of

BSR to derive both the P- and S-wave seismic velocities and hence the Poisson ratio. These are

related to the saturation of gas-hydrates. Based on two different approaches of

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(i) cooperative travel-time inversion and AVO modeling (Ojha and Sain, 2008b) and

(ii) AVO A-B crossplot (Ojha and Sain, 2008c),

followed by rock physics modeling, it is possible to estimate gas-hydrates and free-gas.

Applying tools based on different approaches to the same data set has been an effective strategy

for quantitative assessment of gas-hydrates with great certainty. Fig. 7 shows an example of

travel-time inversion followed by amplitude modelling, and provides an estimate of 12-14.5%

gas-hydrates underlain by 4.5-5.5% free-gas in the Makran accretionary prism of

Fig.7: (a) high reflectivity below the BSR is attributed to the gasrich and gas-poor sedimentary

strata and (b) Blanking above the BSR is caused due to cementation by gas-hydrates in the pore

spaces of sediments (after Ojha and Sain, 2008a) 248

Fig. 8: (a) Velocity-time model derived from travel time inversion, (b) The t–p transformed

seismic data in the Makran accretionary prism. Arrows mark the major reflectors around which

travel time inversion are performed. (c) The reflection coefficients of BSR plotted against the

incidence angles. Red and black open circles represent the directivity corrected and non

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corrected reflections coefficients. The black line is the best-fit model response (calculated using

the Zoeppritz equation) through the average reflection coefficients (blue stars) corresponding to

the directivity corrected reflection coefficients (after Ojha and Sain, 2008b)

the Arabian Sea. As the sophisticated full-waveform inversion is a powerful technique for

delineating the smallscale features and resolving tuning effects, the method has been employed

to determine the fine structure across a BSR at two locations in the Makran offshore (Sain et al.,

2000). The result reveals an unusually thick ‘free-gas’ zone (~200- 350 m) below ~160 m-thick

hydrates bearing sediments, only comparable to the sonic log results of ODP leg 164 at the Blake

Ridge (Holbrook et al., 1996). All these tools are being extensively used at NGRI for appraising

gas-hydrates in the K-G offshore and Andaman regions. The effective medium theory (EMT) is

capable of computing the complete stiffness tensor for a transversely isotropic composite with

several solid phases, pore water and gas, and can include the effects of sediment microstructures.

In recent times, the EMT has been an excellent tool for quantitative assessment of gas-hydrates

and freegas. By employing this theory, Ghosh et al. (2008) appraised the gas-hydrates as 22% for

contact (in which hydrates surround the grains) and 31% for non-contact (in which hydrates

occur away from the grain contacts) models respectively in the Cascadia margin. The study

shows that the nature of distribution of gas-hydrates within the sediments should be known for

true assessment of gashydrates, and there is a strong need to derive both P- and S-wave seismic

velocities (Gupta and Sain, 2005) in a region. Therefore, acquisition of multi-component ocean

bottom seismic data plays a vital role. NGRI has recently procured ocean bottom seismometers

using which the abovementioned problem can be addressed in a more comprehensive way, even

if we don’t have expensive log data.

The work at NGRI has shown that the BSR can be used to derive various parameters like

porosity, density, thermal conductivity, temperature, geothermal gradient, hydrate saturation,

electrical resistivity and heat flow (Ghosh et al., 2006; Vohat et al., 2003). To determine these

parameters by employing relevant techniques at depth below the seafloor is not only difficult but

also expensive. We have also brought out specific character of BSR near mud diapir and other

proxies like pock mark, transparency in sediments, faults, etc which are considered important for

studying gashydrates (Uma Shankar and Sain, 2007). From a theoretical consideration, Dash et

al., (2004) have presented that the AVO data can be used to detect overpressure, which helps

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in planning for drilling to avoid potential geo-hazard due to abnormally high pressures. As the

resistivity of hydrates-bearing sediments is higher than that of normal oceanic sediments within

the zone of gas-hydrates stability, the resistivity as a function of depth can also be used as a

complimentary method for quantifying gas-hydrates. The transient electric dipole-dipole or

electromagnetic method can be aimed at deriving the resistivity-structure of submarine sediments

to demarcate the zone of gas-hydrates-laden sediments. Since the amount of gas in gas-hydrates

depends on its structure, research based on X-ray diffraction, NMR and Raman spectroscopic

methods is needed to understand the crystal structure of gas-hydrates. Experiment with synthesis

of gas-hydrates in the laboratory with different composition and structure will provide basic data

of value in developing natural gas-hydrates as an energy resource. Kinetics of gashydrate

dissociation from the solid to the gaseous form needs to be investigated to understand the

production process. A field situation is to be simulated for studying the physical properties on

formation and dissociation of gas-hydrates and their impact on surroundings. Methods such as (i)

thermal stimulation, (ii) depressurisation and (iii) inhibitor injection (Holder et al., 1984) or a

combination of these can be considered for producing gas from gas-hydrates. However, none of

the methods has so far proved economically viable. It is also thought of that carbon dioxide can

be used as a molecular substitution of methane through a process called carbon dioxide

sequestration. With the fast growth of technology, it is expected that free-gas from below the

gas-hydrates would be exploited soon. It may take some more time to exploit gas from gas-

hydrates economically.

9. CONCLUSIONS

Application of various seismic tools will help in identifying and quantifying gas-hydrates, and

delineating the gas-hydrates-bearing sediments by measuring the seismic velocities. The resource

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estimate is very much required for further research with regard to exploitation. Much of the

deep-water regions within the Indian EEZ are yet to be explored. NGRI is in the process of

acquiring largeoffset MCS and OBS data from the prospective zones in the eastern offshore. The

travel time tomography of large-offset MCS data would assist in demarcating the lateral/areal

and vertical extent of gas-hydrates/’free-gas’-saturated sediments by imaging the velocity ‘build

up’ and ‘drop’. Independent P- and S-wave seismic velocities from OBS data would help to

know the nature of distribution of gashydrates within the sediments. Again, the Poisson ratio

provides important input for assessing gas-hydrates and free-gas much reliably. Pre-stack depth

migration of largeoffset MCS and OBS data using the derived velocity tomogram would provide

an improved structural image of BSR and base of free-gas saturated sediments. This may

delineate features like faults or fractures, which act as migration path for fluid flow and helps to

understand the genesis of gas-hydrates. If exploited properly, gas-hydrates can meet the

overwhelming demand of energy at one hand and reduce the environmental and submarine

hazards on the other. A concerted effort spanning from basic science to technology development

is on to study the gas-hydrates in a systematic way with a view to make India energy self-

sufficiency and world leader in this emerging field of gas-hydrates. It is to be mentioned here

that gas-hydrates are not stable like minerals and their extraction and dissociation have impact on

environment. So the mining strategies are to be evolved keeping in mind their effect on

environment by the exploitation and dissociation of gas-hydrates. The way, the price of oil is

increasing and the reserve of fossil fuels is getting depleted, gas-hydrates may emerge as a viable

sources of energy during the coming decades.

10.REFERENCES

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(1) Clathrate Hydrates of Natural Gases, by E. Dendy Sloan, Jr., Marcel Dekker, Inc., New

York,1998.

(2)Brooks, J.M., Anderson, A.L., Sassen, R., MacDonald, I.R., Kennicutt, M.C., and Guinasso,

N.L., Jr., 1994, Hydrate occurrences in shallow subsurface cores from continental slope

sediments, in Sloan, E.D. Jr., Happel, J., and Hnatow, M.A., eds., International Conference on

Natural Gas Hydrates: Annals of the New York Academy of Sciences, v. 715, p. 381-391.

(3) Ashi, J., Tokuyama, H. and Taira, A., 2002, Distribution of methane hydrates BSRs and its

implications for the prism growth in the Nankai Trough: Marine Geology, v. 187, p. 177-191.

(4) Dash R.K., Sain, K. and Thakur, N.K., 2004, Overpressure detection from seismic AVO

response: an application to gas-hydrates: Current Science, v. 86, p. 985-990. (6) Ginsburg, G. D.

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(5) Ghosh, R., Ojha, M., Sain, K. and Thakur, N.K., 2006, Physical parameters of hydrated

sediments estimated from marine seismic reflection data: a case study: Current Science, v. 90, p.

1421-1430.

(6) Henriet, J. P. and Mienert, J., 1998, Gas Hydrates: Relevance to World Margin Stability and

Climate Change, Geological Society, London, Spec. Pub., 137, p. 338. (9) Sain, K., Thakur N.K.

and Khanna, R.K., 2008, Seismic quality factor observations for gas-hydrates bearing sediments

on western margins of India: Marine Geophysical Researches, submitted after revision..

(7) Malik, S.K. and Varadarajan, S., 2002, Energy and Food Security: Advances in science for

sustainable environment and development in India during the next decade: Indian National

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(8) Wood, W.T. and Ruppel, C., 2000, Seismic investigations of the Blake ridge gas hydrate

area: a synthesis, in Paull, C.K. Matsumoto, R., Wallace P.J. and Dillon, W.P., eds., Proceedings

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(9) Boswell, R., and T. Collett, 2006. The gas hydrates resource pyramid, Fire in the Ice, US

Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, 6(3),

p. 5-7.

http://www.netl.doe.gov/technologies/oil

gas/publications/hydrates/2009Reports/FITI06_Pyramid.pdf

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