GAS PRODUCTION FROM METHANE HYDRATE RESERVOIRS

GAS PRODUCTION FROM METHANE HYDRATE RESERVOIRS

Masanori Kurihara∗ Department of Resources and Environmental Engineering, Waseda University

3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

Hisanao Ouchi (Japan Oil Engineering Company)

Hideo Narita (National Institute of Advanced Industrial Science and Technology)

Yoshihiro Masuda (The University of Tokyo)

ABSTRACT The MH21 Research Consortium in Japan, which was organized to accomplish the exploration and exploitation of methane hydrate (MH) offshore Japan, has been implementing a variety of research projects toward the assessment of MH resources, establishment of MH production methods and examination on environmental impacts of MH development. Through these research projects, noteworthy insights into MH dissociation and gas production has been obtained and accumulated. Especially, state-of-the art numerical simulator for predicting MH reservoir performances (MH21-HYDRES) has been being developed/updated in this consortium, which has been providing instructive simulation results regarding the gas producibility from MH reservoirs. This paper summarizes the results of our past and ongoing researches, including numerical simulation studies, for investigating gas producibility from MH reservoirs. In this paper, following the introduction to MH21-HYDRES, the MH reservoirs confirmed in the world are classified from the viewpoint of reservoir characteristics. The methods for MH dissociation and gas production currently proposed are also briefly exposed. Then, the applicability of these methods mainly to confined pore filling type MH reservoirs, which were discovered in many places in the world including offshore Japan, are discussed referring to MH reservoir properties. At the last of this paper, field case studies targeting MH reservoirs offshore Japan are presented to discuss the feasibility and challenges in the commercial development of MH.

Keywords: methane hydrates, production method, numerical simulation, economics

∗ Corresponding author: Phone: +81 3 5286 2697 Fax: +81 3 5286 3491 Email: [email protected]

INTRODUCTION Methane hydrate (MH) is in the solid state and hence does not have a flowability. Furthermore, since the dissociation of MH is an endothermic reaction, reservoir temperature decreases along with the MH dissociation and gas production. Theses make the dissociation of MH and

production of gas from MH reservoirs extremely difficult as opposed to a simple production of fluids from conventional oil/gas reservoirs. Hence, a variety of researches including laboratory experiments and field production tests have been implemented to seek for technologies to produce a

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

considerable amount of gas from MH reservoirs economically. Among theses researches, numerical simulation plays a critical role and is widely used, because it can predict MH dissociation and gas production performances assuming arbitrary conditions within fairly short time. Therefore, the numerical simulation is utilized not only for the design and the analysis of results of laboratory experiments and field tests but also for comprehending the MH dissociation/production mechanisms and for probing gas production strategies from MH reservoirs. Thanks to the results of these researches, it has been being clarified that the viability of currently proposed production methods highly depends on reservoir characteristics and that simple production methods may be able to be applied to economically produce gas from MH reservoirs if MH reservoir conditions are favorable. This paper describes the production of gas from MH reservoirs, focusing mainly on the relationship between effects of production methods and reservoir characteristics, which are being revealed through a variety of research work. In this paper, following the introduction to the numerical simulator for predicting MH dissociation and gas production behaviors we have been developing, the MH reservoirs confirmed in the world are classified from the viewpoint of reservoir characteristics. The methods for MH dissociation and gas production currently proposed are also briefly exposed. Then, the applicability of these methods mainly to confined pore filling type MH reservoirs, which were discovered in many places in the world including offshore Japan, are discussed referring to MH reservoir properties. At the last of this paper, field case studies targeting MH reservoirs offshore Japan are presented to discuss the feasibility and challenges in the commercial development of MH. NUMERICAL SIMULATOR FOR PREDICTING MH DISSOCIATION AND GAS PRODUCTION The simulator used in a part of this study (MH21-HYDRES) was originally developed by the University of Tokyo and has since been modified and improved by Japan Oil Engineering Co., Ltd., the University of Tokyo, Japan National Oil

Corporation and National Institute of Advanced Industrial Science and Technology [1, 2, 3]. As illustrated in Figure 1, this simulator is able to deal with three-dimensional, five-phase, six-component problems and has the following features: • Three-dimensional (3D) Cartesian and two-

dimensional (2D) radial coordinates can be applied with local grid refinement.

• Six components (methane, carbon dioxide, nitrogen, water, methanol and salt) are available.

• Five phases (gas, water, ice, MH and (deposit) salt) are available.

• Darcy’s law and relative permeability curves are applied to gas and water flows.

• Effective permeability is estimated as a function of MH and ice saturations.

• Endothermic dissociation of MH and ice, and exothermic formation of MH and ice are accounted for.

• Kim-Bishnoi equation [4] is used for MH dissociation kinetics.

• Gas-MH-water or Gas-MH-ice equilibrium pressure is estimated as a function of temperature and methanol/salt concentration.

• Various boundary conditions such as constant-pressure production, constant-rate production, constant surface pressure/temperature and variable surface pressure/temperature can be applied.

Further details on this simulator are given in our previous papers [1, 2, 3].

gas MH

Core Test

Single Well Test

MH

Production well

Free gas

Const. T boundary

overburden

MH reservoir

heat

heat

Horizontal well

Full Field Study

Const. Pboundary

Depres-suriza-tion

2D Radial Coordinate Model 3D Cartesian Coordinate Model

Figure 1 Overview of MH21-HYDRES

MH RESERVOIRS Classification of MH reservoirs MH is formed if methane and water coexist in the high pressure and low temperature conditions satisfying MH stability. Many types of MH

deposits were created depending on the places where methane and water coexisted and on the volumes of water and gas supplied. Three types of sub-surface MH deposits were confirmed to date; pore filling type MH reservoir, naturally fractured type MH reservoir and massive/nodule MH deposit. In pore filling type MH reservoirs, MH is contained in pore spaces of porous media such as sandstones and carbonate rocks, like a typical accumulation of conventional oil and gas. On the other hand, in fractured type reservoirs, MH is contained in fractures/vugs in formations, while in massive/nodule MH deposits, MH is accumulated in the form of lump in fine grained muds due probably to the formation of MH on the surface of sea floor (Figure 2).

Massive MH or concentrations of MH nodules in fine mud

Massive/nodule type

MH nodules Vein fills

MH

Fractured type Pore filling type

Fine MH grains

MH filling massive/small fracture veins

MH grains filling pore space of

permeable rock Figure 2 Type of MH depositions

The Mallik MH reservoirs in Canada where the onshore production tests were carried out with the success of the world first sustainable gas production from MH reservoirs [5], Mt. Elbert MH reservoirs in Alaska North Slope where reservoir characteristics were investigated by analyzing the data acquired through exploratory drilling, coring and well logging [6], and MH reservoirs located in the Eastern Nankai Trough offshore Japan where MH was confirmed by 2D/3D seismic and exploratory drilling campaigns [7] are categorized into the pore filling type MH reservoir. MH is contained in the relatively thick and clean sand layers in Mallik and Mt. Elbert MH reservoirs, while MH is accumulated in thin sand layers of turbidite deposits composed of the alternation of thin sand and mud layers in the Eastern Nankai Trough MH reservoirs. Although fracture type MH reservoirs were discovered offshore India and Korea and massive/nodule MH deposits were confirmed in Gulf of Mexico and in Japan Sea, it is very difficult to produce gas from these types of MH reservoirs/deposits. Hence, as illustrated in Figure 3, these types of MH are ranked a few levels

below pore filling type MH reservoirs as energy resources [8].

Figure 3 MH resource pyramid

The pore filling type of MH reservoirs are further divided into several groups in terms of the conditions of the existence of MH, free gas and free water (Figure 4) [9]. If sufficient methane gas is supplied to the porous media in the MH stability conditions, which are sandwiched between impermeable layers such as shale layers, MH is formed throughout a reservoir and pore spaces are filled with MH and small amount of water that can not contact methane gas (Class 3: confined MH reservoir). Most of MH reservoirs discovered in the Eastern Nankai Trough, Mallik site and Mt. Elbert are categorized into this type of MH reservoir.

Confined reservoir (Class 3)

MH

Shale

Shale

MH

Shale

Shale

Reservoir underlain by free water (Class 2)

Reservoir underlain by free gas (Class 1)

MH

Shale

Free Water

MH

Shale

Free Water

MH

Shale

Free Gas

MH

Shale

Free Gas

Figure 4 Classification of pore filling type MH

reservoirs On the other hand, if the significant amount of gas is supplied to the porous media where the upper part is in the MH stable conditions and the lower part is in the MH dissociation conditions, MH is formed in the upper part of the reservoir, while free gas is accumulated in the lower part of the reservoir. This type of MH reservoir underlain by free gas is called a Class 1 MH reservoir. Messoyakha Field in Russia and Sagavanirktok Formation in Alaska are typical examples of this type of reservoir. If sufficient gas is not supplied or underling free gas in Class 1 reservoirs is dissipated after migration, the MH reservoir is underlain by free water, which is categorized as a Class 2 MH reservoir. A part of MH reservoirs in the Eastern

Nankai Trough and the Mallik site are confirmed to be this type of MH reservoirs. Other than theses 3 classes of MH reservoirs, Moridis et al. classified a MH deposit containing MH sparsely in mud layers as a Class 4 MH reservoir [9]. Reservoir properties In pore filling type of MH reservoirs, the mechanisms of MH dissociation and gas production are highly related to the properties of the porous media containing MH such as sand layers, sandstones and carbonate rocks. Especially, depth, thickness, porosity, permeability, MH saturation, initial pressure and temperature, and thermal conductivity are esseitial parameters in evaluating gas producibility from MH reservoirs. Furthermore, heterogeneities of a reservoir such as spatial variation of permeability and MH saturation, and distribution of impermeable layers are very important factors affecting the gas production remarkably. The properties of pore filling type MH reservoirs range widely; MH reservoirs with depth from 1,000 m to 1,500 m, thickness from a few meters to over 100 m, porosity from a few percent to over 40%, absolute permeability from a few milidarcy to over 1,000 mD, initial effective permeability to water in the presence of MH from almost zero to over 10 mD, MH saturation from almost zero to over 90% have been confirmed. The total thermal conductivity of a MH reservoir typically ranges from 2 W/mK to 4 W/mK. The initial reservoir pressure and temperature relates each other so as to satisfy the MH stability conditions. The initial pressure and temperature already confirmed range from 5 MPa to 15 MPa and from 3°C to 15°C, respectively. GAS PRODUCTION METHODS FROM MH RESERVOIRS In a MH reservoir, pressure and temperature conditions are in the MH stability region in the initial stage. To dissociate MH and produce gas from a MH reservoir, these pressure and temperature conditions should be moved to the MH dissociation region. Therefore, 3 methods of depressurization, thermal and inhibitor injection have been proposed as basic methods for the dissociating MH as depicted in Figure 5. The depressurization method decreases a MH reservoir

pressure below 3-phase (gas-MH-water or gas-MH-ice) equilibrium pressure, while thermal method increases a reservoir temperature above 3-phase equilibrium temperature. In the inhibitor injection method, inhibitors of hydration such as salt and alcohol are injected into a reservoir to shift the 3-phase equilibrium conditions to the high pressure and low temperature side and hence to move a reservoir conditions to the MH dissociation region.

Pres

sure

(MPa

)

Temperature (ºC)0 4 8 12 16

0

5

10

15

20

MH DissociationMH Dissociation

MH StabilityMH Stability

Gas-MH-water equilibrium

(original)

Initial reservoir conditions

DepressurizationMethod

DepressurizationMethod

Thermal Method

Thermal Method

Reservoir conditions

Reservoir conditionsGas-MH-water equilibrium

(including inhibitor)

Inhibitor Injection Method

Inhibitor Injection Method

Figure 5 Principle of MH dissociation

In addition, in order to increase gas production from a MH reservoir, it is proposed to combine these 3 basic methods and/or to apply horizontal wells and hydraulic fracturing, which are widely used in the conventional oil/gas development. Furthermore, the several methods such as injection of gas other than methane and irradiation of ultrasonic wave are investigated especially for the MH dissociation and gas production. The features of these methods are briefly described in the following. Depressurization method In the depressurization method, the bottomhole pressure is reduced by a pump installed in the downhole and this low pressure is transferred to the reservoir to induce the dissociation of MH. As illustrated in Figure 6, by reducing the bottomhole pressure of a well, the reservoir pressure in the vicinity of this well decreases first. The dissociation of MH is induced in the near well regions where the reservoir pressure becomes below the 3-phase equilibrium pressure. In this region, along with the dissociation of MH and hence with the decrease in MH saturation, effective permeability to fluids increases

remarkably. Hence, the low pressure is more easily transferred to the regions more distant from the well. This virtuous cycle (i.e., low pressure transfer, MH dissociation, increase in permeability, pressure transfer to more distant area, more MH dissociation, more increase in permeability, pressure transfer to much more distant area and so forth) is considered to enhance the area of MH dissociation and hence the gas production with time.

Initial condition After MH dissociation

Pres

sure

Distance from well

Sand

face

Radius of MH dissociation

Pressure DistributionPressure Distribution

BHP

Pump

Initial pressure

3-phase equilibrium

MH stability

MH dissociation

MH MH

Dramatic permeability increase in MH dissociation zone

Much better pressure transfer

Figure 6 Concept of pressure reduction and MH dissociation induced by depressurization method

MH StabilityMH Stability

MH Dissociation

MH Dissociation

0 4 80

15

20

5

10

12 16

Reservoir pressure after depressurization

ΔT

Pres

sure

(MPa

)

Temperature (ºC)

Gas-w

ater-M

H

equilib

rium

Initial reservoir condition

Sensible heat consumable for MH dissociation

Figure 7 Temperature reduction along with MH

dissociation in depressurization method In the depressurization method, however, all the MH contained in the region where the pressure has become below the 3-phase equilibrium pressure does not dissociate. The reservoir temperature decreases along with the dissociation of MH, because the dissociation of MH is an endothermic reaction. The dissociation of MH stops when the reservoir temperature becomes identical to the 3-

phase equilibrium temperature corresponding to the reservoir pressure as shown in Figure 7. In other words, the dissociation of MH stops when all the sensible heat of a reservoir between the initial reservoir temperature and the 3-phase equilibrium temperature corresponding to the reduced reservoir pressure has been consumed. After the consumption of all the sensible heat, MH gradually dissociates obtaining a latent heat at a rate corresponding to the heat flux from surroundings by conduction, which is usually extremely slow. Figure 8 illustrates the cross sectional views showing the change in MH saturation in a confined MH reservoir predicted by numerical simulation assuming the application of depressurization method at the constant bottomhole pressure, together with the predicted gas production rate. In this simple simulation, the outer boundary was considered to be closed (i.e., no mass/heat flux) assuming the operation with multiple wells. Soon after the reduction of the bottomhole pressure, MH in the vicinity of the well starts to dissociate reducing the temperature or consuming the sensible heat of this region. Since the heat is supplied from the overburden and underburden, the dissociation of MH is promoted in the regions close to the upper and lower boundaries (Period-1). The effective permeability to fluids increases associated with the MH dissociation, which expands the area of MH dissociation especially in the vicinity of the upper and lower boundaries, resulting in the increase in the gas production rate (Period-2). In the Period-3, the MH dissociation regions near upper and lower boundaries reach the outer boundary (boundary with the areas covered by neighboring wells) and the reduction of the pressure is accelerated. Therefore, gas production rate starts to increase dramatically due not only to the expansion of the MH dissociation area but also to the expansion and withdrawal of the gas remaining in the reservoir. The gas production continues to increase and becomes maximum in the Period-4. After that, because of the reduction of the reservoir pressure and hence of the decrease in the force driving gas toward the well (draw down pressure), the gas production starts to decline. In the Period-5, the pressure has reached the bottomhole pressure throughout the reservoir consuming all the sensible heat available. The gas is then produced at a almost constant low rate, associated with the

MH dissociated by the heat flux from overburden and underburden.

0

20

40

60

80

0 200 400 600 800 1000 1200 1400 1600 1800

(1)

(2) (3)

(4)

Time

Gas

rate

(5)

Gas

Gas

Gas

Heat

Heat

Well (Depressurization)

Gas

Gas

Gas

Heat

Heat

Well (Depressurization)

MH dissociation→Permeability improvement→More pressure transfer→More MH dissociation area→More gas production

Gas

Gas

Gas

Depressurization effect reaches outer boundary→Increase in speed of pressure decline

Well (Depressurization)

Heat Heat HeatHeat

Heat Heat HeatHeat

Gas

Gas

Gas

Fast pressure decline→More MH dissociation→Fast temperature drop→More heat flux from outside→More MH dissociation

Heat Heat HeatHeat

Heat Heat HeatHeatWell (Depressurization)

Gas

Gas

Gas

MH dissociation induced only by heat conduction from outside→Small, stable MH dissociation

Heat Heat HeatHeat

Heat Heat HeatHeat

Well (Depressurization)

MH stability (small effective permeability) MH dissociating (medium permeability) Complete MH dissociation (large permeability)

(1)

(4) (5)

(3)(2)

Figure 8 Mechanism of MH dissociation by

depressurization with multiple wells The production tests were conducted using the depressurization method in the Mallik production program in April 2007 and in Mach 2008 [5]. These tests attained the world first and only successful methane gas production to the surface from a MH reservoir by the depressurization method. Although these tests suggested a promise of gas production from MH reservoirs by the depressurization method, the methane recovery from a MH reservoir by depressurization highly depends on reservoir characteristics and is predicted to be up to about 60% even in the favorable case. Thermal method Thermal method is the general term for the methods promoting the dissociation of MH by increasing the reservoir temperature, which includes thermal stimulation method and thermal flooding method. As depicted in Figure 9, the thermal stimulation methods aim at the increase in the temperature in the vicinity of a well. These methods include a hot water circulation method circulating hot water in a wellbore to increase

bottomhole temperature, wellbore heating method increasing near wellbore temperature by heaters installed in the downhole and hot water huff’n’puff method in which hot water is injected into the reservoir from a well (huff), the well is shut-in for a certain period to sufficiently transfer the heat from the injected hot water to a reservoir (soak) and then gas and water are produced from the same well (puff). On the other hand, in the thermal flooding, the heat such as hot water is injected from a well and is flooded toward other wells increasing the temperature and hence dissociating the MH between wells (hot water flooding method).

MH MH

Hot water injection

HuffHuff SoakSoak PuffPuff HuffHuff

Depressurization

MH MH

Heaterdepressurization

DepressurizationHot water injection

MH MH

InjectorInjector ProducerProducer

Area heated

Re-formation of MH (high P

& low T)

Hot water huff’n’puffHot water Hot water huffhuff’’nn’’puffpuff

Depressurization + wellbore heating

Depressurization + Depressurization + wellbore heatingwellbore heating

Hot water flooding

Hot water Hot water floodingflooding

Figure 9 Overview of a variety of thermal

methods In applying hot water circulation and wellbore heating methods, in the region where reservoir temperature becomes higher than the 3-phase equilibrium temperature, MH dissociates and the pressure increases associated with the gas generated from MH. The drastic dissociation of MH stops in this region when the reservoir pressure becomes identical to the 3-phase equilibrium pressure. Even after the drastic MH dissociation stops, the heat is supplied continuously and MH continues to dissociate steadily, which results in the complete dissociation of MH located in this region. In these methods, however, the expansion of the region with high temperature is extremely slow, since the heat is transferred by thermal conduction. In 2002, the production test was conducted at the Mallik site in Canada, which succeeded the methane gas

production from a MH reservoir for first time in the world by applying the hot water circulation method [10]. On the contrary, in the methods injecting hot water into a reservoir such as hot water huff’n’puff and hot water flooding, much faster propagation of the heat and hence much faster expansion of the MH dissociation area are expected, if the hot water can be injected smoothly. In general, however, it is very difficult to inject hot water at high rate, since the effective permeability to water is very low in the presence of MH with high saturation in the initial stage. Furthermore, in the hot water flooding, the methane gas generated associated with MH dissociation near an injection well is cooled again in the course of the advance toward a production well and (secondary) MH is re-formed between well, which dramatically reduces the permeability and prevents the further smooth injection of hot water. Therefore, it is proposed to apply the hot water injection as a secondary recovery method after dissociating MH to some extent by depressurization and making paths for water movement [11]. In applying a hot water circulation, only a few percent of methane recovery is expected, since the bottomhole pressure should be kept higher than the initial reservoir pressure to circulate water in a wellbore. In the other thermal methods, combining the depressurization method, almost 100% of methane recovery is expected by the synergistic effect of depressurization and heating, if the conditions (e.g., reservoir properties, well spacing, etc.) are favorable. However, since the energy supplied in these methods is quite large, the applicability of the thermal methods is disputed from the viewpoint of energy efficiency. Inhibitor injection method In the inhibitor injection method, inhibitors of hydration such as salt and alcohol are injected into a reservoir to shift the 3-phase equilibrium conditions to the high pressure and low temperature side and hence to move a reservoir conditions to the MH dissociation region. Since the magnitude of this sift is limited, the significant MH dissociation is not expected by applying this method solely. Furthermore, it should be difficult to inject an inhibitor smoothly into a reservoir due to very low initial effective permeability to water. Therefore, it is investigated to inject an inhibitor

together with hot water in applying hot water huff’n’puff or hot water flooding method aiming at the effect similar to increasing the temperature of injected hot water and hence at the improvement of energy efficiency in these methods. The effect of the inhibitor injection method, however, comes under question because of the high cost and dilution/dispersion of inhibitors. Other methods Other than the basic methods introduced above, the following methods are investigated especially for the MH dissociation and gas production, although all of these methods are still in the laboratory stage. • Gas injection method: to inject gas other than

methane (e.g., nitrogen and carbon dioxide) into a MH reservoir to increase the effect of depressurization by reducing the partial pressure of methane without reducing the reservoir pressure significantly.

• CO2 displacement method: to inject carbon dioxide into a MH reservoir to displace MH with CO2 hydrate and to produce the released methane, because CO2 hydrate is generated more easily compared with MH.

• Ultrasonic wave irradiation method: to promote the dissociation of MH with vibration by ultrasonic waves.

• Electrical heating method: to increase the reservoir temperature by transmitting the electrical energy such as electrical current and micro wave to a reservoir via electric probes, which is utilized for the recovery of heavy oil.

• CO2 injection near MH formation: to increase a MH reservoir temperature by nearby CO2 injection, which generates CO2 hydrate with exothermic reaction [12].

In addition, in order to increase gas production from a MH reservoir, it is proposed to combine multiple methods and/or to apply horizontal wells and hydraulic fracturing, which are widely used in the conventional oil/gas development. PRODUCIBILITY OF GAS FROM CONFINED MH RESERVOIRS It was investigated through numerical simulation how MH reservoir properties including thickness, permeability and initial temperature as well as the operation conditions such as bottomhole pressure

and well spacing affected the gas production from MH reservoirs by various production methods introduced in the above. A variety of reservoir models were constructed in this simulation, simplifying the features of typical confined (Class 3) MH reservoirs composed of the alternation of sand and mud layers. For the vertical single well simulation, 2D (r-z) radial coordinate models were built, targeting the area in which a well can produce gas and water exclusively. For the simulation with multiple wells, 3D Cartesian coordinate models were constructed mimicking the area between injector and producer. The specifications of these models used in the base case simulation runs are listed in Table 1 and are illustrated in Figure 10. In addition to the base case simulation, a variety of reservoir properties including pressure, temperature, thickness, absolute permeability, MH saturation, effective permeability to water, sand-mud ratio and external radius were assigned to reservoir models to examine the sensitivity of gas production behavior to these properties.

Grid system 2D radial coordinate

3D Cartesian coordinate

Model area

Near well area with radius of 250 m

Area of 353.6 m×353.6 m square with 2 wells (injector and producer) located at the diagonal corners (well spacing: 500 m)

Number of grids 80×202 =16,160 35×35×202=247,450

Thickness (m) 50.5 Unit sand layer thickness (m) 0.5

Unit mud layer thickness (m) 0.5

Initial pressure (MPa) 14 @ mid depth of MH reservoir

Initial temperature (K)

285.15 @ mid depth of MH reservoir

Porosity (%) 35 (sand layers); 35 (mud layers) Absolute permeability (mD) 500 (sand layers); 1 (mud layers)

Initial effective permeability to water (mD)

1 (sand layers); 1 (mud layers)

Initial MH saturation (%) 65 (sand layers); 0 (mud layers)

Initial water saturation (%) 35 (sand layers); 100 (mud layers)

Rock thermal conductivity (W/mK)

2.915 (sand layers); 1.7 (mud layers)

Initial water salinity (%) 3.5

Table 1. Specifications of 2D radial and 3D Cartesian coordinate models constructed

(base case)

Using these MH reservoir models, MH reservoir performances were predicted for 8 years, assuming the application of the depressurization method, combination of depressurization and wellbore heating methods, hot water huff’n’puff method and hot water flooding method. Analyzing the results of these simulation runs together with the results of our past simulation studies [13], the following are envisaged for the applicability of these methods and for the effect of reservoir properties on MH dissociation and gas production.

250m

Pattern of repeated 5-spot

Model Area

353.6m35

3.6m

3D Cartesian coordinate model2D radial model

Well250 m

50.5 m

Heat

Heat

MH reservoir (alteration of sand and mud layers) 50.5 m

353.6 m

Producer

353.6 m

Heat

HeatInjector

MH reservoir (alteration of sand and mud layers)

Figure 10 Schemata of 2D radial and 3D Cartesian coordinate models constructed

Depressurization method Effects of reservoir properties Investigating the results of the sensitivity simulation as well as the base case simulation, it was revealed that the reservoir properties of thickness, initial temperature, absolute permeability and initial effective permeability to water significantly affected the cumulative gas production for 8 years. It is obvious that the thicker is a reservoir, the more MH is dissociated and hence the more gas is produced. If the initial reservoir temperature is higher, the more sensible heat can be consumed to dissociate MH, which results in the larger gas production (Figure 11). Both the absolute and effective permeability are also essential to transfer the low bottomhole pressure to the inside of the reservoir. High permeability enhances the pressure transfer, which accelerates the dissociation of MH. If the permeability is smaller than some critical value, pressure cannot be transferred throughout the exclusive area of the well within a target production period (8 years in the above simulation). On the other hand, in the cases of the permeability larger than this critical value, the cumulative gas production during a target production period does not increase dramatically

with increase in the permeability (Figure 12). Even if the permeability is extremely high far beyond this threshold value, MH dissociation becomes slow after all the sensible heat available in the exclusive area has been consumed by the endothermic reaction of MH dissociation induced by the reduction of reservoir pressure, as discussed in the above. It is needles to say that this critical permeability value depends on the production period and the size of the exclusive area (i.e., well spacing).

0

20

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60

80

100

120

140

160

180

200

277.15 279.15 281.15 283.15 285.15 287.15 289.15Initital reservoir temperature (K)

Cum

ulat

ive

gas

prod

uctio

n (1

06 m3 )

kwi = 0.1 mDkwi = 1 mD

Figure 11 Effect of initial temperature on gas

production (depressurization; ka=500 mD)

0

20

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100

120

140

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0 500 1000 1500Absolute permeability (mD)

Cum

ulat

ive

gas

prod

uctio

n (1

06 m3 )

kwi = 0.01 mDkwi = 0.1 mDkwi = 1 mDkwi = 10 mD

Figure 12 Effect of permeability on gas

production (depressurization; T=285.15 K) Compared with the properties discussed in the above, the effects of other parameters such as initial pressure and thermal conductivity on MH dissociation and gas production performances are relatively small.

Class-A

Class-B

Class-C

Absolute permeability = 500 mD

Initial methane hydrate saturation = 65%

Porosity = 35%

Salinity = 3.5%

Bottom hole pressure = 3 MPa

Well spacing = 500 m

(a) kwi = 0.01 mD

(c) kwi = 1 mD

(b) kwi = 0.1 mD

(d) kwi = 10 mD

Figure 13 Example charts for grading confined

MH reservoirs (depressurization) In accordance with theses simulation results, confined MH reservoirs were graded into three classes (Class A of cumulative gas production larger than 1x108 m3; Class B of cumulative gas production between 1x108 m3 and 0.5 m3; Class C of cumulative gas production smaller than 0.5x108 m3) in terms of thickness, initial temperature, absolute permeability and initial effective permeability to water. Figure 13 presents the examples of these grading of MH reservoirs with the absolute permeability of 500 mD, assuming the application of the depressurization method. Although the grading of MH reservoirs is not straightforward at all, a reservoir with absolute permeability higher than 500 mD, initial effective permeability higher than 1 mD, initial temperature higher than 10°C and thickness larger than 50 m can be categorized as Class A in general. Note that the effect of salinity of formation water, which should shift the 3-phase equilibrium curve, can be roughly accounted for by adjusting the initial reservoir temperature as NaClCTT 42.0' += , where

'T and T denote the corrected and original initial

reservoir temperatures in K respectively and NaClC stands for the salinity in %. Effects of operations The results of the simulation assuming various bottomhole flowing pressure applied in depressurization revealed that increase in bottomhole flowing pressure significantly reduced the effect of depressurization. Therefore, the skin effect of the well should be as small as possible to effectively transfer the wellbore pressure to the sand face. Since the equilibrium pressure corresponding to 0°C is about 2.7 MPa, ice may be formed in the case of the depressurization with the bottomhole flowing pressure below 2.7 MPa, which may be a hazard for MH dissociation and gas production. As discussed in the above, the optimum well spacing highly depends on the production period, absolute permeability and initial effective permeability to water. Our past study suggested that the horizontal well might be effective if there were no significant barriers to vertical flow, since the total gas production from a horizontal well was expected to be almost proportional to the horizontal length [13]. Our past study also clarified that although the increment of gas production accomplished by hydraulic fracturing might be remarkable in the reservoir with smaller permeability, this effect became insignificant when MH starts to dissociate mainly outside of the fracture region [13]. To maintain the effect of hydraulic fracturing, not a few numbers of fractures but a network of fractures should be generated. Thermal methods Combination of depressurization and wellbore heating Since the area affected by wellbore heating is limited only to the vicinity of a well, the gas production by the combination of depressurization and wellbore heating is almost the same as that by the depressurization merely. The gas production by this combination becomes larger than that by the depressurization merely only in the case that the MH is re-formed near the well due to the temperature reduction caused by the adiabatic expansion of gas. Hot water huff’n’puff In applying hot water huff’n’puff, almost 100% of MH is dissociated in

the area of hot water invasion. This area, however, is limited only to 150 m from the well at maximum in the above simulation cases. Therefore, if the external radius (half of the well spacing) is much larger than 150 m, the volume of dissociated MH and hence the gas production by this method is smaller than that by depressurization in which MH is dissociated throughout the exclusive area of the well. Figure 14 shows the examples of the grading of MH reservoirs with the absolute permeability of 1,000 mD, assuming the application of depressurization, combination of depressurization and wellbore heating, hot water huff’n’puff and hot water flooding. Since the well spacing was assumed to be 500 m in this case, the producibility of gas by hot water huff’n’puff method is smaller than that by depressurization for most of reservoir properties. Even in this case, however, if the reservoir temperature is low (i.e., unfavorable for depressurization), the absolute permeability is high (i.e., favorable for water injection) and the reservoir is thick enough (i.e., relatively small effect of heat transfer from/to overburden and underburden), the hot water huff’n’puff suppresses depressurization in cumulative gas production.

Absolute permeability = 1,000 mD

Initial effective permeability to water = 1 mD

Initial methane hydrate saturation = 65%

Porosity = 35%

Salinity = 3.5%

Bottom hole pressure = 3 MPa

Well spacing = 500 m

(a) depressurization

(c) hot water huff’n’puff

(b) combination of depressurization and wellbore heating

(d) hot water flooding

more gas production than (a)

almost the same as (a)

Class-A

Class-B

Class-C

more gas production than (a)

Figure 14 Example charts for grading confined

MH reservoirs (various methods)

Hot water flooding In the above simulation, it was assumed that the hot water was injected into a reservoir after 4-year depressurization. Since the effective permeability of a MH reservoir is very low due to the presence of MH in the early stage of production, it is impossible to inject water continuously. Hence the hot water flooding was attempted as a secondary production method after 4-year depressurization in which flow paths (high permeability paths) were generated by the dissociation of MH. In the reservoirs favorable for depressurization (e.g., absolute permeability larger than 500 mD, initial effective permeability to water greater than 1 mD, initial temperature higher than 10°C), the flow paths large enough for water flow are generated during depressurization, which increases the effects of hot water flooding and enables the dissociation of MH remaining after depressurization. However, in the case of the reservoirs with the initial temperature higher than 14°C, the effect of hot water flooding after depressurization is quite small, because most of MH can be dissociated by depressurization merely. PRODUCIBILITY OF GAS FROM OTHER TYPES OF MH RESERVOIRS Other classes of pore filling type MH reservoirs Some numerical simulation studies have been conducted for predicting gas production from MH reservoirs defined as Classes 1, 2 and 4 [11, 13]. They indicated the superiority of Class 1 MH reservoirs in terms of the gas production. In this type of reservoirs, the pressure of the free gas zone is extensively reduced along with gas production, which induces the dissociation of extensive MH located near the MH-gas contact, resulting in the large gas production from the MH zone. Hence, in this type of reservoir, MH is dissociated naturally only by producing free gas without the conscious of the complex MH dissociation and gas production. Since neither special methods nor special equipments are needed for MH dissociation and gas production, this type of reservoir has advantages from the viewpoints not only of gas production but also of economics. However, the amount of MH dissociation and hence the contribution of the MH zone to the total gas production depend on the properties of the MH zone such as initial temperature and permeability as in the case of Class 3 reservoirs. Even in a Class 1 reservoir, significant contribution of MH

zone cannot be expected, if the properties of MH zone are unfavorable. In a Class 2 MH reservoir, the bottom water intrudes into the MH zone by coning along with the MH dissociation and gas production by depressurization. This coning water supplies the heat to the MH zone from the aquifer, which promotes the MH dissociation in the lower part of the MH zone (near MH-water contact) and enhances gas production. Therefore, Class 2 reservoirs used to be considered superior to Class 3 reservoirs from the gas production point of view. However, along with the recent advancement of researches taking account of commercial gas production from MH reservoirs, it is viewed with suspicion to produce huge amount of water from Class 2 reservoirs. It has started to consider that some Class 3 reservoirs are superior to those of Class 2 from a practical point of view, because MH can be dissociated and produced with relatively small amount of water production in Class 3 reservoirs. For Class 4 reservoirs, some researches indicated the difficulty in development under any combination of reservoir properties and production technologies currently proposed [11, 14]. MH deposits other than pore filling type Massive/small vein fills contained in naturally fractured reservoirs and massive/nodule MH generally found encased in fine grained muds and shales (Figure 2) are classified in the fourth and fifth tiers of the gas hydrate resource pyramid shown in Figure 3. Unlike the pore filling type MH reservoirs, high matrix permeability cannot be expected, which makes the application of well-based production extremely difficult. Furthermore, the development of MH from these types of MH deposit may induce geomechanical troubles such as the failure of well integrity and formation stability. The development of MH from these types of deposits is problematic unless major technological advancements beyond current production systems should be attained. PREDICTION OF GAS PRODUCIBILITY FROM ACTUAL MH RESERVOIRS The MH21 Research Consortium succeeded to identify MH concentrated zones and MH bearing (non-concentrated) deposits in the Eastern Nankai Trough offshore Japan, by analyzing 2D/3D

seismic survey results as well as the interpretation results of well log and core data acquired in the exploratory drilling [15, 16]. The 2D radial reservoir models with radii of 120 m and 180 m as well as 3D Cartesian coordinate reservoir models were constructed for the vicinities of the exploratory Wells-A and -B drilled in the MH concentrated zones and Well-C drilled in the MH bearing deposit, accurately reflecting well log interpretation results. Numerical simulation was then conducted for predicting 8-year production performances from these wells, assuming the development of MH applying a variety of MH dissociation and gas production methods.

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Rate-(hot water + methanol huff & puff)Rate-(hot water huff & puff)Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Cum-(hot water + methanol huff & puff)Cum-(hot water huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)

Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Rate-(hot water huff & puff)Rate-(hot water + methanol huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)Cum-(hot water huff & puff)Cum-(hot water + methanole huff & puff)

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Rate-(hot water + methanol huff & puff)Rate-(hot water huff & puff)Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Cum-(hot water + methanol huff & puff)Cum-(hot water huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)

Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Rate-(hot water huff & puff)Rate-(hot water + methanol huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)Cum-(hot water huff & puff)Cum-(hot water + methanole huff & puff)

Base

Depressurization +wellbore heating

H&P

H&P (5% MeOH)

Depressurization

Figure 15 Example prediction results for Well-A

in the Eastern Nankai Trough

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Rate-(hot water huff & puff)Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Cum-(hot water huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)

Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Rate-(hot water huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)Cum-(hot water huff & puff)

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Rate-(hot water huff & puff)Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Cum-(hot water huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)

Rate-(Base Case : depressurization)Rate-(depressurization + wellbore heating)Rate-(hot water huff & puff)Cum-(Base Case : depressurization)Cum-(depressurization + wellbore heating)Cum-(hot water huff & puff)

Base

Depressurization +wellbore heating

H&P

Depressurization

Figure 16 Example prediction results for Well-B

in the Eastern Nankai Trough Details of this prediction simulation were presented in our previous paper [11]. It was predicted that the cumulative gas production ranging from 26x106 m3 to 88x106 m3 (average gas production rate of 9,000 m3/d - 30,000 m3/d) or methane recovery ranging from 30% to 60% was accomplished from Wells-A and -B, assuming the application of depressurization with the bottomhole pressure of 3 MPa for 8 years. It was also predicted that at these wells, methane

recovery could be improved up to about 90% by applying thermal methods, especially hot water huff’n’puff and hot water flooding methods (Figures 15 and 16). On the contrary, the cumulative gas production from Well-C was predicted at as small as 5x106 m3 - 13x106 m3, because of the sparse and hence small methane in place near this well. These simulation results were analyzed from the view point of energy efficiency (ratio of energy produced to that consumed) as shown in Figure 17, which revealed that the energy efficiencies by depressurization were the highest among various production methods at all the wells and that energy efficiencies by thermal methods were very low and became below 1 in some cases.

0

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ater h

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-C (B

ase C

as)

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Pa

Re = 18

0 m

depre

ssuri

zatio

n + w

ellbo

re he

ating

Ener

gy E

ffici

ency

Low energy efficiency for thermal methods

Figure 17 Example calculation results for energy

efficiency Furthermore, in accordance with the above simulation results, the simple economic evaluation was conducted. The net present values were roughly calculated for 8 years for various cases, assuming (1) gas price of 0.30 USD/m3, (2) drilling cost of 4,000,00 USD/well, (3) facilities (platform) cost of 10,000,00 USD/well (estimated dividing total cost by expected number of wells), (4) pipeline cost of 1,500,000 USD/well for well spacing of 240 m or 3,375,000 USD/well for well spacing of 360 m (estimated dividing total cost by expected number of wells), (5) operating cost of USD 62.2 per 1000 m3 of gas production as well as costs for pumping and heating water, (6) inflation factor and discount factor of 0%/year and 5%/year, respectively. Figure 18 shows the profile of the net present values for 8 years on a well basis. In applying the depressurization method, only the net present

value in the case of Well-B with the spacing of 360 m becomes greater than zero after 3 years from the start of the production. Although the net present values of the other depressurization cases are negative throughout 8 years, that in the case of Well-A with the spacing of 360 m is close to zero, which may suggests the promise of the depressurization methods at Wells-A and -B by optimizing the operation conditions including well spacing. For these two wells, however, the economics for the thermal methods is far below the feasibility. The feasibility of Well-C cannot be expected even in applying the depressurization method.

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Well-A : Base Case Well-A : Re = 180 mWell-A : Base + wellbore heating Well-A : hot water huff & puffWell-A : hot water flooding Well-B : Base CaseWell-B : Re = 180 m Well-B : Base + wellbore heatingWell-B : hot water huff & puff Well-C : Base CaseWell-C : Re = 180 m Well-C : Base + wellbore heating

Well-B; R=180m

Well-A; R=180m

Well-C; R=180m

Thermal stimulation

Figure 18 Example calculation results for net

present value DISCUSSION As introduced in the above, a variety of MH reservoirs exist having diverse characteristics. It is expected that there are some MH reservoirs among them in which MH is rather easily dissociated applying simple methods like a depressurization method. On the contrary, there are many MH reservoirs in which the development of MH is hopeless under the current technologies. Thanks to the various research work, feasibility of MH dissociation and gas production by depressurization has been clarified, if MH reservoir properties are favorable. However, it is far less economical to dissociate MH by the methods injecting heat or other energies into a reservoir. Before the field production test was conducted at the Mallik site in Canada in 2002 [10], it had been considered impossible to dissociate MH by depressurization because of the extremely low initial effective permeability in the presence of MH. However, in the course of the analyses of the

test results, the MH 21 Research Consortium revealed that the fluid flow could be expected even in the presence of MH with the saturation of as high as 80% in the initial stage. After this discovery, many researches have been conducted focusing on the MH dissociation and gas production by depressurization. It is regretful that the methods superior to the depressurization method have not been developed yet. As presented in Figure 19, in some MH reservoirs having comparatively high permeability, thermal methods such as hot water huff’n’puff and hot water flooding can produce more gas than depressurization. Even in these reservoirs, however, it is difficult to apply thermal methods from the economic point of view.

DepressurizationDepressurization

highPermeabilitylow

Tem

pera

ture

high

lowHot water Hot water huffhuff’’nn’’puffpuff

Hot water Hot water huffhuff’’nn’’puffpuff

Hot water Hot water floodingflooding

in terms only of gas production volume

Wellbore Wellbore heating,heating,

electrical electrical heatingheating

Figure 19 Applicability of MH dissociation and

production methods in terms of reservoir permeability and initial temperature

Since there are some MH reservoirs where MH is expected to be dissociated and produced economically by depressurization, it is highly recommended to start the MH development with MH reservoirs where MH is expected to be dissociated rather easily. Therefore, MH reservoirs of the first target for development include: • Class 1 MH reservoirs • Class 3 MH reservoirs with sufficient

thickness, initial temperature and permeability

• Onshore (permafrost) MH reservoirs with nearby existing infrastructure.

Once we have started the development of MH reservoirs, unexpected troubles may be encountered even during the operations in MH

reservoirs having favorable properties. It is recommended to accomplish the following researches preparing these troubles, which may hasten the start of the commercial gas production from MH reservoirs. • Identification of MH reservoirs to which the

depressurization method can be applied • (long term) field production tests using

depressurization • Clarification and solution of potential

problems (e.g., failure of well integrity and zone isolation, reduction of permeability due to sand migration toward the well and/or to deformation/compaction of a reservoir, etc.)

• Development of methods to be applied to enhance or to follow the dissociation of MH by depressurization

Concurrently with the research work focusing on the depressurization method, sure and steady studies should be carried out continuously, towards the development of MH reservoirs having significant difficulties for the MH dissociation and gas production by depressurization. In parallel with these complex researches, insights into MH dissociation and gas production by depressurization in actual fields are expected to be accumulated, which should be beneficial to promote theses complex researches. There are many opinions on the development of MH including positive and negative ones. All of these opinions, however, are based only on the results of laboratory experiments, numerical simulation and very few short term field tests. Early and multiple field production tests may be essential for reducing uncertainties, improving the quality of research work and accelerating research work in the right direction. CONCLUSIONS The following conclusions are drawn for the gas production from MH reservoirs. • Three types of sub-surface MH deposits are

confirmed to date; pore filling type MH reservoir, naturally fractured type MH reservoir and massive/nodule MH deposit.

• Among these three types of MH deposits, the pore filling type of MH reservoirs may be promising as energy resources. They are

further divided into 4 groups; Class 1 reservoir underlain by free gas, Class 2 reservoir underlain by free water, Class 3 reservoir confined by impermeable layers and Class 4 deposits containing MH sparsely in fine grained mud layers.

• Basically, three methods of depressurization, thermal and inhibitor injection are proposed for the MH dissociation and gas production.

• For a confined MH reservoir, depressurization can be applied feasibly, if the thickness, initial temperature, absolute permeability and initial effective permeability to water of a reservoir are large enough.

• Thermal methods may accomplish the gas production higher than that by depressurization in some conditions such as MH reservoirs with high permeability and small enough well spacing. However, it is not expected to dissociate MH and produce gas economically by thermal methods.

• Since MH is dissociated naturally only by producing free gas without the conscious of the complex MH dissociation and gas production in a Class 1 MH reservoir, this type of reservoir has advantages from the viewpoints not only of gas production but also of economics.

• In a Class 2 MH reservoir, although the coning bottom water supplies the heat to the MH zone from the aquifer promoting MH dissociation and hence gas production, it is viewed with suspicion to produce huge amount of water, from the practical point of view.

• It is hopeless to develop Class 4 reservoirs and MH deposits other than pore filling type reservoirs under any combination of reservoir properties and production technologies currently proposed.

• It is highly recommended to conduct long term field production tests to ensure unexpected problems such as the failure of well integrity and the reduction of permeability that may be encountered in actual field production.

ACKNOWLEDGEMENT This study was financially supported by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) to carry out Japan’s Methane Hydrate R&D

Program by the Ministry of Economy, Trade and Industry (METI). The authors gratefully acknowledge them for the financial support and permission to present this paper. The authors also wish to thank Japan Oil Engineering Co. Ltd., National Institute of Advanced Industrial Science and Technology, the University of Tokyo and Japan Oil, Gas and Metals National Corporation for their technical support. REFERENCES [1] Masuda Y, Naganawa S, Ando S, Sato K. Numerical calculation of gas-production performance from reservoirs containing natural gas hydrates. paper SPE 38291, Proceedings, Western Regional Meeting, Society of Petroleum Engineers, Long Beach, California, 1997. [2] Kurihara M, Ouchi H, Inoue T, Yonezawa T, Masuda Y, Dallimore SR, Collett TS. Analysis of the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate thermal production test through numerical simulation. Geological Survey of Canada, Bulletin 585, 2005. [3] Masuda Y, Konno Y, Iwama H, Kawamura T, Kurihara M, Ouchi H. Improvement of near wellbore permeability by methanol stimulation in a methane hydrate production well. Paper OTC 19433, Proceedings of the Offshore Technology Conference, Houston, Texas, 2008. [4] Kim HC, Bishnoi PR, Heidemann RA, Rizvi SSH. Kinetics of methane hydrate decomposition, Chemical Engineering Science 1987;42(7):1645-1653. [5] Numasawa M, Dallimore SR, Yamamoto K, Yasuda M, Imasato Y, Mizuta T, Kurihara M, Masuda Y, Fujii T, Fujii K, Wright JF, Nixon FM, Cho B, Ikegami T, Sugiyama H. Objectives and operation overview of the JOGMEC/NRCan /Aurora Mallik gas hydrate production test. Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, 2008. [6] Boswell R, Hunter R, Collett TS, Digert S, Hancock S, Weeks M, Mount Elbert Science Team. Investigation of gas hydrate bearing sandstone reservoirs at the Mount Elbert stratigraphic test well, Milne Point, Alaska. Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, 2008. [7] Takahashi H, Tsuji Y. Multi-well exploration program in 2004 for natural hydrate in the Nankai-Trough offshore Japan. Paper OTC 17162 presented at the Offshore Technology Conference, Houston, Texas, 2005.

[8] Boswell R, Collett T. The gas hydrate resource pyramid. Fire In The Ice, NETL Methane Hydrates R&D Program Newsletter, Fall 2006. (http://www.netl.doe.gov/technologies/oil-gas/publications/Hydrates/Newsletter/HMNewsFall06.pdf). [9] Moridis GJ, Collett TS, Boswell R, Kurihara M, Reagan MT, Koh C, Sloan ED. Toward production from gas hydrates: current status, assessment of resources, and model-based evaluation of technology and potential. Paper SPE 114163 presented at the SPE Unconventional Reservoirs Conference, Keystone, California, 2008. [10] Dallimore SR, Collett TS. Summary and implications of the Mallik 2002 Gas Hydrate Production Research Well Program. Geological Survey of Canada, Bulletin 585, 2005. [11] Kurihara M, Sato A, Ouchi H, Narita H, Masuda Y, Saeki T, Fujii T. Prediction of gas productivity from Eastern Nankai Trough methane-hydrate reservoirs. SPE Reservoir Evaluation & Engineering 2009;12(3):477-499. [12] Ikegawa Y, Miyakawa K, Suzuki K, Masuda Y, Narita H, Ebinuma T. Experimental results for long term CO2 injection near methane hydrate formations. Paper OTC 20575 presented at the Offshore Technology Conference, Houston, Texas, 2010. [13] Kurihara M, Funatsu K, Ouchi H, Masuda Y, Narita H. Investigation on applicability of methane hydrate production methods to reservoirs with diverse characteristics. Proceedings of the 5th International Conference on Gas Hydrates, Trondheim, Norway, 2005. [14] Moridis GJ, Sloan ED. Gas production potential of disperse low-saturation hydrate accumulations in oceanic sediments. J. Energy Conversion and Management 2007;48(6):1834-1849. [15] Saeki T, Fujii T, Inamori T, Kobayashi T, Hayashi M, Nagakubo S, Takano O. Delineation of methane hydrate concentrated zone using 3D seismic data in the Eastern Nankai Trough. Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, 2008. [16] Fujii T, Saeki T, Kobayashi T, Inamori T, Hayashi M, Takano O, Takayama T, Kawasaki T, Nagakubo S, Nakamizu M, Yokoi K. Resource assessment of methane hydrate in the Eastern Nankai Trough, Japan. Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, 2008.