Advances inGeo-Energy Research www.astp-agr.com

Vol. 2, No. 1, p. 53-62, 2018

Invited review

Review of exploration and production technology of naturalgas hydrate

Yudong Cui1, Cheng Lu1,2, Mingtao Wu1, Yue Peng1, Yanbin Yao1, Wanjing Luo1*1School of Energy Resources, China University of Geosciences, Beijing 100083, P. R. China

2The Key Laboratory of Unconventional Petroleum Geology, China Geological Survey, Beijing 100029, P. R. China

(Received January 6, 2018; revised January 26, 2018; accepted January 28, 2018; available online February 5, 2018)

Citation:Cui, Y., Lu, C., Wu, M., Peng, Y., Yao,Y., Luo, W. Review of exploration andproduction technology of natural gashydrate. Advances in Geo-EnergyResearch, 2018, 2(1): 53-62, doi:10.26804/ager.2018.01.05.

Corresponding author:*E-mail: luowanjing@cugb.edu.cn

Keywords:Gas hydrateproperty of natural gas hydrateexploration methodsproduction methods

Abstract:Natural gas hydrate is an ice-like substance which is sometimes called “combustibleice” since it can literally be lighted on fire and burned as fuel. Natural gas hydrate ischaracterized by widespread distribution, large reserves and little pollution. This paperintroduced the distributions of hydrate, hydrate reserves and properties of hydrate. Themain exploration methods, such as geophysical exploration and geochemical explorationhave been presented. In addition, the main production techniques of natural gas hydrateincluding depressurization, thermal stimulation and chemical injection have been summedup. Finally, the challenges and outlooks of natural gas hydrate production have beenproposed.

1. IntroductionThe mixture of water molecules and light natural gases

in a crystalline ice-like structure forms gas-hydrate. In thissolid structure, the gaseous components are surrounded by thewater molecules (Makogon, 1997; Sloan and Koh, 2007). Ina hydrate, 80% of the volume is occupied by water and 20%by gas (Makogon, 2007). Methane is the main contributingcomponent in natural gas hydrates (NGH), which are a poten-tially important energy resource for future demand (Demirbas,2010; Moridis et al., 2011). Decomposition of one volume ofmethane gas-hydrate releases approximately 150-180 volumesof methane at standard conditions (Krason and Finley, 1992;Carroll, 2009). The substantial size of the natural hydrateresource is a motivating factor in its development (Moridiset al., 2007, 2011; Demirbas, 2010).

Hydrates find application in energy for flow assurance(Hammerschmidt, 1934) and in resource assessment (Max etal., 2005), as well as in climate change (Kennett et al., 2003),seafloor stability (Ginsburg et al., 1998) and gas transportationand storage (Sanden et al., 2005). There are numerous gashydrate reserves all over the world, especially in permafrostregions and ocean environments (Trehu et al., 2006). Fig. 1

is the map of discovered gas-hydrate deposits (Wang et al.,2017).

Currently, oil and natural gas are the primary fuels. Theabundance of gas hydrate reserves is expected to be more thantwice of the combined carbon of coal, conventional gas andpetroleum reserves (Aregba et al., 2017). Fig. 2 shows thecomparison between hydrate deposits and other sources offuel, using a unit of 1015 g of carbon.

As Fig. 3 shows, clathrate hydrates consist of guest gasmolecules inside hydrogen-bonded water lattices (Lee et al.,2011). NGH can be stable over a wide range of pressuresand temperatures. Three main structures of gas hydrates-cubicstructure I (sI), cubic structure II (sII) and the hexagonalstructure (sH) have been identified (Davidson et al., 1973;Englezos et al., 1993; Sloan et al., 2008; Veluswamy et al.,2014). The properties of ice, structures I and II gas hydratesis showed in Table 1 (Aregba et al., 2017).

Temperature and pressure of NGH are not static param-eters. They are affected by tectonic activity, sedimentation,changes in sea level, and changes in ocean temperature(Bohrmann et al., 2006).

Fig. 4 demonstrates how uplift or changes in sea level will

https://doi.org/10.26804/ager.2018.01.05.2207-9963 c© The Author(s) 2018. Published with open access at Ausasia Science and Technology Press on behalf of the Division of PorousFlow, Hubei Province Society of Rock Mechanics and Engineering.

54 Cui, Y., et al. Advances in Geo-Energy Research 2018, 2(1): 53-62

Table 1. Properties of Ice, structures I and II gas hydrates (After Areba, 2017).

S/N Properties (Unit Cell) Ice Structure I (sI) Structure II (sII)

1 Water molecules number 4 46 1362 Lattice parameters at 273 K, nm a = 0.452, c = 0.736 1.2 1.733 Dielectric constant at 273 K 94 ∼ 58 584 Water diffusion correlation time (µsec) 220 240 255 Water diffusion activation energy (kJ/m) 58.1 50 506 Shear Velocity (Vs), m/s 1949 1963.6 2001.17 Compressional Velocity (Vp), m/s 3870.1 3778 3821.88 Refractive index, 638 nm, -3◦C 1.3082 1.346 1.359 Density, kg/m3 916 912 94010 Poissons Ratio 0.33 ∼ 0.33 ∼ 0.33

11 Bulk Modulus (272 K) 8.8 5.6 -12 Shear Modulus (272 K) 3.9 2.4 -13 Velocity Ratio (comp./shear) 1.99 1.92 1.9114 Linear thermal expn., K−1 (200 K) 56× 10−6 77× 10−6 52× 10−6

15 Heat capacity, J/(kg*K) 3800 3300 360016 Thermal conductivity, W/(m*K) (263 K) 2.23 0.49± 0.02 0.51± 0.02

Location of test exploitation

Gas hydrate sample

Gas hydrate inferred

Lake Baikal

Eileen(Mt Eibert)

Mallik

UBGH1UBGH2

MET1

GMGS-1GMGS-2SHSC

IODP 311

ODP 204ODP 144JIP-1

Messoyakha

NGHP 01

NGHP 02

Fig. 1. Location of test exploitation, sampled and inferred gas hydrate occurrences in oceanic sediments of outer continental margins and permafrost regions(SHSC-Shenhu Test Exploitation; IODP-Integrated Ocean Drilling Program; UBGH-Ulleung Basin Gas Hydrate Expedition; ODP-Ocean Drilling Program;JIP-Joint Industry Project; METI-Ministry of Economy, Trade, and Industry; GMGS-Guangzhou Marine Geological Survey; NGHP-India National Gas HydrateProgram. After Wang et al., 2017).

Cui, Y., et al. Advances in Geo-Energy Research 2018, 2(1): 53-62 55A. G. Aregbe

29

Figure 1. Gas hydrates deposits compared with other fuel resources, units = 1015 g of carbon, [2].

that methane is highly flammable and almost impossible to detect any leak without using odorant.

Hydrates have been one of the flow assurance problems in gas production and transportation. Different models and approaches have been adopted to solve the gas hydrate problems. Hydrates block the conduit of oil and gas pipelines and transportation systems with significant economic impacts. In recent years, hy-drate production has been studied and investigated as potential for sustainable energy resource. The abundance of gas hydrate is estimated to be more than twice the combined carbon of coal, conventional gas and petroleum reserves [3]. Gas hydrate reserves contain enormous source of energy in form of pure me-thane gas which can serve as sustainable energy resource.

Nature of Gas Hydrate Deposits

The hydrates of CH4 and C3H8 gases occur naturally all over the world. The for-mation of Arctic hydrates is usually under the permafrost layer. Oceanic hy-drates are also discovered to be deposited at the continental shelves. In natural settings, such as the ocean bottom, where organic matter are buried and de-composed to methane, the methane formed is dissolved in water to form me-thane hydrates at temperatures around 277 K. Biogenic methane usually dis-solves slowly in water to form hydrate because of mass transfer limitations.

Over geologic time, the estimated methane hydrate in the ocean is approx-imately 2.1 × 1016 standard cubic metres—twice the total energy of all other fos-sil fuels on earth. The amount of methane hydrate in the northern latitude per-mafrost is approximately 7.4 × 1014 standard cubic metres. Hydrates deposits are usually determined by indirect methods such as the seismic reflection method known as bottom simulating reflectors (BSRs). This method uses seismic signals which are caused by velocity inversion because the presence of gas below some high-velocity barriers such as a hydrate deposit.

Significant hydrate sediments in the ocean beds can jeopardize the foundation

Gas hydrate, 10000

Fossil Fuels, 5000

Soil, 1400

Dissolved organic matter in water, 980

Land biota, 830Peat, 500

Others, 67

Fig. 2. Gas hydrates deposits compared with other fuel resources, units=1015 g of carbon (After Tohidi, 2014).

Fig. 3. Schematic Diagram of Natural Gas Hydrate Formation in Molecular Scale (Tetrahedral molecule is methane and dipole molecules are water, afterLee, 2011).

Oceanography Vol. 19, No. 4, Dec. 2006 127

This condition means that gas hydrate

is, generally, restricted to continental

margins and enclosed seas (Figure 2),

where organic matter accumulates rap-

idly enough to support methane produc-

tion by bacteria or where existing free or

dissolved gas is transported into the gas

hydrate stability zone (GHSZ).

Temperature and pressure are not

static parameters. They are affected by

tectonic activity, sedimentation, chang-

es in sea level, and changes in ocean

temperature. As one example, Figure 3

demonstrates how uplift or changes in

sea level will affect the GHSZ (Pecher et

al., 1998, 2001). Similarly, an increase in

the temperature of deep-ocean water will

thin the GHSZ, although such tempera-

ture changes require thousands of years

to propagate into sediments (Xu, 2004;

Mienert et al., 2005; Bangs et al., 2005).

Short-term changes in bottom water

temperature and pressure due to tides,

currents, or deep eddies can also affect

gas hydrate deposits (e.g., Ruppel, 2000;

MacDonald et al., 1994, 2005).

Both biogenic and thermogenic

sources produce hydrocarbons that

are incorporated into gas hydrate de-

posits. Biogenic methane can originate

wherever organic matter occurs in the

presence of a suitable microbial consor-

tium (Davie and Buffett, 2001). Offset-

ting methane production are microbial

processes that consume methane (e.g.,

Boetius et al., 2000; Orphan et al., 2001a,

2001b; Luff et al., 2005). Within marine

sediments, anaerobic oxidation of meth-

ane (AOM) by sulfate is largely respon-

sible for the methane-free zone found

from the seafl oor to subseafl oor depths

as great as hundreds of meters in some

settings. The thickness of this meth-

ane-depleted zone, the base of which

coincides with depletion in sulfate, is a

valuable proxy for methane fl ux in diffu-

sion-dominated systems (e.g., Borowski

et al., 1996). However, gas hydrate de-

posits have been observed at and near

the seafl oor (e.g., Brooks et al., 1984;

Suess et al., 2001; Chapman et al., 2004).

Formation and maintenance of these

deposits require rapid methane fl ux and

many of these deposits show geochemi-

cal indicators of thermogenic gas that

has migrated from subseafl oor depths

greater than 2 km.

A critical factor in formation of gas

hydrates is the concentration of meth-

ane with respect to methane solubility

in the sediment pore water, which is a

function of temperature and pressure

(Claypool and Kaplan, 1974). Methane

BSRP

BSR

ThermodynamicRe-equilibration

Tectonic Uplift orSea Level Fall

BSR

Dep

th

Temperature

Hydrate &Sediment

Initial Condition

Seawater

Free Gas

BSR

Sediment

HydrateStability

Temperature

Figure 3. Eff ect on gas hydrate stability of tectonic uplift or sea level fall. Th e resultant decrease in pressure will, in either case, result in destabilization

of any gas hydrate present near the base of the former gas hydrate stability zone. If the gas hydrate content in this zone is high enough, this may re-

sult in large pore pressures and induce slope instability or initiation of “gas chimneys.” In some cases, this gas hydrate dissociation leaves a “paleoBSR”

(BSRp) in the geologic record. After Bangs et al. (2005).

Fig. 4. Effect on gas hydrate stability of tectonic uplift or sea level fall (After Trehu, 2006).

56 Cui, Y., et al. Advances in Geo-Energy Research 2018, 2(1): 53-62

continental margins. In-order to establish a geological framework for the region

where gas hydrates are found; one needs to study the distribution and variability of

BSR, sediment deposition and distribution, diapirism, fluid-migration features.

There are different geological indicators which help in studying the gas hydrate

provinces. These indicators have been found worldwide and explained here under;

Fig. 5.1 Location of known and inferred gas hydrate occurrences in deep marine and arctic

permafrost environments. Courtesy of Council of Canadian Academics

Source: http://www.scienceadvice.ca/uploads/eng/assessments and publications and news releases/

hydrates

0.751076 CDP

Sea Floor

BSR

0.80

0.90

Tim

e (s

)

0.95

1.00

1.05

Fig. 5.2 A classic example of identified BSR from Hydrate Ridge Offshore USA. (After Rajput

2009, reproduced with permission)

5.2 Geological Indicators 101

Fig. 5. A classic example of identified BSR (After Thakur, 2010).

affect the gas hydrate stability zone (Pecher et al., 1998,2001). Similarly, an increase in the temperature will thin thegas hydrate stability zone, although such temperature changesrequire thousands of years to propagate into sediments. Short-term changes in bottom water temperature and pressure canalso affect gas hydrate deposits (Ruppel, 2000; MacDonald etal., 2005).

China’s marine geological science and technology person-nel first drilled high-purity gas hydrate samples in the easternwaters of the Pearl River Estuary in Guangdong Province andobtained considerable control reserves through drilling. May10, 2017, the China Geological Survey mined out natural gasfrom the South China Sea. July 9, 2017, accumulated naturalgas production exceeded 300, 000 m3 in a continuous test for60 days, and the well has been officially shut down. Acquired6.47 million scientific experiment data set has laid a solidfoundation for further work.

2. NGH Exploration Methods

2.1 Geophysical Exploration

As an important future energy, NGH’s exploration is ofgreat significance (Moridis et al., 2008; Lee et al., 2011). Gashydrate deposits are identified principally on the basis of theiracoustic expression. The phase boundary between methanehydrate and methane plus water gives rise to a prominentnegative-polarity event known as a BSR. Besides, the additionof hydrate to pore fluids has been interpreted to cause acousticblanking, a suppression of sediment reflectivity. Fig. 5 is aclassic example of identified BSR.

Previous studies suggest that most of the BSR amplitudeis due to the velocity reduction of the underlying free gas(Singh et al., 1993; MacKay et al., 1994; Hyndman et al.,2001; Pecher et al., 2001). The presence of a free-gas zone(FGZ) is an important part of the gas hydrate system, and isparticularly important if the presence of gas hydrate is to be

In the 2-D velocity model shown in Figure 6, the most prom-inent feature is a high-velocity layer between interfaces 5 and 6,with a thickness of w110 m and an average velocity of 1900 m/s.Refractions produced at the top of this layer are observed on almostall OBSs. The velocity of the layer is significantly higher than theaverage velocity (1600m/s) between the seafloor and the top of thislayer (w220 mbsf). The base of this high-velocity layer, at

310e320 mbsf, corresponds to the BSR identified on the SCSreflections sections (Fig. 2a e reflection 6) and the 2-D MSCreflection data (Mosher et al., 2004).

Below interface 6, a velocity decrease of w130 m/s wasmodelled for a layer that has almost the same thickness as the layerabove (w100 m) (Fig. 6). The results are similar to those obtainedfrom the 1-D velocity models (Fig. 4), except that the 1-D velocities

1.40

1.50

1.60

1.70

1.80

1.90

Distance (km)4 5 6 7

Dep

th (k

m)

1.6

1.8

2.0

2.2

1.4

2004

SF

BSR

(km/s)NS

9 12345678

2.4

Velocity8

12

3

45

67

8

9

Figure 6. 2004 2-D velocity model from travel-time tomography using arrivals from the nine OBS stations (triangles) and the corresponding 2-D SCS vertical incidence profile(Fig. 2a). The BSR marks the transition to a layer of reduced velocity (1590 m/s) below a high-velocity zone (average 1900 m/s). The numbers indicate the eight main reflections thatwere used in the travel-time inversion modelling. The velocity model shows a 4 km long section of the whole 12 km long profile that indicates the region of full ray-coverage wherethe travel-time arrivals provide control on velocities.

Figure 7. 2006 2-D velocity model from travel-time tomography using arrivals from the nine OBS stations (diamonds) of 2006 and the corresponding 2-D SCS vertical incidenceprofile. The travel-time inversion results show a small velocity contrast at the BSR depth. Refractions are produced below interface 5 at 210e220 mbsf, where the velocity increasesby 200 m/s. The unshaded area indicates the region of ray-coverage, where the travel-time arrivals provide control on velocities. (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)

A. Schlesinger et al. / Marine and Petroleum Geology xxx (2012) 1e116

Please cite this article in press as: Schlesinger, A., et al., Seismic velocities on the Nova Scotian margin to estimate gas hydrate and free gasconcentrations, Marine and Petroleum Geology (2012), doi:10.1016/j.marpetgeo.2012.03.008

Fig. 6. Reduced velocity of BSR (After Schlesinger, 2012).

inferred by observation of a BSR (Haacke et al., 2007).Fig. 6 shows that the BSR marks the transition to a layer

of reduced velocity below a high-velocity zone (Schlesingeret al., 2012). The saturation of the free-gas is less sensitiveto the seismic amplitude, but the amplitude of natural gashydrate with different saturation is quite different (Boswellet al., 2016).

Both methane hydrate and free gas exist even where a clearBSR is absent (Holbrook et al., 1996). The low reflectance, orblanking, above the BSR is caused by lithologic homogeneityof the sediments rather than by hydrate cementation. Theaverage methane hydrate saturation above the BSR is relativelylow (5 to 7 percent of porosity).

2.2 Geochemical Exploration

Geochemical exploration is performed by analyzing actualcore samples. However, geochemical exploration serves animportant role in gas hydrate exploration combined withgeophysical exploration. Geochemical indicators of hydrateinclude dissolved gas distribution, gas and organic compoundcomposition in sediments, chlorinity, salinity, alkalinity of porefluid, and the isotope compositions of water. These indicatorsinform us about the origins of natural gases and gas hydrateformation mechanisms.

During gas hydrate formation process dissolved ions suchas Na+and Cl− are excluded from the hydrate structure andonly water molecules crystallize into cubic lattice structure(Hesse and Harrison, 1981). Surrounding pore waters initiallybecome more saline during the process of hydrate forma-tion. During hydrate crystallization the exclusion of salinity(chlorine) and intake of δ18O is due to the solidfluid isotopefractionation that causes preferential uptake of the heavyisotope δ18O in the solid phase and depletion in the fluid(Hesse, 2003). During the dissociation of hydrates the releaseof pure water in the host sediments reduces the chlorinity.During the dissociation of hydrates release of heavy isotopewater and its mixing up with lower order isotopic watercomponent results in enrichment of δ18O in the host sediments

Cui, Y., et al. Advances in Geo-Energy Research 2018, 2(1): 53-62 57

Overburden Overburden Overburden

UnderburdenUnderburdenUnderburden

Free gas zone Water zone

Hydrate-bearinglayer

Hydrate-bearinglayer

Hydrate-bearinglayer

Hydrate-bearinglayer

( a ) ( b ) ( c ) ( d )

Fig. 7. Classes of Hydrate Reservoir (a) Class 1, (b) Class 2, (c) Class 3, (d) Class 4.

(a) (b) (c)

Gas Gas Gas Hot Water

HydrateDissociation

HydrateDissociation

HydrateDissociation

Heater

Inhibitor

Fig. 8. NGH Production Methods (a) Depressurization, (b) Thermal Stimulation, (c) Inhibitor Injection (After Lee et al., 2011).

pore waters. The δ18O values increase with increasing depthand appreciable higher values recorded in the hydrate bearingsediments. The observed combined anomalies of chlorinityand δ18O in the pore water chemistry of the host sedimentsprovides a reliable measure for identification of gas hydrates.Lower values of chlorine concentration in the hydrate stabilityzone may suggest the occurrence of hydrates even whenthe prominent geophysical signature (BSR) is found missing(Holbrook et al., 1996).

3. Production Methods

3.1 Classification of Gas Hydrate Reservoirs

Gas hydrate reservoirs have been classified according to thegeologic setting of the reservoir for the purpose of developingoptimal production strategies for each geologic condition(Moridis and Sloan, 2007; Moridis and Collett, 2013). Thefour classes are depicted in Fig. 7.

Class 1-3 has overburden and underburden layers in thegeologic setting. Class 1 reservoir consists of a hydrate-bearinglayer with underlying free gas zone, class 2 reservoir consistsof a hydrate-bearing layer with underlying water zone, andclass 3 reservoir consists of one hydrate-bearing layer withoutunderlying free-fluid phase zones. Class 4 reservoir consistsof single hydrate-bearing layer.

3.2 Techniques of gas production

Release and production of methane from hydrate-bearingformations are associated with hydrate dissociation. Differ-ent methods have been proposed to enhance gas production(Pooladi-Darvish, 2004; Sloan and Koh, 2007; Demirbas,2010). The most prominent methods are (i) lowering the reser-voir pressure below the hydrate equilibrium pressure at theprevailing temperature, known as depressurization; (ii) raisingthe temperature above the hydrate equilibrium temperature atthe existing pressure, normally known as thermal stimulation;and, (iii) shifting the thermodynamic equilibrium curve ofhydrate by means of injection of inhibitors, such as methanolor brine, to decompose the hydrate. Combinations of thesemethods can also be used. In general, where conditions aresuitable, it is believed that the depressurization techniquewould be the most economical and practical method in theproduction from gas-hydrate deposits (Grace et al., 2008).

3.2.1 DepressurizationThe depressurization method is currently regarded as the

most promising method among others. However, it still ex-ists several inevitable issues, such as subsidence during thedepressurization and hydrate reformation due to endothermicdepressurization event. All the problems should be solved in

58 Cui, Y., et al. Advances in Geo-Energy Research 2018, 2(1): 53-62

13

Figure 2-1: Conceptual model for methane recovery from hydrates using depressurization

Figure 2-2: Methane hydrate equilibrium curve

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

269 271 273 275 277 279 281 283 285 287

Pre

ssu

re (

kP

a)

Temperature (K)

Gas + Water

Hydrate + WaterHeat Transfer

(pi,Ti)

(ps(t),Ts(t))(pr,Tr)

Fig. 9. Conceptual model for methane recovery from hydrates using depres-surization (After Tabatabaie, 2014).

13

Figure 2-1: Conceptual model for methane recovery from hydrates using depressurization

Figure 2-2: Methane hydrate equilibrium curve

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

269 271 273 275 277 279 281 283 285 287

Pre

ssu

re (

kP

a)

Temperature (K)

Gas + Water

Hydrate + WaterHeat Transfer

(pi,Ti)

(ps(t),Ts(t))(pr,Tr)

Fig. 10. Menthane hydrate equilibrium curve (After Tabatabaie, 2014).

the future. The problems could be relieved by using combinedmethods. Such as the combination of the depressurization andthe gas swapping for mitigating the subsidence, and the com-bination of the depressurization and the thermal stimulationfor mitigating hydrate reformation.

Class 1 reservoir is considered as the most promisingreserve since the underlying free gas zone guarantees gasproduction and the T/P condition is close to the hydrateequilibrium condition (Moridis et al., 2007). In most cases,the depressurization method is the most promising methodbecause it is the most simple and cost effective (Moridis etal., 2007). Class 2 is less effective in propagating depressur-ization front since the underlying free water zone continuouslysupply a large body of water (Moridis and Kowalsky, 2007).Class 3 reservoir would exhibit even slower propagation ofdepressurization front due to the absence of free gas or waterzone that is more permeable than hydrate-bearing layers andcan serve as a conduit for depressurization front (Moridis etal., 2007). Class 4 reservoir has been reported as the least effe-

0 2 0 4 0 6 0 8 0 1 0 00

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

Gas P

roduct

ion(m

L/min)

T i m e / m i n

Fig. 11. Production rate curve of depressurization method (After Hao, 2016).

ctive in depressurization, producing very large amount of waterand small amount of gas (Moridis and Sloan, 2007).

Fig. 9 shows a schematic representation of the depres-surization method of gas production from Class 1 hydratereservoirs. A well is completed in the free-gas zone. The depthof the hydrate layer can be found by superimposing the gas-hydrate equilibrium curve and the geothermal gradient (Demir-bas, 2010). The base of the hydrate zone is at thermodynamicequilibrium pressure and temperature.

As the gas is removed, the pressure in the free-gas zoneis reduced to some pressure below the equilibrium pressurecorresponding to initial temperature, causing the hydrate todecompose (Hong and Pooladi-Darvish, 2005; Moridis et al.,2007). Fig. 10 shows a gas-hydrate phase diagram illustratingthe dissociation mechanism. As the picture depicts, hydrate ex-ists only in a high pressure and low temperature environment.Once the pressure is lower than critical pressure (i.e. equilib-rium pressure) or temperature is beyond critical temperature(i.e. equilibrium temperature), solid hydrate will decomposeand produce natural gas and water. Hydrate decompositionneeds to absorb heat from the surrounding environment. Thetemperature of the hydrate bearing layers will be reduced ifthere is a single depressurization.

The NGH dissociation by depressurization can be dividedinto three stages:

(1) Free gas produce in the early stage. With the reductionof system pressure, the associated free gas is rapidly producedand the gas production rate increases rapidly.

(2) Gas release from hydrate dissociation in the secondstage. Natural gas hydrate decomposition rate is larger becauseof the larger fractional surface area.

(3) Surplus free gas release in the third stage. Decomposi-tion rate gradually decreases with the smaller decompositionof the surface area.

3.2.2 Thermal StimulationSimple depressurization appears promising in Class 1 hy-

drates, but its appeal decreases in Class 2 hydrates. The mostpromising production strategy in Class 2 hydrates involves co-

Cui, Y., et al. Advances in Geo-Energy Research 2018, 2(1): 53-62 59

0 5 1 0 1 5 2 0 2 5 3 00 . 0

0 . 5

1 . 0

1 . 5

2 . 0

2 . 5

W i t h d i s s i p a t i o n B C 2 , Q = 0 W i t h d i s s i p a t i o n B C 1 , Q = 1 0 0 0 W i t h o u t d i s s i p a t i o n B C 2 , Q = 1 0 0 0 W i t h d i s s i p a t i o n B C 2 , Q = 1 0 0 0

T i m e ( i n d a y s )

Gas P

roduct

ion (S

TDCC

)X 1 0 4

Fig. 12. Cumulative production of methane with time (After Kumar, 2014).

ombinations of depressurization and thermal stimulation.(Moridis et al., 2002).

The concept of thermal stimulation is straightforward.Natural gas hydrates are heated in situ till the local temperatureis away from the hydrate stability region. When hydratedecomposes, the entrapped gas is released from water cagesand flows through the wellbore to be recovered. External heatis supplied through wellbore or point sources. The thermalstimulation method dissociates hydrates by raising temperatureabove the hydrate equilibrium temperature. In the thermalstimulation method, the energy that takes in the dissociationand the production should not exceed the energy we can getfrom produced gases to meet a simple economic principle.The energy needed for the hydrate dissociation is governedby thermal characteristics of the hydrate-bearing region. Fig.12 shows that the use of heat source increased the productionsignificantly.

The thermal stimulation method has the disadvantage oflosing substantial amounts of the introduced energy in theinjection path and surroundings. As a result, in this technique,only a small fraction of injected energy is usefully employedto dissociate the hydrate.

3.2.3 Chemical InjectionAs compared to depressurization and thermal stimulation,

chemical inhibition as a recovery method has been relativelyless studied. The chemical injection method injects inhibitorsof gas hydrates to dissociate hydrates in the reservoir. Chemi-cal inhibitor injection works by shifting the equilibrium curvetowards higher pressures and lower temperatures, therebydestabilizing hydrate at natural conditions. There are two maintypes of inhibitors, namely thermodynamic inhibitors whichalter the hydrate equilibrium conditions and kinetic inhibitorswhich slow down the rate of hydrate formation.

Thermodynamic inhibitors are of particular interest to theapplications for gas production. Two of the most commonthermodynamic inhibitors used are methanol and ethyleneglycol (Demirbas, 2010). Ethylene glycol (EG) is more com-monly studied due to its higher availability in the market,

lower toxicity and better performance in inducing hydratedissociation owing to its higher density compared to methanol(Dong et al., 2009). Several factors have been identified tocontrol the rate of dissociation, including the concentrationand temperature of inhibitor solution, inhibitor injection rate(Fan et al., 2005), pressure, and hydrate-inhibitor interfacial(contact) area (Sira et al., 1990). Apart of EG and methanol,NaCl also possesses inhibitory properties and it is widelyfound in nature (Qi et al., 2012). A molecular dynamic studyhas provided an insight into the possible roles of Na+ and Cl−

on methane hydrate recovery (Yi et al., 2014). The possibilityof using brine injection to recover gas from methane hydratein pure water and Berea sandstone was also explored (Lee,2009). The salt also inhibits hydrate formation and dissociationkinetically.

3.2.4 CO2 InjectionThe thermodynamic feasibility of CO2-CH4 replacement

was fully proven through numerous studies. From the thermo-dynamic point of view, both pure CO2 and CH4 moleculestypically form hydrates under corresponding conditions, andthe enthalpy of CO2 hydrate formation (about -57.98 kJ/mol)is lower than that of CH4 hydrate formation ( -54.49 kJ/mol). Itmeans CO2 hydrate is more stable than CH4 hydrate under thesame condition of temperature and pressure (Li et al., 2016).Yuan (2012) proposed a conceptual mechanism for CO2-CH4

replacement in hydrate-bearing sediments.The schematic diagram for CO2-CH4 replacement in the

hydrate is shown in Fig. 13, which is divided into four steps:(a) Diffusion of CO2 molecule to the surface of CH4

hydrates in porous media, which would disturb the stabilityof the methane hydrate structure.

(b) Dissociation of CH4 hydrate and CH4 molecule es-capes from the hydrate cage. Both the large and small cagesof methane hydrate rupture, and CH4 molecule escapes fromthe hydrate cage.

(c) Rearranging of CH4 and CO2 in hydrate cage. In the re-formed hydrate, CO2 molecules mainly enter the large cagesof the hydrate, and CH4 molecules can enter both the smallcages and the large cages of the hydrate, but mainly enter thesmall cages.

(d) Diffusion of CH4 molecule from the hydrate surfaceto gas phase and CO2 molecule diffusion into deeper hydrate-bearing sediment layer. CO2 molecules diffuse into deeperhydrate layer and the replacement reaction continues.

The experimental results show that the method of CH4

replaced with gaseous CO2 is not suitable for the reservoir,in which hydrate layer with absence of underlying zones ofmobile water. For high water saturation reservoir, gaseous CO2

will react with free water firstly which results in the inferioreffect of replacement. In comparison, for low water saturationreservoir, the replacement effect is perfect and encouraged inCH4 hydrate exploitation.

The techniques have their individual limitations such ashigh energy consumption, low gas recovery, low gas produc-tion rate and environmental issues. The combination of thosetechniques is considered as a better technique to enhance the

60 Cui, Y., et al. Advances in Geo-Energy Research 2018, 2(1): 53-62

a b c d

CO2 CH4 Large cage Small cage

Fig. 13. Schematic diagram for CO2-CH4 replacement in hydrate (After Yuan, 2012).

efficiency of gas production, lowering the energy consumption,accelerate the gas production, etc. Compared with thermalstimulation or depressurization, the combination technique isproven to be more efficient in natural gas production fromNGH reservoirs (Li et al., 2016).

4. Challenges and OutlookWith the gradual depletion of traditional energy such as oil

and coal, natural gas hydrate has a wide range of distribution,huge amount of resources and low pollution characteristicswhich make it become “future energy”. The abundance ofgas hydrate reserves is expected to be more than twice ofthe combined carbon of coal, conventional gas and petroleumreserves. Natural gas hydrate has changed the pattern of theworld’s energy. The challenges and problems for gas hydrateexploration and development are listed as following:

(1) For NGH’s exploration, although a potential for char-acterizations of hydrate reservoir using seismic indicators hasbeen reported in literatures, characterizing based only onseismic survey results is less reliable.

(2) For NGH’s development, it’s difficult to producemethane from the NGH reservoir and assess hydrate recov-ery rates. Besides, locating potential resources, quantifyinghydrate content, keeping process safe from geomechanicalimpacts from hydrate dissociation are worth considering.

(3) For the environment and application, Hydrate devel-opment should not only consider its economic benefits, butalso the ways of transporting the extracted natural gas to themarket. Whats more, development of natural gas hydrate maydo more harm to environment. Methane gas greenhouse effectis much greater than the carbon dioxide gas.

In the future, the researcher should pay more attention onhow to develop nature gas from hydrate, such as do more workon hydrate simulation and hydrate production test. Besides,the properties of hydrates should be studied. The kineticsarena will represent the largest challenge for advancing theinformation on hydrates. Although we know quite a lot aboutwhat hydrates are, the question of how hydrates form is still

very much unanswered. Finding the answers to such questionsprovides the intrinsic motivation for future research.

AcknowledgmentsThis work was supported by the National Natural Science

Foundation of China (Grant No. 51674227) and the Funda-mental Research Funds for the Central Universities (Grant No.2652015133).

Open Access This article is distributed under the terms and conditions ofthe Creative Commons Attribution (CC BY-NC-ND) license, which permitsunrestricted use, distribution, and reproduction in any medium, provided theoriginal work is properly cited.

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