Advances in Geo-Energy Research

Advances in Geo-Energy Research

Vol. 2, No. 1, p. 53-62, 2018 www.astp-agr.com

Invited review

Review of exploration and production technology of natural gas hydrate Yudong Cui1 , Cheng Lu1,2 , Mingtao Wu1 , Yue Peng1 , Yanbin Yao1 , Wanjing Luo1 * 1 2

School of Energy Resources, China University of Geosciences, Beijing 100083, P. R. China

The 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:

Abstract:

Cui, Y., Lu, C., Wu, M., Peng, Y., Yao, Y., Luo, W. Review of exploration and production technology of natural gas hydrate. Advances in Geo-Energy Research, 2018, 2(1): 53-62, doi: 10.26804/ager.2018.01.05.

Natural gas hydrate is an ice-like substance which is sometimes called “combustible ice” since it can literally be lighted on fire and burned as fuel. Natural gas hydrate is characterized by widespread distribution, large reserves and little pollution. This paper introduced the distributions of hydrate, hydrate reserves and properties of hydrate. The main exploration methods, such as geophysical exploration and geochemical exploration have been presented. In addition, the main production techniques of natural gas hydrate including depressurization, thermal stimulation and chemical injection have been summed up. Finally, the challenges and outlooks of natural gas hydrate production have been proposed.

Corresponding author: *E-mail: [email protected] Keywords: Gas hydrate property of natural gas hydrate exploration methods production methods

1. Introduction The mixture of water molecules and light natural gases in a crystalline ice-like structure forms gas-hydrate. In this solid structure, the gaseous components are surrounded by the water molecules (Makogon, 1997; Sloan and Koh, 2007). In a hydrate, 80% of the volume is occupied by water and 20% by gas (Makogon, 2007). Methane is the main contributing component in natural gas hydrates (NGH), which are a potentially important energy resource for future demand (Demirbas, 2010; Moridis et al., 2011). Decomposition of one volume of methane gas-hydrate releases approximately 150-180 volumes of methane at standard conditions (Krason and Finley, 1992; Carroll, 2009). The substantial size of the natural hydrate resource is a motivating factor in its development (Moridis et al., 2007, 2011; Demirbas, 2010). Hydrates find application in energy for flow assurance (Hammerschmidt, 1934) and in resource assessment (Max et al., 2005), as well as in climate change (Kennett et al., 2003), seafloor stability (Ginsburg et al., 1998) and gas transportation and storage (Sanden et al., 2005). There are numerous gas hydrate reserves all over the world, especially in permafrost regions 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. The abundance of gas hydrate reserves is expected to be more than twice of the combined carbon of coal, conventional gas and petroleum reserves (Aregba et al., 2017). Fig. 2 shows the comparison between hydrate deposits and other sources of fuel, using a unit of 1015 g of carbon. As Fig. 3 shows, clathrate hydrates consist of guest gas molecules inside hydrogen-bonded water lattices (Lee et al., 2011). NGH can be stable over a wide range of pressures and temperatures. Three main structures of gas hydrates-cubic structure I (sI), cubic structure II (sII) and the hexagonal structure (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 hydrates is showed in Table 1 (Aregba et al., 2017). Temperature and pressure of NGH are not static parameters. 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. c The Author(s) 2018. Published with open access at Ausasia Science and Technology Press on behalf of the Division of Porous 2207-9963 Flow, 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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Water molecules number Lattice parameters at 273 K, nm Dielectric constant at 273 K Water diffusion correlation time (µsec) Water diffusion activation energy (kJ/m) Shear Velocity (Vs), m/s Compressional Velocity (Vp), m/s Refractive index, 638 nm, -3◦ C Density, kg/m3 Poissons Ratio Bulk Modulus (272 K) Shear Modulus (272 K) Velocity Ratio (comp./shear) Linear thermal expn., K−1 (200 K) Heat capacity, J/(kg*K) Thermal conductivity, W/(m*K) (263 K)

4 a = 0.452, c = 0.736 94 220 58.1 1949 3870.1 1.3082 916 0.33 8.8 3.9 1.99 56 × 10−6 3800 2.23

46 1.2 ∼ 58 240 50 1963.6 3778 1.346 912 ∼ 0.33 5.6 2.4 1.92 77 × 10−6 3300 0.49 ± 0.02

136 1.73 58 25 50 2001.1 3821.8 1.35 940 ∼ 0.33 1.91 52 × 10−6 3600 0.51 ± 0.02

Eileen (Mt Eibert)

Mallik

Lake Baikal

Messoyakha IODP 311 UBGH1 UBGH2

ODP 204

JIP-1

ODP 144

MET1 GMGS-1 GMGS-2 SHSC

NGHP 01 NGHP 02

Location of test exploitation Gas hydrate sample Gas hydrate inferred

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 Hydrate Program. After Wang et al., 2017).


A. G. Aregbe

55

67 Dissolved Land biota, 830 Peat,Others, 500 organic matter in water, 980

Soil, 1400

Fossil Fuels, 5000

This condition means that gas hydrate is, generally, restricted to continental

Gas hydrate, 10000

Short-term changes in bottom water temperature and pressure due to tides,

settings. The thickness of this methane-depleted zone, the base of which

margins and enclosed seas (Figure 2), currents, or deep eddies can also affect coincides with depletion in sulfate, is a where organic matter accumulates rapgas hydrate deposits (e.g., Ruppel, 2000; valuable proxy for methane flux in diffu15 Fig. 2. Gas hydrates deposits compared with other fuel resources, units=10 g of carbon (After Tohidi, 2014). Gas hydrates deposits etcompared with other fuel resources, units(e.g., = Borowski idly enoughFigure to support1. methane producMacDonald al., 1994, 2005). sion-dominated systems tion by bacteria existing free or 1015orgwhere of carbon, [2]. dissolved gas is transported into the gas

Both biogenic and thermogenic et al., 1996). However, gas hydrate deposits have been observed at and near sources produce hydrocarbons that hydrate stability zone (GHSZ). are incorporated into gas hydrate dethe seafloor (e.g., Brooks et al., 1984; that methane is are highly flammable almost impossible to2001; detect any Temperature and pressure not posits. Biogenicand methane can originate Suess et al., Chapman et al.,leak 2004). static parameters. They are affected by wherever organic matter occurs in the Formation and maintenance of these without using odorant. tectonic activity, sedimentation, chang- presence of a suitable microbial consordeposits require rapid methane flux and

Hydrates have inbeen flow assurance problems production and es in sea level, and changes oceanone of tiumthe (Davie and Buffett, 2001). Offsetmanyin of gas these deposits show geochemitemperature. As one example, Figure 3 ting methane production are microbial cal indicators of thermogenic gas that transportation. Different models and approaches have been adopted to solve the demonstrates how uplift or changes in processes that consume methane (e.g., has migrated from subseafloor depths seagas level will affect theproblems. GHSZ (Pecher etHydrates Boetius etblock al., 2000;the Orphan et al., 2001a, greater thangas 2 km.pipelines and hydrate conduit of oil and al., 1998, 2001). Similarly, an increase in 2001b; Luff et al., 2005). Within marine A critical factor in formation of gas transportation systems with significant economic impacts. In recent years, hythe temperature of deep-ocean water will sediments, anaerobic oxidation of methhydrates is the concentration of meththin the GHSZ, although such has tempera(AOM) byand sulfateinvestigated is largely respon- as ane with respectfor to methane solubility drate production been ane studied potential sustainable

ture changes require thousands of years sible for the methane-free zone found in the sediment pore water, which is a energy resource. The abundance of gas hydrate is estimated to be more than to propagate into sediments (Xu, 2004; from the seafloor to subseafloor depths function of temperature and pressure as hundreds of meters in some Mienert et al., 2005; Bangs et al., 2005). and Kaplan, 1974). Methane twice the combined carbonasofgreat coal, conventional gas and(Claypool petroleum reserves [3]. Fig. 3. Schematic Diagram of Natural Gas Hydrate Formation in Molecular Scale (Tetrahedral molecule is methane and dipole molecules are water, after Lee, 2011). Gas hydrate reserves contain enormous source of energy in form of pure me-

thane gas which can serve as sustainable energy resource. Temperature Nature of Gas Hydrate Deposits

Depth

The hydrates ofTemperature CH4 and C3H8 gases occur naturally all over the world. The formation of Arctic hydrates is usually under the permafrost layer. Oceanic hySeawater BSR Hydrate Free Gas shelves. In natural Stability drates are also discovered to be deposited at the continental BSR BSR settings, such as the ocean bottom, where organic matter are buried and decomposed Hydrate & to methane, the methane formed is dissolved in water to form meSediment thane hydrates at temperatures around 277 K. Biogenic methane usually disBSR Sediment solves slowly in water to form hydrate because of mass transfer limitations. Tectonic Uplift or hydrate in the Thermodynamic Initial Condition Over geologic time, the estimated methane ocean is approxSea Level Fall Re-equilibration 16 Figure 3. Eff2.1 ect on × gas 10 hydrate stability of tectoniccubic uplift or sea level fall. The resultant decrease pressure energy will, in either case, destabilization imately standard metres—twice theintotal of result all inother fosany gas hydrate present the base of theor former hydrate If the gas hydrate content in this zone is high enough, this may reEffect on gasof hydrate stability of near tectonic uplift sea gas level fall stability (After zone. Trehu, 2006). in largeon pore pressures induce slope instability or of “gas chimneys.” In some cases, this gasnorthern hydrate dissociation leaves a “paleoBSR” sil sult fuels earth.andThe amount ofinitiation methane hydrate in the latitude per(BSR ) in the geologic record. After Bangs et al. (2005). mafrost is approximately 7.4 × 1014 standard cubic metres. Hydrates deposits are usually determined by indirect methods such as the seismic reflection method P

Fig. 4.

p

Oceanography

Vol. 19, No. 4, Dec. 2006

127

Fig. 5.1 Location of known and inferred gas hydrate occurrences in deep marine and arctic permafrost environments. Courtesy of Council of Canadian Academics 56 Cui, Y., et al. Geo-Energy Research 2018, 2(1): 53-62 Source: http://www.scienceadvice.ca/uploads/eng/assessments andAdvances publicationsinand news releases/ hydrates 6

CDP

1.4 Depth (km)

0.80

0.90

Distance (km) 6

8

7

S SF

N 9 8 7 6 5 4

3

2 1

1.40

3

1.50

6

BSR

8

1.70

1.80

2.2 2.4

1.00

1.60

7

BSR 0.95

Velocity (km/s)

1 2 4 5

1.8

1.6

Sea Floor

Time (s)

5

4

0.75

2.0

1076

A. Schlesinger et al. / Marine and Petroleum Geology xxx (2012) 1e11

1.90

9 2004

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 profi (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 th were 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 whe the travel-time arrivals provide control on velocities.

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

1.05

In the 2-D velocity model shown in Figure 6, the most prom-

310e320 mbsf, corresponds to the BSR identified on the SC 2-D MS

inent feature a high-velocity Fig. 5.2 A Fig. classic identified BSR fromBSR Hydrate Offshore USA.is(After Rajputlayer between interfaces 5 and 6, reflections sections (Fig. 2a e reflection 6) and the 5. example A classicofexample of identified (AfterRidge Thakur, 2010). reflection data (Mosher et al., 2004). with a thickness of w110 m and anby average velocity of 1900 inferred observation ofm/s. a BSR (Haacke et al., 2007). 2009, reproduced with permission) Below interface 6, a velocity decrease of w130 Refractions produced at the top of this layer are observed on almost

m/s wa

Fig. 6 shows that the BSR marks transition to athelayer modelled the for a layer that has almost same thickness as the lay above (w100 m) (Fig. 6). The results are similar to those obtaine of reduced velocity below a high-velocity zone (Schlesinger continental margins. In-order to establish a geological framework for the region from the 1-D velocity models (Fig. 4), except that the 1-D velocitie affect the gas hydrate stability zone (Pecher et al., 1998, et al., 2012). The saturation of the free-gas is less sensitive where gas hydrates are found; one needs to study the distribution and variability of 2001). Similarly, an increase in the temperature will thin the to the seismic amplitude, but the amplitude of natural gas BSR, sediment deposition and distribution, diapirism, fluid-migration features. gas hydrate stability zone, although such temperature changes hydrate with different saturation is quite different (Boswell There are different geological indicators which help in studying the gas hydrate require thousands of years to propagate into sediments. Shortal., 2016). provinces. These indicators have been found worldwide and explained here et under; term changes in bottom water temperature and pressure can Both methane hydrate and free gas exist even where a clear also affect gas hydrate deposits (Ruppel, 2000; MacDonald et BSR is absent (Holbrook et al., 1996). The low reflectance, or al., 2005). blanking, above the BSR is caused by lithologic homogeneity China’s marine geological science and technology person- of the sediments rather than by hydrate cementation. The nel first drilled high-purity gas hydrate samples in the eastern average methane hydrate saturation above the BSR is relatively waters of the Pearl River Estuary in Guangdong Province and low (5 to 7 percent of porosity). obtained considerable control reserves through drilling. May 10, 2017, the China Geological Survey mined out natural gas 2.2 Geochemical Exploration from the South China Sea. July 9, 2017, accumulated natural gas production exceeded 300, 000 m3 in a continuous test for Geochemical exploration is performed by analyzing actual 60 days, and the well has been officially shut down. Acquired core samples. However, geochemical exploration serves an 6.47 million scientific experiment data set has laid a solid important role in gas hydrate exploration combined with foundation for further work. 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 inciden geophysical exploration. indicators hydrate profile. The travel-time inversion results show a small velocity contrast at the BSRGeochemical depth. Refractions are produced below interface 5of at 210e220 mbsf, where the velocity increas by 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 dissolved gas distribution, gas and organic compound this figure legend, the readerinclude is referred to the web version of this article.) 2. NGH Exploration Methods composition in sediments, chlorinity, salinity, alkalinity of pore Please cite this article in press as: Schlesinger, A., et al., Seismic velocities on the Nova Scotian margin to estimate gas hydrate and free ga fluid, and the isotope compositions of water. These indicators 2.1 Geophysical Exploration concentrations, Marine and Petroleum Geology (2012), doi:10.1016/j.marpetgeo.2012.03.008 inform us about the origins of natural gases and gas hydrate As an important future energy, NGH’s exploration is of formation mechanisms. great significance (Moridis et al., 2008; Lee et al., 2011). Gas During gas hydrate formation process dissolved ions such hydrate deposits are identified principally on the basis of their as Na+ and Cl− are excluded from the hydrate structure and acoustic expression. The phase boundary between methane only water molecules crystallize into cubic lattice structure hydrate and methane plus water gives rise to a prominent (Hesse and Harrison, 1981). Surrounding pore waters initially negative-polarity event known as a BSR. Besides, the addition become more saline during the process of hydrate formaof hydrate to pore fluids has been interpreted to cause acoustic tion. During hydrate crystallization the exclusion of salinity blanking, a suppression of sediment reflectivity. Fig. 5 is a (chlorine) and intake of δ 18 O is due to the solidfluid isotope classic example of identified BSR. fractionation that causes preferential uptake of the heavy Previous studies suggest that most of the BSR amplitude isotope δ 18 O in the solid phase and depletion in the fluid is due to the velocity reduction of the underlying free gas (Hesse, 2003). During the dissociation of hydrates the release (Singh et al., 1993; MacKay et al., 1994; Hyndman et al., of pure water in the host sediments reduces the chlorinity. 2001; Pecher et al., 2001). The presence of a free-gas zone During the dissociation of hydrates release of heavy isotope (FGZ) is an important part of the gas hydrate system, and is water and its mixing up with lower order isotopic water particularly important if the presence of gas hydrate is to be component results in enrichment of δ 18 O in the host sediments all OBSs. The velocity of the layer is significantly higher than the average velocity (1600 m/s) between the seafloor and the top of this layer (w220 mbsf). The base of this high-velocity layer, at


Overburden

Overburden

Overburden

Hydrate-bearing layer



Free gas zone

Water zone

Underburden

Underburden

(a)

57


Underburden

(b)

(c)

(d)

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

Gas

Hydrate Dissociation

(a)

Hot Water

Gas

Heater

Inhibitor

Gas



(b)

(c)

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

pore waters. The δ 18 O values increase with increasing depth and appreciable higher values recorded in the hydrate bearing sediments. The observed combined anomalies of chlorinity and δ 18 O in the pore water chemistry of the host sediments provides a reliable measure for identification of gas hydrates. Lower values of chlorine concentration in the hydrate stability zone may suggest the occurrence of hydrates even when the 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 the geologic setting of the reservoir for the purpose of developing optimal production strategies for each geologic condition (Moridis and Sloan, 2007; Moridis and Collett, 2013). The four classes are depicted in Fig. 7. Class 1-3 has overburden and underburden layers in the geologic setting. Class 1 reservoir consists of a hydrate-bearing layer with underlying free gas zone, class 2 reservoir consists of a hydrate-bearing layer with underlying water zone, and class 3 reservoir consists of one hydrate-bearing layer without underlying free-fluid phase zones. Class 4 reservoir consists of single hydrate-bearing layer.

3.2 Techniques of gas production Release and production of methane from hydrate-bearing formations are associated with hydrate dissociation. Different 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 reservoir pressure below the hydrate equilibrium pressure at the prevailing temperature, known as depressurization; (ii) raising the temperature above the hydrate equilibrium temperature at the existing pressure, normally known as thermal stimulation; and, (iii) shifting the thermodynamic equilibrium curve of hydrate by means of injection of inhibitors, such as methanol or brine, to decompose the hydrate. Combinations of these methods can also be used. In general, where conditions are suitable, it is believed that the depressurization technique would be the most economical and practical method in the production from gas-hydrate deposits (Grace et al., 2008).

3.2.1 Depressurization The depressurization method is currently regarded as the most promising method among others. However, it still exists several inevitable issues, such as subsidence during the depressurization and hydrate reformation due to endothermic depressurization event. All the problems should be solved in

58


3 0 0

G a s P r o d u c tio n (m L /m in )

2 5 0

2 0 0

1 5 0

1 0 0

5 0

0 0

2 0

4 0

6 0

8 0

1 0 0

T im e /m in

Fig. 11. Production rate curve of depressurization method (After Hao, 2016). Figure 2-1: Conceptual model for methane recovery from hydrates using depressurization

Fig. 9. Conceptual model for methane recovery from hydrates using depressurization10000 (After Tabatabaie, 2014).

Figure 2-1: Conceptual model forHydrate methane+recovery Water from hydrates using depressurization 9000 Heat Transfer 8000 (pi,Ti)

Pressure (kPa)

10000 7000

Hydrate + Water Heat Transfer

9000 6000

Pressure (kPa)

5000 8000

(pi,Ti)

4000 7000 3000

6000

2000

(pr,Tr)

(ps(t),Ts(t))

5000

Gas + Water

1000

4000 0 271 273 275 277 279 281 283 285 287 3000 269 (p (ps(t),Ts(t)) r,Tr) Temperature (K) 2000 Figure 2-2: Methane hydrate equilibriumGas curve+ Water

1000 0 269

271

273

275 13 277

279

281

283

285

287

Temperature (K)

Figure 2-2: Methane hydrate equilibrium curve

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

13

the future. The problems could be relieved by using combined methods. Such as the combination of the depressurization and the gas swapping for mitigating the subsidence, and the combination of the depressurization and the thermal stimulation for mitigating hydrate reformation. Class 1 reservoir is considered as the most promising reserve since the underlying free gas zone guarantees gas production and the T/P condition is close to the hydrate equilibrium condition (Moridis et al., 2007). In most cases, the depressurization method is the most promising method because it is the most simple and cost effective (Moridis et al., 2007). Class 2 is less effective in propagating depressurization front since the underlying free water zone continuously supply a large body of water (Moridis and Kowalsky, 2007). Class 3 reservoir would exhibit even slower propagation of depressurization front due to the absence of free gas or water zone that is more permeable than hydrate-bearing layers and can serve as a conduit for depressurization front (Moridis et al., 2007). Class 4 reservoir has been reported as the least effe-

ctive in depressurization, producing very large amount of water and small amount of gas (Moridis and Sloan, 2007). Fig. 9 shows a schematic representation of the depressurization method of gas production from Class 1 hydrate reservoirs. A well is completed in the free-gas zone. The depth of the hydrate layer can be found by superimposing the gashydrate equilibrium curve and the geothermal gradient (Demirbas, 2010). The base of the hydrate zone is at thermodynamic equilibrium pressure and temperature. As the gas is removed, the pressure in the free-gas zone is reduced to some pressure below the equilibrium pressure corresponding to initial temperature, causing the hydrate to decompose (Hong and Pooladi-Darvish, 2005; Moridis et al., 2007). Fig. 10 shows a gas-hydrate phase diagram illustrating the dissociation mechanism. As the picture depicts, hydrate exists only in a high pressure and low temperature environment. Once the pressure is lower than critical pressure (i.e. equilibrium pressure) or temperature is beyond critical temperature (i.e. equilibrium temperature), solid hydrate will decompose and produce natural gas and water. Hydrate decomposition needs to absorb heat from the surrounding environment. The temperature of the hydrate bearing layers will be reduced if there is a single depressurization. The NGH dissociation by depressurization can be divided into three stages: (1) Free gas produce in the early stage. With the reduction of system pressure, the associated free gas is rapidly produced and the gas production rate increases rapidly. (2) Gas release from hydrate dissociation in the second stage. Natural gas hydrate decomposition rate is larger because of the larger fractional surface area. (3) Surplus free gas release in the third stage. Decomposition rate gradually decreases with the smaller decomposition of the surface area.

3.2.2 Thermal Stimulation Simple depressurization appears promising in Class 1 hydrates, but its appeal decreases in Class 2 hydrates. The most promising production strategy in Class 2 hydrates involves co-


G a s P r o d u c tio n (S T D C C )

2 .5

X 1 0

4

2 .0

1 .5

1 .0 W it W it W it W it

0 .5

h d is h d is h o u t h d is

s ip a s ip a d is s s ip a

tio n tio n ip a t tio n

B C 2 B C 1 io n B B C 2

,Q = ,Q = C 2 ,Q =

0 1 0 0 0 ,Q = 1 0 0 0 1 0 0 0

0 .0 0

5

1 0

1 5

2 0

2 5

3 0

T im e (in d a y s )

59

lower toxicity and better performance in inducing hydrate dissociation owing to its higher density compared to methanol (Dong et al., 2009). Several factors have been identified to control the rate of dissociation, including the concentration and 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 widely found in nature (Qi et al., 2012). A molecular dynamic study has provided an insight into the possible roles of Na+ and Cl− on methane hydrate recovery (Yi et al., 2014). The possibility of using brine injection to recover gas from methane hydrate in pure water and Berea sandstone was also explored (Lee, 2009). The salt also inhibits hydrate formation and dissociation kinetically.

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

3.2.4 CO2 Injection 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 temperature is away from the hydrate stability region. When hydrate decomposes, the entrapped gas is released from water cages and flows through the wellbore to be recovered. External heat is supplied through wellbore or point sources. The thermal stimulation method dissociates hydrates by raising temperature above the hydrate equilibrium temperature. In the thermal stimulation method, the energy that takes in the dissociation and the production should not exceed the energy we can get from produced gases to meet a simple economic principle. The energy needed for the hydrate dissociation is governed by thermal characteristics of the hydrate-bearing region. Fig. 12 shows that the use of heat source increased the production significantly. The thermal stimulation method has the disadvantage of losing substantial amounts of the introduced energy in the injection path and surroundings. As a result, in this technique, only a small fraction of injected energy is usefully employed to dissociate the hydrate.

3.2.3 Chemical Injection As compared to depressurization and thermal stimulation, chemical inhibition as a recovery method has been relatively less studied. The chemical injection method injects inhibitors of gas hydrates to dissociate hydrates in the reservoir. Chemical inhibitor injection works by shifting the equilibrium curve towards higher pressures and lower temperatures, thereby destabilizing hydrate at natural conditions. There are two main types of inhibitors, namely thermodynamic inhibitors which alter the hydrate equilibrium conditions and kinetic inhibitors which slow down the rate of hydrate formation. Thermodynamic inhibitors are of particular interest to the applications for gas production. Two of the most common thermodynamic inhibitors used are methanol and ethylene glycol (Demirbas, 2010). Ethylene glycol (EG) is more commonly studied due to its higher availability in the market,

The thermodynamic feasibility of CO2 -CH4 replacement was fully proven through numerous studies. From the thermodynamic point of view, both pure CO2 and CH4 molecules typically form hydrates under corresponding conditions, and the enthalpy of CO2 hydrate formation (about -57.98 kJ/mol) is lower than that of CH4 hydrate formation ( -54.49 kJ/mol). It means CO2 hydrate is more stable than CH4 hydrate under the same 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 stability of the methane hydrate structure. (b) Dissociation of CH4 hydrate and CH4 molecule escapes from the hydrate cage. Both the large and small cages of methane hydrate rupture, and CH4 molecule escapes from the hydrate cage. (c) Rearranging of CH4 and CO2 in hydrate cage. In the reformed hydrate, CO2 molecules mainly enter the large cages of the hydrate, and CH4 molecules can enter both the small cages and the large cages of the hydrate, but mainly enter the small cages. (d) Diffusion of CH4 molecule from the hydrate surface to gas phase and CO2 molecule diffusion into deeper hydratebearing sediment layer. CO2 molecules diffuse into deeper hydrate 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 of mobile water. For high water saturation reservoir, gaseous CO2 will react with free water firstly which results in the inferior effect of replacement. In comparison, for low water saturation reservoir, the replacement effect is perfect and encouraged in CH4 hydrate exploitation. The techniques have their individual limitations such as high energy consumption, low gas recovery, low gas production rate and environmental issues. The combination of those techniques is considered as a better technique to enhance the

60


a

b

CO 2

c

CH 4

Large cage

d

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 thermal stimulation or depressurization, the combination technique is proven to be more efficient in natural gas production from NGH reservoirs (Li et al., 2016).

4. Challenges and Outlook With 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 characteristics which make it become “future energy”. The abundance of gas hydrate reserves is expected to be more than twice of the combined carbon of coal, conventional gas and petroleum reserves. Natural gas hydrate has changed the pattern of the world’s energy. The challenges and problems for gas hydrate exploration and development are listed as following: (1) For NGH’s exploration, although a potential for characterizations of hydrate reservoir using seismic indicators has been reported in literatures, characterizing based only on seismic survey results is less reliable. (2) For NGH’s development, it’s difficult to produce methane from the NGH reservoir and assess hydrate recovery rates. Besides, locating potential resources, quantifying hydrate content, keeping process safe from geomechanical impacts from hydrate dissociation are worth considering. (3) For the environment and application, Hydrate development should not only consider its economic benefits, but also the ways of transporting the extracted natural gas to the market. Whats more, development of natural gas hydrate may do more harm to environment. Methane gas greenhouse effect is much greater than the carbon dioxide gas. In the future, the researcher should pay more attention on how to develop nature gas from hydrate, such as do more work on hydrate simulation and hydrate production test. Besides, the properties of hydrates should be studied. The kinetics arena will represent the largest challenge for advancing the information on hydrates. Although we know quite a lot about what hydrates are, the question of how hydrates form is still

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

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51674227) and the Fundamental Research Funds for the Central Universities (Grant No. 2652015133). Open Access This article is distributed under the terms and conditions of the Creative Commons Attribution (CC BY-NC-ND) license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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