THE BASIC KNOWLEDGE

ABOUT GAS HYDRATE

Hailong Lu

Steacie Institute for Molecular Sciences

National Research Council Canada

Gas hydrate (GH) P, T Gas + Water

The occurrence of natural gas hydrate around the world (Updated

from Collett, 2002)

Natural gas hydrate: the largest organic carbon reserve on the Earth

Slope failure caused by mass dissociation of gas

hydrate (Kvenvolden, 1993)

Pingo on the shelf of Beaufort Sea,

N.W.T.

Gas hydrate is taken as a reason for the

formation of Pingo (Paull et al., 2007)

Pipeline blockage caused by the formation of gas hydrate (AGAR Corporation)

Source: CBC TV

Gas hydrate formers

(Updated from Sloan &

Koh, 2007)

5 2/3 H2O

5 3/4 H2O

7 2/3 H2O

17 H2O

C6: cyclohexane, methyl-cyclopentane

C7: methyl-cyclohexane

cyclopentane

+

+

+

+

sI

sII

sH

dodecahedron

tetrakaidecahedron

hexakaidecahedron

icosahedrondodecahedron

Water molecules

Fig. 1 The most stable (lowest energy) hydrogen-bond arrangement

for the cluster geometries with eight, ten and twelve water molecules

as obtained from B3LYP/cc-pVDZ computations. (Lenz and Ojamae,

2005）

Fig. 4 The structures with the

most stable hydrogen-bond

arrangements for the

different shapes and cluster

sizes from B3LYP/cc-pVDZ

optimizations. (Lenz and

Ojamae, 2005)

FIG. 6. Density distribution of the orientations of a water molecule’s dipole moment

vector in the hydration shell around methane in aqueous solution. (Koh et al. 2000)

The formation of methane hydrate

Methane gas + water

↓

Methane dissolving in water

↓

Cage constructed with 16±1 water molecules surrounding

methane molecule

↓

Aggregation of these cages into methane hydrate (20 water

molecules in small cage，24 water molecules in large cage）

Koh et al. (2000), Dec and Gill (1985), Bridgeman et al. (1996)

Pm3n

a=12.0 Å

51262

512

Cubic structure I

Fd3m

a=17.0 Å

51264

512

Cubic structure II

P6/mmm

a=12.0 Å

c=10.0 Å

51268

512 435663

Structure H

Schicks &

Ripmeester, 2004

Chou et al. 2000

Figure 2. Packing and

schematic view of the new

space-filling polyhedron.

(Kurnosov, 2004)

At superhigh pressure（0.8 GPa)

THF hydrate appearing with a new

strcuture, orthorhombic system

Pnma， a=12.54, b=11.44, c= 6.60 A.

At super-high pressure crystalline gas hydrate can be

converted to amorphous material（Tanaka & Amano, 2002)

At certain ratio mixed CH4-C2H6 hydrate can be changed

from sI to sII (Subramanian et al. 2000)

The unit-cell parameters of methane hydrate changing with

pressure（Klapproth et al. 2003)

CO2 occupancy rates in cages changing with pressure

(Klapproth et al. 2003)

Occupancy rates of methane hydrate changing with

pressure (Klapproth et al. 2003)

Table 1-2 Ratios of molecular diameters to cavity diameters for natural gas

Molecules including natural gas hydrate formers ( Cited from

Sloan, 1990)

Molecule

Guest

Size(A°)

Molecular diameter/Cavity diameter

Structure I Structure II

(2)*512 (6)*51262 (16)*512 (8)*51264

CH4 4.36 0.886 0.757 0.889 0.675

H2S 4.58 0.931 0.795 0.934 0.708

CO2 5.12 1.041 0.889 1.044 0.792

C2H6 5.5 1.118 0.955 1.122 0.851

C3H8 6.28 1.276 1.090 1.280 0.971

c-C3H6 5.8 1.178 1.007 1.182 0.897

i-C4H10 6.5 1.321 1.128 1.325 1.005

0

10

20

30

40

50

60

70

250 260 270 280 290 300 310

T (K)

1

2

3

45

6

1. CH4 (sI)

2. CH4-methylcyclopentane (sH)

3. natural gas hydrates (sII-sH)

4. 95.2% CH4-propane (sII)

5. CH4-6% isobutane (sII)

6. CH4-cyclopentane (sII)

Figure 3 The dissociate conditions of gas hydrates from Barkley Canyon, offshore

Vancouver Island.

Lu and

Matsumoto

(2001)

Gas hydrate (GH) ← Gas + Water

Gas hydrate

2 Phases (gas + liquid)

1 Phase

Gas phase （gas + water

vapor）

Liquid （gas dissolved in

water）

In natural environment gas hydrate can form either from 2

phases or 1 phase

Seafloor

BSR3 phases （GH+G+L)

2 phases （GH+L) or

3 phases （GH+G+L)

3 Phases （GH+G+L)

Sea surface

The occurrence of gas hydrate in natural

environment

Gas Hydrate

Materials

TimeStability (P-T)

Space

Conditions to be met for the formation of gas hydrate

• Materials: gas, water

• Time: experienced for pore water to

be saturated with hydrocarbon

• Space: sediment type, sediment

structure

• Stability condition: gas composition,

sediment, pore water composition, and

mineral composition.

Materials

Water: always available in marine environment

Gas: biogenic, thermogenic. It seems most of the recovered gas

hydrates are composed of biogenic gas as indicated by gas

composition and isotope composition, however it doesn’t mean

hydrate is formed with the in situ gas.

Most of the gas hydrate is formed with the gas transported from

deeper sediments, either biogenic or thermogenic.

Wallman et al

(2006)

Profiles of CH4 and SO42- along sediment section (Schmidt et al., 2005)

Sulfate-

Methane

Interface

Sea surface

Sea floor

Sediments

Sea water

BSR

(Base of gas

hydrate zone)

GH

The occurrence of gas hydrate in marine environment

Most of the recovered natural gas hydrate were composed

dominantly CH4, however hydrates with other hydrocarbons

of C2+, up to C7, did be recognized existing in natural

environment.

Because C2+ hydrocarbons are thermogenic origin, there are

more opportunities to find gas hydrate in deep sediments.

0

100

200

300

400

500

600

10 20 30 40 50

2theta (degree)

R694

Figure 1. XRD spectra of the gas hydrates from Barkley Canyon, offshore Vancouver

Island (Lu et al., 2007)

Time

In laboratory:

Time is referred to Induction time, needed for hydrate nucleate

since experiment starts, on a scale of minutes, hours, sometimes

days.

In nature:

As compared with the scale of geological time, years, even

million years, the induction time can be negligible. However, 1)

for hydrate formation a condition, pore water to be saturated

with hydrocarbon, has to be met. For this certain time is needed;

2) Time issue will be met upon discussing the kinetics of gas

hydrate growth.

Space

1. Space availability determines the appearance of gas

hydrate in sediments,

2. Sediment, where gas hydrates exist, controls the

saturation level of gas hydrate in sediments.

Visible gas hydrate

Massive gas hydrate occurring

at seafloor, Barkley Canyon,

Cascadia (Chapman et al.,

2004)

Massive gas hydrate

occurring in sediments at

a seepage site, Joetsu

Basin, Japan Sea

Nodular gas hydrate

occurring in

sediments at a cold

vent field, Cascadia

Vein-like gas

hydrate in silty clay,

K-G Basin, offshore

India

Thin film-like gas

hydrate occurring at

the bedding plane in

silty clay, K-G Basin,

offshore India

Invisible gas hydrate – In-pore gas hydrate

Gas hydrate in volcanic

ash, offshore Andaman

Island

Gas hydrate in silty clay,

K-G Basin, offshore

India

Gas hydrate in sand,

Mallik, Mackenzie

Delta, N.W.T., Canada

Gas hydrate in sand,

Nankai Trough, offshore

Japan

Gas hydrate in clay silt,

South China Sea

The types of natural gas hydrates

1）in pore gas hydrate

2) locally aggregated gas hydrate

In-pore gas hydrate

• dispersed

• pore-filling

Existing in the pore of sediments, no altering of original sediment

structure. Because the resolution of human eyes is ~150 μm and

generally the pore size of sediments is at the scale of μm and even

nm, in-pore gas hydrate is not identifiable by naked eyes.

Locally aggregated gas hydrates

Gas hydrates that are identifiable by eyes directly and appear as

certain shape, for example nodular, platy, vein-like, massive, etc.

The size of gas hydrate aggregate is obviously larger than the

normal pore size of sediments, and the original structure of

sediments has been altered, generally related to faults,

fractures, and other sediment structures with large space.

Collet et al. (2000) revised from Malone (1990)

Can gas hydrate grow over the confinement of sediment particles?

Sediment particles

Gas hydrate

?

Due to the density difference between sI gas hydrate (~0.93 g/cm3) and pore

water (~1), the formation of gas hydrate will result in a volume increase. This

volume increase may cause the sediment particles over pressurized

However in most cases, due to the slow growth, the over pressure is so

small that it is not strong enough to push sediment particles away to create

larger space for hydrate growth.

The occurrence of gas hydrate in natural environment

Hydrocarbon flux

Pore filling

gas hydrate

Massive gas hydrate

Vein-like gas hydrate

Massive gas hydrate

Not to scale

Nodular, tabular

gas hydrate

0 5 10 15 20 25 30

T (degree celsius)

Seafloor

Water-temperature profile

Stability curve of the

sII & sH mixture

Stability curve of methane

hydrate in seawater

Geothermal gradient

BGHS-1

BGHS-2

0

200

400

600

800

1,000

1,200

1,400

Lu and

Matsumoto

(2005)

Lu and Matsumoto (2002)

40

50

60

70

80

90

100

6 7 8 9 10 11

P (

ba

r)

T( C)

CH4 hydrate in pure water

(Sloan, 1998)

CH4 hydrate in Nankai Trough sediments (#49)

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

T (C)

Methane hydrate stability condition in

water-saturated Na-montmorillonite

Methane hydrate stability condition in

water (Sloan, 1998)

Figure 3. Methane hydrate stability condition in water-saturated Na-montmorillonite.

80

85

90

95

100

105

110

10 11 12 13 14 15

T (C)

Pure water

Water-saturated kaolinite

Figure 4. The stability condition of methane hydrate in water saturated kaolinite.

85

90

95

100

105

10 11 12 13 14 15

T (C)

Figure 5. The stability condition of methane hydrate in water-saturated Ca-Montmorillonite.

Water saturated Ca-montmorillonite

(this research)

Pure water (Sloan, 1998)

85

90

95

100

105

10 11 12 13 14 15

T (C)

Figure 5. The stability condition of methane hydrate in water-saturated Ca-Montmorillonite.

Water saturated Ca-montmorillonite

(this research)

Pure water (Sloan, 1998)

0

20

40

60

80

100

120

0 2000 4000 6000 8000 1 104

1.2 104

Hyd

rate

sa

tura

tio

n (

%,

po

re s

pa

ce

)

T2 (us)

Silica sands

Natural sediments

The relationship between hydrate saturation and water

relaxation time (T2)

Thanks!