gas hydrates as potential energy resource and trigger of … · methane hydrate (ch 4 nh 2 o)...
TRANSCRIPT
Gas Hydrates as Potential Energy Resource
and Trigger of Submarine Slope Failure
Nagoya Institute of Technology
Prof. (Assistant)
Hiromasa Iwai
The First JSCE-CICHE Joint Workshop
~Sustainability and Disaster Reduction~
May 22-23, 2016
@Garden Villa Hotel, Kaohsiung, Taiwan
1/24
What is Gas Hydrate ?
Gas Hydrate consists of guest gas trapped
inside cage-like structures of water molecules.
: H
: C
: O
n=5.75 for structure I hydrate
4 2 ( )CH nH O hydrate
2 4( ) ( )nH O water CH gas
Burning MH
The gas interacts with the water under the
conditions of low temperature and high
pressure to form ice-like structure.2/24
Gas Hydrate dissociation process
4 2 4 2( ) ( ) ( )CH nH O hydrate CH gas nH O liquid
1m3 165m3 0.8m3
A unit volume of gas hydrate dissociates into approximately 165 times the
volume of guest gas and 0.8 times of water (at 0C and atmospheric
pressure).
Solid Gas Water(Liquid)
Methane Hydrate (CH4nH2O)
Carbon Dioxide Hydrate (CO2nH2O)
: Recognized as a potential future energy resource
: Recognized as a new material for CCS
Gas hydrate can exist only in a low temperature and high pressure
conditions. The gas hydrate will dissociate by heating and depressurizing
3/24
MH production test in Japan
The first offshore production test site.
North latitude: 3356
East latitude: 13719
Japan Oil Gas, and Metals National Corporation (JOGMEC) conducted the
first offshore methane hydrate production trial in March 2013 in the eastern
Nankai-Trough, Japan.
In the test, depressurization method was
adopted for methane gas production.
MH in sand layer
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MH-bearing layer
Water flow
Gas flow
Dissociation frontMH-bearing layer
Water flow
Gas flow
Settlement
Dissociation frontMH-bearing layer
Water flow
Gas flow
Dissociation front
Settlement
Excess pore pressure
Sand
Cracks
Depressurization method
1,000m
300m
50~60m
The seabed ground surface is at a
water depth of about 1,000m in
eastern Nankai-Trough.
MH-bearing layer lies at a depth of
300m from the top of the ground
surface with a thickness of 50 or 60m.
270 275 280 285 290 295 300 305 310 3150
5
10
15
20
25
30
gas water
hydrate
Po
re p
ress
ure
(M
Pa)
Temperature (K)
Equilibrium curve of
Methane Hydrate
Pump up the sea-water inside the
production well, and the pore
pressure in the MH-layer will decrease.
5/24
Gas hydrates-bearing layer
Slope failureBubbles
Turbidity current
Tsunami
Production well
Settlement
×Cut submarine cables
and pipelines
Geological and Geotechnical problemsIn contrast to the aspects of the expected energy source, gas hydrates have
been believed to play a important role in the failure of submarine sediments.
The slides can cause tsunami…
“In 1979 a 0.15-km3 slide off Nice airport caused a tsunami that killed 11people.”
Nisbet, E.G. and Piper, D.J.W. 1998, Nature, 392(6674), pp.329-330.
Gas hydrates break down was involved in the creation of the megaturbidite.
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Before dissociation
Soil particlesPore water
Gas hydrate
After dissociation
Gas hydrate morphology in sediments
Gas hydrate in the pores works as
additional soil particles.
Bond between soil particles.
Hydrate morphology affects the stiffness,
strength, and dilation.
Dissipation of the hydrate particles will
cause decrease of the material density.
Lost of bonding effects or load bearing
effects will reduce the stiffness and
strength.7/24
We have developed a temperature pressure controlled triaxial apparatus,
which can recreate low-temperature and high-pressure environment like
deep seabed-ground.
We have conducted a series of undrained triaxial compression tests and
dissociation tests on gas hydrate-containing sand specimens.
CO2-hydrate is used instead of methane hydrate considering safety of the
experiment, because it is non-flammable gas.
For the purpose of safe and economical production of methane gas from
the MH reservoir, further investigations related to mechanical properties
of gas hydrate-bearing sediments are required
In addition, it is essential to understand dissociation-deformation
behavior, in order to clarify the relationship between gas hydrate
dissociation and submarine slope failure.
Main objectives for this research
8/24
Temperature and pressure controlled triaxial apparatus
Testing Apparatus
9/24
Confining fluid
thermal control tank
-35℃~ +50℃ CO
2 g
as
Confin
ing p
ressure in
tensifier
Volume change
Po
re pressu
re inten
sifier
35mm
70mm
Cell pressure
transducer
Pore pressure
transducer
Therm
oco
uple
Co
olin
g flu
id
Load cell
200kN
P
Gas flow meter
Po
re pressu
re inten
sifier
①② ③
CO
2 d
issolv
ed
water tan
k
Schematic diagram of the apparatus
10/24
• Toyoura sand for the host material.
• Initial moisture content : 11%
• Initial void ratio : 0.67
compacted in a stainless mold using the moist tamping
method with a diameter of 35mm and height of 70mm.
Material properties
Density of soil particles 2.64
D50 (mm) 0.197
Diameter (mm) 35
Height (mm) 70
Initial void ratio 0.67
Density of CO2 hydrate 1.12
Target value of initial hydrate saturation 0.55
Initial moisture content w (%) 11.0
3cms g
0e
3cmH gH
rS
*Hydrate saturation
HH
r V
VS
V
φ= 35mm
H = 70mm
Sample preparation
11/24
Formation process
-5 0 5 10 15 200
2
4
6
8
10
12
(1C, 10.0MPa)
(c)
CO2 gas-CO
2 liquid
water(ice)-CO2 gas-hydrate
(1C, 2.3MPa)(b)
Po
re p
ress
ure
[M
Pa]
Temperature [C]
hydrate stability region
(a)
(10C, 2.3MPa)
12/24
Case-No. Void ratio Back pressure Initial mean effective stress Temperature Hydrate saturation
Case-1 0.74 10 1.0 1.0 0.0
Case-2 0.72 10 2.0 1.0 0.0
Case-3 0.72 10 3.0 1.0 0.0
Case-1H 0.76 10 1.0 1.0 34.6
Case-2H 0.73 10 2.0 1.0 27.8
Case-3H 0.73 10 3.0 1.0 28.5
0e0 [MPa]wu 0 [MPa]p o[ C]T
H
rS
Experimental conditions
Case-1, Case-2, Case-3 : water saturated sand specimens (without hydrate)
Case-1H, Case-2H, Case-3H contain CO2-hydrate in the pores.
Initial mean effective stresses are set to be 1.0MPa, 2.0MPa, or 3.0MPa.
Strain rate is set to be 0.1%/min among all the cases.
6 cases of undrained triaxial compression tests are conducted.
13/24
0 1 2 3 4 5 60
1
2
3
4
5
6
min/%1.0a
1.20
1
Dev
iato
r st
ress
q [
MP
a]
Mean effective stress p' [MPa]
Case-1, e=0.74, SH
r=0.0 %
Case-1-H, e=0.76, SH
r=34.6 %
Case-2, e=0.72, SH
r=0.0 %
Case-2-H, e=0.73, SH
r=27.8 %
Case-3, e=0.72, SH
r=0.0 %
Case-3-H, e=0.73, SH
r=28.5 %
0 5 10 15 200
1
2
3
4
5
6
min/%1.0a
Dev
iato
r st
ress
q [
MP
a]
Axial strain a [%]
Case-1, e=0.74, SH
r=0.0 %
Case-1H, e=0.76, SH
r=34.6 %
Case-2, e=0.72, SH
r=0.0 %
Case-2H, e=0.73, SH
r=27.8 %
Case-3, e=0.72, SH
r=0.0 %
Case-3H, e=0.73, SH
r=28.5 %
Stress-strain relationship & stress path
Hydrate-bearing sand
Water-saturated sand
Both stiffness and strength in hydrate-bearing sand are greater than that of water-
saturated sand specimens.
The stress paths in hydrate-bearing specimens express larger dilation than that of
water-saturated specimens in each confining pressure.
The hydrate may act like additional small particles in the sand structure.14/24
0 10 20 30 40 50
100
200
300
400
500
600 Case-1, p
'
0=1.0MPa Case-1H, p
'
0=1.0MPa
Case-2, p'
0=2.0MPa Case-2H, p
'
0=2.0MPa
Case-3, p'
0=3.0MPa Case-3H, p
'
0=3.0MPa
Sca
nt
modulu
s E
50 [M
Pa]
Hydrate saturation SH
r [%]
0 10 20 30 40 500.5
1.0
1.5
2.0
2.5 Case-1,Case-1H, p
'
0=1.0MPa Case-2, Case-2H, p
'
0=2.0MPa
Case-3, Case-3H, p'
0=3.0MPa
Ehyd
rate
50
/Esa
nd
50
Hydrate saturation SH
r [%]
Scant modulus E50
0 5 10 15 200
1
2
3
4
5
6D
evia
tor
stre
ss q
[M
Pa]
Axial strain a [%]
Case-1, e=0.74, SH
r=0.0 %
Case-2, e=0.72, SH
r=0.0 %
Case-3, e=0.72, SH
r=0.0 %
0 5 10 15 200
1
2
3
4
5
6
Dev
iato
r st
ress
q [
MP
a]
Axial strain a [%]
50
sandE
50
hydrateE
15/24
0 5 10 15 20-2
-1
0
1
2
min/%1.0a
Exce
ss p
ore
wat
er p
ress
ure
uw [
MP
a]
Axial strain a [%]
Case-1, e=0.74, SH
r=0.0 %
Case-1-H, e=0.76, SH
r=34.6 %
Case-2, e=0.72, SH
r=0.0 %
Case-2-H, e=0.73, SH
r=27.8 %
Case-3, e=0.72, SH
r=0.0 %
Case-3-H, e=0.73, SH
r=28.5 %
0 5 10 15 200.0
0.5
1.0
1.5
2.0
Case-1, e=0.74, SH
r=0.0 %
Case-1-H, e=0.76, SH
r=34.6 %
Case-2, e=0.72, SH
r=0.0 %
Case-2-H, e=0.73, SH
r=27.8 %
Case-3, e=0.72, SH
r=0.0 %
Case-3-H, e=0.73, SH
r=28.5 %min/%1.0a
Str
ess
rati
o q
/p'
Axial strain a [%]
Excess P.W.P. vs strain, Stress ratio vs strain
The reduction in the pore water pressure
becomes larger in the specimen with
hydrate.
The P.W.P profile in the hydrate-bearing
specimen behaves like a dense sand
comparing the one without hydrate.
In the case of the same confining
pressure, the maximum stress ratio of
the hydrate-bearing specimen becomes
larger than that of the water-saturated
specimen.
In the critical state, the stress ratio
settled at the same value, namely, 1.2,
among all the cases.
16/24
0 5 10 15 20 25 30 35 400
5
10
15
2.0 MPa
Case-3-H
Case-2-H
Case-1-H
min/%1.0aStr
ess
rati
o i
ncr
easeR
[
%]
Hydrate saturation SH
r [%]
Constant strain rate test (0.1 %/min)
Strain rate change test (0.1 %/min)
max
max
1 100H
R
0 5 10 15 20 25 30 35 400
5
10
15
20
25
30
Case-1-H
2.0 MPa
Case-1-H
Case-2-H
Str
ess
rati
o i
ncr
easeR
q [
%]
Hydrate saturation SH
r [%]
Constant strain rate test (0.1 %/min)
Strain rate change test (0.1 %/min)
Case-3-Hmin/%1.0a
max
max
1 100H
q
qR
q
Strength increase ratio vs Hydrate saturation
Increase ration in the maximum deviator stress
Increase ration in the maximum stress ratio
Increase ration in the maximum deviator stress
seems to depend on both the confining
pressure and the hydrate saturation.
Not only sand particles but also gas
hydrate might have the dependency
on the confining pressure.
Increase ration in the maximum stress ration
increases with increase in the hydrate
saturation.
17/24
Summary – undrained triaxial tests –
In order to clarify the mechanical properties of gas hydrate-bearing sediments,
we conducted undrained triaxial compression tests with different confining
pressure and different hydrate saturation.
- Conclusions -1. Both stiffness and strength of hydrate-bearing specimen becomes larger
comparing with that of the specimens without hydrate.
2. In the case of hydrate-bearing sand, the excess pore water pressure is
reduced largely. It may be because that the hydrate acts like additional
small particles, and the apparent density of the sample increases.
3. Increase ratio in the maximum deviator stress may depend on both the
confining pressure and hydrate the saturation.
4. Increase ratio in the maximum stress ratio increase with increase in the
hydrate saturation.
18/24
Case-No.Initial
void ratio
Initial pore
pressure
Effective confining
pressureTemperature
Hydrate
saturation
Case-0 0.69 2.0 0.1 18.0 0.0
Case-1 0.75 4.0 3.0 18.0 39.1
Case-2 0.75 4.0 3.0 18.0 42.2
Case-3 0.79 4.0 3.0 18.0 15.4
Case-4 0.76 6.0 3.0 18.0 32.1
H
rS0e [MPa]iniP [MPa]co[ C]T
Experimental conditions
Case-0 is the one without hydrate
Case-1~Case-3: the initial pore pressure is 4.0MPa
Case-6: the initial pore pressure is 6.0MPa
Case-1~Case-4: the effective confining pressure are the same; 3.0MPa
Undrained conditions for water and gas
19/24
Experimental results -Pore pressure-
0 1 2 3 4 5
0
1
2
3
4
5
6
7 Case-0, S
H
r= 0.0%, Case-1, S
H
r=39.1%, Case-2, S
H
r=42.2%
Case-3, SH
r=15.4%, Case-4, S
H
r=31.9%
Ex
cess
po
re p
ress
ure
[M
Pa]
Time [hour]
I. In Case-0(without hydrate) , any buildup of the pore pressure was not observed.
II. The pore pressure is almost constant until one hour. After 1-hour the pore pressure
begins to increase drastically, in Case-1~Case-4.
III. In Case-2, increase rate of the pore pressure seems to be the largest. It is because
that the hydrate saturation is the largest among the cases
I
II
III
20/24
-5 0 5 10 15 200
2
4
6
8
10
12 CO
2 liquid boundary, CO
2 hdrate boundary
Case-1 (SH
r=39.1%), Case-2 (S
H
r=42.4%),
Case-3 (SH
r=15.4%), Case-4 (S
H
r=32.1%)
end
Pore
pre
ssure
[M
Pa]
Temperature [C]
Initial
The first increase in the pore pressure might be caused
by gasification of remaining liquid CO2 .
Little increase in the pore pressure can be found until the
temperature reaching the CO2-hydrate boundary.
Temperature-pore pressure path
21/24
0 1 2 3 4 5 6 7 8-4
-3
-2
-1
0
1
2
3
4
5Excess pore pressure: Case-3, Case-4
Axial strain: Case-3, Case-4
Time [hour]
Ex
cess
po
re p
ress
ure
[M
Pa]
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
Ax
ial
stra
in [
%]
Compression
Expansionpeak
MAX: 2.8MPa
MAX: 0.45% in expansion
Tensile strain can be observed with increase in the excess pore pressure.
When the pore pressure reaches at the maximum, the tensile strain also reached at
the peak.
Under undrained conditions, the hydrate dissociation may lead the large build up
of the pore pressure, and decrease in the density of the sediments, simultaneously.
Excess pore pressure and axial strain
22/24
1. We have developed a temperature and pressure controlled triaxial
apparatus.
2. This new triaxial cell can make the environment where gas
hydrates can form, and we have carried out formation and
dissociation tests using carbon dioxide hydrate.
In order to investigate the fundamental behavior of the gas hydrates
containing soils associated with hydrates dissociation …
- Conclusions -1. The hydrates dissociation will cause a significant increase in the pore
pressure, and result in a significant reduction in the effective confining
pressure under undrained conditions.
2. The reduction in the effective confining pressure is a kind of liquefaction
phenomena, which may lead to instability of the marine sediments.
3. The result indicates that both increase in the pore pressure and decrease
in the density of the sediments might be caused simultaneously.
Summary – dissociation tests –
23/24
Thank you for your kind attention.
24/24