EFFECT OF GAS HYDRATES DISSOCIATION ON SEAFLOOR SLOPE
STABILITY.
N. SULTAN, P. COCHONAT, J.P. FOUCHER
IFREMER, BP 70, 29280 Plouzané Cédex, France
J. MIENERT
Department of Geology, University of Tromsø, N-9037 Tromsø, Norway
H. HAFLIDASON, H.P. SEJRUP
Department of Geology, University of Bergen, N-5007 Bergen, Norway
Abstract
We present a theoretical study of the thermodynamic chemical equilibrium of gas
hydrate in soil by taking into account the influence of temperature, pressure and pore
water chemistry. The second part of the paper shows an application of the model
through a back-analysis of the giant Storegga Slide on the Norwegian margin. Two of
the most important changes during and since the last deglaciation (hydrostatic pressure
due to the change of the sea level and the increase of the sea water temperature) were
considered in the calculation.
1. Introduction
Quantitative studies of the dynamics of gas hydrate in marine sediments can be grouped
into two categories. The first of these models take into account methane conservation,
methane advection-diffusion coupled with heat transfer (Rempel and Buffett (1998) and
Xu and Ruppel (1999)). An alternative simpler category, deals with models accounting
for thermal non steady-state regime (Chaouch and Briaud (1997) Delisle et al. (1998)).
By neglecting the effect of gas components and concentration on the gas hydrate
stability law, the second category of models are inadequate by regarding mainly the
following three points:
- The low concentration of gas in the upper meters of sediment, which can be
related to the methane exchange between bulk water and seawater column, can
prevent the formation of gas-hydrates. Thus, the gas-hydrate stability zone
from p-T conditions does not coincide with the hydrate occurrence zone.
- The hydrate fraction, which depends on the gas concentration within the
sediment column, is often improperly considered as constant.
- The excess pore pressure generated by the melting of the gas hydrate depends
on i) the hydrate fraction and ii) on the gas solubility. By considering only the
energy conservation equation, it is impossible to evaluate this excess pore
pressure.
Therefore, a numerical model of the formation or dissociation of gas hydrate, which
takes into account the influence of temperature, pressure, pore water chemistry, and the
pore size distribution of the sediment is developed. This model fully accounts for the
103
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Sultan et al.
latent heat effects. The model allows for the evaluation of the excess pore pressure
generated during hydrate melting using the Soave’s equation of state.
2. Thermodynamic model for gas hydrate phase stability.
For material undergoing a phase transformation, the conservation of energy of twophase mixture can be expressed in terms of temperature and total volumetric energy.
The form of the conservation of energy is given by the following equation:
∂η
C p ∂T =∇ κ ∇T +φL
[1]
∂t
∂t
(
)
where L is the latent heat and C p and κ are respectively the volumetric heat capacity
and the thermal conductivity of the medium.
For equations [1] φ is porosity and η is the hydrate fraction which is calculated from the
two-phase chemical potential equilibrium equation:
2σVl
∆µ H + ∆µ W =−
cosθ
[2]
rRT
where µ W is the chemical potential of water in the aqueous liquid which is independent
of sediment pore size, µ H is the chemical potential of water in hydrates (Van der Waals
and Platteeum model 1959) and the right-hand term of equation 2 corresponds to the
capillary effects on the hydrate phase equilibrium condition (Henry 1999). σ is the
surface tension of water-ice interface (Henry,1999), θ is the porous host-water contact
angle, r is the pore radius and Vl is the molar volume of water.
In the following, the effect of gas components and salinity on the gas hydrate phase
stability is studied.
In the first calculation, three gas components were tested (1: for pure methane, 2: 98%
of methane and 2% of ethane, 3: 96.7% of methane and 3.2% of ethane). Simulation
results of the stability curve of the three cases are shown in Figure 1. One can see that
the effect of the gas components cannot be neglected in the thermodynamic formulation
of the gas hydrate phase stability. Indeed for a temperature of 280°K, a change of
around 18% of the pressure of hydrate formation was observed between the first and the
third case. In the second calculation, the effect of the salinity on the stability curve of
methane hydrate is considered. Three cases were tested using the developed model (1:
0% weight of NaCl, 2: 3% wt NaCl and 3: 3.5% wt NaCl). Once again, simulation
results presented in Figure 1-b show the high sensitivity of the gas hydrate stability
curve to the change of salinity. For a constant temperature of 280 K, a change of around
15% of the pressure of hydrate formation was observed between the first and the third
case. Experimental results obtained by Dickens and Quinby-Hunt (1994) for methane
hydrate stability conditions in seawater (Salinity in 3.35% wt) are presented in Figure 1b. For all the pressure range, the temperature is 1.1 ºC higher for the seawater compared
to that of pure water. Figure 1-b shows that the thermodynamic predictions obtained
with the model developed in this paper are consistent with the experimental results of
Dickens and Quinby-Hunt (1994).
In this work, the excess pore pressure generated by the melting of gas hydrate was
calculated from gas solubility and the ratio of gas to water in the hydrate phase. Handa
Effects of gas hydrates dissociation on seafloor slope stability
105
(1989) showed that the ratio of methane to water in the hydrate phase are around 150
times greater than the ratio of methane to water in the aqueous solution. Therefore, the
melting of the hydrate phase will generate a huge quantity of methane, which is much
higher than the solubility of the dissolved gas in the aqueous solution. As a
consequence, at short term, and under the assumption of no gas diffusion and no volume
change of the sediment, it is possible to evaluate the excess pore pressure generated by
the hydrate melting using Soave’s (1972) state equation.
8
8
(a)
COMPONENTS
(b)
COMPONENTS
100% Methane
100% Methane + 0% wt NaCl
98% Methane 2% Ethane
100% Methane + 3% wt NaCl
96.7% Methane 3.2% Ethane
100% Methane + 3.5% wt NaCl
6
P (MPa)
P (MPa)
6
4
4
OBSERVED (Dickens et al. 1994)
Pure water
Sea water
2
2
272
274
276
278
T (K)
280
282
272
274
276
278
280
282
T (K)
Figure 1. a) Effect of gas components on the stability law of gas hydrate and b) effect of salinity on the
stability law of methane hydrate.
3. Storegga Slide
3.1
GEOLOGICAL SETTING AND GEOTECHNICAL DATA
The Storegga Slide covers an area of 85-90,000 km2, which is one of the world’s biggest
underwater slides with its overall volume of 3300 km3 and an estimated area of slide
scar of 30000 km2 (Haflidason et al. 2002). This complex slide has earlier been
interpreted to be the product of three slide events (Bugge 1983), but during the last
couple of years, extensive stratigraphical and chronological studies aimed to understand
the continental margin stability and the sedimentary processes within the Storegga Slide
area were carried out by the University of Bergen. A number cores (gravity and Selcore)
have been collected both inside and outside the slide area for this purpose. The objective
of the dating project was to verify the age of the main morphological slide structures of
the Storegga Slide area. Haflidason et al. (2001, 2002) show that the Storegga Slide has
been activated/ mobilised within the same age interval 7.300 radiocarbon age (14C) BP
or ca. 8150 cal. yrs BP. The trigger mechanisms that initiate the Storegga slide in this
area are not well understood. While some authors associate the Storegga slide to the
excess pore pressures caused by gas-hydrate dissociation after a thermal warming since
last deglaciation, other authors consider that the Storegga Slide may have been triggered
by offshore earthquakes. Indeed, the Storegga area has occasionally been susceptible to
high-magnitude seismicity. On the other hand, the hypothesis of the gas hydrate at the
origin of the Storegga slide is defensible. This is supported by a well-defined bottom-
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Sultan et al.
simulating reflection (BSR) identified on seismic profiles from the northern flank of the
Storegga Slide (Bugge 1983; Mienert and Bryn, 1997, Mienert et al. 1998, Posewang
and Mienert 1999, Bouriak et al. 2000 among others).
In this paper, the scenario of the gas hydrate melting as the origin of the Storegga Slide
(Mienert et al., 2001) is tested. The design geotechnical and geochemical data used in
the calculation are presented in Table 1. These design geotechnical data were
determined from the available geotechnical data (from ODP sites and boreholes of the
“failure layer” consortium).
Table 1. Design parameters using in the calculation. c’: is cohesion, ϕ : is internal friction angle, φ : is
porosity , γ : is bulk density
1
DEPTH (m)
0
γ (g/cm3)
0.6
φ
0.6
c’ (kPa)
7
ϕ (°)
28
20
20
0.831
1.01
0.46
0.394
7
7
28
28
50
50
0.795
0.92
0.520
0.438
7
7
28
28
Top
80
80
1.11
1.11
0.342
0.342
7
10
28
29
Bottom
300
1.11
0.342
10
29
LAYER
Top
Bottom
2
Top
Bottom
3
Top
Bottom
4
3.2
FORMATION AND DISSOCIATION OF GAS HYDRATES OVER THE
RECONSTRUCTED STOREGGA SLOPE SINCE LAST DEGLACIATION.
The reconstructed seafloor topography proposed by Bouriak et al. (2000) is used in this
paper in order to study the dynamics of gas hydrate formation and dissociation since the
last deglaciation in the storegga area (Figure 2). The sediment layers are considered
parallel to the reconstructed seafloor topography. The depths of the different layers are
presented in Table 1.
Depth (km)
0.5
RECONSTRUCTED SEAFLOOR
(from BOURIAK et al. 2000)
1.0
1.5
0
5
10
15
20
25
Distance (km)
Figure 2. Reconstruction of the seafloor topography over the line PSAT69 (from Bouriak et al. 2000). Profile
oriented N-S perpendicular to the margin.
Effects of gas hydrates dissociation on seafloor slope stability
107
The dynamics of the hydrate stability on the Storegga slope were studied under the
effects of: 1- hydrostatic pressure due to the change of the sea level and 2- the increase
of the sea water temperature. Two factors, which are well-known to have changed since
the end of last glaciation. Estimation of the sea level change since the last glaciation was
taken from Bard et al. (1990). From paleotemperature estimates made on cores extracted
from the Eastern Norwegian Sea Koc et al. (1993) conclude that during the last
glaciation and the Younger Dryas (13.4 kyr – 10.2 kyr), relatively extensive sea ice
cover was present over the Mid Norwegian Margin. Thus, in our calculation we have
considered that during this period (12kyr-10.2kyr) the temperature was constant and
equal to –1.9°C. Between 10.2 kyr and 9 kyr, the temperature profile was taken as the
mean value of the temperature of –1.9°C and the present temperature profile. Since 9
kyr onwards the temperature was taken equal to the present temperature profile.
3.2.1
Simulation Results
Figures 3 and 4 present respectively the hydrate fraction and the excess pore pressure
over the reconstructed slope for 2 different time steps (11.85 kyr and 9 kyr). The low
concentration of methane in the upper 50 meters of sediment prevent the formation of
gas-hydrates in the upper sediment layers (Figure 3-a). The maximum hydrate fraction
over the reconstructed slope is around 1% of the pore volume (Figure 3). The increase
of the sea level induces an increase of the hydrostatic pressure of around 400 kPa. At
higher hydrostatic pressure and higher temperature, the solubility of the methane will
increase. Thus the new hydrostatic pressure and temperature conditions will induce a
dissociation of the gas hydrate at the top of the gas hydrate layer in order to establish the
chemical potential equilibrium between the hydrate phase and the liquid phase. The
melting of the gas hydrate during this period will induce a decrease of the hydrate
fraction (Figure 3) and generate an excess pore pressure (Figure 4). It is important to
mention that the excess pore pressure is generated at the top of the hydrate layer. The
stability of the reconstructed slope was evaluated at each time step. The details of the
stability study of the reconstructed slope will be presented in the next section. The
seafloor topography in Figure 3 and Figure 4 was update at each time step by taking out
the geometry of the failure surface. At the time step of 9kyr (Figure 3-b and Figure 4-b),
the increase of the seawater temperature and the hydrostatic pressure induce the melting
of the gas hydrate over a layer of around 15 m. The result is a generation of excess pore
pressure and a decrease of the soil resistance due to the disappearing of hydrates, which
bond the sediments in which it occurs. The hydrate fraction at 9kyr is presented in
Figure 3-b. One can see a sudden decrease of the hydrate fraction over the updated slope
(Figure 3-b). The hydrate fraction is less than 0.6% of the pore volume. The maximum
excess pore pressure generated over the reconstructed slope is around 38 kPa (Figure 4b).
3.3 EVALUATION OF THE INSTABILITY OF THE RECONSTRUCTED
STOREGGA SLOPE SINCE LAST DEGLACIATION.
Different methods were proposed in the literature to solve the problem of slope stability
analysis. However, the limit equilibrium methods are commonly used because of the
simplicity with which complex geometry, soil heterogeneity and pore water pressure
conditions can be taken into account. In this paper, we used the generalised limit
equilibrium method (Sultan et al. 2001).
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Sultan et al.
The geotechnical parameters (undrained shear strength Su, cohesion c’, internal friction
angle ϕ’, hydraulic diffusivity Dh, permeability k, etc.) are affected to each node. These
methods require postulating a collapse mechanism by which failure can occur. By
examining a number of different mechanisms, the critical one where the safety factor is
minimal is found. Thus, all the possible concave failure surfaces in a vertical crosssection are automatically generated.
Figure 5-a, b show the factor of safety calculated for 2 different time steps (11.85 kyr,
and 9 kyr). At 11.85 kyr, the slope is safe and the minimum factor of safety over the
slope is around 13 (Figure 5-a). An intermediate calculation at 10 kyr, shows that a
section of the reconstructed slope has already slipped due to the decrease in soil
resistance and an increase in excess pore pressure. The new seafloor topography was
updated (Figure 5-b) by removing the area where the factor of safety is less than 1. At 9
kyr, a new critical failure surface (FOS<1) is observed at the steepest part of the slope.
A comparison between the present slope and the slope obtained by back calculation
show a good agreement in the lower part of the reconstructed slope (Figure 5-b). As it
was shown previously, the failure surfaces are initiated at the top of the hydrate layer
and not at the bottom of the hydrate as it is often suggested.
-600
0.010
a) t = 11.85 kyr
0.009
-800
Seafloor
0.008
000
0.007
0.006
200
0
1000
2000
3000
4000
5000
6000
7000
8000
9000 10000 11000 12000 13000 14000
0.005
DISTANCE
000
b) t = 9 kyr
0.004
200
0.003
0.002
400
0.001
600
0
1000
2000
3000
4000
5000
6000
7000
DISTANCE
8000
9000 10000 11000 12000 13000 14000
0.000
η (-)
Figure 3. Hydrate fraction over the reconstructed slope as a function of time. For the 9kyr graphs, the seafloor
was updated from the resulting failure surface after calculation of the slope stability.
Effects of gas hydrates dissociation on seafloor slope stability
600
a) t = 11.85 kyr
109
32
30
800
28
26
24
000
22
20
200
18
0
1000
2000
3000
4000
5000
6000
7000
8000
9000 10000 11000 12000 13000 14000
DISTANCE
000
16
14
b) t = 9 kyr
12
10
200
8
6
4
400
2
0
600
0
1000
2000
3000
4000
5000
6000 7000 8000
DISTANCE
9000 10000 11000 12000 13000 14000
∆u (kPa)
Figure 4. Distribution of the excess pore pressure over the reconstructed slope as a function of time. For the
9kyr graphs, the seafloor was updated from the resulting failure surface after calculation of the slope stability.
4. Conclusion
In this paper, we have developed a theoretical model of the thermodynamic chemical
equilibrium of gas hydrates in sediment, which is based on models previously reported
by Handa (1989), Sloan (1998) and Henry (1999). This model fully accounts for the
latent heat effects, as done by Chaouch and Briaud (1997) and Delisle et al. (1998). The
model uses a new formulation based on the enthalpy form of the law of conservation of
energy. The model allows the evaluation of the excess pore pressure generated during
gas hydrate dissociation using the Soave’s (1972) equation of state. Fluid flow and gas
flow within the sediment are simulated.
A parametric study showed that neglecting the gas effect on the gas hydrate stability law
induces:
- An overestimation of the thickness of the hydrate zone;
- A significant error concerning the stability curve of the hydrate equilibrium;
- A wrong hypothesis by considering a constant hydrate fraction of the pore
volume within the sediment;
- The impossibility to estimate the excess pore pressure generated by the melting
of the gas hydrate.
An application of the numerical model developed in the present work allowed a backanalysis of the giant Storegga slide on the Norwegian margin. Hydrostatic pressure due
to the change of sea level and an increase of the sea water temperature were considered
in the calculation. Simulation results show that melting of gas hydrates can be at the
origin of a retrogressive failure in the Storegga slope. Moreover, and due to the gas
solubility, the failure interface is initiated at the top of the hydrate layer and not at the
level of the BSR.
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Sultan et al.
-600
a) t = 11.85 kyr
-800
-1000
-1200
0
1000
2000
3000
4000
5000
6000 7000 8000
DISTANCE
9000 10000 11000 12000 13000 14000
6000 7000 8000
DISTANCE
9000 10000 11000 12000 13000 14000
-1000
b) t = 9 kyr
-1200
present slope
-1400
-1600
0
1000
2000
3000
4000
5000
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
FOS (-)
Figure 5. Factor Of Safety (FOS) over the reconstructed slope as a function of time.
5. Acknowledgements
This work has been developed within the European project COSTA (EVK3-199900028). The comments made by Roger Urgeles during the evaluation of the paper were
greatly appreciated.
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