EFFECT OF GAS HYDRATES DISSOCIATION ON SEAFLOOR SLOPE STABILITY. J. MIENERT Department of Geology, University of Tromsø, N-9037 Tromsø, Norway

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1 EFFECT OF GAS HYDRATES DISSOCIATION ON SEAFLOOR SLOPE STABILITY. N. SULTAN, P. COCHONAT, J.P. FOUCHER IFREMER, BP 70, 90 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

2 10 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.. 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: T η Cp = ( 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: σ µ H Vl + µ W = cosθ [] 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 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 V l 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, : 98% of methane and % of ethane, 3: 96.7% of methane and 3.% 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 0 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, : 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 0 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 (199) for methane hydrate stability conditions in seawater (Salinity in 3.35% wt) are presented in Figure 1- b. 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 (199). 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

3 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 (197) state equation. 8 COMPONENTS (a) 8 COMPONENTS (b) 100% Methane 100% Methane + 0% wt NaCl 98% Methane % Ethane 100% Methane + 3% wt NaCl 96.7% Methane 3.% Ethane 100% Methane + 3.5% wt NaCl 6 6 P (MPa) P (MPa) T (K) OBSERVED (Dickens et al. 199) Pure water Sea water 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 km, which is one of the world s biggest underwater slides with its overall volume of 3300 km 3 and an estimated area of slide scar of km (Haflidason et al. 00). 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. (001, 00) show that the Storegga Slide has been activated/ mobilised within the same age interval radiocarbon age ( 1 C) BP or ca 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-

4 106 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. 000 among others). In this paper, the scenario of the gas hydrate melting as the origin of the Storegga Slide (Mienert et al., 001) 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 3 LAYER DEPTH (m) γ (g/cm 3 ) φ c (kpa) ϕ ( ) Top Bottom Top Bottom Top Bottom Top Bottom FORMATION AND DISSOCIATION OF GAS HYDRATES OVER THE RECONSTRUCTED STOREGGA SLOPE SINCE LAST DEGLACIATION. The reconstructed seafloor topography proposed by Bouriak et al. (000) 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 ). The sediment layers are considered parallel to the reconstructed seafloor topography. The depths of the different layers are presented in Table Depth (km) 1.0 RECONSTRUCTED SEAFLOOR (from BOURIAK et al. 000) Distance (km) Figure. Reconstruction of the seafloor topography over the line PSAT69 (from Bouriak et al. 000). Profile oriented N-S perpendicular to the margin.

5 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 - 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. kyr 10. kyr), relatively extensive sea ice cover was present over the Mid Norwegian Margin. Thus, in our calculation we have considered that during this period (1kyr-10.kyr) the temperature was constant and equal to 1.9 C. Between 10. 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 Simulation Results Figures 3 and present respectively the hydrate fraction and the excess pore pressure over the reconstructed slope for 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 00 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 ). 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 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 -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 - b). 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. 001).

6 108 Sultan et al. The geotechnical parameters (undrained shear strength Su, cohesion c, internal friction angle ϕ, hydraulic diffusivity D h, 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 different time steps (11.85 kyr, and 9 kyr). At 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 a) t = kyr Seafloor b) t = 9 kyr 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.

7 Effects of gas hydrates dissociation on seafloor slope stability a) t = kyr b) t = 9 kyr 600 u (kpa) Figure. 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 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 (197) 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.

8 110 Sultan et al a) t = kyr b) t = 9 kyr present slope 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 (EVK ). The comments made by Roger Urgeles during the evaluation of the paper were greatly appreciated. 6. References Bard, E., Hamelin, B., Fairbanks, R. G., and Zindler A., Calibration of the 1C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals. Nature 35, p Bouriak, S., Vanneste, M. and Saoutkine, A., 000. Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Vøring Plateau, offshore Norway, Marine Geology, Volume 163, Issues 1-, p Bugge, T., Submarine slides on the Norwegian continental margin, with special emphasis on the Storegga area. Continental Shelf and Petroleum Research Institute (IKU) Publication 110, Trondheim, Norway, 15 pp Bugge, T., Belderson, R.H. & Kenyon, N.H., The Storegga slide. Philosophical Transactions of the Royal Society of London, A 35, p Chaouch, A., & Briaud, J.-L., Post melting behavior of gas hydrates in soft ocean sediments, OTC- 898, in 9th offshore technology conference proceedings, v. 1, Geology, earth sciences and environmental factors: Society of Petroleum Engineers, p Delisle, G.; Beiersdorf, H.; Neben, S.; Steinmann, D., The geothermal field of the North Sulawesi accretionary wedge and a model on BSR migration in unstable depositional environments. in Henriet, J.-P.; Mienert, J. (Ed.): Gas hydrates: relevance to world margin stability and climate change. Geological Society Special Publication, 137. The Geological Society: London, UK, p Dickens, G.R. & Quinby-Hunt, M.S., 199. Methane hydrate stability in seawater. Geophysical Research Letters, 1 (19), p

9 Effects of gas hydrates dissociation on seafloor slope stability 111 Haflidason, H., Sejrup, H. P., Bryn, P. and Lien, P., 001. The Storegga Slide; Chronology and Flow Mechanism, EUG XI Abstracts, p. 70. Haflidason, H., Sejrup, H.P., Bryn, P., Lien, R, Masson, D., Jacobs, C., Huehnerbach, V. and Berg, K. 00. The architecture and slide mechanism of the Storegga Slide, Mid Norwegian margin. The Norwegian Petroleum Society, Annual Meeting in Trondheim October 00.NGF Abstracts and Proceedings No., 00, Handa,Y.P., Effect of Hydrostatic Pressure and Salinity on the Stability of Gas Hydrates. J.Phys.Chem., Vol.9, p Henry, P., Thomas, M.; Clennell, M.B., Formation of Natural Gas Hydrates in Marine Sediments. Thermodynamic Calculations of Stability Conditions in Porous Sediments, J. Geophys. Res., 10, p Koc, N., Jansen,E., and Haflidason,H., Paleoceanographic reconstructions of surface ocean conditions in the Greenland, Iceland and Norwegian Seas through the last 1ka based on diatoms. Quaternary Science Reviews 1, Mienert,J, and Bryn, P., Gas hydrate drilling conducted on the European Margin. Eos 78, No.9, 567,571. Mienert, J., Posewang, J. & Baumann, M., Gas hydrates along the north-eastern Atlantic margin: possible hydrate bound margin instabilities and possible release of methane. in Henriet, J.-P. & Mienert, J. (eds); Gas hydrates: Relevance to world margin stability and climatic change, Geological Society of London, Special Publication, 137, p Posewang, J. and Mienert, J., 1999.The enigma of double BSRs: Indicators for changes in the hydrate stability field. Geo-Marine Letters 19: Mienert, J., Posewang,J., Lukas, D Changes in the hydrate stability zone on the Norwegian Margin and their consequences for methane and carbon releases into the oceanosphere. In: Schäfer,P., Ritzrau,W., Schlüter,M., Thiede, J. (eds.), The northern North Atlantic: A changing environment, Springer, Berlin, pp Rempel, A.W. & Buffett, B.A., Mathematical models of gas hydrate accumulations. in Henriet, J.-P. & Mienert, J. (eds); Gas hydrates: Relevance to world margin stability and climatic change, Geological Society of London, Special Publication, 137, Sloan, E.D. Jr., Clathrate hydrates of natural gases. Marcel Dekker Inc., nd edition, New York, pp Soave G, 197. Equilibrium constants from a modified Redlich-Kwong equation of state. Chemical Engineering Science, 7, Sultan N., Cochonat P., Bourillet J.F., Cayocca F., 001. Evaluation of the risk of marine slope instability: a pseudo- 3D approach for application to large areas. Marine Georesources and geotechnology, 19, Van der Waals, J.A. & Platteeuw, J.C., Adv. Chem. Phys., p Xu, W. & Ruppel, C., Predicting the occurrence, distribution and evolution of methane gas hydrates in porous marine sediment. Journal of Geophysical Research, 10, p