Dynamics of gas hydrate: case of the Congo continental slope

Abstract

A numerical model is developed to study the temperature effect on stability and phase transformation of gas-hydrate. The model uses a mathematical formulation based on the enthalpy form of the conservation law of energy. The use of the enthalpy form instead of the temperature form as often done in the literature has made the problem numerically simpler. The model is then applied to describe the effect of sea bottom temperature variations on the stability of gas hydrate occurrences and on the seafloor reflectivity in sediments of the Congo continental slope. Indeed, a migrating seafloor reflectivity front is observed on 3D seimic images from two surveys performed 6 months apart and interpreted as a migrating gas hydrate stability zone. Seawater temperature variations were recorded over a 230-day period. At the seafloor, the amplitude of these variations could explain a 75m shift in water depth of the upper limit of the gas hydrate stability zone. However, the response of the sediment to a perturbation applied at the seafloor is not immediate because of the effects of thermal diffusion and of latent heat of hydrate dissociation. Thermal modelling shows that the depth of penetration of these perturbations is around 2 m for the saturated sediment and around 6 m for hydrate-bearing sediments (hydrate fraction of 0.1). The switch between gas and gas hydrate generated by these temperature fluctuations concern only the first meter of sediment. These appear too small to explain the migration of the seafloor reflectivity, considering that the wavelength of the seismic signal is significantly larger (around 10 m). However, for temperature fluctuations with larger wavelength the switch between gas and gas hydrate will be more important and consequently could explain the reflectivity contrast as a possible migrating gas or gas hydrate front.

Empirical expressions for gas hydrate stability law, its volume fraction and mass-density at temperatures 273.15K to 290.15K

“…It shows that the predicted gas hydrate formation conditions from the empirical expression for pure methane and pure water system are well consistent with the experimental measurements. The predicted results are also generally in agreement with other models (Sultan et al, 2004b;Sloan, 1998;Handa, 1990), especially at low temperatures below 290 K (Fig. 1).…”

“…These results show that with the increase of salinity the ARDPs gradually increase. When compared with other models (Sultan et al, 2004b;Sloan, 1998), it can still be seen that they are in agreement with them within the given temperature range (Fig. 4).…”

“…However, at lower temperatures, the differences between them seem smaller than they are at higher temperatures. When compared with other models, our predicted results are intermediate between models of Sultan et al (2004b) and Sloan (1998), suggesting a general agreement between them.…”

“…At high ethane percentage of the mixture in pore water, the increasing difference between the prediction and the experimental data is probably caused by gas hydrate structure transformation, which is reported by experimental results (Maekawa, 2001). When compared with other models (Sultan et al, 2004b;Sloan, 1998), it is shown that the differences between the predicted results and the models are not big. It should be pointed out that among all the empirical expressions some can not be validated due to lack of experimental data and thus need to be further confirmed.…”

Synthesising bushmeat research effort in West and Central Africa: A new regional database

“…A third and final potential marine source for methane outgassing is the presence of gas hydrates, which have been inferred on the Northern Congo fan from mud volcanoes and along the Congo continental slope near the presence of pockmark structures (Charlou et al, 2004;Sultan et al, 2004). Gas hydrates have also been recovered from the shallow subsurface at water depths of 3100 m in the vicinity of ODP Site 1075 (Sahling et al, 2008).…”