In the first part of the paper, we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in sediment, which is based on models previously reported by Handa (1989), Sloan (1998) and Henry (1999). This model fulIy accounts for the latent heat effects. It 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. In the second part of the paper, we present and discuss an application of the numerical model developed in the present work through a back-analysis of an observed slide in the Lower Congo Basin. Two scenarios were tested in order to identify the possible trigger mechanism of the observed slide instability 1) under an increase of the seawater temperature and 2) under an increase in the deep thermal gradient. Simulation results show that for the two scenarios, the slide will occur at the top of the gas hydrate layer.
Quantitative studies of kinetics of gas hydrate formation and dissociation in marine sediments can be grouped into two categories. The first of these models take into account methane conservation (Rempel and Buffett, 1997; 1998; Xu & Ruppel, 1999), methane advection-diffusion coupled with heat transfer (Rempel and Buffett (1998) and Xu and Ruppel (1999)), and pore chemistry in terms of chloride (Egeberg and Dickens (1999) and Davie & Buffett (2001)). 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, gas-hydrate stability zone from p-T conditions does not coincide with the real hydrate occurrence zone.
The hydrate fraction, which depends on the gas concentration within the sediment column, is often improperly considered, by the last category of model, as constant.
The excess pore pressure generated by the melting of the gas hydrate depends on
the hydrate fraction and
on the gas solubility.
Therefore, by considering only the energy conservation equation, it is impossible to evaluate this excess pore pressure.
Therefore, in this paper, 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 latent heat effects. The model allows for the evaluation of the excess pore pressure generated during gas hydrate dissociation using the Soave's equation of state.
The two-phase chemical potential equilibrium relation involving hydrate phase is governed by the following key equation: