Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

3D-numerical model, simulation guidelines

A simulator was developed, which was originally based on a numerical model for the description of the precipitation of minerals in porous media.

The simulation work was mainly oriented on the conditions prevailing in the Anaximander area. To identify guidelines for future simulation work, directed on the description of the generation or decomposition of gas hydrate deposits in past times, sensitivity studies were performed.

The 3D-model includes the following topics:
- Methane fugacity at gas hydrate ls-equilibrium;
- Growth and decomposition of gas hydrates;
- Changes of transmissibilities due to gas hydrate content;
- Transport by convection and diffusion;
- Heat transfer including enthalpies of formation;
- Aerobic and anaerobic methane consumption.

The Kihara-parameter used for the calculation of ls-equilibria was adjusted to include structural changes at low ethane concentrations.

To fit the parameter data sets of ethane/methane- and propane/methane-mixtures including the pure components were applied from literature.

For the growth of gas hydrates in pore space of consolidated sediments, with permanent pore geometry, different modules were developed and used to describe the changes of transmissibilities due to gas hydrate content. This is especially essential for the simulation of the growth behaviour in millimetre length scale.

The stochastic generation of gas hydrate nuclei is incorporated into the model, to apply a more realistic scenario for the initial condition of the growth of gas hydrates in sediments.

A reaction kinetic module was implemented, to study the effect of the microbial activity on the release of methane from the sediment into the overlying seawater column.

The following conclusion can be drawn from the simulation work and are to be taken in account for future simulations:
- The slow growth processes are governed by the slow mass transport. The habitus of the hydrates is interconnected with the coupling of convection and diffusion at a millimetre scale.

- For acceptable predictions a large set of data is needed. The sensitivity studies point out, that it is paramount to describe the vertical flux properly. With large distances in the range of 100 m the mass transport is governed by convection at low velocities in the range of 1 x 10-7-1 x 10-9 m x s-1, valid for mud volcanoes without heavy eruptions.

Therefore detailed information about the permeability variation in the sediment is needed. In case of free mobile gas the relative permeabilities must be accurately evaluated.

- The distribution of gas hydrates in deposits is strongly dependent on the nucleation probability. The knowledge about the nucleation probability, which is responsible for the distribution of hydrates in the sediment, is poor. Especially for porous media, the dependencies on concentration and pore size distribution are unknown.

- The microscopic growth behaviour derived for porous media of permanent geometry must be taken into account, to evaluate the time dependent transport properties of larger blocks with deep lying sediments.

- The description of the quasi-steady state of gas hydrate deposits are related to states close to equilibrium and slow mass transfer. Therefore reliable ls-equilibria calculations are essential including structural changes.

- The modelling of field data underline, that the reduction of the methane release from the sediment due to microbiological activity becomes only relevant at rather low convection velocities less than 1 x 10-8 m x s-1.

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