HYDRATE STABILITY ZONE IN POROUS MEDIA, EXPERIMENTAL MEASUREMENT AND THERMODYNAMIC MODELLING

Gas hydrates could form when water (or ice) and suitably sized guest molecules are brought together under favourable temperature and pressure conditions. Suitable conditions for gas hydrate formation can be found in some marine sediments and in the subsurface of permafrost regions, where methane hydrates are known to form naturally.

The depth of the hydrate stability zone is generally estimated from the intersection of the geothermal temperature profile with the calculated hydrate stability zone of methane, taking into account the effect of salinity of pore water. Field measurements (e.g., ODP 164) showed significant deviations between the measured and predicted hydrate stability zones.

It is known that gas hydrate stability zone in sub-sea sediments is a function of pore size, fluid saturations, local stresses and sediment mineralogy. In this communication, details of an experimental set-up designed to investigate the effect of pore size and saturation on hydrate stability zone has been described. Hydrate stability zones for methane and CO2 have been measured in 251 Å, 128 Å, 82 Å porous glass beads, using step-heating techniques. The results are compared with the literature data, demonstrating the reliability of the experimental set-up and test procedures.

The effect of pore water salinity on hydrate stability in porous media has been simulated by using an aqueous methanol solution with an inhibition strength equivalent to normal sea water. The results demonstrate the importance of mass transfer of excluded solutes on the kinetics of gas hydrate formation and dissociation.

An in-house thermodynamic model has been extended to take into account the effect of porous media and fluid saturations. The predictions of the thermodynamic models are compared with the experimental data with very good agreement. The results of this work can explain some of the deviations observed between measured and calculated hydrate stability zones at ODP drill sites. The extended model can also provide a more reliable prediction of the hydrate stability zone in sub-sea sediments.