Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 060
Effect of Salinity on Hydrate Phase Equilibria in Porous Media
Østergaard, K. K., Llamedo, M., Tohidi, B., and Anderson, R.
EUG XI Meeting, Strasbourg, France, 8-12 April (2001).
The huge amounts of hydrates in deep sea sediments represent a potential source of energy as well as a safety hazard to drilling operations. Furthermore, they influence ocean margin stability and global climate changes. It is generally accepted that small diameter capillary media have an inhibiting effect on hydrate stability. The pore spaces of fine-grained marine sediments can be viewed as such a media. Marine sediments are very complex systems, and in addition to a pore size distribution control on hydrate equilibria, mineral surface properties and the presence of saline water could also play an important role. The large number of factors involved makes it difficult to accurately predict hydrate phase behaviour in marine sediments. Considering the ever-increasing importance of gas hydrates in sediments, it crucial to gain a better understanding of their formation and decomposition in porous media. Nevertheless, before hydrate formation and decomposition in complex marine sediment systems can be successfully described, it is necessary to understand the basic interactions between fluids, hydrates and the porous medium. Such information can be obtained by performing experiments in the controlled laboratory environment using well defined porous media and fluids. The available hydrate equilibrium data in tight porous media are sparse and inconsistent. Recently, we have presented an improved test procedure and subsequent measurements obtained from porous glass beads. In this paper we focus on the effect of saline water on the hydrate equilibria in porous media. Experimental results on gas hydrate equilibria of methane in the presence of inhibited water (representing salinities typical to marine sediments) in porous glass with 251 Å, 128Å, and 82 Å pore sizes are presented. Finally, a thermodynamic model taking into account the effect of pore size as well as the presence of saline water is constructed. The model, which is based on comprehensive thermodynamics, enables successful prediction of the experimental data. The model can also be used as a tool to simulate, and consequently to gain better understanding of, hydrate equilibria in marine sediments.