Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications



Abstract 058
Effect of Fluid Saturation on the Hydrate Stability Zone in Porous Media
Llamedo, M., Østergaard, K. K., Tohidi, B., Anderson, R., and Burgass, R. W.
EAGE 9th Annual Research Review, Edinburgh, UK, 14 February (2001).
Gas hydrates are crystalline compounds formed through a combination of gas and water molecules under low temperature and high pressure conditions. These conditions exist in many marine sediments where there is a supply of gas, e.g., methane. Seismic reflection profiles across continental margins indicate the frequent occurrence of gas hydrates within the upper few hundred metres of sea-floor sediments. An important milestone in the history of gas hydrates happened in mid 1960's, when large amounts of gas in the form of gas hydrates were found in permafrost regions and deep-sea sediments. The existing estimates of the quantity of gas captured in submarine hydrates varies between 1015 and 7.6x1018 m3. This is equivalent to 535 to 4,000,000 Gigaton carbon compared to 5,000 Gigaton carbon in fossil fuels. In the UK sector, deep water exploration is likely to encounter hydrates. The gas hydrate stability zone in porous media is known to be affected by many factors, including pore size, fluid saturation, local stresses, and sediment mineralogy. In the previous communications we reported a detailed account of experimental set-up and test procedures for measuring the hydrate phase boundary in porous media. In this presentation, the hydrate phase boundary of methane and CO2 in 251 Å, 128 Å, and 82 Å porous glass beads are reported. In addition, the effect of fluid saturation on the hydrate phase boundary has been investigated. As expected, the hydrate phase boundary was found to be a function of pore size. The work provided reliable and consistent data (compared to those reported in the literature), which were used to tune a thermodynamic model for these systems. The results on under-saturated porous glass beads were somewhat unexpected. Here, two dissociation points were observed for all systems investigated. The in-house thermodynamic model was extended to take into account the effect of porous media on the hydrate stability zone. A multi-stage tuning procedure was implemented to match the experimental data generated in this work. The results identified the shortcomings of existing models and provided interesting information on the surface forces involved. Various fluid-solid saturation scenarios were assumed for predicting the two hydrate dissociation points in under-saturated systems. The predictions are compared with the experimental data generated in this work.