Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 084
Experimental Investigation of Gas Hydrate Formation and Dissociation in Unconsolidated Porous Media
Yang, J., Llamedo, M., and Tohidi, B.
AGU/EUG/EGS Meeting, Nice, France, 6-11 April (2003).
There are large quantities of natural gas hydrates in marine sediments and permafrost regions. It is believed that natural gas hydrates could play a major role in the supply of methane to the atmosphere affecting global climate changes. Various scenarios have been suggested for the dissociation of methane hydrates and the release of methane to the atmosphere, e.g., diffusion and subsea landslides. In this presentation we report on the results of our experiments on forming gas hydrates in unconsolidated porous media and dissociating hydrates using step-depressurisation. In our tests packed glass beads of 0.1 mm in diameter were used to act as marine sediments, and certain overburden pressures were exerted to simulate natural conditions. Two different procedures of gas injection were applied to form gas hydrates in the porous media. In the first method gas was injected into the system after initial water injection. Whereas in the second method gas was injected first, followed by water injection. Methane and a typical natural gas were used in the tests. A method based on measuring system volume and mass balance was used to determine the hydrate saturation in the sediments. Better gas/hydrate distribution and higher hydrate concentration was achieved in the second method. The results on hydrate dissociation showed that upon depressurisation (and hydrate dissociation) water is produced first, until the gas phase becomes mobile. The results could have significant implications on the mechanical properties of hydrate hosting sediments and the dominant mechanisms of methane release to the atmosphere. The tests with the natural gas demonstrated the effect of gas fractionation due to hydrate formation. In the tests, although the system was cooled-down to temperatures much lower than the hydrate stability zone of the natural gas, some gas was present inside the porous media. This could be explained with the fact that hydrate formation removes heavier hydrocarbons from the gas phase. Therefore, further cooling is required to convert the remaining gas phase (which is mainly methane with some nitrogen) into hydrates. At the end of the hydrate formation experiments a mixture of methane and nitrogen would remain in the gas phase inside the porous media. The depressurisation tests support the above phenomenon, as upon depressurisation, hydrates are dissociated at much higher pressures than natural gas dissociation pressure at the system temperature.