(63a) Gas Hydrates in Porous Media

Description: Natural gas hydrates occur in large quantities below the seafloor and in permafrost regions. The amount of hydrocarbon-hydrates deposits worldwide is enormous: this makes them one of the most important future energy resources. In this paper, we present an experimental study on the formation of gas hydrates in porous media. Non-destructive X-ray Computed Tomography techniques are used for visualization of the gas hydrate formation in porous media.

Applications: The data of this study give insight into the gas hydrate formation in porous medium. The data are compared to experimental data on hydrate formation in the bulk phase. With this knowledge differences in the physical-chemical behavior in the bulk and the porous medium can be identified and analyzed. This forms the base for modification of existing models so that they are applicable for porous medium. Additionally, it will give insight into the possibility to sequestrate CO2 in the form of gas hydrate.

Observations: The gas hydrate formation in porous medium is studied on a well defined model porous medium made of uniformly sized glass beads. The core is first filled with water up to a representative water saturation of 70%. Then gas, either methane or carbon dioxide, is injected into the core at a constant flow rate. Due to gravity segregation the gas mainly occupies the upper part of the core. The core is then shut in at the desired pressure. After shutting-in the core the gas hydrates starts to form which is visualized with the help of CT-scan images. CT images show that gas hydrates are mainly formed in the upper part of the core where initially gas was present. In the lower part hydrate could not be detected. The experiments show that formation of gas hydrate in porous media is slower than in the bulk. The retarding of the process can be explained by the fact that in porous medium the transfer of the gas to the water to allow the formation of gas hydrates is dominated by diffusion. Further on, in porous medium the formation of CH4 hydrates is order of magnitude slower than the formation of carbon dioxide; just as it was observed during bulk phase experiments.

Conclusion: The process of gas hydrate formation in a water-saturated core is a diffusion limited crystal-growth process. It was found out that a fast pressure build up in the core by injecting the gas without controlling the flow rate affects the hydrate formation kinetics as well as the rate of formation. Fast and uncontrolled injection appears to shorten the formation time before a constant pressure is established (equilibrium). However, the data are not very reproducible. If gas is injected slowly and with a constant flow rate the data are reproducible. The final (equilibrium) pressure is the same as when injecting uncontrolled but the time to reach the equilibrium pressure is by a factor of 3 shorter. It can be concluded from the experiments in the core and in the bulk phase that CO2 hydrate formation is faster and at lower pressures (at the same temperature) than methane hydrate formation. This supports the idea of sequestrating CO2 into the reservoir after producing methane from the hydrates.