Research On The Permeability Of Methane Hydrate-bearing Porous Media

Natural gas hydrate exists widely in the permafrost and submarine sediments with a very large reserve, which makes it become one of the potential energy resources in the future. Seepage in the reserve has a great effect on the flow characteristic of gas and water during the exploitation of natural gas hydrate, which determine the technical feasibility and economic viability of exploitation.In this work, a device for hydrate formation and measurement of in situ permeability was designed and developed. Glass beads with different particle sizes were used to simulate the reservoir under effective stress. Hydrate was formed by water and excessive methane under a constant pressure, then in-situ permeability measurement was performed. The results showed that with the increasing of hydrate saturation, permeability of the porous media reduced significantly, but the trend of attenuation under different hydrate saturations are different. There is a certain critical saturation value. When the saturation is larger than this value, the permeability decayed significantly faster than that in lower saturation. The results of the experiments was compared with hydrate permeability models. It was concluded that when the hydrate saturation is lower than the critic saturation value, hydrate formed in the type of grain-coating. With the increasing of saturation, hydrate growth will tend to pore-filling, leading to a much faster decay in permeability. The value of the critical saturation is between 30 to 40%. Fitting the results of the experiments in different saturation range, the corresponding permeability decay index was obtained.Magnetic resonance imaging (MRI) was used to visualize the flow characteristic in the hydrate-bearing porous media. Due to the inherent instability of hydrate, the process of one phase displaced by another is not easy to achieve in unsteady state method to measure relative permeability. For this problem, we attempted to validate which gas should be better to achieve the displacement. The results showed that nitrogen failed to achieve the displacement because it would cause hydrate dissociation; while, methane could achieve the displacement with suitable experimental operations, A phenomenon that the heterogeneous distribution of hydrate in porous media was also found in the experiments, which would lead to the non-uniformity of permeability in the reservoir. Thus, the unevenness would cause the randomness and dispersion of the measured permeability.