[H-SC07] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC07-11] 2D CFD analysis of permeability change of gas-hydrate-bearing sediments

*Hiroya Takahisa¹, Yoshihiro Konno¹, Zelin Xu¹ (1.University of Tokyo)

Keywords:methane hydrate , CCS, micromodel , seabed layer

Predicting the permeability of gas-hydrate-bearing sediments is important in the development of methane hydrate and in the storage of carbon dioxide in methane hydrate concentrated zones. Several mathematical models have been proposed to express permeability as a function of hydrate saturation, and it has been found that these models agree well with laboratory-synthesized gas-hydrate-bearing sediments; however, those models underestimate the permeability for natural gas-hydrate-bearing sediments. In this study, we considered that the heterogeneity of gas hydrates distribution may be the cause of the underestimation and examined its influence through numerical analysis. To reduce the computational burden, the computational domain was set to two dimensions. Two types of gas hydrate morphology were assumed: the grain coating type, in which gas hydrates are formed on the surface of particles of the porous media, and the pore filling type, in which gas hydrates are formed at the center of the pores. For each type, analysis was conducted in two cases: homogeneous hydrate distribution and heterogeneous hydrate distribution. In the heterogeneous cases, the degree of heterogeneity was varied by changing the thickness and grain size of the gas hydrates and the location of formation. The differential pressure and temperature were fixed, and the permeability was derived from the calculated average flow velocity. When gas hydrates were homogeneously distributed, the permeability of the grain coating type was higher than that of the pore filling type at the same hydrate saturation, as reported in previous studies. In the case of heterogeneous gas hydrate distribution, the permeability was generally higher as the heterogeneity of gas hydrate increased when compared for the same formation type. This was due to an increase in the number of connecting passages. However, when comparing the homogeneous and heterogeneous cases, the relationship between the grain coating and pore filling types in terms of permeability was reversed, with the grain coating type showing a lower permeability than the homogeneous distribution. This is thought to be due to the fact that in the grain coating type, blockage of the flow path occurs continuously when individual gas hydrates become larger than a certain size. However, this phenomenon occurred when the width and branches of the flow path were regularly configured, and the size of each gas hydrate was uniform. It suggests that the results can significantly change by distributing the size of hydrates or changing the shape of hydrates or the flow paths. On the other hand, for the pore filling type, which is common in nature, it was found that the permeability was higher for the heterogeneous distribution than for the homogeneous distribution, supporting the previous experimental results. Acknowledgments This study was conducted as a part of an activity of MH21-S R&D consortium (MH21-S) as planned by the Ministry of Economy, Trade and Industry (METI), Japan.