Numerical simulation of microscopic CO2 hydrate formation in sandy sediment with two-phase flow

•A simulation method for CO2 hydrate growth in two-phase flow was developed.•The effects of hydrate formation rate and flow speed on permeability were studied.•The morphological changes in hydrate growth due to two-phase flow were discussed.•The difference in permeability with and without the flow was quantitatively small. This study provides a numerical simulation method of CO2 hydrate formation within microscopic porous media with the presence of two-phase flow of liquid CO2 and water. The influence of the hydrate formation rate and the two-phase flow speed on permeability reduction were studied numerically. The results showed that, when the hydrate saturation is small, say 0.2 or less, whatever the water shape is, grain-coating hydrate is dominant and the difference in flow speed hardly affect permeability reduction. When the hydrate saturation is about 0.3 or more, the larger the ratio of the flow speed to the hydrate formation rate, the larger the solid-phase surface area and the smaller the pore size are: that is, the smaller the permeability is. As the hydrate grows further, most of the water turns into hydrate and the difference in effective permeability due to the flow speed becomes noticeable as the water shape changes over time. It is thought that, at a high flow speeds, the flow purges more water from sand surfaces and places the purged water in the wake of sand grains, resulting in more pore-filling hydrate rather than the grain-coating type. However, such a difference is not so large and it seems that the existing mathematical models of permeability reduction due to hydrate formation can be applied even when there is two-phase flow.