Research
Methane Hydrate Transport
Methane hydrates are ice-like crystals consist of methane gas molecules trapped inside the water cages. They are stable under the low temperature and high pressure regions such as in arctic regions under permafrost and in ocean sediments along the continental slopes. Possible severe risks caused by the formation of methane hydrate inside the deepwater oil and gas pipelines motivated the study in the behavior of methane hydrate formation.
I used the data collected at the Ulleung Basin Sites [PHTK2016] for following simulations in 1D.
I used the data collected at the Ulleung Basin Sites [PHTK2016] for following simulations in 1D.
Advection of Methane Gas in GHSZ for Homogeneous Sediment
Gas hydrate stability zone (GHSZ) is where only liquid and hydrate phases are present. I assumed the constant supply of methane gas from the center of the earth (x increases towards the seafloor) and no other source of methane gas. When the mass fraction of methane gas in the liquid phase reached its maximum solubility (depends on pressure, temperature, salinity, and rock type), all excessive methane gas would form hydrate. I used the first-order Godunov's scheme to get the total mass contents per mass of liquid phase. Then I used the local phase behavior solver to get the mass fraction of methane in the liquid phase and hydrate saturation.
Advection of Methane Gas in GHSZ for Heterogeneous Sediment
This example is inspired by the 1D case study and model given by Daigle and Dugan [DD2010]. For simplicity, I used the model data instead of the real data. Each layer has different physical property such as porosity and permeability which implies to have different maximum solubility of methane gas.
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Methane Gas Transport at Darcy Scale
(Diagram on the left) Schematic diagram of heterogeneous rock. Assume the constant supply of methane gas from the center of Earth through BHSZ. Sand, fine sand, silt, and chimney have different permeabilities.
As time goes, the mass fraction of methane gas dissolved in the liquid phase propagates in an upward direction towards the seafloor. Since the permeability in the chimney is much larger than other rock types, we can observe the high mass concentration in the region of chimney compare to the surroundings. As consequences, we first see the hydrate formation in the chimney as maximum solubility can be reached at the earlier stage than in sands. Moreover, since the permeability of silt is greater than that of fine sand, more methane concentration left behind in the sand on the right side of chimney than the left side of chimney. Thus, hydrate forms in the sand on the right side of chimney before forming on the left. Note that the 'spiky' shapes in black are due to the low-resolution.
As time goes, the mass fraction of methane gas dissolved in the liquid phase propagates in an upward direction towards the seafloor. Since the permeability in the chimney is much larger than other rock types, we can observe the high mass concentration in the region of chimney compare to the surroundings. As consequences, we first see the hydrate formation in the chimney as maximum solubility can be reached at the earlier stage than in sands. Moreover, since the permeability of silt is greater than that of fine sand, more methane concentration left behind in the sand on the right side of chimney than the left side of chimney. Thus, hydrate forms in the sand on the right side of chimney before forming on the left. Note that the 'spiky' shapes in black are due to the low-resolution.
Hydrate Formation in Sand with and without the Phase-Field Model
The methane gas transport in GHSZ is highly diffusive flow. Hence, if we simulate the advection-diffusion transport in the sand, it is unlikely to see any hydrate formation in pores. Above figures have rock in white and methane fraction in color (blue to red). Upper row of figures show the evolution of methane concentration over 100 kyrs. Since the methane concentration never reaches the maximum solubility, we don't see any hydrate formation. In reality, hydrate can present in the pores. To simulate hydrate formation in the pores, we run the simulation of methane gas transport using a coupled transport-phase-field model. The bottom 5 figures show the evolution of methane concentration over 100 kyrs. When methane concentration exceeds maximum solubility, hydrate forms by consuming nearby methane gas. Blue colored region shows region of consumed methane gas due to hydrate formation.