Fig. 4 . Breakthrough of hydrate–containing sealing layer in Case 2
In Case 3, the gas–water flow rate was 1–0.25 ml/min. It could be found the inlet pressure kept increasing, so as the pressure difference between reactor inlet and outlet. It was obvious there was a fluctuation in pressure increase rate curve (231 min, and the inlet pressure was 5.98 MPa), and the curve tended to be stable before 231 min, but it got larger after 231 min. After 231 min, the seawater can’t flow through the reactor, the hydrate reservoir possessed sealing effect to fluids after 231 min. The pressure increase rate at \(i\) min can be calculated by Eq. (4):
\(\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }R_{i}=\frac{P_{i}-P_{i+t}}{t}\bullet\%\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \)(4)
The inlet pressure increased to 13.0 MPa at last, due to the limitation of experimental, thus, at least 9.0 MPa pressure difference existed on both sides of the hydrate reservoir. It’s important to emphasize this pressure difference was not caused by gravity, and the sealing effect of hydrate–bearing reservoir, which is shown as pressure difference, represents the ability of hydrate reservoir to trap underlying gas. In addition, this pressure difference providing the development condition of higher–pressure gas reservoir under hydrate layer, and the pressure difference between hydrate zone and gas zone must be considered during hydrate exploitation process.
The reservoir condition could be visually observed by MRI, because the MRI can only acquire images of the 1H cantained in liquid water (Wang et al. , 2020), and the hydrate saturation could be calculated using Eq. (1). Fig. 5(a) shows the characteristics of water distributions in FOV, and Fig. 5(b) reflects the real–time characteristics of hydrate saturation. When the gas–water flow rate was 4–1 ml/min (Case 1), the MRI images brightened gradually after 52 min, and the hydrate saturation decreased, indicating the hydrate dissociated during the gas–water flow process. The reservoir condition (274.15 K, 3.5 MPa) was in hydrate thermodynamic stable area, thus, the driving force for hydrate dissociation was the chemical potential difference between seawater phase and hydrate phase (Chen et al. , 2019a;Chen et al. , 2019b). The chemical potential difference is caused by the inadequate dissolution of methane gas (Sun et al. , 2020b;Zheng , 2007). Hydrate dissociation is the reason that the inlet pressure decreased after 52 min. Thus, hydrate dissociation will reduce or even disappear the sealing effect of hydrate–bearing sediments. For Case 2, the MRI images had no noticeable darkening area, and hydrate saturation curve fluctuated, indicating small amount of hydrate forms. In the gas–water flow rate of 2–0.5 mi/min, hydrates were not always formed or decomposed, it was in an approximate equilibrium state. Thus, the hydrate–containing sealing layer was broken through under large pressure difference. For Case 3, it can be seen that the MRI images going to darken gradually over time, and the hydrate saturation curve presented an increasing trend. Hydrate formed during the gas–water flow process at the flow rate of 1–0.25 ml/min. The inlet pressure was also increased to 13.0 MPa, it could be concluded that hydrate formation enhanced the sealing effect of hydrate–bearing sediments. Hydrate formed continuously even though the sealing effect had been existed in hydrate reservoir. Furthermore, the continuously increasing inlet pressure provided larger driving force for hydrate formation. The hydrate formation before the turning point formed the hydrate–containing sealing layer; the hydrate formation before the turning point further enhanced the hydrate–containing sealing layer. After the turning point (231 min), the seawater can’t flow through the reactor, indicating the reservoir had sealing effect. The mechanism of sealing effect is because that the sediment pores were occupied by more hydrate and became smaller, the residual water in the sediment created capillary forces in the narrow pore space, which blocked the gas–water migration.