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.