Fig. 11 . MRI images and inlet pressure and hydrate saturation
changes in Cases 6 and 7
Fig. 11 shows MRI images in FOV and the characteristics of hydrate
saturation during the whole flow process. In the stage 1 (\(S_{1}\)),
the inlet pressure of both cases remained stable around 4.0 MPa.
However, the inlet pressure is slightly higher than the back pressure
(about 4.2 MPa), because the contact of seawater and hydrate causes the
capillary force in the
sediments. The MRI images of \(S_{1}\) brightened, indicating the
hydrate dissociated during the gas–water flow process with the flow
rate of 1–2 ml/min. Because the large amounts of seawater flow enhanced
the mass transfer between hydrate phase and seawater phase, the driving
force is the chemical potential difference caused by low methane
concentration in seawater (Sun et al. , 2020a; Yang et al. ,
2019). Meanwhile, the random distribution of hydrate in porous media was
the reason that the seawater–hydrate interface in Case 6 and Case 7 was
different (Zhang et al. , 2019). The hydrate dissociation areas in
Fig. 11(a) could be considered the chimney in the hydrate reservoir
(Torres et al. , 2004), the reservoir had no sealing effect and
showed well permeability characteristics.
In the stage 2, the gas–water flow in hydrate–bearing sediments with
chimney due to hydrate dissociation in stage 1, it was obvious that the
reactor inlet pressure was increasing, shown in Fig. 11(b), and the
corresponding MRI images went dimmed, hydrate saturation increased. As
the seawater flow rate decreased and methane gas flow rater increased,
the methane dissolved more fully in seawater. Thus, the hydrate formed
inside the hydrate thermodynamically stable area, the gas–water flow
process in \(S_{2}\) was the formation process of hydrate–containing
sealing layer. In Case 6, no more seawater flowed out of the reactor
after 246 min, hydrate–containing sealing layer had sealing effect at
this moment. The inlet pressure fluctuated in 274~279
min, and seawater flowed into the FOV. Under the driving of pressure
difference, the hydrate–containing sealing layer was partially
destroyed and seawater migrated in the reservoir, but the seawater did
not break through the sealing layer. The hydrate continuously formed
after 279 min, the pore volume is reduced, thus the inlet pressure
increased faster. In Case 6, the turning point appeared at 202 min, the
pressure difference of in and out is 2.34 MPa in Case 5 and 2.71 MPa in
Case 6, respectively. They were all close to the mean value of pressure
different (2.47 MPa), the pressure difference of in and out could be a
marker to judge the formation of hydrate–containing sealing layer. In
hydrate–bearing sediments, the hydrate–containing sealing layer can be
formed when hydrate reforms in hydrate reservoir, even though hydrate
has dissociated before.