Fig. 9 . (a) multi–level MRI images in Case 4; (b) hydrate
saturation in different areas
Fig. 9(a) was the multi–level MRI images of reactor after 158 min in
Case 4. The camera on the MRI device remained stationary, and moved the
reactor 30 mm upwards after each shot, for the reason the FOV was 30 mm\(\times\) 30 mm, complete reservoir status is presented by this
operation. The gas–water injection was stopped and the inlet pressure
was constant at 13.0 MPa during the whole shooting process. Significant
brightness differences could be found in the hydrate reservoir. By
comparing the hydrate saturation in different areas, as shown in Fig.
9(b), the hydrate saturation was higher in farther area alone the
gas–water injection direction. The pore seawater in area C was almost
consumed up, tiny amount of seawater still remained in the hydrate
reservoir and it is too saline to form hydrate, it was hydrate zone. The
brighter area (areas A and B) compared with area C was water zone
(seawater, hydrate, gas coexisted). The free gas was hindered out of the
reactor inlet. Thus, the hydrate reservoir can be divided into three
layers. This is very similar to the hydrate reservoir with three
structures in the South China Sea: gas hydrate layer; mixing layer
consisting of hydrates, free gas and seawater; free gas layer coexists
from top to bottom (Qin et al. , 2020). And the experimental
phenomenon is consistent with the model for free gas migration presented
by Liu et al (Liu and Flemings , 2006).