Fig. 7 . Pressure difference values at onset time of sealing layer formation
Fig. 8 shows the variation characteristics of hydrate saturation in FOV during the gas–water injection process of cases 4 and 5. The MRI images is getting darker in Case 5 and hydrate saturation present an increasing trend, thus, hydrate formed during the gas–water flow process and hydrate–bearing reservoir had sealing effect. However, hydrate saturation didn’t show an obviously increasing trend in Case 4, though the sealing effect existed after 94 min. It could be found that there was a significant change in the lower left corner of FOV in Case 4, so we analyzed the change in MI value of Line 1. The small distance of MI value (0~4.2 mm) is obviously reduced, indicating that hydrate formed in this area and consumed the pore water. The reason for sealing effect exists in Case 4 was also caused by hydrate formation, but hydrate formation was not concentrated in the FOV, and thought it was below the FOV. To further analyze the reservoir state, multi–level MRI image (4 layers) was carried out by adjusting the position of the reactor.