Fig. 12 . Saturations of hydrate and water phases below hydrate zone at sealing onset in different cases.
The hydrate–containing sealing layer formed in cases 3, 4, 5, 6, 7 for the reason that hydrate reformed in hydrate–bearing sediments. Hydrate saturation is the most direct criterion to judge whether hydrate reservoir has sealing effect. Seawater is almost consumed up of Area A in Fig. 9(a) and hydrate saturation is close to 100%, thus, we choose Area B in Fig. 9(a) as reference. Hydrate saturation at the turning point for cases 3, 4, 5, 6, 7 are shown in Fig. 12. It is obvious that hydrate saturation is above 51.61%, we consider this is the lowest hydrate saturation that reservoir has sealing effect, and the water saturation is between 32.08% and 37.27%. If the hydrate saturation is higher than 37.27%, sealing effect of hydrate–containing sealing layer may not exist due to insufficient conversion of water to hydrate, and the water saturation will decrease as the hydrate continues to form in hydrate–containing sealing layer.
Conclusion
In this study, the existence of hydrate–containing sealing layer is firstly experimentally confirmed using MRI visualization device by injecting methane gas and seawater into hydrate reservoirs at 3.5 and 4.0 MPa and 274.15 K. The reformation of hydrates at low gas–water flow rate or high reservoir pressure environment (hydrate saturation is higher than 51.61%) is confirmed contributed to the occurrence of sealing effect that hinders the external fluids flowing through the reservoir. Near all of pore water has been consumed to form hydrates in the sealing layer. In addition, the same pressure difference of approximately 2.47 MPa between reactor inlet and outlet was found in the different experimental cases that process sealing layers and could be a significant marker suitable for hydrate reformation. This is also a novel method, put forward in this study, to form hydrate–containing sealing layer in experimental condition. When the hydrates dissociate by high–rate gas–water flow, the sealing effect does not exist any longer. The entire natural gas hydrate reservoir, which includes three–layer distribution of hydrate, water, and gas from top to bottom, recognized by BSR is perfectly demonstrated by MRI images in this study, and A pressure difference of at least 9.0 MPa is found existing on both sides of the sealing layer, providing the development condition of higher–pressure gas reservoir under hydrate layer. The results of this study reveal the key reason why free gas can exist stably underling hydrate layer in nature and can guide the safe joint exploitation of hydrate and gas reservoirs.