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.