Key points:
• First–time experimental
visualization on the theoretic sealing effect of natural gas hydrate on
its lower gas by MRI.
• Further formation or dissociation of initial formed hydrate under
excess water is critical to whether the sealing effect works.
• At least 9.0 MPa pressure
difference can exist on both sides of the manmade methane
hydrate–containing sealing layer.
Abstract :High–pressure methane gas generally exists stably under methane hydrate
stability zone at several hundred meters cutting through the marine
sedimentary strata. The usually employed bottom simulating reflector
(BSR) for hydrate recognition represents the interface between hydrate
and fluid areas in typical natural methane hydrate reservoir system with
hydrate, water and gas layers. In this study, the gas–seawater
migration in hydrate reservoir was simulated through gas–seawater
injection, and the existence of hydrate–containing sealing layer was
experimentally confirmed. The hydrate reformation was observed by
magnetic resonance imaging (MRI) during the gas–water injection process
above the methane hydrate phase equilibrium pressure and it is the
fundamental reason that hydrate reservoir has sealing effect on free
gas. As the decrease of pore spaces in sediments, the interaction of
seawater and hydrate in the reservoir products capillary sealing in the
narrow space, thus the free gas and seawater migration are inhibited and
the free gas exited stably underlying the hydrate layer. However, low
methane concentration in seawater caused by high gas–water flow rate
(4–1 ml/min) resulted in the hydrate dissociation, the hydrate–bearing
sediments can’t produce the sealing effect. Hydrate further forms in the
sealing layer and leads to seawater depletion until it is too salty to
form hydrate. Finally, the gas layer, water layer and hydrate layer
coexist under the seabed. In addition, the hydrate–containing sealing
layer could be broken through, and the breakthrough pressure is a
significant parameter for hydrate reservoir.
Keywords: Natural hydrate reservoir; Sealing effect;
Gas–water flow; MRI