MRI insight on natural methane hydrate reservoir system with hydrate,
water and gas layers: Development basis of higher-pressure gas reservoir
under hydrate 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 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.