MRI insight on multiphase flow in hydrate-bearing sediment and development mechanism of hydrate seal

Gas and water migration through the hydrate-bearing sediment are characteristic features in marine gas hydrate reservoirs worldwide. However, there are few experimental investigations on the effect of water-gas flow on the gas hydrate reservoir. In this study, gas-water migration in gas hydrate stability zone (GHSZ) was investigated visually employing a high-resolution magnetic resonance imaging (MRI) apparatus, and the formation of hydrate seal was experimentally investigated. Results revealed that normal flow of gas-water at the low flow rate of 1–0.25 mL/min will induce the hydrate reformation. Conversely, higher gas-water flow rates (at 2–0.5 and 4–1 mL/min) need higher reservoir pressure to induce the hydrate reformation. In addition, the hydrate reformation during the gas-water flow process produced the hydrate seal, which can withstand an over 9.0 MPa overpressure. This high overpressure provides the development condition for the underlying gas and/or water reservoir. A composite MRI image of the whole hydrate seal was obtained through the MRI. The pore difference between hydrate zone and coexistence zone produces a capillary sealing effect for hydrate seal. The hydrate saturation of hydrate seal was more than 51.6%, and the water saturation was more than 19.3%. However, the hydrate seal can be broken through when the overpressure exceeded the capillary pressure of the hydrate seal, which induced the sudden drop of reservoir pressure. This study provides a scientific explanation for the existence of high-pressure underlying gas below the hydrate layer and is significant for the safe exploitation of these common typical marine hydrate reservoirs.