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.

Pore-based architecture and representative element volume evaluation in artificial sand packs and natural rock cores

The phenomenon of multiphase flow in porous media is confronted in various fields of science and industrial applications. Owing to the complicated porous structure, the flow mechanisms are still not completely resolved. A critical and fundamental question is the variation of pore structure and REV sizes among different types of porous media. In this study, a total of 22 porous samples were employed to systematically evaluate the pore-based architecture and REV sizes based on X-ray CT image analysis and pore network modelling. It is found that the irregular grain shapes give rise to large specific areas,narrow and thin throats in identical sand packs. The packs with more types of sands, or composited by the sands with larger difference in diameter, have larger specific area, smaller tortuosity and pore spaces.Based on the REV measurement through porosity solely, the REV sizes of sand packs are generally in the order of magnitude of 10^(-2) m L, while it is at least one order of magnitude smaller in rock cores. Our result indicates that the combination of porosity and Euler number is an effective indicator to get the REV sizes of porous samples.