Quantitative Analysis of phase saturation distribution during hydrate formation and dissociation under high water saturation condition using low-field NMR

Natural gas hydrates are known to exist in vast quantities beneath the permafrost and deep-sea sediment layers worldwide, transcending national borders, making them a promising future energy resource. While test productions have been conducted by a few countries such as Japan, China, and the United States, a commercially viable production method has yet to be established. The technologies developed so far have limitations, as laboratory-scale experiments and computational models often fail to accurately predict production behavior in actual field conditions. To achieve reliable predictions of production patterns, it is crucial to understand the changes in phase distribution within hydrate-bearing sediment layers and the corresponding multiphase fluid behavior. This study utilizes low-field Nuclear Magnetic Resonance (NMR) to quantitatively analyze phase distribution, saturation, and pore occupancy changes within rock samples during hydrate formation and dissociation processes. Particularly, experiments on hydrate formation and dissociation, along with NMR signal analysis, were conducted under conditions where water is abundant to investigate the role and influence of excessive water. In cases of high initial water saturation (presence of movable water), it was observed that the phase saturation distribution during hydrate formation becomes heterogeneous due to water migration in an unexpected manner. During the depressurization-driven dissociation process, a quantifiable increase in water saturation due to dissociated water was observed, revealing the occurrence of hydrate reformation even during the dissociation process. This research provides a methodology and analytical data to understand phenomena that are challenging to predict during hydrate formation and dissociation processes.