Characterizing methane hydrate phase transitions and multiphase flow behavior in porous media: Insights from low-field nuclear magnetic resonance measurements

Methane hydrate was viewed as a potential clean energy resource and has attracted extensive research attention in the energy field. The hydrate exploiting phase transition has complex characteristics, which was generally a coupling process of temperature, pressure, and multiphase flow field in reservoir. It is necessary to reveal the effect of reservoir environment on high-efficient exploitation of hydrate. In this study, a nuclear magnetic resonance (NMR) experimental system was used to analyze the phase transition characteristics of methane hydrate under different conditions (constant pressure, gas migration, and gas-water flow). The water migration behavior and water distribution evolution were also investigated in the hydrate-free and hydrate-bearing reservoir during the gas-water flow process. The findings showed that methane hydrate formation under gas migration and constant pressure conditions was divided into three stages, included the hydrate nucleation induction, mass growth, and stabilization. Compared to constant pressure condition, gas migration resulted in a shorter induction time for hydrate formation. Additionally, the water distribution difference between hydrate-free and hydrate-bearing sediments was revealed. In the hydrate-bearing reservoir, hydrate phase transition induced the uneven water distribution, while the even distribution of free water was observed in the hydrate-free reservoir. Furthermore, water phase flow rate was confirmed the one of the key factors for promoting hydrate decomposition. A threefold increase in water phase flow rate led to an 80% increase in the hydrate decomposition rate. These results provided valuable insights for the efficient extraction of methane hydrates in natural reservoir.