Evolution characteristics of pore water during methane hydrate dissociation by depressurization in unconsolidated sand

The content and distribution of pore water affects hydrate dissociation, gas migration and gas deliverability in the process of natural gas hydrate exploitation. Understanding the evolution characteristics of pore water during the hydrate dissociation is important for extracting natural gas from hydrate-bearing sediments (HBSs). In this work, methane hydrate formation and dissociation experiments were completed in unconsolidated sand with different initial water contents by using a self-designed low-field nuclear magnetic resonance monitoring apparatus. The evolution laws of water content were revealed during the process of methane hydrate dissociation by depressurization. The distribution characteristics of water were analyzed in the water-rich and the gas-rich sediments. Results show that the unclathrated water content is affected by mineral surface in gas-rich HBSs. It is affected by hydrate barrier effects in water-rich HBSs. The hydrate distribution is controlled by the initial water distribution in gas-rich HBSs. It is homogeneous in water-rich HBSs. The pore water content first decreases and then increases during the hydrate dissociation by depressurization in water-rich HBSs, while the pore water content keeps increasing in gas-rich HBSs. Compared with gas-rich HBSs, the migration efficiency of substances is lower,and the hydrate dissociation rate is slower in water-rich HBSs. The increase of water relative permeability and the decrease of pore temperature cause the reduction in growth rate of pore water content in the middle stage of hydrate dissociation by depressurization. For relatively closed HBSs, the pore water distribution after the hydrate exploitation by depressurization is similar to that before the hydrate formation. • Revealed control mechanism of unclathrated water content in HBSs. • Obtained pore water evolution law during hydrate dissociation by depressurization. • Analyzed water/hydrate distribution differences in water-rich and gas-rich HBSs.