During the depressurization extraction of natural gas hydrates, the dynamic evolution of reservoir permeability is decisive for gas production efficiency. Current research on sensitivity mechanisms in hydrate reservoirs primarily focuses on stress sensitivity, whereas an understanding of flow velocity sensitivity remains less explored. This study investigates the effects of flow velocity gradient and fluid media (gas/water) on sediment permeability, using both clayey silt sediments from the South China Sea and synthetic quartz-montmorillonite mixtures with varying montmorillonite concentrations. Experimental results indicate that under gas displacement conditions, permeability decreases as flow velocity increases for both sediment types. This trend is characterized by a rapid decline at lower flow velocities, followed by gradual stabilization at higher velocities. For water flow, the permeability of the natural clayey silt sediment decreased by approximately 64.3% as the flow velocity increased from 0.02 to 0.08 mL/min, while the quartz-25% montmorillonite mixture exhibited a permeability reduction of about 30.0% when the flow velocity was raised from 0.5 to 3.5 mL/min. This behavior is attributed to the preferential mobilization and migration of weakly cemented fine particles at lower flow velocities, which progressively depletes the reservoir of migratable particles as the velocity increases. Variations in the flow velocity gradient showed no significant effect on the permeability. Furthermore, the relationship between permeability and flow velocity is well described by an exponential function. This study enhances our understanding of flow velocity sensitivity in hydrate reservoirs and provides crucial experimental evidence for predicting permeability evolution during hydrate production via depressurization in the South China Sea. This study presents the first systematic comparison of natural South China Sea sediments with synthetic quartz-montmorillonite mixtures under both gas and water flow conditions and establishes an empirical exponential model describing the permeability–velocity relationship. These findings provide crucial experimental evidence for predicting permeability evolution during hydrate production via depressurization in the South China Sea.
Liu et al. (Wed,) studied this question.