ABSTRACT
It is a known and experimentally verified fact that the flow of pressure-driven nanoconfined fluids cannot be accurately described by the Navier-Stokes (NS) equations with non-slip boundary conditions, and the measured volumetric flow rates are much higher than those predicted by macroscopical continuum models. In particular, the flow enhancement factors (the ratio between the flow rates directly measured by experiments or simulations and those predicted by the non-slip NS equation) reported by previous studies have more than five orders of magnitude differences. We showcased an anomalous phenomenon in which the flow enhancement exhibits a non-monotonic correlation with fluid pressure within the carbon nanotube with a diameter of 2 nm. Molecular dynamics simulations indicate that the inconsistency of flow behaviors is attributed to the phase transition of nanoconfined fluid induced by fluid pressures. The nanomechanical mechanisms are contributed by complex hydrogen-bonding interactions and regulated water orientations. This study suggests a method for explaining the inconsistency of flow enhancements by considering the pressure-dependent molecular structures.
ABSTRACT
Both intra-pore hydrate morphology and inter-pore hydrate distribution influence the physical properties of hydrate-bearing sediments, yet there has been no pore-scale observations of hydrate habit under pressure in preserved pressure core samples so far. We present for the first time a pore-scale micro-CT study of natural hydrate-bearing cores that were acquired from Green Canyon Block 955 in UT-GOM2-1 Expedition and preserved within hydrate pressure-temperature stability conditions throughout sub-sampling and imaging processes. Measured hydrate saturation in the sub-samples, taken from units expected to have in-situ saturation of 80% or more, ranges from 3 ± 1% to 56 ± 11% as interpreted from micro-CT images. Pore-scale observations of gas hydrate in the sub-samples suggest that hydrate in silty sediments at the Gulf of Mexico is pore-invasive rather than particle displacive, and hydrate particles in these natural water-saturated samples are pore-filling with no evidence of grain-coating. Hydrate can form a connected 3D network and provide mechanical support for the sediments even without cementation. The technical breakthrough to directly visualize particle-level hydrate pore habits in natural sediments reported here sheds light on future investigations of pressure- and temperature-sensitive processes including hydrate-bearing sediments, dissolved gases, and other biochemical processes in the deep-sea environment.
