Pore-Scale\nAnalysis of the Permeability and Effective\nThermal Conductivity of Hydrate-Bearing Sediments Based on a High-Pressure\nMicrofluidics Approach

Methane hydrate (MH), recognized for its extensive resource\nvolume\nand high energy density, is a viable future clean energy source. This\nstudy introduces a novel high-pressure microfluidics approach to examine\nthe pore-scale behavior of MH formation and dissociation in hydrate-bearing\nsediments (HBS). Utilizing this technique, we identified two distinct\nMH morphologies: needle-like or fan-shaped MH crystals formed from\ndissolved CH<sub>4</sub>, and a porous structure MH coexisting with\nCH<sub>4</sub> gas formed from CH<sub>4</sub> gas bubbles. In addition,\nthe growth of crystal-type MH prompts sudden nucleation of the porous-type\nMH. During MH dissociation via depressurization, we observed two distinct\npaths of gas bubbles development: micro gas bubbles accumulating and\ncoalescing at the porous-type MH dissociation interface and several\ngas bubbles expanding with a water layer at the crystal-type MH dissociation\ninterface. Subsequent image segmentation of the high-resolution MH\nmorphology images, processed using an algorithm developed in-house,\nfacilitated the construction of a geometric model for fluid flow and\nthermal conduction simulations. Computational fluid dynamics (CFD)\nsimulations revealed that decreasing MH saturation (<i>S</i><sub>H</sub>) from 37.9% to 0 leads to a 3 order of magnitude increase\nin normalized permeability, while reducing <i>S</i><sub>H</sub> from 46.4% to 0 decreased the effective thermal conductivity\nfrom 1.13 to 0.22 W/m/K. Moreover, empirical models were developed\nfor the normalized permeability and effective thermal conductivity\nyielded from the CFD simulations, respectively. The study provides\nfundamental direct visual evidence on MH morphology evolution at the\npore scale during MH dissociation. The method of pore-scale CFD modeling\nbased on real-time MH morphology acquired by high-pressure microfluidics\nis instrumental to understanding the thermophysical properties of\nHBS.