Pore-Scale Analysis of the Permeability and Effective Thermal Conductivity of Hydrate-Bearing Sediments Based on a High-Pressure Microfluidics Approach

Methane hydrate (MH), recognized for its extensive resource volume and high energy density, is a viable future clean energy source. This study introduces a novel high-pressure microfluidics approach to examine the pore-scale behavior of MH formation and dissociation in hydrate-bearing sediments (HBS). Utilizing this technique, we identified two distinct MH morphologies: needle-like or fan-shaped MH crystals formed from dissolved CH4, and a porous structure MH coexisting with CH4 gas formed from CH4 gas bubbles. In addition, the growth of crystal-type MH prompts sudden nucleation of the porous-type MH. During MH dissociation via depressurization, we observed two distinct paths of gas bubbles development: micro gas bubbles accumulating and coalescing at the porous-type MH dissociation interface and several gas bubbles expanding with a water layer at the crystal-type MH dissociation interface. Subsequent image segmentation of the high-resolution MH morphology images, processed using an algorithm developed in-house, facilitated the construction of a geometric model for fluid flow and thermal conduction simulations. Computational fluid dynamics (CFD) simulations revealed that decreasing MH saturation (S H) from 37.9% to 0 leads to a 3 order of magnitude increase in normalized permeability, while reducing S H from 46.4% to 0 decreased the effective thermal conductivity from 1.13 to 0.22 W/m/K. Moreover, empirical models were developed for the normalized permeability and effective thermal conductivity yielded from the CFD simulations, respectively. The study provides fundamental direct visual evidence on MH morphology evolution at the pore scale during MH dissociation. The method of pore-scale CFD modeling based on real-time MH morphology acquired by high-pressure microfluidics is instrumental to understanding the thermophysical properties of HBS.