Electrical properties of multi-component methane hydrate systems, a potential tool for constraining volumetric distribution of fluids in high-saturation formations

Controlled source electromagnetic surveys (CSEM) is a promising geophysical method for remotely detecting naturally formed gas hydrate in marine settings, with sensitivity to concentration and distribution of hydrate/fluid saturations within seafloor sediment. However, overall electrical conductivity of these systems is a complicated product of multiple components. Pure methane hydrate has relatively low electrical conductivity, on the order of 10^-4 to 10^-5­ S/m, although sand can increase this by an order of magnitude due to the presence of impurities. Even higher concentrations of salt can stabilize a secondary hydro-salt phase at low temperature (T) that melts incongruently into highly conductive brine. Here we present the electrical properties of various sediments (sand, silt, or glass beads) in semi-porous mixtures with methane hydrate formed from either high-purity H₂O ice or flash-frozen seawater in a pressurized CH₄ gas environment. Hydrate synthesis was achieved through multiple T cycles (-24 to +15°C) with in situ monitoring of electrical impedance. Samples were then quenched for cryogenic scanning electron microscopy and energy dispersive spectroscopy (cryo-SEM/EDS). Frozen brine was distinguished from gas hydrate or sediment by texture, as well as its Na and Cl contents. Equivalent circuit modeling was used to isolate sample conductivity from systemic effects such as electrode polarization, to monitor changes to the impedance structure, and proved to be sensitive to the various phase boundaries traversed during T cycling. High salinity samples formed from flash-frozen seawater saw a marked increase in electrical conductivity and diminished electrode polarization at higher frequency. In these cases, cryo-SEM/EDS verified that brine was present at sufficient volumes to cross the percolation threshold. Electrical conductivity of samples with well-connected brine was largely controlled by solute content, whereas conductivity of lower salinity samples was lower with greater T dependence. These results suggest that CSEM in regions with high gas hydrate saturations, like those found offshore of Japan, may exhibit high contrast boundaries associated with this percolation threshold that could be used to constrain the fluid volume ratio and the overall salinity of these formations.