Laboratory Electrical Conductivity of Marine Gas Hydrate

Abstract

Methane hydrate was synthesized from pure water ice and flash frozen seawater, with varying amounts of sand or silt added. Electrical conductivity was determined by impedance spectroscopy, using equivalent circuit modeling to separate the effects of electrodes and to gain insight into conduction mechanisms. Silt and sand increase the conductivity of pure hydrate; we infer by contaminant NaCl contributing to conduction in hydrate, to values in agreement with resistivities observed in well logs through hydrate‐saturated sediment. The addition of silt and sand lowers the conductivity of hydrate synthesized from seawater by an amount consistent with Archie's law. All samples were characterized using cryogenic scanning electron microscopy and energy dispersive spectroscopy, which show good connectivity of salt and brine phases. Electrical conductivity measurements of pure hydrate and hydrate mixed with silt during pressure‐induced dissociation support previous conclusions that sediment increases dissociation rate.

Electrical Properties of Carbon Dioxide Hydrate: Implications for Monitoring CO2 in the Gas Hydrate Stability Zone

“…(2019) and Constable et al. (2020) in which ECM fitting was performed using EIS Spectrum Analyzer software (Bondarenko & Ragoisha, 2005). Briefly, the ECM fits indicate that different conduction mechanisms exist for CH 4 hydrate assemblages (i.e., CH 4 hydrate with impurities, sediments, or fluids present) at different temperatures.…”

“…Recent advances in controlled‐source EM technology now allows measurement of even small variations in conductivity; for example, Kannberg and Constable (2020) resolved seafloor conductivity by a factor of two. We have previously demonstrated that EM studies in the field can benefit from salinity‐conductivity insights gained from laboratory measurement of σ on pure CH 4 hydrate and hydrate ± variable amounts of ionic impurities, sediments, or brines (Constable et al., 2020; Du Frane et al., 2011, 2015; Lu et al., 2019). Our measurements of methane hydrate formed in the presence of NaCl, for example, compare well with field measurements from logging‐while‐drilling data collected from seafloor chimneys off eastern Japan containing high saturations of gas hydrate (Matsumoto et al., 2017).…”

“…Examples at three representative temperatures spanning the step-dwell sweep are shown in Figure 3b. For all but the simplest complex Z measurements, equivalent circuit modeling (ECM) is needed to model sample impedance; we cover this topic in Lu et al (2019) and Constable et al (2020) in which ECM fitting was performed using EIS Spectrum Analyzer software (Bondarenko & Ragoisha, 2005) S1). Six thermal cycles (labeled 1-6) correspond to those in Figure 1, followed by two "step-dwell" heating cycles, the latter in the CO 2 hydrate + CO 2 gas field (hours 223-242, green box) where final Z measurements were collected.…”

“…The residual gas-filled porosity is only expected to contribute a near-negligible effect on sample conductivity (Du Frane et al, 2011). CH 4 hydrate samples that contain a secondary fluid/brine component as shown in Lu et al (2019) and Constable et al (2020), however, can be several orders of magnitude more conductive than pure CO 2 hydrate.…”

“…Frequency = 10-300 kHz. These are the simplest ECMs required to obtain acceptable fits with <5% error in the fitted resistance values (seeConstable et al, 2020;Lu et al, 2019). (c) Example of complex Z and ECM fit for pure methane hydrate fromLu et al (2019) for comparison.…”