Electrical properties of methane hydrate + sediment mixtures

Abstract

Knowledge of the electrical properties of multicomponent systems with gas hydrate, sediments, and pore water is needed to help relate electromagnetic (EM) measurements to specific gas hydrate concentration and distribution patterns in nature. Toward this goal, we built a pressure cell capable of measuring in situ electrical properties of multicomponent systems such that the effects of individual components and mixing relations can be assessed. We first established the temperature‐dependent electrical conductivity (σ) of pure, single‐phase methane hydrate to be ~5 orders of magnitude lower than seawater, a substantial contrast that can help differentiate hydrate deposits from significantly more conductive water‐saturated sediments in EM field surveys. Here we report σ measurements of two‐component systems in which methane hydrate is mixed with variable amounts of quartz sand or glass beads. Sand by itself has low σ but is found to increase the overall σ of mixtures with well‐connected methane hydrate. Alternatively, the overall σ decreases when sand concentrations are high enough to cause gas hydrate to be poorly connected, indicating that hydrate grains provide the primary conduction path. Our measurements suggest that impurities from sand induce chemical interactions and/or doping effects that result in higher electrical conductivity with lower temperature dependence. These results can be used in the modeling of massive or two‐phase gas‐hydrate‐bearing systems devoid of conductive pore water. Further experiments that include a free water phase are the necessary next steps toward developing complex models relevant to most natural systems.

Laboratory Electrical Conductivity of Marine Gas Hydrate

“…This is the Arrhenius equation, which can be linearized by plotting the logarithm of σ versus 1/ T to create an Arrhenius plot, the slope of which gives E a . Figure 3 presents an Arrhenius plot for the samples introduced in this paper, along with hydrate synthesized from pure water (MH) and the salt‐bearing samples MH + NaCl0.25 and MH + NaCl1.0 from Lu et al (2019) and the sand‐bearing samples MH + Sand10 and MH + Sand45 from Du Frane et al (2015) for comparison. Du Frane et al (2015) did not carry out ECM modeling on MH + Sand45, but calculated conductivity from the impedance with the smallest angle from the real axis, which approximated the value of R1.…”

Vulcan: A deep‐towed CSEM receiver

“…The resistivities of 30 Ωm are significantly higher than have been previously observed in marine EM hydrate studies [ Goto et al ., ; Weitemeyer et al ., ; Schwalenberg et al ., ; Goswami et al ., ; Gehrmann et al ., ]. Hydrate and sediment mixtures without free water have a resistivity of about 2000 Ωm [ Du Frane et al ., ], so a simple Archie's Law calculation using this matrix resistivity, a pore space filled with seawater, and a porosity exponent of 2 suggests a liquid water content of only about 10%. Although there are as yet no laboratory conductivity data on methane hydrate/sediment mixes containing sea water, and the use of Archie's Law is subject to errors [e.g., Lee and Collett , ], the conclusion is that these high resistivities suggest high levels of hydrate saturation.…”

The Effect of Brine on the Electrical Properties of Methane Hydrate

“…For example, impedance spectroscopy can help reveal variations in electrical behavior, conduction mechanisms/pathways, composition distributions, and other internal changes in a material system just by varying current frequencies (Roberts, 2002;Roberts & Tyburczy, 1999). Gas hydrate/sediment mixtures are dielectrics with electrical behavior that is strongly dependent on temperature (Du Frane et al, 2015), and obtaining quantitative information on the subtleties that contribute to the overall electrical response of gas hydrate mixtures is crucial for understanding more complex systems.…”