Electrical characterization of methane hydrate with coexisting brine
Abstract
Electromagnetic sensing measurements have proven effective as a complementary tool for mapping specific gas hydrate concentrations in nature. However, empirical information on gas hydrate petrophysics is still needed to better correlate field data with lab results. In this work, we investigated the electrical properties of polycrystalline methane hydrate with coexisting NaCl-bearing brine. Methane hydrate samples were synthesized from pure (triple-distilled) H2O seed ice mixed with finely ground (<75 µm) NaCl salt, plus CH4 gas, and impedance was measured in-situ in a custom pressure vessel throughout synthesis. Conductivity was deduced from impedance spectroscopy results for temperatures between -20°C and +15°C. The addition of low total NaCl concentration ( 0.25wt% NaCl) was found to effectively dope the resulting methane hydrate, eliciting a small but measurable increase in conductivity relative to pure methane hydrate by 1 log unit across the full temperature range. Higher total NaCl content (≥1.0wt% NaCl), however, resulted in production of a coexisting fluid component (brine) that in some regions enveloped methane hydrate grains in a partially interconnected manner, as revealed by cryogenic SEM imaging of quenched samples. Our measurements indicate that at low temperatures where brine is unconnected or partially frozen, methane hydrate is the primary conduction medium, whereas at higher test temperatures for high salt content samples, the connected brine network provides an additional conduction path for current flow. In this presentation we quantify the effect of NaCl on methane hydrate electrical conductivity and show that our interpretations are consistent with equivalent circuit modeling of impedance results. Comparing electrical conductivity of all samples at the geologically-relevant temperature of 5°C shows a roughly log-linear relation between conductivity and NaCl concentration, with conductivity values for hydrate mixtures ranging from 3.1x10-5 S/m (no NaCl added) to 6.4x10-3 S/m (2.5wt% NaCl added).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMOS51F1317L
- Keywords:
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- 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 3004 Gas and hydrate systems;
- MARINE GEOLOGY AND GEOPHYSICSDE: 3022 Marine sediments: processes and transport;
- MARINE GEOLOGY AND GEOPHYSICSDE: 3036 Ocean drilling;
- MARINE GEOLOGY AND GEOPHYSICS