The Effect of Brine on the Electrical Properties of Methane Hydrate

Abstract

Gas hydrates possess lower electrical conductivity (inverse of resistivity) than either seawater or ice, but higher than clastic silts and sands, such that electromagnetic methods can be employed to help identify their natural formation in marine and permafrost environments. Controlled laboratory studies offer a means to isolate and quantify the effects of changing individual components within gas-hydrate-bearing systems, in turn yielding insight into the behavior of natural systems. Here we investigate the electrical properties of polycrystalline methane hydrate with ≥25% gas-filled porosity and in mixture with brine. Initially, pure methane hydrate was synthesized from H 2 O ice and CH 4 gas while undergoing electrical impedance measurement, then partially dissociated to assess the effects of pure pore water accumulation on electrical conductivity. Methane hydrate + brine mixtures were then formed by either adding NaCl (0.25-2.5 wt %) to high-purity ice or by using frozen seawater as a reactant. Conductivity was obtained from impedance measurements made in situ throughout synthesis while temperature cycled between +15°C and −25°C. Several possible conduction mechanisms were subsequently determined using equivalent circuit modeling. Samples with low NaCl concentration show a doping/impurity effect and a log linear conductivity response as a function of temperature. For higher salt content samples, conductivity increases exponentially with temperature and the log linear relationship no longer holds; instead, we observe phase changes within the samples that follow NaCl-H 2 O-CH 4 phase equilibrium predictions. Final samples were quenched in liquid nitrogen and imaged by cryogenic scanning electron microscopy (cryo-SEM) to assess grain-scale characteristics. Key Points:• Electrical properties of a multicomponent methane hydrate system synthesized from a mixture of ice, NaCl, and CH 4 gas were examined • Electrical conductivity of samples increases nonlinearly with increasing salt content for temperatures above −15°C • No secondary NaCl-bearing phases were detected in samples with <1.0 wt % salt due to possible incorporation into the hydrate structure Supporting Information:• Supporting Information S1

Laboratory Electrical Conductivity of Marine Gas Hydrate

“…The shading is a result of the low angle of the incident radiation. Additional SEM images of methane hydrate ± quartz sand ± NaCl‐bearing phases, including hydrate formed from seawater ice, are shown in Lu et al (2019).…”

“…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.…”

“…Sand was Oklahoma #1, >99% purity SiO 2 with 84% grain‐size distribution of 106–250 μm, also used in Du Frane et al (2015). In addition to the four samples described above, Table 1 lists six previous experiments from Du Frane et al (2015) and Lu et al (2019) relevant to the current work and discussion. The sample naming convention is MH for hydrate synthesized from high purity ice, and SH for hydrate synthesized from seawater, followed by the added product (silt, sand, or NaCl) and the percentage by weight or volume.…”

“…For each run and each temperature, complex electrical impedance was measured at 27 frequencies between 0.5 and 300 kHz, evenly spaced in log (frequency) and limited by the impedance meter, as described in Lu et al (2019), and plotted as "Cole-Cole" plots in the complex plane. Figure 2 shows examples of such plots at representative temperatures for four runs considered here.…”

“…The interpretation of CSEM and well log data requires the support of laboratory studies of conductivity (e.g., Lee et al, 2010; Li et al, 2012; Priegnitz et al, 2015; Spangenberg & Kulenkampff, 2006). In a series of previous papers we investigated electrical conductivity as a function of temperature for pure methane hydrate (Du Frane et al, 2011), hydrate with sand (Du Frane et al, 2015), and hydrate with salt (NaCl) (Lu et al, 2019). Here we conclude these studies by comparing the effect of sand and silt with hydrate synthesized from pure water and flash frozen seawater, and examining conductivity during pressure‐induced dissociation.…”