Abstract
AbstractMethane hydrate saturation estimates from remote geophysical data and borehole logs are needed to assess the role of hydrates in climate change, continental slope stability, and energy resource potential. Here we present laboratory hydrate formation/dissociation experiments in which we determined the methane hydrate content independently from pore pressure and temperature and from electrical resistivity. Using these laboratory experiments, we demonstrate that hydrate formation does not take up all the methane gas or water even if the system is under two phase water‐hydrate stability conditions and gas is well distributed in the sample. The experiment started with methane gas and water saturations of 16.5% and 83.5%, respectively; during the experiment, hydrate saturation proceeded up to 26% along with 12% gas and 62% water remaining in the system. The coexistence of hydrate and gas is one possible explanation for discrepancies between estimates of hydrate saturation from electrical and acoustic methods. We suggest that an important mechanism for this coexistence is the formation of a hydrate film enveloping methane gas bubbles, trapping the remaining gas inside.
Highlights
Hydrate is a naturally occurring ice-like, crystalline solid comprising a hydrogen-bonded water lattice with trapped gas molecules, that forms in seafloor sediments at high pressures and low temperatures (Kvenvolden, 1993)
Has been inferred in several locations away from seabed methane plumes (e.g., Guerin et al, 1999; Milkov et al, 2004; Lee and Collett, 2006; Miyakawa et al, 2014). Such field studies have attributed this presence of gas within the gas hydrate stability zone (GHSZ) to: (i) influx of gas into the GHSZ along fracture/faults (Gorman et al, 2002; Lee & Collett, 2006b; Smith et al, 2014); (ii) local deviations from two phase water-hydrate stability conditions resulting in local hydrate dissociation within the GHSZ (Guerin et al, 1999; Milkov et al, 2004); or (iii) hydrate formation kinetics (Torres et al, 2004)
Even though we allowed enough time (80 - 180 h) for hydrate formation to continue, and there was always stoichiometrically sufficient methane gas and brine available for more methane hydrate formation, the reaction stabilized at a maximum methane hydrate saturation between 23
Summary
Methane hydrate saturation estimates from remote geophysical data and borehole logs are needed to assess the role of hydrates in climate change, continental slope stability, and energy resource potential. We present laboratory hydrate formation/dissociation experiments in which we determined the methane hydrate content independently from pore pressure and temperature, and from electrical resistivity. Using these laboratory experiments, we demonstrate that hydrate formation does not take up all the methane gas or water even if the system is under two phase water-hydrate stability conditions and gas is well distributed in the sample. The experiment started with methane gas and water saturations of 16.5% and 83.5% respectively; during the experiment, hydrate saturation proceeded up to 26% along with 12% gas and 62% water remaining in the system. We suggest that an important mechanism for this co-existence is the formation of a hydrate film enveloping methane gas bubbles, trapping the remaining gas inside
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