Susan Circone

Scanning Electron Microscopy investigations of laboratory-grown gas clathrate hydrates formed from melting ice, and comparison to natural hydrates

Abstract Scanning electron microscopy (SEM) was used to investigate grain texture and pore structure development within various compositions of pure sI and sII gas hydrates synthesized in the laboratory, as well as in natural samples retrieved from marine (Gulf of Mexico) and permafrost (NW Canada) settings. Several samples of methane hydrate were also quenched after various extents of partial reaction for assessment of mid-synthesis textural progression. All laboratory-synthesized hydrates were grown under relatively high-temperature and high-pressure conditions from rounded ice grains with geometrically simple pore shapes, yet all resulting samples displayed extensive recrystallization with complex pore geometry. Growth fronts of mesoporous methane hydrate advancing into dense ice reactant were prevalent in those samples quenched after limited reaction below and at the ice point. As temperatures transgress the ice point, grain surfaces continue to develop a discrete “rind” of hydrate, typically 5 to 30 μm thick. The cores then commonly melt, with rind microfracturing allowing migration of the melt to adjacent grain boundaries where it also forms hydrate. As the reaction continues under progressively warmer conditions, the hydrate product anneals to form dense and relatively pore-free regions of hydrate grains, in which grain size is typically several tens of micrometers. The prevalence of hollow, spheroidal shells of hydrate, coupled with extensive redistribution of reactant and product phases throughout reaction, implies that a diffusion-controlled shrinking-core model is an inappropriate description of sustained hydrate growth from melting ice. Completion of reaction at peak synthesis conditions then produces exceptional faceting and euhedral crystal growth along exposed pore walls. Further recrystallization or regrowth can then accompany even short-term exposure of synthetic hydrates to natural ocean-floor conditions, such that the final textures may closely mimic those observed in natural samples of marine origin. Of particular note, both the mesoporous and highly faceted textures seen at different stages during synthetic hydrate growth were notably absent from all examined hydrates recovered from a natural marine-environment setting.

Laboratory synthesis of pure methane hydrate suitable for measurement of physical properties and decomposition behavior

Gas hydrates are an intriguing class of nonstoichiometric compounds that have significant commercial and scientific applications both as an energy resource and as a manufactured material. The last half-century has witnessed a marked escalation in the scope of experimental research on gas hydrates, particularly directed towards the determination of their phase equilibria, formation kinetics, crystallographic and structural properties, transport and thermal properties, effects of inhibitors, and a number of related geochemical topics.

The effect of elevated methane pressure on methane hydrate dissociation

Abstract Methane hydrate, equilibrated at P, T conditions within the hydrate stability field, was rapidly depressurized to 1.0 or 2.0 MPa and maintained at isobaric conditions outside its stability field, while the extent and rate of hydrate dissociation was measured at fixed, externally maintained temperatures between 250 and 288 K. The dissociation rate decreases with increasing pressure at a given temperature. Dissociation rates at 1.0 MPa parallel the complex, reproducible T-dependence previously observed between 250 and 272 K at 0.1 MPa. The lowest rates were observed near 268 K, such that >50% of the sample can persist for more than two weeks at 0.1 MPa to more than a month at 1 and 2 MPa. Varying the pressure stepwise in a single experiment increased or decreased the dissociation rate in proportion to the rates observed in the isobaric experiments, similar to the rate reversibility previously observed with stepwise changes in temperature at 0.1 MPa. At fixed P, T conditions, the rate of methane hydrate dissociation decreases monotonically with time, never achieving a steady rate. The relationship between time (t) and the extent of hydrate dissociation is empirically described by: Evolved gas (%) = A·tB (1) where the pre-exponential term A ranges from 0 to 16% s-B and the exponent B is generally <1. Based on fits of the dissociation results to Equation 1 for the full range of temperatures (204 to 289 K) and pressures (0.1 to 2.0 MPa) investigated, the derived parameters can be used to predict the methane evolution curves for pure, porous methane hydrate to within ± 5%. The effects of sample porosity and the presence of quartz sand and seawater on methane hydrate dissociation are also described using Equation 1.

Methane Hydrate Dissociation Rates at 0.1 MPa and Temperatures above 272 K

Abstract: We performed rapid depressurization experiments on methane hydrate under isothermal conditions above 272 K to determine the amount and rate of methane evolution. Sample temperatures rapidly drop below 273 K and stabilize near 272.5 K during dissociation. This thermal anomaly and the persistence of methane hydrate are consistent with the reported recovery of partially dissociated methane hydrate from ocean drilling cores.

Measurement of gas yields and flow rates using a custom flowmeter

A simple gas collection apparatus based on the principles of a Torricelli tube has been designed and built to measure gas volume yields and flow rates. This instrument is routinely used to monitor and collect methane gas released during methane hydrate dissociation experiments. It is easily and inexpensively built, operates at ambient pressures and temperatures, and measures gas volumes of up to 7 L to a precision of about 15 ml (about 0.0025 mol). It is capable of measuring gas flow rates varying from more than 103 to less than 10−1 ml/min during gas evolution events that span minutes to several days. We have obtained a highly reproducible hydrate number of n=5.891 with a propagated uncertainty of ±0.020 for synthetic methane hydrate.