Stoichiometry measurements are ideally made on samples that are dissociated in situ directly after synthesis by the temperature-ramping procedures described above. This method permits stabilization of samples for an extended time at 0.1 MPa and at T < 194 K prior to dissociation, allowing the release of any residual pore gas or adsorbed methane on grain surfaces. Such release is easily detectable by baseline shifts recorded by the flow meter.
Seven samples tested in this manner confirmed the high reproducibility of sample composition produced by the prescribed growth methods; stoichiometry number n of all the test specimens measured at 5.89 ± 0.01 (Fig. 7A). This measured stoichiometry is slightly closer to ideal than that which we reported previously (6.1 ± 0.1, on samples that contained 0 to 3 % unreacted ice, Stern et al., 1996) due to the greatly improved analytical and measurement capabilities provided by the internal thermocouples and the gas collection apparatus, and the current ability to detect very small amounts of unreacted ice. Samples are also now routinely held at the highest P-T conditions during synthesis for several hours longer than previously, to insure reaction of the last several percent of ice to hydrate. (The difference between n of 5.89 vs. 6.1 in methane hydrate samples corresponds to 3.2 vol. % unreacted ice.)
As our synthesis procedures form hydrate at conditions of P and T that are significantly overdriven with respect to the equilibrium curve, it may be desirable to anneal the as-grown hydrate at low PCH4 or at equilibrium conditions prior to measurement of certain physical properties. Annealing is easily accomplished through resetting the P-T conditions on the samples to those more relevant to geologic settings, or to conditions close to or directly on the methane hydrate equilibrium curve (Fig. 1). Annealing effectively removes some methane from the hydrate structure (when made by the methods given here), as lower P stabilizes a less ideal stoichiometry. Kinetic factors are also important in reaching an equilibrium composition. On the laboratory time scale, there is a definite trade-off in achieving equilibrium compositions by reducing P at cold temperatures to very low equilibrium pressures, compared to annealing at warmer temperatures to the somewhat higher equilibrium pressures. While this effect has not yet been quantified, preliminary tests show that, as expected, annealing at warmer temperatures promotes a more rapid re-equilibration of n. Allowing a fully-reacted sample (n = 5.89) to anneal at 274 K and 4.5 MPa for 6 days, for instance, results in an increase in the stoichiometry number n to 6.01. Annealing at 250 K for 12 days at 2 MPa, however, only produces an increase in n to 5.94 even though the annealing pressure is lower (see Fig. 1).
- top thermocouple
- middle -bottom
180 200 220 240 260 280 Temperature, K
Figure 7. A typical dissociation profile (A) and flow-rate profile (B) for a sample of methane hydrate grown from 26 g of H20 ice and dissociated by temperature-ramping procedures (Fig. 6A) by slowly warming the sample above dissociation conditions at a rate of approximately 11 K/hr. This sample yielded 0.244 moles of CH4 gas, corresponding to a stoichiometry CH4»5.89H20. Most of the gas evolves over the temperature range 200 to 215 K, and the final 2-5% hydrate is not released until the sample is warmed through the H20 melting point.
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