Dissociation kinetics and phase stability

The methods described above for in situ synthesis and stoichiometry measurement are particularly well suited for determination of effects of various pressure-temperature-time (P-T-t) paths on hydrate dissociation kinetics and phase stability, due to the ease of controlling such parameters as pressure, peak temperature, temperature-ramping rate, sample volume, sample composition, and stoichiometry. Below we present some representative results from nearly 100 experiments that demonstrate several applications of the flow meter to the investigation of hydrate dissociation kinetics and phase stability behavior over the temperature range 190 to 290 K at 0.1 MPa, as methane hydrate dissociates to either ice + CH4 gas (T< 273 K) or liquid water + gas (T > 273 K).

3.4.1. Methane hydrate —> ice + gas A suite of methane hydrate samples were removed from initially stable conditions of low T or elevated P by either the temperature-ramping method or the rapid-depressurization method discussed above. Samples decomposed by the temperature-ramping method exhibited highly reproducible decomposition behavior in which approximately 95% of the expected amount of gas evolved over the temperature range 195 K < T < 220 K, as shown in Figure 7. The remaining gas was then only released from the samples (on the laboratory time scale) by warming through 273 K. This resistance to decomposition displayed by the residual 3-5% is likely due to its entrapment along ice grain boundaries or within grain interiors, as it is subsequently released by the melting of the encapsulating ice.

When rapidly depressurized at isothermal conditions ranging from 204 K to 240 K, methane hydrate samples exhibit systematically increasing dissociation rates with increasing temperature, as expected with the increasing thermal overstep of the stability field (Fig. 8A). Samples tested at 204 K dissociated over ~ 3 hours, while those tested at 239 K dissociated within 7 minutes. These times correspond to the duration of the main dissociation event, or about 88-90 % reaction, and the remaining hydrate did not dissociate until the samples were warmed through 273 K and all the ice product had melted.

\ 225 K Continuous dissociation at

—1 " 211 K moderate to rapid rates

\ 225 K Continuous dissociation at

—1 " 211 K moderate to rapid rates

0 20 40 60 80 100 120 140 160

0 20 40 60 80 100 120 140 160

Figure 8. Representative curves from rapid depressurization tests, showing the unusual temperature dependency of methane hydrate dissociation kinetics as it decomposes to ice + gas. (A) Dissociation rates of samples tested over the temperature range 204 to 270 K. Up to 240 K, methane hydrate dissociates at increasing rates with increasing T. Only the isothermal portion of the tests are shown here; heating through 273 K then releases all remaining gas, usually totaling very close to 100% of the expected amount. From 250 to 270 K however, dissociation rates become highly suppressed, and large fractions of the hydrate can be "anomalously" preserved for at least many days. (B) Profile of a rapid depressurization test at 268 K, showing 60% of the hydrate preserving at 1 atm after 6 days, at temperatures 73 K above the equilibrium dissociation temperature. The expected remainder of gas was then recovered by heating the sample through 273 K.

"Anomalous" preservation" behavior. When methane hydrate samples are rapidly depressurized at isothermal conditions between 250 and 270 K, an anomalous preservation effect can be invoked (Fig. 8; also Stern et al., 1998c). This effect is most prominent at temperatures approaching 270 K, where significant amounts of hydrate can remain metastable due to greatly suppressed dissociation rates, even in tests lasting over 6 days (Fig. 8B). Warming of the metastable hydrate through 273 K then promotes full dissociation and rapid release of all remaining gas. Tests in which "preserved" samples were cooled to 190 K and then slowly rewarmed, however, showed that this preservation effect is thermally irreversible, as the samples then dissociated over the interval 198218 K as observed in the temperature-ramping tests discussed above.

Based on textural observations of quenched samples and the lack of appreciable ice in them, it appears that warm-temperature preservation may be at least partially due to grain surface effects, grain boundary mobility, or structural changes within the hydrate grains. This is in contrast to the residual hydrate in temperature-ramping tests that is released at 273 K, that is expected to be "preserved" by the encapsulating ice. Other tests conducted on unconsolidated methane hydrate and on hydrate + sediment ± seawater mixtures, all samples that were rapidly depressurized at 268 K, demonstrated that while dissociation rates were less suppressed than those from more compacted or pure samples, the rates were still orders of magnitude slower than those predicted by extrapolation of dissociation rates from the lower temperature (204 to 240 K) regime. These results indicate that while the warm-temperature preservation effect is enhanced by grain boundary contacts, it is largely a structural or intrinsic property of methane hydrate.

We still do not fully understand the physical chemistry involved in this preservation effect, but have found it to be highly reproducible. Descriptions of other gas hydrate "self' preservation effects have also been documented by Davidson et al. (1986), Yakushev & Istomin, (1992), Gudmundsson et al. (1994), and Dallimore & Collett (1995).

3.4.2. Methane hydrate -» water + gas. Rapid-depressurization tests conducted at temperatures above 273 K show that dissociation proceeds in a systematic manner in which rates increase with increasing external temperatures (Fig. 9). In all tests conducted at T > 273 K, sample interior temperatures plummet and buffer at 272.5 K as dissociation proceeds (Fig. 9; also see Fig. 1 in Circone et al., 2000), an effect similar to that observed on recovered natural hydrate from oceanic drill core material (Kastner et al., 1995). Tests on methane hydrate + sediment ± seawater samples also show that the addition of particulates and seawater both measurably increase the dissociation rate compared to pure hydrate samples. Additional details are provided by Circone et al., (2000) and increase systematically (shown by arrow). The two runs conducted at 273.6 K and 273.8 K demonstrate the excellent reproducibility of the test

Figure 9. Dissociation rates of rapid depressurization tests conducted at T > 273 K, as pure methane hydrate decomposes to water + gas (from Circone et al., 2000, Fig. 2). With increasing external bath temperature (TeKt), dissociation rates procedures and results.

Figure 9. Dissociation rates of rapid depressurization tests conducted at T > 273 K, as pure methane hydrate decomposes to water + gas (from Circone et al., 2000, Fig. 2). With increasing external bath temperature (TeKt), dissociation rates procedures and results.

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