Thermal Diffusivity

Thermal diffusivity a (units of m2 s"1) is related to thermal conductivity K through the relationship a=K/(pCp), where p denotes density and Cp represents heat capacity. The thermal diffusivity of hydrate can be measured during the determination of thermal conductivity by using two needle probes separated by a known distance (Drury, 1988) or in specialized experiments using the Angstrom method (e.g., Durham et al., 1987). Due to the interdependence of thermal conductivity, thermal diffusivity, and heat capacity, combining laboratory thermal diffusivity determinations with thermal conductivity results should yield an estimate of heat capacity, a fundamental thermodynamic parameter that has only been the subject of a few studies to date (e.g., Ross and Anderson, 1982). There are no published direct measurements of the thermal diffusivity of hydrate although efforts are currently underway to determine this quantity for compacted methane hydrate under pressure conditions characteristic of those in marine sediments on continental margins.

3. STABILITY CONDITIONS

As discussed above, the stability of hydrate depends on pressure and temperature and on gas solubility as a function of these two variables. This section focuses on some of the physical and chemical factors that promote or inhibit hydrate stability. These factors, whose effects on the stability field are summarized in Figure 3, include: (a) gas properties, (b) pore fluid properties, (c) sediment composition, and (d) sediment physical properties. Although the effects of these factors on hydrate formation may to first order be kinematic, thermodynamic, or purely physical or chemical, these effects are cast here only in terms of their impact on the temperature of hydrate stability. It should also be noted that, at least anecdotally, many hydrate reservoirs appear to be associated with overpressured sediments. Compared to the tens of megapascals of ambient hydrostatic pressure in most marine systems though, the degree of overpressure in shallow hydrate reservoirs is typically not large enough to significantly displace the stability curve in P-T space. The effect is therefore not considered here.

m 3100-

Q 3400-

METHANE HYDRATE

Cations

METHANE HYDRATE

METHANE GAS

Most gases Clays?

METHANE GAS

Most gases Clays?

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Temperature (°C)

Figure 3. Stability of methane hydrate for pure water (solid line) and seawater (dashed line) systems. Arrows schematically show how various physical and chemical factors affect the stability of hydrate. After Ruppel (1997).

The phase equilibria shown in Figure 3 apply to pure methane systems and Structure I gas hydrate in the presence of fresh or saline water. The presence of even a fraction of a percent of a Structure II hydrate-former (e.g., ethane, propane) will produce Structure II hydrate, which is stable to higher temperatures than Structure I at a given pressure. Even when the concentration of a gas other than methane is not high enough to affect the structure that forms, addition of even very minor amounts of C02, ethane (C2H6), H2S, and high-order thermogenic hydrocarbons may shift the stability curve toward higher temperatures, increasing the stability of methane hydrate.

In contrast, the presence of dissolved ions (Na2+, K+, Mg2+, Ca2+) in pore fluids inhibits the stability of gas hydrate. Laboratory and theoretical thermodynamical calculations (de Roo et al., 1983; Dickens and Quinby-Hunt, 1994; Dickens and Quinby-Hunt, 1997) confirm a -1.1°C offset between methane hydrate dissociation temperatures in 33.5%o NaCl seawater vs. pure water. For the entire range of naturally-occurring seawaters (33-37%o NaCl), the dissociation temperature does not vary by more than 0.14°C. The inhibitory effect that ionic compounds exercise on the stability of hydrate has critical implications for the evolution and long-term stability of hydrate deposits in areas characterized by salt tectonism, evolution of seafloor brine basins, and circulation of briny fluids (e.g., Gulf of Mexico; Sassen and Macdonald, 1997).

The composition of the sediment matrix may also exercise a strong influence on the temperature of hydrate dissociation in some settings. As hydrophilic substances with unbalanced surface charge and large surface area, clays are the most likely minerals to influence hydrate formation by sorbing water and providing nucleation surfaces for hydrate crystals. Laboratory experiments on mixtures of water + bentonite or montmorillonite ± other compounds have yielded contradictory results: Clays did not affect hydrate stability (Kotkoskie et al., 1990; Englezos and Hall, 1994) in some experiments, but appeared to promote hydrate formation both thermodynamically and kinetically in other experiments (Cha et al., 1988).

Physical properties of the sediment may play a critical role in the stability of hydrate as well. In fine-grained materials, large capillary forces will arise and may inhibit the entry of fluids into the interstices between grains. Theoretical calculations (Clennell et al., 1999) and laboratory measurements using finegrained (clay-sized) particles (Handa and Stupin, 1992; Zakrzewski and Handa, 1993; Melnikov and Nesterov, 1996) indicate that capillary forces may significantly inhibit hydrate stability, depressing hydrate dissociation temperatures.

4. IN-SITU THERMAL REGIMES IN HYDRATE RESERVOIRS

Direct measurements of in situ equilibrium temperatures using sensors placed in the sediments at the bottom of Ocean Drilling Program boreholes is the best method for characterizing the thermal state of the hydrate reservoir. Such measurements have now been conducted in hydrate-bearing sediments in a number of settings, including both active (Cascadia, Chile, and Costa Rica; Hyndman et al., 1992; Brown et al., 1996; Ruppel and Kinoshita, 2000) and passive (Blake Ridge; Paull et al., 1996) margins.

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