The theoretical approach of Hunt (1979) for detecting gas hydrate in a pressurized sediment core can be illustrated by considering pressure-temperature, pressure-composition (molality), and pressure-volume phase diagrams for the CH4-water system (Fig. 1, Fig. 2, and Fig. 3). Figure 1 is a temperature-pressure phase diagram for CH4 and water at high gas concentration. Figure 2 is a concentration (molality)-pressure phase diagram for CH4 and water at constant temperature. Figure 3 is a volume-pressure plot concerning the expected volume of CH4 gas released from a pressurized core of known volume at constant temperature. For simplicity and convenience, these figures pertain to the pure CH4-pure water and pure CH4-seawater (S = 35) systems (Handa, 1990; Dickens and Quinby-Hunt, 1994; Tohidi et al., 1995). The ensuing treatise is provided because the technique of Hunt (1979) has not been thoroughly discussed in the literature, and because it is necessary for understanding limitations to the approach.
A core with sediment and pore water (pure water or seawater in this discussion) is collected at depth in a pressure container of known volume at an initial time t0 with a given pressure, temperature, and CH4 quantity. At temperatures and pressures in the hydrate stability field and sufficiently high CH4 concentration, the core at t0 should contain CH4 hydrate and water saturated with CH4 (Path A, Fig. 1, Fig. 2). Alternatively, under the same pressure and temperature conditions, if CH4 concentration is low, the core at t0 should contain water undersaturated with CH4 (Path B, Fig. 2 [cannot be shown in Fig. 1]). Although a pressurized core at moderate temperature and very high CH4 concentrations could contain CH4 hydrate, free CH4 gas, and solid salt in the pure water-seawater system (Handa, 1990), this case is unlikely in the marine environment because the gas to water ratio will be relatively low (except, perhaps at the microscopic scale or in unusual environments like mud volcanoes).
As pressure is initially released from either a hydrate-bearing or hydrate-free core under isothermal conditions (t0 to t1 in Fig. 1, Fig. 2), concentration of CH4 should not decrease because all CH4 remains in CH4-saturated water or CH4 hydrate. However, once a pressure along the hydrate saturation curve is reached (t1 in Fig. 1, Fig. 2), cores containing CH4 hydrate should display a different behavior during isothermal degassing than cores with only dissolved CH4. Consider a core containing hydrate at a temperature and pressure on the CH4 gas-CH4 hydrate-water equilibrium curve (Fig. 1, Fig. 2). As pressure is released across the three-phase equilibrium curve, hydrate dissociates to CH4 gas and water. Because of the large volume increase in going from solid CH4 hydrate to CH4 gas, degassing of CH4 can occur under isobaric and isothermal conditions from t1 to t2 provided infinitely small volumes of CH4 gas are released from the container (Fig. 1, Fig. 2). In contrast, degassing of CH4 cannot occur isobarically for a core that does not initially contain hydrate (Fig. 2).
At t2, temperature, concentration and pressure are on a CH4-water saturation curve (Fig. 2). As pressure is released at this point for a core with gas hydrate (At2, Path A), the remaining quantity of hydrate dissociates to CH4 gas, and then there is only gas saturated water, a situation similar to that for a core that did not have gas hydrate in the first place (Bt2, Path B). Continued decompression will move the system down the gas-saturation curve (Fig. 2).
Concentration of CH4 and pressure should decrease under isothermal conditions from t2 to t3 (Fig. 2). The reason is simple: pressure is directly proportional to gas concentration at these pressure and temperature conditions (Henry's Law).
Characteristic volume-pressure plots (Fig. 3) should result after slow and isothermal release of CH4 from pressurized cores that contained sediment, pore water, and CH4 at initial experimental conditions (Hunt, 1979; Kvenvolden et al., 1983). Minimal volumes of CH4 should be released from containers during the pressure drop between t0 and t1 for cores with gas hydrate or t0 and t2 for cores without gas hydrate (Fig. 3). The reason is twofold: (1) essentially all CH4 during the pressure drop is in hydrate or dissolved in water; and (2) a two phase CH4-CH4 hydrate saturation curve is nearly vertical in concentration-pressure space (Fig. 2; Handa, 1990; Tohidi et al., 1995). Volumes of CH4 should be released from containers with gas hydrate at constant pressure during the concentration drop between At1 and At2 (Fig. 3), because gas removal is driving hydrate dissociation. Volumes of CH4 should be released from containers at decreasing pressure during the concentration drop between t2 and t3 (Fig. 3), because gas removal causes gas to be released from solution. Actual volumes of CH4 released during these pressure and concentration changes depend on the volume of water inside of the container (Fig. 3).
In theory, amounts of CH4 in hydrate and dissolved in water could be determined from volume-pressure plots. The quantity of CH4 stored in hydrate would be the volume released under isobaric conditions (t1 to t2); the quantity of CH4 dissolved in water would be the volume released under decreasing pressure conditions (t2 to t3). In principle, in situ pore-water salinity (at least the effective salinity) also could be determined from volume-pressure plots because pressure at t1 and t2 depends on the activity of water (e.g., Handa, 1990; Dickens and Quinby-Hunt, 1997). However, it would be difficult to assess this subtle effect, which would further be complicated by additional gas components (e.g., CO2).