The Hydrate Pressure-Temperature Stability Envelope

There are four requirements for generation of natural gas hydrates (1) low temperature, (2) high pressure, (3) the availability of methane or other small nonpolar molecules, and (4) the availability of water. Without any one of these four criteria, hydrates will not be stable. As indicated in both the previous section and in Section 7.4.3, the third criteria for hydrate stability—namely methane availability—is the most critical issue controlling the occurrence of natural gas hydrates. Water is ubiquitous in nature so it seldom limits hydrate formation. However, the first two criteria are considered here as an initial means of determining the extent of a hydrated reservoir. 

For methane hydrate, the minimum water depth is 381 m in freshwater and upto 436 m in seawater, respectively, at 277 K. In the world s oceans at water depths greater than 600 m, the temperature is typically uniform at 277 K, due to the density maximum in seawater. Lower bottom water temperature exceptions can be found with strong subbottom currents from Antarctic and Arctic environments such as the north of Norway or Russia. Methane-phase equilibrium data in Chapter 6, indicate that 3.81 MPa are required to stabilize methane hydrates at 277.1 K. Using the rule of thumb 1 MPa = 100 m water, hydrates in pure water would be stable at depths greater than 381 m. 

At a sea salt concentration totaling 3.5 wt%, using the thermodynamic data of Dholabhai et al. (1991) in Chapter 6, a pressure of 4.364 MPa (a minimum seawater depth of 436 m—about 55 m deeper than in pure water) is required to stabilize hydrates at 277 K. Further corrections to the phase boundary are required considering effects of (1) hydrocarbons other than methane, (2) salt concentrations other than 3.5 wt%, and (3) sediment pores or capillary pressure, as indicated in Chapter 5. 

Second, the hydrates at the lower stability depth are the most easily dissociated because they are at the phase boundary. At some constant depth, above the lower intersection of the two lines, the hydrates (along with their encasing sediments) that exist at the geothermal gradient must be heated to the phase boundary resulting in a loss of recovery efficiency, due to the requirement of heating the hydrated sediment before dissociation. 

Example What Fraction of Hydrate Reserves are Economical to Recover for Energy  

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