Page 229 - Handbook Of Multiphase Flow Assurance

228 10. Research methods in flow assurance

confirmed spikes of methane content in atmosphere over the area coinciding with approxi-
mate time of the event, substantiating this hypothesis. Unexplained ice also has been found
by explorers at the bottom of the several noticed craters, further substantiating this hypothesis
because hydrate dissociation is endothermic, and upon dissociation at atmospheric pressure
water released from hydrate would convert to ice. Warming of the atmosphere may gradually
bring more dissociation of some portion of the natural gas hydrate deposits existing onshore
which are estimated at 3% of the global hydrate amount releasing methane and setting off a
chain reaction. Eventual warming of the oceans may similarly destabilize, with time, some
of the oceanic sediment natural gas hydrate, estimated at 97% of the global hydrate amount.
However, if other natural phenomena such as methane solubility in seawater plays a role,
the process would eventually over geologic times balance out. Makogon et al. (1972) showed
that at T and P corresponding to the hydrate equilibrium, the solubility of methane in wa-
ter decreases abruptly, by a factor of 3 to 5. Makogon et al. (2004) confirmed these data and
3
showed that at a pressure of 75 bar the solubility of methane in water changes from 4 cm /g
3
without hydrate to 0.22 cm /g above hydrate. This difference in concentrations of methane in
water creates the driving force for methane diffusion from the atmosphere into the hydrate.
3
Based on gas solubility without hydrate, seawater can dissolve 4 cm methane / gram water.
3
24
Estimating global ocean at 1.34 × 109 km = 1.34 × 10 g, and global reserve of gas in natural
3
16
3
22
hydrate deposits at 1.5 × 10 m = 1.5 × 10 cm , then 0.28% of the ocean can dissolve all gas
released from all the natural hydrate. Nonetheless, dynamics of gas dissolution may be slow
and depend on pressure and temperature. While the above estimate is encouraging that the
ocean can dissolve the methane from hydrate, more detailed research is needed to confirm
the rate of methane dissolution in the ocean because global ocean temperatures vary laterally
and with depth.
Properties and structures of gas hydrates
A gas hydrate is a crystalline compound in which water molecules enclathrate one or more
types of guest molecules. Such inclusion compounds are formed when the appropriate ther-
mobaric conditions were applied to the gas-water system. An extensive review of common
types of hydrate crystals and their properties is available (Sloan, 1990). Also a list of the more
rare hydrate structures was presented by Dyadin et al. (1991).
Hydrate crystalline structures are composed of guest and water molecules. Water mole-
cules arrange themselves in polyhedra encapsulating the guest molecules. Oxygen atoms of
water molecules are positioned in the vertices of such polyhedra (Fig. 10.6).
Such polyhedra or cavities share faces to form the crystalline lattice. Different combina-
tions of cavities produce different hydrate structures (Fig. 10.7). Geometric properties of hy-
drate crystals are presented in Table 10.1.
Most often in nature hydrates of cubic structure I (sI), (Fig. 10.8) and cubic structure II (sII),
(Fig. 10.9) are formed. These two hydrate structures are formed with three types of cavities:
12
12 2
12 4
5 , 5 6 , and 5 6 . These numbers represent the types of faces and numbers of these faces
12
forming a cavity. Thus, 5 represents twelve pentagonal faces forming a dodecahedron.
12 2
12
Cubic structure (sI) hydrate is formed when 5 and 5 6 cavities come together. sI hydrate
may be formed with molecules ranging in size from methane (0.436 nm diameter) to trieth-
ylene oxide (0.61 nm diameter).