uGeneral statement

uFigure captions

uFormation of hydrate

uHydrate vs LNG energy density

uConclusion

uReferences

uGeneral statement

uFigure captions

uFormation of hydrate

uHydrate vs LNG energy density

uConclusion

uReferences

uGeneral statement

uFigure captions

uFormation of hydrate

uHydrate vs LNG energy density

uConclusion

uReferences

uGeneral statement

uFigure captions

uFormation of hydrate

uHydrate vs LNG energy density

uConclusion

uReferences

uGeneral statement

uFigure captions

uFormation of hydrate

uHydrate vs LNG energy density

uConclusion

uReferences

Figure and TableCaptions

Formation of Hydrate 

Let’s consider how the formation of hydrate causes a high concentration of natural gases by comparing the physical sizes of a sediment grain size and a unit-volume of hydrate. A unit-volume of Structure I hydrate is shown as Figure 1. Limited page space does not permit the unit-volume geometries of Structure II and Structure H hydrates to be illustrated. This crystalline structure is called a “unit-volume” because Structure I hydrate grows in increments of this fundamental building block. This unit-volume consists of eight cages of structured water that can each trap one gas molecule.  

Dendy Sloan at the Colorado School of Mines (e.g., Sloan, 2003, 2004) defines the diameter of each cage of this unit-volume as a length that varies from 8 to 10 angstroms. Because any arbitrary diameter across this unit-volume will span no more than six cages (probably no more than four cages, actually), the diameter of a unit-volume of hydrate is less than 60 angstroms (6 x 10^-9 m).  

For a size comparison, sedimentologists define the low end of very fine-grained sand to be fragments of sediment that have diameters of about 60 microns (micrometers) (6 x 10^-5 m). The diameter of very fine-grained sand is thus larger than the diameter of a unit-volume of hydrate by a factor of 10⁴.  

Because the volume of an object is proportional to (diameter)³, if we ratio the volume of a very fine-grained sand and the volume of a unit-volume of hydrate, we find that this 10⁴ difference in diameters means that 10¹² unit-volumes of hydrate can fit into the space occupied by one very fine sand grain. Assuming that on average only 80 percent of the eight cages in a unit-volume manage to trap a gas molecule, hydrate formation causes approximately 6 x 10¹² gas molecules to be compressed into a volume equal to that of a single very fine sand grain.

Gas Hydrate vs LNG Energy Density 

This simple arithmetic supports the statement by Pellenbarg and Max (2001) that hydrate has the highest energy density (184,000 BTU/ft³) of any form of biogenic or thermogenic gas found naturally. By comparison, liquid natural gas (LNG) represents the highest energy density (430,000 BTU/ft³) of natural gas that humans can create using cryogenic technology.  

Hydrate thus has an energy density E_gh that is 0.42 that of the energy density E_LNG of LNG. For any deepwater, near-seafloor sediments where clusters of hydrate unit-volumes can be assumed to be distributed throughout the sediment pore space, the energy density of the gas trapped in the structured-water prison cells of this dispersed hydrate can be expressed as:

E_gh = (0.42fC_gh)E_LNG

where f is the porosity of the host sediment containing the hydrate.

Table 1 shows how E_gh is related to E_LNG for:

1)      Common porosity ranges found in deepwater, near-seafloor sediment.

2) That range of C_gh that causes the reaction “Too bad the concentration is so low.”

Now let us consider a specific example: If a hydrate system has a porosity of 0.5 and a hydrate concentration of 0.5, how large does a hydrate accumulation have to be in order to have an amount of stored energy that equals the energy stored in one LNG tanker? Simply solve the energy-balance equation:

E_gh(hydrate reservoir volume) = E_LNG(tanker volume)

Using the ratio E_gh/E_LNG = 0.1 from Table 1 for f = 0.5, and C_gh = 0.5, the result is:

Hydrate reservoir volume = 10 LNG tanker volumes.

If we assign length, width and depth dimensions of 600, 100 and 50 feet to our hypothetical LNG tanker volume, the size of the hydrate reservoir that has the same equivalent stored energy is 826 acre-ft (~10⁶ m³). Thus, when the hydrate concentration in deepwater sediments is only 0.5 of the available pore space, we see that there is a tremendous amount of gas in a small volume of sediment. 

When Mother Nature causes hydrate to form, the result is an impressive concentration of energy that is independent of burial depth. Because structured-water hydrate has a large bulk modulus and is difficult to compress, hydrate will have the same crystalline structure, and thus the same energy density of entrapped gas, whether at the seafloor or buried deeply beneath. 

This concept about the relationship between gas concentration and confining pressure differs from the logic that has to be used in dealing with compressible gases found in conventional reservoirs. Therefore, there may be a large number of LNG tanker equivalents awaiting the bold who initiate deepwater hydrate production.

Conclusion 

The only intent of this discussion is to illustrate that a tremendous amount of energy is stored in deepwater hydrate. What has been avoided is any discussion of the challenges of trying to produce that hydrate.

As a geophysicist, I have two observations about strategies for producing deep-water hydrate:

1) Hydrate production is the engineer’s problem.

2) I am glad I am not the engineer assigned to the problem.

For geophysicists, it is exciting to try to unravel the mysteries of deepwater hydrate systems using 4C OBC seismic data and rock physics theory. Refer to Hardage and Murray (2006), if you wish to know how some of this multi-component seismic research is being done.

References 

Hardage, Bob A., and Paul E. Murray, 2006, High resolution P-P imaging of deepwater near-seafloor geology and High resolution P-SV imaging of deepwater near-seafloor geology: Search and Discovery Article #40200 (2006) (http://www.searchanddiscovery.net/documents/2006/06054hardage06/index.htm)

Pellenbarg, Robert E., and Michael D. Max, 2001, Gas hydrates: From laboratory curiosity to potential global powerhouse: Journal of Chemical Education, v. 78, p. 896-900.

Sloan, E.D., 2003, Fundamental principles and applications of natural gas hydrates: Nature (invited review), v. 426, p. 353-359.

Sloan, E.D., 2004, Introductory overview: hydrate knowledge development," American Mineralogist, v. 89, p. 1155-1161.

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