Hydrates represent gas source, drilling hazard

Elchin Bagirov
Azerbaijan Academy of Sciences
Baku
Ian Lerche
University of South Carolina
Columbia

It is widely known that gases transform into liquids and then into solid crystals under conditions of low temperature and high pressure. However, if the gases are combined with other substances, then solid crystals can be formed under less harsh conditions.

One of such forms is named gas hydrates, which could also be called "explosive ice." The name "hydrate" shows that such formations consist of water and gases. Gas hydrates look like ordinary ice. However, if a piece of such "ice" is put into warm water its behavior will be different from the ordinary melting of normal ice. In contrast, gas hydrates cause bubbles in the warm water, which indicates the high content of gas in the hydrate crystals.

A look at the crystalline structure of hydrates reveals that it is formed by water molecules that make a loose "cage" surrounding the molecules of gas. Therefore the crystalline net is formed by the water molecules, which give a relatively high melting temperature, while the gas molecules are located in the free spaces left by the crystalline net (Fig. 1 [59,616 bytes]).

Generally, all gases (except hydrogen, helium, and neon) can form hydrates. However, most often one is faced in nature with hydrocarbon hydrates (methane and ethane hydrates). To understand what can happen we have to look at the conditions that create hydrates. The presence of four components is required: gas itself, water, high pressure, and low temperature.

How hydrates form

The first time the people in the former Soviet Union were faced with the problems of methane hydrates was in 1929 in Kazakhstan. Gas in pipelines transformed into hydrates, which slowly blocked the pipeline. To prevent this effect one had to either heat the pipeline or treat the gas before compressing to exclude water molecules from the mixture. The second way is cheaper.

In nature, hydrates can form in the north Arctic zone, especially in zones of permafrost, which itself is a good seal and can form good traps for gas accumulations. So gas, slowly moving towards the surface, is stopped and accumulates at the permafrost region. Naturally the near-surface temperature is then low, and hydrate crystals start to form (Fig. 2 [22,162 bytes]).

The thickness of the hydrate zone is defined by the geothermal gradient, or how fast the earth warms with depth. Estimates indicate that such zones can contain trillions of cubic meters of methane. And this is a tremendous energy reserve which people can use in the future.

Another zone favorable for hydrate formation is the sea bottom. The pressure in the sea increases with depth by about 1 atm for every 10 m of water column. So, by a depth of 500 m the water pressure will reach more than about 50 atm. At that depth the sea bottom temperature usually does not exceed 4-6° C.

As a result of biological degradation of organic matter (consisting of dead sea plants and animals), the sediments at near sea bottom are saturated by carbon dioxide, hydrocarbon, and sulfur gases. So all four components are present for hydrate formation, and in this way layers of hydrates do form in the sediments near sea bottom. These layers are covered by a new layer of sediments, again containing organic matter; and the cycle continues. The lower boundary of the hydrate formation zone is defined by the intensity of the heat flow coming from the deep zones of the earth.

For instance, in the American shelf of the Atlantic near the coast of South Carolina (southeast of the U.S.), seismic investigations detected a massive concentration of hydrates. The reserves of the gas here are estimated to be 16,000 times the annual production of gas in the U.S.

If people can learn how to produce these resources and effectively convert them to energy (even more effectively than natural gas is used), then the energy problems would be solved for at least a couple of centuries.

Hydrates stability

Hydrates can exist not only in deepwater conditions. In the Gulf of Mexico in high temperature waters (20° C.) hydrates have been found at shallow water depths and even as much as 2,200 m of water. An explanation is that the precise chemical composition of hydrates has a major role to play in determining the stability domain of a hydrate. For instance, ethane addition to methane allows hydrates to exist at much lower pressures and higher temperatures than for a pure methane hydrate. This tremendous stability sensitivity to the hydrate composition is important because hydrates can then exist at shallower overlying water depths and warmer sea-bottom temperatures than for pure methane hydrates.

Even 10% of ethane in the gas mixture makes a hydrate stable at 6 atm of pressure (60 m of water column) and 6° C., while a 100% methane hydrate is stable only at pressures exceeding about 40 atm (400 m of water column).

But hydrates should not be considered only as interesting phenomena that can be useful in the future. Hazards arise from the fact that hydrates are only quasi-stable; if the temperature is increased at a fixed pressure or the pressure decreased at a fixed temperature, or both, then it is easy to pass out of the stability field of hydrates.

The dissociation of gas hydrates can be slow, or explosive. Which happens depends on the chemical content and concentration of the hydrates and, what is more significant, how fast the pressure-temperature conditions change.

For instance, on the edges of continental shelves very often we can observe massive landslides toward the deep ocean zones. These will certainly change the pressure of the hydrate formations; dissociated gases from hydrates will escape (Fig. 3 [37,294 bytes]). Such zones have been detected near the South Carolina coast. In the zone of a huge landslide (40 miles wide), the seismic information shows the absence of hydrates in the cross-section, while a massive hydrate formation is located in the adjacent area. Escaped hydrate gas will go into solution with the overlying water and so decrease the density of the water.

Buoyancy effects of hydrates in unconsolidated sediments are also relevant. Hydrate density is estimated to be 0.8-0.9 gcm³. But to achieve a buoyancy effect in sediments, one needs the volume fraction of hydrate admixture to be 70% in the sediments, which is higher than observed. However, Ginsburg et al.¹ report finding crystals of hydrate (5 cm x 2 cm area, 2-4 mm thick) sitting on the ocean floor in 500 m of water. And these crystals are clearly buoyant relative to overlying sea water.

Because the hydrate crystals are buoyant there must then be a binding affinity of hydrates to the sediments that exceeds 10 atm. When sediments are disturbed by mud flows, landslides, etc., it is interesting to speculate that if the binding affinity is removed, then hydrate buoyancy can lead to a rapid rise of hydrate crystals through the overlying water until the critical pressure depth is reached (about 100-200 m depth of water depending on hydrate composition), when hydrate crystals will dissociate, releasing methane and ethane gas bubbles that continue their rise to the ocean surface.

This bubbling water presents a real hazard to ocean vessels and even aircraft. The decreasing density of water leads to the decrease of buoyancy and flotation capabilities of ships. And, when released into the air, methane and ethane lower the air density making low-flying aircraft lose lift ability. According to McEiver, as reported in a London Channel Four Television production,² the mystery of the Bermuda Triangle can be explained by this phenomenon.

But the hydrate can also dissociate explosively when the pressure is released very fast or the temperature increases rapidly. Indeed, such explosive dissociation of hydrates has been inferred in the Barents Sea (after recent ice removal of about 3 km thickness) by the presence of numerous large craters on the sea-floor.³

The free energy of hydrates upon dissociation is about 4-8 kcal/mole of gas, and there is roughly a ratio of 1 gas molecule for every 6 water molecules. Note that an energy of 1 kcal/mole, if all converted to kinetic energy of gas molecules, corresponds to about 500° K.; while if energy equipartition occurs between gas and water molecules in the hydrate at dissociation then the corresponding temperature equivalent is about 70° K. per kcal/mole, yielding between 280-560° K. rise in temperature. Thus the explosive mix would be at a temperature of 290-570° C. The explosive pressure is often enough to blow a hole in the ocean.

Given that the air/ methane self-ignition temperature is about 537° C. it follows that:

Water would convert to steam.
A high dynamic pressure is available to blow a hole in the ocean, releasing gas from the hydrates to combine with air and self-ignite.
A quantity of superheated steam will also be produced.

South Caspian hydrates

One of the hazardous zones in the planet is the offshore South Caspian basin, a unique place from the geological structure viewpoint. A huge amount of sediment (over 30 km thickness) and super high rates of sedimentation (about 10 km deposited during the last 5 million years) lead to large overpressures in the sedimentary formations and to such phenomena as mud diapirism and mud volcanism.

Mud diapirs are geological bodies made up of unconsolidated rocks, water, and gas and penetrate the sedimentary formations. Such pillar-shaped intrusions can reach 30 km in height and be tens of kilometers in diameter. The surface expressions of mud diapirs in the South Caspian are usually described as mud volcanoes.

Mud volcanoes are the vents of the diapirs. Gas, water, and mud are released from channels located on the body of the volcano. In the quiet stages of the volcanoes, every day hundreds and tens of hundreds of cubic meters of gas are emitted into the air and ocean water. During a single eruption tens of millions of cubic meters of natural gas, mostly pure methane, are ejected. So mud diapirs are a very significant source of methane and other hydrocarbon gases. Besides, because of the low thermal conductivity of the zones of mixed mud, water, and gas, diapirs are cool. All of these factors work together to create favorable conditions for hydrate existence.

Direct gravity-core sampling of hydrates has been made at or near the crests of the Buzdag, Elm, and Abikh mud volcanoes^{1, 4} in water depths of 480 m, 660 m, and 600 m, respectively. For Buzdag, seven gas hydrate samples indicate a methane content from 59-87% and an ethane content of 10-19%, with higher homologues making up the balance;⁴ while at Elm and Abikh, the methane concentration in hydrates is between 81-96%, with ethane and traces of higher homologues picking up the remaining 4-19%. Also, direct sampling of gas hydrate waters at the three sites indicates that it is water of mud volcano origin, and not sea water, that is incorporated in the hydrates.¹

Hydrate concentration in the sediments cored from the Vezirov diapir varies from 5-35% at water depths of about 500 m; while in the case of the Elm hydrates of the Azizbehov structure, hydrate concentration in the sediments reaches 15-20%^{1, 4} in water depths of 660 m.

The stability field of the known hydrates would appear to indicate that around 60-100 m of water depth (at 6° C.) is sufficient to provide stability of an ethane/ methane hydrate, that hydrates in gas-charged diapirs are likely to be a fairly common occurrence, and to be a fairly rich (5-35%) component in surficial and near-surficial sediments.

In the case of the offshore South Caspian basin, regional temperature gradients would seem to be about 1.8-2° C./100 m, at least as deep as the Productive Series of the Plio-Pleistocene,⁵ while undisturbed waterbottom temperatures are around 4-6° C. However, the presence of gas in mud diapirs is known to lower the temperature in the crestal domain of a diapir and local surrounds by up to 30-50° C. relative to regional measurements at the same sediment depth.⁵ For a mud diapir of typical diameter 10 km, this lowered temperature corresponds to a lowering of the vertical temperature gradient by about 0.3-0.5° C./100 m, giving a total temperature gradient in and near a diapir of around 1.3-1.7° C./100 m (Fig. 4 [31,273 bytes]).

Consider for illustration the Vezirov diapir in 500 m of water, where the waterbottom temperature is 4° C. For a 10% ethane-90% methane hydrate, the corresponding hydrate potential thickness is about 500 m, at which depth into the sediments the temperature is about 11° C. and the dissociation pressure is about 130 atm. From an explosive decompression viewpoint it then would seem that a mud-diapir of radius R, filled to a volume fraction f with hydrates to a depth z*, has an explosive volumetric potential, V, of VpR²z* f. For a 20% volume fraction (f) of hydrate (10% ethane-90% methane) in a 5 km radius (R) mud diapir, with the hydrate extending to its maximum sediment depth (z*) of about 500 m, the total hydrate volume is V 8x10⁹m³. Because the volume V is roughly 1/7 hydrate gases, a total gas volume of

Vgas 10⁹m³ is available.

Hydrates hazards

So hydrates in the South Caspian can provide a substantial hazard for people, ships, pipelines, and drilling platforms.

It would appear that hydrates can occur in as shallow as 50 m of water, if the hydrate composition is dominated by ethane, while 150-250 m of water is needed at a 90% methane composition.

It would appear that stability of hydrates can occupy as much as about 500 m of sediments when overlain by about 400-500 m of water. The cooling of the crestal regions of a mud diapir by the low thermal conductivity of gas-charged sediments is partly responsible for this large thickness of potential hydrates.

The corresponding dissociation pressure is about 130 atm, quite capable of blasting a hole through the overlying 500 m of water, which only provides a retaining pressure of 50 atm, leaving a residual 80 atm of dynamic drive pressure. Note that if the 80 atm of pressure is converted to a dynamic drive on the mud, then a column of mud to a height of about 800 m can be produced. Thus, on the order of 1 km (or a little less) can be extruded from the mud diapir at a maximum speed of around 0.01-0.1 km/sec, so that a hazard event would take on the order of 10-102 sec to become established.

This speed of hazard development is so fast as to require constant monitoring for its actualization probability. Dissociation energy is sufficiently large to produce temperatures of 290-570° C., enough to convert sea water to superheated steam, and enough for self-ignition of released methane in contact with air.

Dynamic motion of a mud diapir and sea level changes can also lead to explosive dissociation of a hydrate-bearing mud diapir. Of even greater concern is the fact that submarine mud diapirs are known to exhibit flows down their flanks, thereby changing their topography. Internal sediment failure of the unconsolidated mud can then lead to pressure and temperature changes within the sediments, which may inhibit or enhance hydrate dissolution (Fig. 5 [66,638 bytes]).

Catastrophic hydrate hazard associated with the reshaping of a mud diapir would seem to be a problem of pressing concern, but one which is difficult to evaluate from the viewpoints of either time-of-occurrence, or interval-between-occurrences.

The natural hazard of submarine methane hydrates is already substantial in the South Caspian basin, as attested to in records of offshore flaming eruptions.⁴ But man-made triggering of the hydrate hazard must also be considered. For instance, drilling into a mud diapir will raise the mud/hydrate temperature either by direct drill-bit heating or by warm circulating mud heating. In either event, heating hydrate-bearing mud above about 11° C. will dissociate the hydrates, leading to explosive release of pressure, and to possible rig annihilation.

Accordingly, this hazard is perhaps one that should be of serious concern to oil companies drilling in offshore regions of the South Caspian basin.

Acknowledgments

The work reported here was supported by the Industrial Associates of the Basin Modeling Group at USC. The authors appreciate the managing director of Geofilms company for providing video copies of the films "Hot Ice" and "Bermuda Triangle."

References

Ginsburg, G.D., Guseynov, R.A. Dadashev, A.A., Ivanova, G.A., Kazantsev, S.A., Solov'yev, V.A., Telepnev, E.V., Askeri-Nasirov, R. Ye., Yesikov, A.D., Mal'tseva, V.I., Mashirov, Yu.G., and Shabayeva, I. Yu., Gas hydrates of the South Caspian, International Geology Review, Vol. 34, 1992, pp. 765-782.
The Bermuda Triangle, Equinox Science Series-Channel Four Television, London, produced by Geofilms Ltd., Oxford, U.K., 1992.
Solheim, A., and Elverhoi, A., Gas-related seafloor craters in the Barents Sea, Geo-Marine Letters, Vol. 21, 1993, pp. 12-19.
Dadashev, F.G., Guseynov, R.A., and Aliyev, A.I., Map of mud volcanoes of the Caspian Sea (explanatory notes), Geological Institute of the Azerbaijan Academy of Sciences, Baku, 1995, 20 p.
Lerche, I., Bagirov, E., Nadirov, R., Tagiyev, M., and Guliev, I., (eds.), 1996, Evolution of the South Caspian Basin: Geologic risks and probable hazards, Azerbaijan Academy of Sciences, Baku, 1996, 625 p.

Bibliography

Hand, J.H., Katz, D.L. and Verma, V.K., Review of Gas Hydrates with Implication for Ocean Sediments, in Natural Gases in Marine Sediments (ed. I.R. Kaplan), Plenum Press, New York, 1972, pp. 179-194. "Hot Ice," "Antenna" series of BBC-2, produced by Geofilms Ltd., Oxford, U.K., 1991. Lerche, I., and Bagirov, E., Hydrate hazards in the South Caspian basin, Izvestiya Akademii Nauk Azerbaijana (in Russian), 1997.

The Author

Elchin Bagirov has worked at the University of South Carolina since 1994 as a post-doctoral research associate. From 1989-94 he held the position of senior researcher in the geological institute at the Azerbaijan Academy of Science and worked at Azerbaijan State Oil Academy as a part-time docent (associate professor). He has a degree in geology from Azerbaijan Institute of Oil & Chemistry and a PhD in mathematics from Steklov Mathematical Institute of the Russian Academy of Sciences. He has published two books dealing with the South Caspian basin. E-mail: [email protected]

Ian Lerche has been professor of geology in the department of geological sciences at the University of South Carolina since 1984. He was also an associate chairman of the geology department from 1984-89. He worked as a senior research geophysicist and then as a research associate for Gulf Oil Corp. during 1980-84. He received a BS in physics in 1962 and a PhD in astronomy in 1964 from the Univeresity of Manchester. He has written several books and technical papers on geology, basin analysis, and oil exploration.