Introduction

In the early 1820's, John Faraday, working in England, was investigating the newly discovered gas, chlorine. He easily repeated the earlier experiments of Humphrey Davy (Davy, 1811) in which gaseous chlorine and water formed solid chlorine hydrate upon cooling in the "- late cold weather -". Faraday's lab curiosity chlorine hydrate has water as the host molecule, and chlorine molecules as the guest. These pioneering syntheses experiments are the first reported reference to a class of associative compounds now known as gas hydrates (Faraday, 1823, wvusd. 2000). Chlorine hydrate has persisted as a laboratory curiosity (Pauling et al., 1994) in part because its ease of formation lends it to laboratory demonstration. A variety of other molecules can form hydrates specifically and a variety of clathrates in general. The non-bonding uniqueness of clathrates as "chemicals" has interested scientists for almost two centuries.

A clathrate is a compound formed by the inclusion of molecules of one kind within cavities in the crystal lattice of another (Webster, 1994). The generic name, clathrate, is taken from the Latin word 'clathratus', which means, 'enclosed by bars or grating' (Barrer and Stuart, 1957; Brown, 1962). Clathrates display no chemical bonding between the host and guest molecules, a condition which is a key characteristic of clathrates. Some clathrates can form without water being present where non-water molecules form the molecular structural array. There are many examples of clathrates (Table 1), which are also known as container compounds (Cram, 1992).

Conventional chemical wisdom holds that a chemical compound consists of atoms bonded to one another in a fixed ratio, yielding an atomic structure for the molecule. Thus, salt, or sodium chloride is NaCl, hexane is C6H14, or a chain of 6 carbon atoms with 14 hydrogen atoms attached. Either ionic (as in sodium chloride) or covalent (as in hexane) bonds serve to hold these and other molecular entities together, either as gases, liquids, or solids. Ordering on the molecular level is manifested by a crystal form, a material characteristic in all compounds save certain supercooled fluids such as glass. Some crystals of simple compounds, such as salt, are dense solids, and consist essentially of a series of spheres laid adjacent to one another in a three dimensional array. However, a variety of molecules will crystallize under certain conditions of temperature and pressure to give a rather open structure. The molecules of the clathrate-forming substances in question form an open lattice in the form of a rigid three-dimensional structure, rather like balled-up chicken wire, with open space (voids) regularly distributed within the lattice.

Host

Guest

Urea

Straight chain hydrocarbons

Thiourea

Branched chain and cyclic hydrocarbons

Dinitrodiphenyl

Derivatives of diphenyl

Phenol

Hydrogen chloride, sulfur dioxide, acetylene

Water (ice)

Halogens, noble gases, sulfur hexaflouride, low molecular weight hydrocarbons, C02, S03, N2, etc.

Nickel dicyanobenzene

Benzene, chloroform

Clay minerals (molecular sieves)

Hydrophilic substances

Zeolites

Wide range of adsorbed substances

Graphite

Oxygen, hydrocarbons, alkali metals (in sheet-like cavities and buckyballs)

Cellulose

Water, hydrocarbons, dyes, iodine

Table 1. Common clathrates: Various hosts and guests.

Table 1. Common clathrates: Various hosts and guests.

Often the open space in the crystal lattice remains just that: open space. For example, many silicate minerals (especially zeolites) crystallize into solids with linear channels, or planar sheets of open volume in the crystal. However, under certain conditions, these crystalline voids can be occupied by foreign (guest) molecules of such size and configuration that the guest molecule fits into the crystalline voids formed by the host molecule. Since a host lattice has a well defined structure with a fixed void volume in the lattice, a clathrate can exhibit a definite formula, if the voids are completely occupied by guest molecules, quite analogous to that of a true chemical (bonded) compound. This state of affairs, that is, a combination of atoms with a fixed ratio in the mix implies a strong chemical bonding, a state which commonly does not exist in the case of clathrates.

Clathrates can form spontaneously under certain pressure temperature conditions. A host material, which can crystallize into an open lattice structure, is first needed, then a guest molecule of suitable size and molecular conformation to fit into the lattice voids is required to complete the clathrate crystalline structure. As an example, methane hydrates are dependant on pressures that are usually in excess of one atmosphere (STP), which forces a reordering of the lattice water molecules into their 3-dimensional array and inserts the guest methane molecule into the structure.

2. METHANE HYDRATE, A CLATHRATE SUBSPECIES

Hydrates are a subgroup of clathrates. The term "hydrate" is applied to clathrates in which the structural molecules are water (H2O) and guest sites are occupied by gas molecules. Water, of course, is ubiquitous on Earth, and gas hydrate can form easily wherever conditions are suitable. Only methane hydrates are discussed here and elsewhere in this book. For information on other hydrates, see Sloan (1998).

Figure 1. Photo of model of Type 1 hydrate lattice (atomic models, NRL Chemistry Division) showing water cage and adjacent methane molecule (atomic radii not to scale but size of lattice voids and guest methane molecule are to proportional scale.).

Crystallization forces methane molecules into tightly packed lattice sites, compressing the methane. Methane hydrate has the highest energy density of any naturally occurring form of methane (184,000 btu/ft3 for the hydrate, and 1,150 btu/ft3 for methane gas; in contrast LNG, which is an cryogenic industrial liquid form of methane, is about 430,000 btu/ft3). Clearly, methane hydrate is an attractive economic target as a source of methane (i.e., energy), especially when it occurs relatively close to the Earth's or seabed surface.

The density of methane hydrate is about 0.9 g/cm3; density may vary minutely according to the degree of methane saturation of the hydrate lattice and the local incorporation of other molecules (e.g., H2S) taking the place of methane in the lattice. Note, however, that the molecular pressure of gas in the hydrate lattice can reach several kilobar with increasing saturation. The heat of hydrate formation and the heat of hydrate dissociation are equal in absolute magnitude but are of opposite sign. When hydrate forms heat is released from the system (exothermic) and when hydrates dissociate, heat is taken into the system (endothermic).

Property

Ice

Hydrate

Dielectric constant at 273 °K

94

-58

NMR rigid lattice 2nd moment

32

33 ±2

of H20 protons(G2)

Water molecule reorientation

21

-10

time at 273 °K (usee)

Diffusional jump time of water molecules at 273 °K (usee)

2.7

>200

Isothermal Young's modulus at

9.5

-8.4

268 °K(109Pa)

Speed of longitudinal sound at 273

°K

Velocity (km/sec)

3.8

3.25-3.6

Transit time (|isec/ft)

3.3

92

Velocity ratio Vp/Vs at 272 °K

1.88

1.95

Poisson's ratio

0.33

-0.33

Bulk modulus (272 °K)

8.8

5.6

Shear modulus (272 °K)

3.9

2.4

Bulk density (gm/cm3)

0.916

0.912

Adiabatic bulk compressibility at 273 °K 10-1 lPa

12

=14

Thermal conductivity at 263 °K (W/m-K)

2.25

0.49±0.02

Heat of fusion (kJ/mol)

6

54 (meas), 57 (calc)

Table 2. Physical properties of water ice and methane hydrate (after Davidson, 1983; Prensky, 1995; Sloan, 1997 & Franks, 1973)

Table 2. Physical properties of water ice and methane hydrate (after Davidson, 1983; Prensky, 1995; Sloan, 1997 & Franks, 1973)

A nominal value for methane hydrate formation enthalpy at 273 °K is 54 kJ/mol (measured, Sloan, 1990; 1997). Hydrates have a constant pressure heat capacity of 257 kJ/mol (Handa, 1986). The heat of solution (absorption) of methane gas is 13.26 kJ/mol (Franks and Reid, 1973). The thermal conductivity of a hydrate-sediment mixture is 2.2-2.8 W/m-°K (Watt per meter-degree Kelvin) range (Sloan, 1997). For comparison the conductivity of a water-ice sediment mixture is 4.7-5.8 W/m-°K (Table 2).

Conductivity for a gas-sediment mixture, such as exists in natural gas pools is very low, in the 0.05-0.4 W/m-°K . Because hydrate is rarely developed uniformly and in a solid mass in nature, this high conductivity will rarely be attained. Taking into account physical heat transport by geothermal circulation cells, some free gas and a rapidly varying porosity and permeability, 1.5 W/m-°K, about 50% higher than for tight sediment with no hydrate, is a reasonable working estimate for modeling thermal conductivity where hydrate is widely developed in the pore space of sediments.

Methane hydrate occurs both in permafrost areas and in the marine sediment in the oceans and deep lakes where pressure-temperature conditions are suitable (Fig. 2), and where sufficient methane is delivered to the zone of hydrate stability in the uppermost sediments. Hydrate, especially in marine sediments, is composed of methane that has been produced largely by biogenic activity at relatively low temperatures and pressures (Chapters 7 & 8) and not through the same processes that produced most conventional gas and oil deposits (Max and Lowrie, 1993). This renders the methane in oceanic hydrate deposits virtually free from liquid petroleum and condensates and allows them to be considered as having a very low potential for pollution hazard.

Hydrate occurs in a relatively narrow zone termed the hydrate stability zone (HSZ or GHSZ) that lies about parallel to the terrestrial or seabed surface both in permafrost regions and in the oceans. In permafrost terrane, the extraordinary cold of the surface layers hydrate stabilizes relatively close to the surface (Chapters 2 & 5). In the oceans (Chapters 2 & 6), the pressure exerted by water stabilizes the hydrate from the surface downward to some depth determined by increasing temperature related to the geothermal gradient (temperature usually increases downward in the range of about 3 to 4 degrees C per 100 meters in continental slope sediments).

Hydrate in the HSZ bears a striking resemblance to well-known strata-bound mineral deposits such as lead-zinc deposits that are found in Carboniferous limestone host rocks (Max, 1997; Max and Chandra, 1998). Hydrate appears to be deposited slowly from groundwater. Hydrate forms in primary porosity sites and may cement sediment grains. Hydrate also forms in secondary porosity regimes created both by faulting and gas-charged fluid movement in finer grained sediments. Hydrate can occur as massive deposits, in fracture fill, and is often recovered in cores in apparent nodules and disseminated both randomly and along specific horizons where its development was apparently controlled by bedding porosity.

Figure 2. Stability fields of methane hydrate. Log plot carried to 10 km. Gas hydrate phase diagram, showing the stability fields of the water ice-methanehydrate system with respect to temperature and total pressuret. The presence of CO2, H2S, ethane and/or propane with methane in the hydrate will have the effect of shifting the hydrate phase boundary line to the right, thus increasing the P-T field in which methane hydrate is stable (Kvenvolden, 1993). Permafrost and oceanic hydrate fields of stability are the main potential economic regions in P-T space.

Figure 2. Stability fields of methane hydrate. Log plot carried to 10 km. Gas hydrate phase diagram, showing the stability fields of the water ice-methanehydrate system with respect to temperature and total pressuret. The presence of CO2, H2S, ethane and/or propane with methane in the hydrate will have the effect of shifting the hydrate phase boundary line to the right, thus increasing the P-T field in which methane hydrate is stable (Kvenvolden, 1993). Permafrost and oceanic hydrate fields of stability are the main potential economic regions in P-T space.

Hydrate is found in the seafloor because of a coincidence of rising pressure and diminishing temperature with increasing water depth. Open oceans are characterized by having warmer water at the surface (at least during summers) than at the seafloor. The change of temperature with depth is called the hydrothermal gradient, which varies considerably depending on latitude and local heating attributes of bodies of water, such as the Gulf of Mexico. Generally, the rate of change of temperature with depth is more rapid near the surface. The rapid temperature decrease with depth often changes from a high rate of change to a lower rate of change at the 'thermocline' (Chapter 6, Fig. 2). Temperatures and pressure depths conducive to the formation of hydrate in the oceans are usually found below this thermocline. In the Polar regions and in restricted bodies of water such as the Mediterranean Sea, however, there is often no thermocline, especially in the winter. In the Arctic hydrate is stable at shallower depth than in the more temperate open oceans because of the very cold water while in the Mediterranean-type water masses, hydrate is only stable at pressure-depths greater than the open oceans because of abnormally warm water at depth.

Pressure-depths and temperatures conducive to the formation of hydrate in open oceans are usually found just below the slope break between continental shelves and slopes. The cold sea water cools the sediments and rocks in the upper seafloor regime, and provides a lower temperature limit, which varies from place to place, from which temperature increases downward into the sediment along the local geothermal gradient (Chapter 6, Fig. 2). The base of the HSZ is located where the effect of increasing temperature renders the hydrate unstable, even though pressure is increased. The base of the HSZ occurs where the geothermal gradient, whose uppermost position is coincident with the deepest water temperature, intersects the local projected hydrate phase boundary. Thus, the base of the HSZ is itself a phase boundary. The thermal structure and exact position of the actual base of the hydrate is subject to many local controls (Chapters 3 &4). Because the pressure at the seafloor, which is the top of the HSZ, increases with water depth (Miles, 1999), where similar geothermal gradients are found along a continental margin, the HSZ thickness increases with increasing water depth (Chapter 6).

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