Natural Gas Hydrate

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.

Interest in natural gas hydrate is increasing rapidly as the multiple implications of its presence in the shallow geosphere are being recognized. The large amount of hydrate methane that is sequestered in shallow terrestrial and marine sediments makes this methane an attractive target for those concerned about future energy requirements and resources. The fact that natural gas hydrate is metastable and affected by changes in pressure and temperature makes any released methane an attractive agent that could globally affect oceanic and atmospheric chemistry and ultimately global climate. And finally this characteristic of metastability could explain ma or seafloor instabilities resulting in submarine slides and slope failures. Thus these ramifications of natural gas-hydrate occurrence all have potential effects on future human welfare, and hence explain the increasing worldwide interest. This chapter introduces natural gas hydrate, provides a background for understanding its occurrence, relates the early history of discovery, and describes the hydrate gas compositions that have been found. Following chapters will deal with the important aspects of natural gas hydrate as a potential (a) energy resource, (b) factor in global change, and (c) submarine geohazard.

Accurate and precise prediction of the temperature and pressure (P-T) conditions at the boundary of the methane hydrate stability field is an essential component of a variety of endeavors in the field of geochemistry. Kvenvolden (1988), Gornitz and Fung (1994) and others have used knowledge of the P-T stability conditions to define the geophysical limits of gas hydrates and thereby estimate the size of the global reservoir. As the thermal signature of global warming penetrates into the ocean (Levitus et al., 2000), precise knowledge of the stability of gas hydrates will be required to assess the risks of decomposition in this reservoir. Recently, Ruppel (1997) has suggested that a discrepancy exists between in situ temperature measurements on the Blake Ridge and the predicted base of the hydrate stability zone. This claim is based in part upon P-T predictions of gas hydrate stability. In our own research, we have conducted a series of in situ deep-sea gas hydrate synthesis experiments (Brewer, et al., 1998) and have begun using an ROV to prospect for gas hydrate out-crops and undersea gas vents, which potentially result from decomposing gas hydrate deposits. One of the goals of this field work is to explore for gas hydrates close to the limit of the stability zone and this creates the need for accurate and precise predictions. Given the small temperature gradients with depth in the deep-sea, an error of 0.5°C, could mean a depth error of more than 100 meters. With a shallow sloping bottom (1% grade), one could easily be ten kilometers or more off target if the wrong temperature is used.

The stability of gas hydrate is dependent on pressure (P), temperature (7), and the solubility of gas (e. g., Handa, 1990; Zatsepina and Buffett, 1997) as a function of pressure and temperature in the system. As illustrated in Chapter 1, the stability of hydrate is more susceptible to changes in temperature than pressure. Measurements that constrain thermal regimes in hydrate reservoirs therefore provide fundamental information about one of the most basic parameters controlling the stability of the deposits.

Gas hydrate in onshore arctic environments is typically closely associated with permafrost. It is generally believed that thermal conditions conducive to the formation of permafrost and gas hydrate have persisted in the Arctic since the end of the Pliocene (about 1.88 Ma). Maps of present day permafrost reveal that about 20 percent of the land area of the northern hemisphere is underlain by permafrost (Fig. 1). Geologic studies (MacKay, 1972; Lewellen, 1973; Molochushkin, 1978) and thermal modeling of subsea conditions (Osterkamp and Fej, 1993) also indicate that permafrost and gas hydrate may exist within the continental shelf of the Arctic Ocean. Subaerial emergence of portions of the Arctic continental shelf to current water depths of 120 m (Bard and Fairbanks, 1990) during repeated Pleistocene glaciations, subjected the exposed shelf to temperature conditions favorable to the formation of permafrost and gas hydrate. Thus, it is speculated that “relic” permafrost and gas hydrate may exist on the continental shelf of the Arctic Ocean to present water depths of 120 m. In practical terms, onshore and nearshore gas hydrate can only exist in close association with permafrost, therefore, the map in Figure 1 that depicts the distribution of onshore continuous permafrost and the potential extent of “relic” sub-sea permafrost also depicts the potential limit of onshore and nearshore gas hydrate.

Many gas hydrates are stable in deep-ocean conditions, but methane hydrate is by far the dominant type, making up >99% of hydrate in the ocean floor (Chapter 2). The methane is almost entirely derived from bacterial methanogenesis, predominantly through the process of carbon dioxide reduction. In some areas, such as the Gulf of Mexico, gas hydrates are created by the rmogenically-formed hydrocarbon gases, and other clathrate-forming gases such as hydrogen sulfide and carbon dioxide. Such gases escape from sediments at depth, rise along faults, and form gas hydrate at or just below the seafloor, but on a worldwide basis these are of minor volumetric importance compared to microbial and the rmogenic methane. Methane hydrate exists in several forms in marine sediments. In coarse grained sediments it often forms as disseminated grains and pore fillings, whereas in finer silt/clay deposits it commonly appears as nodules and veins. Gas hydrate also is observed as surface crusts on the sea floor. Methane hydrate samples have been obtained by drilling.

The microbiological role in the production of methane and the formation and stability of methane hydrates is critical to our understanding of ocean carbon cycling and global warming, and has important ramifications for sources of alternative energy, and the global economy. The methane hydrate reservoir vastly exceeds other carbon energy reservoirs (Kvenvolden, 1988). The amount of methane that is present in the ocean floor depends on the distribution of hydrates and the methane content. The estimated range of ocean gas hydrates is 26.4 to 139.1 × 10¹⁵ m3 (Gornitz and Fung, 1994). The maximum content in 1 m3 of hydrate is calculated to be 164 m3 methane and 0.8 m3 water (Kvenvolden et al., 1993; Max and Lowrie, 1997). Variability in hydrate methane content is controlled by geothermal gradients, methane and other hydrocarbon gas contents and by the rate of biological formation. Currently only the sketchiest details of the ocean carbon cycle in the sediment and water column are understood. This is a primary emerging research topic in ocean science.

Methane is an important product of anaerobic bacterial metabolism. Bacterial methane makes a substantial contribution to global methane reserves. Methanogenesis is the final step in the anaerobic degradation of organic matter, and can continue in deeply buried sediments. Methane can also be produced abiologically at elevated temperatures and pressures e. g., thermal breakdown of organic matter, crustal and hydrothermal processes. The boundary between biological and abiological processes is not always clear. Bacteria can be active at temperatures up to 113°C and pressures in excess of 1000 atm, and abiological processes can produce energy sources for bacterial methanogenesis. In addition, deep sourced thermogenic methane can diffuse to the surface, and under certain conditions, biogenic methane can have a chemical and stable isotope signature indicating an abiological origin.

Hydrates may occur where thermodynamic conditions permit and where methane concentration in the water exceeds a threshold level, but they will only concentrate where gas flow is focused. Existing models of submarine gas hydrate occurrence encapsulate the system of transport and reactions into a one dimensional model (e. g. Rempel and Buffet 1998, Zatsepina and Buffett 1998, Xu and Ruppel 1999). With this simplification we can constrain key parameters, but it is difficult to capture the geological complexity of real systems. To predict the spatial distribution of hydrates we need to account for the range of mechanisms by which methane can move though the sediments.

The estimated amount of gas in the hydrate accumulations of the world greatly exceeds the volume of known conventional gas reserves. However, the role that gas hydrate will play in contributing to the world’s energy requirements will depend ultimately on the availability of sufficient gas hydrate resources and the “cost” to extract them. Yet considerable uncertainty and disagreement prevails concerning the world’s gas hydrate resources. Disagreements over fundamental issues such as volume of gas stored within delineated gas hydrate accumulations and the concentration of gas hydrate within hydrate-bearing reservoirs have demonstrated that we know very little about gas hydrate.

Gas hydrate occurrence in the sediments of the outer continental margins is sustained in place for relatively long periods by high hydrostatic pressure and low ambient temperature. Most naturally occurring hydrate is composed of molecules of methane trapped in an ice cage of water molecules. Thus, the breakdown of hydrate in response reduced hydrostatic pressure or increased bottom-water temperature can potentially introduce significant quantities of this potent greenhouse gas in the water column and atmosphere, encouraging accelerated warming. At higher latitudes hydrate also occurs in association with permafrost at depths ranging from 130 to 2000 m. Here methane is held captive in the clathrate enclosure by frigid temperatures. An increase in the mean temperature of the higher latitudes, therefore, also has the potential to dissociate the hydrate and emit methane directly into the atmosphere.

Gas hydrate decomposition is hypothesized to be a factor in generating weakness in continental margin sediments that may help explain some of the observed patterns of continental margin sediment instability. The processes associated with formation and decomposition of gas hydrate can cause the strengthening of sediments in which gas hydrate grow and the weakening of sediments in which gas hydrate decomposes. The weakened sediments may form horizons along which the potential for sediment failure is increased. While a causal relationship between slope failures and gas hydrate decomposition has not been proven, a number of empirical observations support their potential connection.

One of the few attempts to date to map gas hydrate over a large area has been made on the Atlantic continental margin of the United States (Dillon et al., 1993, 1994, 1995). This work has resulted in the production of an extensive data base of seismic reflection lines including both single and multichannel lines, and complete GLORIA sidescan sonar coverage. This work was part of the assessment of the U.S. EEZ and was carried out by the U.S. Geological Survey. Earlier efforts were made by Tucholke et al. (1977) and Shipley, et al. (1979). Research along the U.S. SE continental margin of the U.S. is continuing.

The northern North Atlantic and Arctic oceans are morphologically and geologically complex. The constructive axial plate margin of the northern North Atlantic is propagating through Fram Strait, forming a young oceanic crust in the Nansen Basin of the Eurasian end of the deep water Arctic Ocean (Fig. 1). A complex transform along the continental margin of the Laptev Sea is the present termination of this Atlantic-Arctic Ocean spreading center. The North American end of the Arctic Ocean is floored by older oceanic crust carrying a thick sediment prism in the western end of the Canada Basin. The Barents Sea, like the other wide shallow water margins of the Asian Arctic Ocean and narrower continental shelf elsewhere around the Arctic margin, is an epicontinental sea (Eldholm & Talwani, 1977).

Natural gas hydrate was first recognized on the Cascadia margin in 1985 through the characteristic bottom-simulating reflector (BSR) on conventional multichannel seismic data (Davis and Hyndman, 1989, Davis et al., 1990). Since then, the Cascadia accretionary margin has received the most intensive studies of any convergent margin for determination of the in-situ properties of marine gas hydrate. Key control for understanding the properties and formation processes of hydrate has been derived from drill holes of the Ocean Drilling Program (ODP) Leg 146, carried out in 1992. Estimates of hydrate concentration were provided through analysis of downhole seismic and resistivity logs and through measurement of chlorinity in pore fluids from recovered sediment core samples.

The Antarctic continent is for the most part surrounded by passive margins, except for a restricted segment along the northern termination of the Antarctic Peninsula. Extensive single-channel and multi-channel seismic reflection surveys carried out in the last two decades, have clarified many aspects of the structure and stratigraphic setting of these margins, and highlighted the importance of the seismic facies analyses for the reconstructions of ice sheet history (i. e., the ANTOSTRAT program, Cooper et al., 1995). Due to the hostile local environment, the knowledge of the Antarctic margins is still uncomplete, allowing for regions adequately covered by seismic surveys to be adjacent to unexplored areas.

There is an increasing gap between the demand for natural gas and its availability in India, which is endowed with only limited conventional methane deposits. In 1997 India produced about 70% of its own methane. Today India produces less than 50% of its own methane. In 2005 it is anticipated that India will produce no more than 36% and by 2010 no more than about 25% of its own methane demands unless new indigenous sources of methane can be identified. Presently the production of gas in India is around 58 million m³/day, and demand is likely to expand to about 285 million m³/day. As there is thought not to be a high likelihood of finding new conventional methane sources in the foreseeable future, India will have to either considerably scale back plans for industrialization and suppress consumer demand or meet its energy requirements from some other source, such as nuclear energy. India could also import methane or develop indigenous methane from unconventional sources, such as: (i) Coal-bed methane; (ii) Gas hydrate; (iii) In-situ coal gasification.

Japan is heavily industrialized, has a high standard of living, and has a relatively high demand for energy. Yet Japan has never had abundant indigenous energy sources. Because Japan has very limited conventional hydrocarbon resources and potential, the country has traditionally imported virtually all of its energy supplies since the beginning of its industrial revolution in the latter part of the last century. Supplies of energy have always ben one of the foremost concerns of industry and government at the highest levels.

In 1998, the Naval Research Laboratory compiled a digital bathymetric database for use in environmental analyses of the seafloor morphology, physiography, and surficial sediments in the northern sector of the South China Sea (McDonnell, et al., 1998). Both detailed bathymetric surveys and random-track bathymetric sounding data were used. Most of this data was obtained from the United States National Geophysical Data Center (NGDC) and the United States National Imaging and Mapping Agency (NIMA), as well as other data not held by these agencies. Compilation created a new set of digital bathymetric contours (Fig. 1).

Estimating the in situ methane hydrate volume from seismic surveys requires knowledge of the rock physics relations between wave speeds and elastic moduli in hydrate/sediment mixtures. The elastic moduli of hydrate/sediment mixtures depend on the elastic properties of the individual sedimentary particles and the manner in which they are arranged. In this chapter, we present some rock physics data currently available from literature. The unreferenced values in Table I were not measured directly, but were derived from other values in Tables I and II using standard relationships between elastic properties for homogeneous, isotropic material. These derivations allow us to extend the list of physical property estimates, but at the expense of introducing uncertainties due to combining property values measured under different physical conditions. This is most apparent in the case of structure II (sII) hydrate for which very few physical properties have been measured under identical conditions.

The geographic extent of hydrate deposits and their distribution within the sediment column are still relatively undefined. Both single and multi-channel seismic reflection surveys remain the principle methods of identifying the presence of hydrate but the effects of methane flux through the seabed is best visualized with several other acoustic sensors, namely side-scan. The strong acoustic impedance contrast reflector at the base of the layer — the bottom simulating reflector or BSR — is normally seen where free methane is present beneath the hydrate. Hydrate formation may also cause blanking of the sediment acoustic stratigraphy through cementation of the sediment structure.

Seismic methods are the most widely used approach for indirect detection and quantification of gas hydrate in marine sediments. Historically, the presence of methane hydrate has been inferred on the basis of bottom simulating reflections (BSRs), which mark the phase boundary between hydrate and the underlying free gas zone (e. g. Shipley et al., 1979). In addition to their association with BSRs, hydrates and the underlying free gas affect the elastic properties of the host sediment in ways that are seismically detectable. Partial replacement of pore fluid by rigid gas hydrate causes an increase of both compressional wave velocity (Vp) and shear wave velocity (Vs) (Chapter 20), while the presence of free gas will strongly decrease Vp. Compressional- and shear-wave attenuation (Q_p ^-1, Q_s ^-1) may also prove to be hydrate/gas indicators: hydrate may increase both Q_p and Q_s, while gas certainly decreases Q_p (Wood and Ruppel, 2000). Accurate, detailed images of the elastic properties of hydrate deposits therefore hold great promise for remotely quantifying hydrate and gas occurrence and concentrations.

The occurrence, distribution, properties, and hydrocarbon reservoir potential of natural gas hydrates in marine sediments continue to be enigmatic questions in marine geoscience. The fact that natural gas hydrate is metastable and affected by changes in pressure and temperature makes its observation and study difficult under laboratory conditions.

Gas-hydrate samples have been recovered at about 16 areas worldwide (Booth et al., 1996). However, gas hydrate is known to occur at about 50 locations on continental margins (Kvenvolden, 1993) and is certainly far more widespread so it may represent a potentially enormous energy resource (Kvenvolden, 1988). But adverse effects related to the presence of hydrate do occur. Gas hydrate appears to have caused slope instabilities along continental margins (Booth et al., 1994; Dillon et al., 1998; Mienert et al., 1998; Paull & Dillon, (Chapter 12; Twichell & Cooper, 2000) and it has also been responsible for drilling accidents (Yakushev and Collett, 1992). Uncontrolled release of methane could affect global climate (Chapter 11), because methane is 15–20 times more effective as a “greenhouse gas” than an equivalent concentration of carbon dioxide. Clearly, a knowledge of gas-hydrate properties is necessary to safely explore the possibility of energy recovery and to understand its past and future impact on the geosphere.

Gas hydrates are an intriguing class of nonstoichiometric compounds that have significant commercial and scientific applications both as an energy resource and as a manufactured material. The last half-century has witnessed a marked escalation in the scope of experimental research on gas hydrates, particularly directed towards the determination of their phase equilibria, formation kinetics, crystallographic and structural properties, transport and thermal properties, effects of inhibitors, and a number of related geochemical topics.

Ever since the discovery of natural methane hydrate in the late 1960’s (Makogon et al, 1972), and the subsequent growing awareness of the enormous quantities of methane locked in them (e. g., Kvenvolden, 1993), research scientists have dreamt about recovering this gas to help satisfy the world’s demand for energy. The transition from a petroleum to a methane energy economy would also be environmentally positive because methane burns very cleanly, and produces less CO₂ per unit of energy than any other fossil fuel. In view of the ever-increasing concern about the environment, methane will likely become the single most important fuel for many decades to come. Natural methane hydrate potentially holds the promise of (i) energy independence to various countries including the USA, India and Japan, and (ii) enabling extending the use of methane and of the existing gas infrastructure.

The possibility that hydrate may supply extremely large volumes of methane over a long period of time will be the main motivation for carrying out research into the naturally occurring hydrate system. Despite the possible significance of hydrate to ocean/atmosphere chemistry and its impact upon climate and seafloor stability, the amount of money available for research into that facet of hydrates which may generate economic return is potentially large enough to support very substantial research efforts. Thus, examining economic potential of hydrate is important.

Over 70 locations with gas hydrate samples and indirect indications of hydrate have been identified in the sea floor by the beginning of 2002 (Fig. 1, also see Kvenvolden, this volume). In 23 of them, gas hydrates have been recovered from the sea floor sediments by drilling or gravity coring. All submarine gas hydrates are 1. related to infiltration of gas-containing fluids in and through the temperature-pressure field of the gas hydrate stability zone (HSZ); 2. distributed mainly as accumulations; and 3. can be subdivided into two groups: Accumulations at or just below the sea floor and accumulations situated from tens to hundred meters below the sea floor. Most accumulations of the first group are related to focused fluid discharge at the sea floor. The second group of accumulations is controlled by the general migration of gas-rich pore water or by gas diffusion in pore fluids.