Previous Article : Natural Gas Hydrates : Basic Principles.

Introduction:

Availability of energy is key to wealth, political and military power, and living standards. Energy availability and consumption is one of the most reliable measure of an economy. There is a direct relationship between energy consumption and countries Human Development Index (HDI). Energy security, which is the relative certainty that energy supplies for a country will be available, constitutes a primary security concern for countries with high energy demands and countries with increasing energy use.

The potentially largest natural gas resource remaining on Earth, oceanic natural gas hydrate (NGH), may substantially supplement the natural gas supply far into the future. An additional benefit is that natural gas produces less CO2 than Coal or Oil and also has a much lower pollution potential than any other combustion fuel. Natural gas is the clean hydrocarbon fuel that will reach into the renewable energy future. Its continued availability at affordable prices becomes increasingly important as coal and oil power plants are retired and energy demand becomes increasingly filled by development of renewable or intermittent power sources.

Characteristics of Deepwater Natural Gas Hydrate:

A brief remind; we use the term ''Natural Gas Hydrate'' or NGH as a general term to denote a clathrate of methane of any crystal structure. The term NGH encompasses compound hydrate containing two or more natural gases and is synonymous with ''gas hydrate'' or ''methane hydrate''.

NGH is one of the major naturally occurring fossil resources from which natural gas may be produced, and it may be the largest. Gases in NGH are in general the same as those found in natural gas used by consumers or by power plants as a fuel, or even as an industrial feedstock for the petrochemical industry, although methane is by far the commonest natural gas present. NGH is a naturally-occurring storage medium that concentrates natural gas in host sediments in relatively shallow marine sediments in deep and ultra-deep seas.

NGH as a storage media. Methane is the dominant gas in NGH. It is formed by a simple molecule with one carbon and four hydrogen atoms. It is stable chemically and does not have molecular substitution or polymorphs, and is also stable in high temperatures approaching 1000 °C, in the absence of oxygen that would cause combustion.

NGH is a solid, crystalline compound composed of water molecules forming interlocking ‘bucky-ball’-type cage structures surrounding voids that are predominantly occupied by hydrocarbon gas molecules in a body centered cubic mineral system. An important aspect of the NGH system is that the reactions of growth and dissolution are highly reversible. Bonding of the water molecule cage structure and gas molecules in the cages in NGH is accomplished by the weak electrical van der Waals bonding. This renders the system highly responsive to changes in temperature and pressure so that it will either spontaneously grow and dissociate as slips in and out of stability. Formation of NGH can be a very slow process, especially in low gas flux in which dissolved concentration results in only a weak driving force of crystallization.

A single cubic foot of NGH comprised only of methane gas will yield up to approximately 164 cubic feet of gas with a higher heating value or HHV (The HHV of a fuel is defined as the amount of heat released at 25 °C and the products returned to a temperature of 25 °C, which takes into account the latent heat of water vaporization) energy density of about approximately 180,000 Btu/ft3. Even small amounts of higher density hydrocarbon gases can substantially raise the energy density of gas produced from NGH and render the NGH more valuable.

The nucleation and growth of NGH involves a chemical reaction between two chemical components (gas and water). NGH also has a much higher heat of transformation than ice. When with NGH, a calculated 57 kJ/mol of heat has to be absorbed by the environment in which it forms (with water 6 kJ/mol). This is a controlling factor, along with elevated pressure, of why NGH is associated with permafrost and in the deep oceans. Dissociation, in contrast, is an endothermic reaction to which heat has to be supplied from its surroundings. When dissociation is forced by depressurization, heat must be applied to the system to prevent cooling that would cause temperature to fall and force dissociation to cease. 

Solution concentration and crystallization. Similar to many other chemical reactions, crystallization is governed by diffusional processes. The relative concentration of dissolved hydrate-forming gas in pore water, in contact with NGH determines a tendency to crystallize, dissolve, or remain relatively the same. Therefore, without a strong flux of methane and other hydrocarbon gases, no hydrate will be present in sediments. Even though there may be a zone of hydrate stability in the sense that pressure and temperature are suitable for hydrate formation and perseverance, without sufficient hydrate-forming gas in solution, the concentration of dissolved natural gas will be too low to allow nucleation and growth. (see figure below)

          *Growth/dissociation tendencies controlled by relative dissolved concentration gas [3]

The hydrate reaction for hydrocarbon and other common hydrate-forming gases is very reversible. That is, very little change in pressure and temperature across the phase boundary can alter the stability of NGH so that it can change from formation to dissociation and vice versa, very quickly. Although compound hydrate composed of more than one hydrate-forming gas can react more complexly, an essentially single gas NGH such as methane hydrate is very sensitive to its environment. In addition, NGH that is in equilibrium with its pore water media will either grow or dissociate with relatively small changes in dissolved concentration of the hydrate-forming gas.

NGH Stability. Temperature and pressure govern NGH stability growth and dissociation so long as the concentration of dissolved hydrate-forming gas in the enclosing pore water is high enough to drive NGH crystallization. The stability field of methane hydrate is usually taken as the gold standard or reference point for the stability of NGH. When a pore-water solution containing sufficient dissolved gas is introduced to conditions of NGH stability, NGH will spontaneously nucleate and grow. 

*Experimental determination of methane hydrate stability [3]

NGH can form spontaneously under the right combinations of pressure and temperature conditions when there is sufficient natural gas flux. The temperature of the water column in the open ocean decreases with depth, which renders NGH stable in the sediments from water depths of about 500 m and greater. In the Arctic Ocean, the minimum depth of NGH stability is at about 250 m water depth. The figure above shows the methane hydrate stability field as a combination of colder and higher-pressure conditions found in permafrost regions on land and in marine sediments in the oceans. We focus on oceanic NGH because Permafrost NGH occurs in geological traps rather than the thermodynamic traps common to oceanic NGH, oceanic NGH is estimated to contain about 95 % of the entire resource, and it thus is the most economic target.

The Gas Hydrate Stability Zone (GHSZ). The term GHSZ was defined as an economic geological term that referred to the zone of NGH stability within seafloor sediments in which large concentrations of NGH could form and from which the natural gas might be recovered. The top of the zone is at the seafloor, in the absence of biogeochemical action. The base of the GHSZ is defined by temperature. The depth at which the geothermal gradienti ntersects the phase boundary defines the lower limit of hydrate stability. This is the depth below seafloor at which rising temperature defines a lower limit to NGH stability, and is also a phase boundary. The two phase boundaries together fully encompass the field of pressure-temperature NGH stability. 

*Stability field of Methane hydrate. GHSZ is thicker or thinner depending on location, water depth, seafloor temperature and geothermal gradient. [3]

The GHSZ is a worldwide feature of deep continental shelves, slopes and rises to some abyssal regions that vary in thickness depending on sediment thickness, seafloor temperature, and pressure related to water depth. The GHSZ is the focus for exploration because NGH only occurs within it. The seaward thinning of the GHSZ usually indicates thinner sediments and a lower likelihood of NGH concentrations because the sediments tend to be muddy and have poor reservoir character. As the geothermal gradient varies considerably, the thickness of the GHSZ varies on a global scale. Rapid lateral changes in the thickness of the GHSZ are rare except near salt diapirs over which, and in the vicinity of, heat flows may be abnormally high even over short distances laterally. Locally, high heat flow associated with venting may warp the base of the GHSZ to the surface upward, but NGH concentrations associated with venting are dependent on transitory mineralizing solution delivery and are less likely to be associated with large NGH concentrations that may have commercial potential. The Polar oceans, and the continental margins in the Indian Ocean, and the southeast margin of Australia, have the thickest and most prospective GHSZ. (see Figure below)

*Methane Stability zone thickness. [3]

Because a certain amount of sediment overburden is required to support a depressurization conversion of the NGH in a production scenario, we would regard the ~200 m thick GHSZ as a minimum safe thickness.

NGH Paragnesis. Paragenesis is an economic geological term that pertains to the origin of minerals or mineral deposits and the order in which the minerals have crystallized. NGH is a diagenetic crystalline material, which distinguishes it from other hydrocarbon fossil fuel deposits. Geology is the key to understanding the principal sources of methane. NGH paragenesis can be described as having its own ‘petroleum system’ that is useful for exploration, but it can also be described as a mineral deposit that concentrates natural gas from mineralizing solutions through a chemical reaction.

Natural gas can be captured and sequestered over a long period of time in the GHSZ. The gas can be produced locally or be transported in aqueous systems from considerable depths and laterally, given the right set of conditions. Geology governs the degree to which NGH might reach sufficient concentrations to be a natural gas resource, From the point of view of the likelihood of first order, high grade NGH concentration formation, the nature of the marine sediment is the primary governing factor. NGH is found in two basic types of deposits, dispersed NGH in a generally fine-grained muddy sediment host, and concentrated NGH occurring in permeable sediment.

Classification of Oceanic NGH. A classification of NGH based on thermodynamic modeling of production characteristics is presented in the table below: [3]

Class 1 NGH concentrations are typically found in permeable sand beds. Class 2 NGH concentrations are found in the same geological situation but are underlain within a permeable horizon containing NGH by pore water that is saturated in hydrate-forming gas. Class 4 and 5 are essentially the same in that they occur in muddy sediments that have no obvious gas production methodology available. Class 3 deposits have been modeled for their production characteristics. Class 3 deposits have no mobile phase and are generally restricted to permafrost-related occurrences where water is contained in the form of either ice or NGH. In this classification, only class 1 and 2 are likely to have a production petential.

NGH Potentially the Largest Natural Gas Ressource on Earth:

Early estimates of the amount of oceanic NGH were unreasonably large.The total volume of NGH in permafrost and marine sediments is colossal, probably going to several hundred thousand trillion cubic feet. Most NGH occurs in low concentrations (1–5 %, rarely to 8–10 %) in shale and muddy sediments. And those deposits do not constitute gas resources. Early estimates concerned only potential gas-in-place and took little notice of either recoverability or commerciality of the resource. There was little known about pore space occupation, gas flux, the ease with which NGH could form, its mechanisms of nucleation and growth, and other aspects governing NGH paragenesis, volumes, and concentration. As with any resource that becomes better understood with time, volume estimates have become more precise and, hopefully, accurate. The estimates for total gas in place have a very wide range from 0.1 to 1.1 million trillion cubic feet. Most of the NGH, however, occurs in marine sediments that have limited permeability or in seafloor and near-seafloor nodules and veins.

*NGH ressources pyramid in comparison of Conventional ressources. [3]

*Estimates of NGH in marine sands by country and region. Copyright-free DOE publication

Next part : Exploration.... available soon.

References:

1. M. Atilhan, S. Aparicio, F. Benyahia, E. Deniz ''Natural Gas Hydrates''

2. E. Dendy Sloan, C.A Koh ''Clathrates Hydrates of Natural Gases'' Third Edition

3. M. D. Max, A. H. Johnson ''Exploration and Production of Oceanic Natural Gas Hydrate''

4. J. Caroll ''Natural Gas Hydrates : A Guide for Engineers'' Second Edition

5. A. Arora and al. ''Techniques for Exploitation of Gas Hydrate (Clathrates) an Untapped Resource of Methane Gas'' Journal of Microbial & Biochemical Technology