What is gas hydrate?

Gas hydrates are solid crystalline compounds formed when gas molecules occupy the void space of the water molecule. The solid compound resembles ice or wet snow. They are included in a general class of compounds called clathrates.

Natural gas hydrates are formed when natural gas, mainly methane, ethane, propane, isobutane, hydrogen sulfide, carbon dioxide, and nitrogen takes the position within the water lattice, and occupies the vacant space. The phenomenon causes water to freeze at temperatures significantly higher than the freezing point of water.

Where are these gas hydrates formed?

Naturally occurring hydrates exist abundantly in two types of environments - arctic permafrost and deepwater oceanic sediments. The majority of the hydrates occurs in oceanic sediments as these are the zones of active production of methane through the process known as methanogenesis in marine sediments.

The methane formed in the marine sediments then comes in contact with pore water and forms methane hydrate under optimum pressure and temperature conditions. As there is little data available on the distribution of gas hydrate deposits in the ocean, it is tough to estimate the actual volumes of natural gas trapped inside the hydrates beneath the sea floor.

How natural gas is extracted from their hydrates?

There is no proved commercial method to produce gas from gas hydrates. Most of the projects are in pilot phase, so the methods developed for extraction of gases may or may not be successful on commercial scale. Few of the known methods of extraction of gas from hydrates are-

• Thermal Recovery Method
• Depressurisation Method
• Carbon Dioxide Injection Method

In thermal method, hot water is supplied to the deposit of gas hydrate. Temperature of the deposit is increased to such an extent that the hydrate stability is broken and natural gas and water is separated.

Depressurisation, as the name suggests, involves lowering of the pressure of the deposit through drilling. The hydrate deposits sit under significant pressure due to overlying sediments and water. Drilling releases the pressure similar to a tire, where pressure could be reduced through a sudden puncture.

The pressure release breaks the stability of the gas and water system and in due process, the gas is released.

Carbon dioxide can be infused into the hydrate structure, causing the release of trapped gas and infusion of the carbon dioxide. The CO₂ makes stronger bond with water with respect to hydrocarbon. Also, this method could ease  CO₂ sequestration and help fight global warming.

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How much gas is available in form of gas hydrates?

The volume of methane available in the form of hydrates has been estimated through several studies. The highest estimates are based on the hypothesis that fully dense clathrates could be spread on the entire floor of the deep ocean.

As more clarity came on clathrate chemistry and enhanced knowledge of sedimentology, it was found that hydrates only occur in a narrow depth ranges like continental shelves. They typically occur in low concentrations (0.9-1.5% by volume).

Constrained by direct sampling method, the recent estimates of the global inventory lie between one and five million cubic kilometres. The corresponding hydrate resource is estimated to be around 500-2500 gigatonnes carbon (Gt C). The estimates are lower than 5000 Gt C for all other known fossil fuel resources, but significantly higher than ~230 Gt C other natural gas sources.

For the sake of comparisons, the total carbon present in the atmosphere is around 700 gigatons.

How are hydrate deposits classified?

Scientists developed a classification system for hydrate-bearing geologic media. The classification of the hydrate deposits is done in four classes - Class 1, Class 2, Class 3, and Class 4. When hydrate-bearing layer is underneath the zone of mobile water and free gas, it is classified as Class 1 system.

Class 2 system includes the hydrate-bearing layer, which is underneath of water. Class 3 systems are those in which a single hydrate-bearing layer exists with no underlying mobile fluids. The Class 4 hydrate deposits are typical of oceanic-hydrate accumulations. These are low-saturation deposits with no bounding formations.

Another classification system is used in parallel to the above-discussed one. The system is based on the geological occurrence of the deposit. Most of the hydrate deposits can be classified as either structural or stratigraphic in the marine environment. Sometimes combination of both, structural and stratigraphic deposits, can be observed.

Structural hydrate deposits are generally formed by the thermogenic gases. These gases migrate from the deeper subsurface to the hydrate stability zone along the faults or permeable channels, gas chimneys above petroleum reservoirs, or mud volcanoes. Then they react with the water in the hydrate stability zone and form hydrates.

The factors which control the hydrates in structural deposits are heat flow, variations in the salinity levels of the sediments, and the presence of permeable pathways.

Gas hydrates can be found concentrated locally around the faults and mud volcanoes. The northwestern Gulf of Mexico is one of the examples of structural gas hydrate accumulation. Other examples are Hydrate Ridge (offshore Oregon) and the Haakon Mosby mud volcano (offshore Norway).

Stratigraphic hydrate deposits are formed mainly by the biogenic gas in marine sediments. These types of deposits come into existence in a low-fluid flux environments or diffusion dominated environments. Hydrates are located beneath the seafloor, having large areal extent, and may occur in very low saturations.

Combination of the two deposits systems is found where hydrates occur in permeable strata. The flow or movement of the gas for hydrate formation occurs along the conductive faults or diapirs. Scientists recently introduced a new system to classify hydrate deposits into four major categories. The geological framework and lithology of the hydrate-bearing sediments are main factors in this type of classification.

According to these researchers, the four major plays where hydrates are found are - sand-dominated plays, fractured clay-dominated plays, massive gas-hydrate formations exposed at the seafloor, and low-concentration hydrates disseminated in a clay matrix.