Gas Hydrates, its structure, stability zones, formation, classification and global occurrence

 Introduction:

Natural gas hydrates (NGH) are crystalline substances produced when water molecules bind to natural gas molecules. The reactions that lead to the production of methane hydrate are depicted in this diagram.

For methane hydrates, NH is the hydration number, which is roughly equal to 6. The hydrate formation reaction is an exothermic (heat-producing) event, while the hydrate dissociation reaction is an endothermic (heat-consuming) reaction (absorbs heat). H1 = 54.2 kJ/mol for the production of methane hydrate from methane and liquid water, and H2 = 18.1 kJ/mol for the synthesis of methane hydrate from methane and ice. Clathrates, which meaning "cage-like structures," are a group of chemicals that include NGHs. Some liquids, such as tetrahydrofuran (THF), can also produce hydrates when they react with water.

Pressure, temperature, gas composition, and the presence of inhibitors such as salts all play a role in the development of natural gas hydrates. NGHs may be found in the subsurface in two different types of habitats: arctic permafrost and deep water marine environments. Hydrates have been researched in the oil and gas sector since (Hammerschmidt, 1934) revealed that hydrate development may cause pipeline blockage. Certain chemicals can be put into pipes to either inhibit hydrate formation or prevent them from adhering to the pipeline walls.

Structure of Gas Hydrates:

Structure I (sI), Structure II (sII), and Structure H (sH) are the three fundamental crystalline structures of gas hydrates. The composition of a gas mixture determines whether it forms sI, sII, or sH.

sI hydrate is formed from pure methane and ethane. sII hydrate is produced for components bigger than ethane (propane, butane). Isopentane and other bigger gas molecules, as well as smaller molecules, can be accommodated in sH hydrate (C1-C4).

Figure 2 Structure of gas hydrates form of cages and unit cell divisions.

Figure 3 Generalized structure of gas hydrate.

Figure 4 Cage types and the number of individual cages forming the three common hydrate crystal structures. The circled numbers denote the numbers of the cages used to form the hydrate structure, (From Center for Gas Hydrate Research – Heriot Watt University, 2007). 

The most prevalent structure in nature is sI hydrate, which is followed by sII hydrate. sH hydrates are more uncommon, and they have just recently been discovered in natural systems. At normal temperature T and pressure P, one cubic foot of methane hydrate may encapsulate up to 164 cubic feet of methane. The high concentration of methane in methane hydrate, along with the huge quantity of global hydrate inventory, has pushed the topic of using natural hydrates as an energy resource to the forefront, and it is the driving factor behind most of the current study on naturally occurring hydrates.

Controls on Hydrate Stability:

Pressure, temperature, gas composition, and the presence of inhibitors all influence the stability of hydrates (such as salts). For most of the naturally occurring temperature ranges, the methane hydrate equilibrium curve with pure water. The remaining portion of the hydrate stability spectrum, below the ice-point, is not included in the pressure and temperature circumstances examined in Figure below.

With increasing concentrations of salt or other chemicals in the water, the equilibrium curve moves to the left (red arrow in Figure); salts and alcohols function as hydrate inhibitors. Alcohols such as methanol and ethylene glycol are put into oil and gas pipelines to prevent hydrate formation. Makogon performed significant research into the effects of alcohols and salts on the development of hydrates. When heavier hydrocarbons, as well as methane, participate in hydrate production, the equilibrium curve moves to the right (green arrow).

Figure 5 Methane hydrate equilibrium curve.

Hydrate Stability Zone:

Hydrates are stable in offshore settings at depths of 200 to 600 metres, depending on the gas composition and seabed temperatures. The "methane-water-hydrate" phase boundary is really an equilibrium curve that is affected by gas composition and pore water salinity. The name "hydrate stability zone" (HSZ) does not imply that hydrates will always be present in that location, but rather that hydrates will be stable there if they occur. 

Apart from pressure, temperature, gas composition, and salinity, the methane supply in marine sediments is the other regulating parameter for hydrate formation. To produce hydrates, methane must be present above the solubility limit at the appropriate pressures and temperatures.

Figure 6 Hydrate stability zone in offshore environments.

Along Continental Margins:

Because of the high sediment flux and thus rapid burial of organic content at these locations, gas hydrates are common along the Continental margins. The fast burial of organic matter generates an anoxic environment, which encourages the conversion of organic matter to methane. Along the continental margin (Figure below), which comprises a Continental shelf, a Continental slope, and a Continental rise, the thickness of the hydrate stability zone will vary. As the water depth grows, the thickness of the hydrate stability zone increases along the slope, but geothermal gradients stay constant across the Continental margin.

Figure 7 Hydrate stability zone along the continental margins.

Figure 8 Methane hydrate stability zones (blue) for (a) permafrost and (b) oceanic environment, Sara H Harrison-Stanford University,2010.

Formation of Gas Hydrates:

Hydrate formation was averaged for 20 natural gases at a typical deep seabed temperature of 39°F and an average formation pressure of 181 psi. The lowest hydrate-formation pressure was 100 psig for a gas containing 7 mol% propane, while the maximum was 300 psig for a gas containing 1.8 mol% propane. Because systems typically run at considerably greater pressures than 181 psi to achieve an economically viable energy density, hydrates can form when tiny hydrocarbons (n-butane or smaller) come into contact with water.

Figure 9 Formation mechanism of gas hydrate.

When water molecules come into touch with gas molecules at low temperatures and high pressures, they produce geometric shapes that aren't hexagonal. The water molecules act as hosts, forming cage lattices that can accommodate gas molecules as guests. Due to the presence of gas molecules, these cage-like crystalline formations are less dense than crystalline water structures. The hydrogen bonds of the water molecules hold the gas hydrate together, and the Vander Waals forces that hold the gas and water molecules together stabilize it.

The Vander Waals force is responsible for the gas hydrate's stability, making it even more stable than ordinary ice produced by water. Gas hydrates come in a variety of shapes, which are defined by the form of their cages.

Figure 10 Process of formation of gas hydrates.

Classification of Gas Hydrates:

According to the classification method, most hydrate deposits in offshore settings may be categorized as structural or stratigraphic. They can also be a combination of both of these options.

Structural Hydrate Deposits:

Thermogenic gases from the deeper subsurface move to the hydrate stability zone (HSZ) through faults or permeable channels, gas chimneys above petroleum reserves, or mud volcanoes, forming structural hydrate deposits. The water in the hydrate stability zone reacts with these gases to produce hydrates.

Heat movement, salinity changes in the sediments, and the presence of permeable pathways are the major determinants of hydrates in structural deposits and their distribution in sediments.

Example:

Localized concentrations of gas hydrates can be found along faults and mud volcanoes. One example of structural gas hydrate buildup occurs in the northwest Gulf of Mexico.

Hydrate Ridge (offshore Oregon) and the Haakon Mosby mud volcano are two more examples (offshore Norway). The structural accumulations often develop in thick layers at high fluid flow conditions. The HSZ, on the other hand, is entirely eliminated due to rapid fluid flow and high pore water salinity.

Stratigraphic Hydrate Deposits:

Stratigraphic hydrate accumulations are hydrate deposits in marine sediments produced by biogenic gas. These deposits form in settings with minimal fluid flow or where diffusion is dominant. Hydrates are found deep beneath the bottom and cover a wide area, although they may also exist at extremely low saturation levels. Under stratigraphic accumulations, BSRs are more common than at structural accumulations.

Combination of Both Deposits:

Combination accumulations arise when hydrates form in permeability strata while the gas supply for the production of the hydrates occurs along conductive faults or diapirs.

Boswell's team has proposed a new approach for categorizing hydrate deposits into four primary groups. Their categorization method is based on the hydrate-bearing sediments' geological framework and lithology. Sand-dominated plays, fractured clay-dominated plays, enormous gas-hydrate formations exposed at the seabed, and low concentration hydrates dispersed in a clay matrix are the four primary hydrate plays.

Figure 11 Structural, stratigraphic and combination of both hydrate deposits.

Global Occurrence of Gas Hydrates:

Currently, government initiatives for the research and production of natural gas from gas-hydrate resources exist in several nations.

Over 220 gas hydrate deposits have been found, over a hundred wells have been sunk, and kilo metres of hydrated cores have been examined as a consequence. The properties of hydrated cores have been studied, effective techniques for recovering gas from hydrate deposits have been created, and novel technology for exploring gas-hydrate fields has been devised. Commercial generation of natural gas from gas-hydrates has been successful for many years.

The volume of hydrated gas that can be profitably generated (17–20 percent of potential) is more essential than the amount of prospective reserves. The world's gas hydrates may contain more organic carbon than the world's coal, oil, and other types of natural gas combined, according to the United States Geological Survey. Natural gas hydrate resources have been estimated to range from 10,000 trillion cubic feet to more than 100,000 trillion cubic feet. Deepwater gas hydrates are being produced by countries like as the United States and Japan.

Figure 12 Global occurrence of gas hydrates.

Potential of Gas Hydrates:

The amount of methane stored in methane hydrate across the globe is enormous.

The worldwide methane hydrate reserve is estimated to be worth 20,000 trillion cubic metres, or 700,000 Tcf. However, only a small fraction of this vast resource will likely be used as a source of energy. If current and new technology can be used cheaply to the production of methane hydrate as a natural gas source, the United States' dependency on foreign energy sources may be considerably reduced.

Several countries that presently rely heavily on imported energy may be able to become more self-sufficient. A early evaluation of the in-place methane hydrate resource in the Gulf of Mexico was issued by the US Minerals Management Service (now the Bureau of Ocean Energy Management, Regulation, and Enforcement, or BOEMRE) in 2008.

This study, which does not examine whether the resource is technically or economically viable, estimates that the northern Gulf of Mexico has between 11,000 and 34,000 Tcf of methane in hydrate form, with a mean value of 21,444 Tcf.

Figure 13 Subset of global in-place gas hydrates that appear to occur at high concentrations in sand-rich reservoirs.