Definition
Hydrates are inorganic and organic substances that contain water and are easily formed by less polarized molecules that would fit into a clathrate water cage. The water molecule is combined in a definite ratio as an essential part of the whole crystal constituting the substance.
Abstract
Natural gas hydrates have always puzzled scientists, researchers, and major stakeholders in the petroleum industry; hydrates have been viewed as future sources of energy though there are challenges on how to contain the gases in combating the associated global warming potential. The report seeks to highlight various identified types of hydrates in the petroleum industry, their formation, application, measures of preventing their formation, and why they need for prevention.
Introduction
Natural gas hydrates are highly available in different geographical locations, provided conditions are right and favor their formation. They are known to exist naturally a clear implication that if well researched and tapped, can offer a long and stable source of energy. This would definitely solve some problems experienced with the application of the current sources of energy like emission of pollutant gases into the atmosphere which includes carbon dioxide, sulfur, and so on.
Conditions Favoring Hydrates Formation
Hydrates form at temperatures greater than the freezing point of water, the more the reason as to why they exist naturally on the seafloor, in ocean sediments, and in deep lake sediments. They are stable at high pressures, usually, but not always, greater than atmospheric pressure, natural gas hydrates are stable at the conditions of deep seafloor and beneath land in the arctic where temperatures are supposedly very low, this implies, hydrates would normally form at diverse conditions where a solid would not otherwise be expected.
In summary, hydrates would only be formed in the following circumstances:
- Water should be present (free water) which acts as a host molecule.
- Low temperatures and high operating pressures,
- High Velocities or turbulence, like the agitation in the transmission of oil through pipelines.
The presence of either hydrogen sulfide or carbon dioxide favors the formation of hydrates due to their relatively higher solubilities compared with hydrocarbons; the two would have a catalytic effect (speeding up) on the formation of hydrates.
Gas (small molar mass gas) + Water = Natural gas hydrate.
The product will depend on the type of hydrate applied or in the equation.
Theoretical Background
Gas hydrates are crystalline forms of matter, a mixture of water and low molar mass gases, which are slightly soluble in water, they are readily found in nature, including natural gas hydrates are readily found on the seafloor, in the arctic, and in ice cores (Sloan, 2003, pp. 426,353 – 363), methane clathrates which are the most common forms of natural gas hydrates are mostly found in polar continental sedimentary rocks where surface temperatures are less than 0o C or in sediments in deep waters where temperatures are approximately 2o C.
“Hydrates are stable only under specific pressure and temperature conditions, under appropriate pressure; they can exist at temperatures above the freezing point of water, salinity can also influence their stability” (Edmonds et al., 1996, pp. 50 &68), but the key factors are pressure and temperature.
Water molecules are adjoined by hydrogen bonds to form a pattern with large cavities in which low molar mass gas molecules enter and get linked through van der Waals forces.
The study of gas hydrates spans back to 1823 when the first recorded observations were made by Humphrey Davy and Michael Faraday1 through bubbling chlorine gas in water where a solid material formed on cooling or at temperatures slightly above the normal freezing point of water (Bishinoi and Clarke, 2005), throughout the years since, natural gas hydrates have been a center of focus on more molecules forming hydrates, their stabilities and how they can be incorporated into the energy sector.
Though seen to be the future source of petroleum, it is very expensive to extract and mostly is in form of deposits in oceans and Polar Regions. Methane forms thermogenic hydrates from a petroleum-based source, petroleum deposits sometimes leak methane gases and so does natural gas deposits, into the surrounding sediments providing enough for hydrates formation, this implies that thermogenic hydrates are more prevalent in areas where there are more petroleum deposits thus a greater tantalizing future prospect for clean energy from methane hydrates. The methane hydrates would either support local oil and gas field operations or be commercially traded to yield revenues and that is why elaborate research and development efforts would be necessary to characterize hydrates in their oil and gas leases. Methane informs of sediments combined with water under right conditions deep into the oceans, approximately about 300meters below the surface to form the hydrate which scatters at various depths and concentrations in seafloor sediments and arctic permafrost, an accidental release of all the deposited methane can cause a major climate change. Methane is a known powerful greenhouse gas though with a short atmospheric half-life of 7years, it is thought to have a global warming potential, a sudden increase in atmospheric methane concentration will cause rapid heating of the earth through depletion of the ozone layer, which causes dissociation of more methane hydrates (Osegovic, 2008, pp. 1&2). This would be achieved through dehydration implying more and more methane in the atmosphere in the long run which would ultimately have diverse effects on the climate.
If methane gas was also to be released from methane hydrates during drilling escapades, there are fears that the buoyancy would potentially cause losses of oil drilling platforms nearby; this poses great safety fears in hydrates harvesting.
Nature of Hydrates
Organic hydrates are formed by the addition of water to their molecule or to its element. For example, ethanol can be considered as a hydrate of ethylene, CH2=CH2, formed by the addition of H to one C and OH to the other C. another example is chloral hydrate, CCl3–CH(OH)2, which can be formed by the reaction of water with chloral, CCl3–CH=O.
“Gas hydrates are stable at moderate to high pressures and low temperatures, above and below the ice point, these ice lattices are stable only when the cages contain a gas molecule” (Claypool and Kaplan, 1974, pp. 99 – 139). Hydrates are normally held in equilibrium in the hydrate region depending on the amount of water present:-
- A large amount of water means the equilibrium is between water and the hydrate,
- A small amount of water means the equilibrium is between gas and the hydrate,
- If there is an extreme amount of water, then the hydrate does not form at all, even though the conditions are in the hydrate region,
- If the mixture is very lean in water, no hydrate forms even though the conditions are in the hydrate region, (Claypool and Kaplan, 1974, pp. 99 – 139).
Types of Hydrates
The crystal structures of hydrates are three dimensional usually two crystallographic and a third hexagonal structure, though rear; they are classified according to the polyhedral cages they contain thus forming the following three categories:
Type I
Usually formed by smaller molecules, examples of molecules forming this type of hydrates include methane, ethane, carbon dioxide, and hydrogen sulfide. Type I hydrates mostly consist of 46 water molecules forming two types of cages arranged in the body-centered packing, made up of up to 8 polyhedral cages; 6 large ones and 2 small ones, but not all cages are occupied thus forming a theoretical composition of 8X · 46 H2O or X · 5 3/4 H2O, where X is the guest molecule (Carroll, 2008, p. 426).
Example, kCH4 (g) + yH2O(l) 8CH4. 46H2O where k and y are stochiometric proportions.
Type II
Normally formed by larger molecules and constitute 136 water molecules in two cages; a small one and a large one. Molecules that would form this type of hydrates include: propane, isobutene, Oxygen, and Nitrogen, relatively small molecules, Type II hydrates are made up of 24 polyhedral cages 8 large ones and 16 small ones, have a theoretical composition of 24 X · 136 H2O or X · 5 2/3 H2O, If only the large cages are occupied, which is typical, then the theoretical composition is 8 X · 136 H2O or X · 17 H2O (Stokelberg & Muller, 1954, pp. 1, 16, 83).
Example, kO2(g) + yH2O(l) 24O. 136H20 where k and y are stochiometric proportions.
Type H
These types of hydrates require a stable corporation of two guests and are formed by larger molecules but only in the presence of a smaller molecule like methane, implying their formation would only happen in the presence of both the large and small molecules (Sloan, 1998, p. 368), they are the rarest among the three types. Formation of this type of hydrates requires a light gas and molecules typically present in oil and condensates the more the reason they are considered related with petroleum production. Type H hydrates are constituted in three cages; two small cages but of different types and one huge one, the huge cavity allows type H hydrates to incorporate large molecules such as butane and other larger hydrocarbons, they are made up of 34 water molecules and have a theoretical composition of X · 5 Y · 34 H2O where X is the large molecule and Y is the small (Sloan, 1998, p. 368), Examples of molecules forming this type of hydrates include: 2-methyl butane, methyl cyclopentane, methylcyclohexane, and cyclooctane,
Example in an equation, 2-methylbutane + water = Structure H hydrate,
kCH3CHCH3CH2CH3 + yH2O CH3CH2CH2CH3.5CH4. 34H2O where k and y are stochiometric proportions.
this type of hydrates are mostly suggested to be available in the Gulf of Mexico; in all these examples the hydrocarbon molecule resides in the large type H cage, structure H hydrate need to be stabilized and the molecules are believed to contain a small fraction of petroleum thus a high possibility of structure H hydrate in natural and artificial environments like crude oil reservoirs.
The use of hydrates in the petroleum industry is limited due to environmental and economical concerns though there has been the development of concepts aimed at their utilization, the most common one being hydra flow which is meant to offer a solution to gas transportation problem through encouraging gas hydrate formation but preventing their agglomeration and pipeline blockage by using either chemical or mechanical means, this process aims at eliminating or greatly minimizing the gas phase by converting it into hydrates in oil or in the aqueous phase (Azarinezhad & et. al., 2008). Hydraflow would convert gas into hydrates and transport it as a slurry in a liquid phase and this may be a major milestone in providing a solution for gas utilization for fields where the ambient temperature and pipeline pressure are inside the hydrate stability zone (Azarinezhad & et. al., 2008).
Prevention of Hydrates Formation
Hydrate have been costing researchers and major stakeholders in the oil industry millions in crafting ways of preventing their formation, natural gas hydrates are said to plug casing and pipelines through the formation of sticky, wet snow like crystals which can grow into large balls thus affecting the flow of oil within the pipes, this can be avoided either, by injecting a thermodynamic inhibitor which prevents the formation of the hydrates, removing water, the addition of ethylene glycol or methanol which work through depressing temperatures at which hydrates form, the latter is mostly preferred when dealing with low gas volumes or when hydrates problem is mild, infrequent or even periodic, the plugging would also cause problems in production, transmission, and processing of natural gas and with even the right conditions the hydrates would form in the reservoirs affecting storage operations.
Inhibitors, heat application, or dehydration are the three most common methods of preventing hydrates formation, for inhibitors to work properly in reducing the temperature at which anhydrate would form, it has to be insufficient or right amounts in order to effectively inhibit the formation examples include; kinetic hydrate inhibitors and anti- agglomerates which would not prevent hydrates from forming but prevent them from sticking together to block equipment.
For hydrates to effectively form the presence of water is vital in the sense that it offers cavities into which guest molecules enter, dehydration deprives guest molecules of water thus preventing them from joining together.
Heat application can be used to drive out water from the molecules reducing chances of hydrate formation through rendering the conditions unfavorable.
Application of hydrates
Hydrates can be used to store natural gas forming a future solution to the transportation of the same using specialized ships or even to be used in natural gas vehicles where currently the gas has to be compressed into cylinders (Energy API, 2009).
Hydrated lime and quick lime are used or can be used in the treatment of many industrial sludges which cause blockages in the major application or in areas of disposals, through the following:
- by correcting pH,
- neutralizing acidic wastes and
- removing contaminants,
- specifically hydrated lime and quick lime can be used for sulfite or sulfate sludges and petroleum wastes.
Natural gas hydrates are common in the oil industry due to their unique properties like the fact that they are nonflowing crystalline solids that are denser than typical fluid hydrocarbons and also the fact that the gases contained are easily and effectively compressed make them useful in gas pipelines; energy recovery and transportation (Sloan, 2003, pp. 353 – 368).
Passive fire protection
This is an essential part of fire protection and seeks to craft a means of containing fire or preventing its spread, when a building catches fire, very high temperatures are realized, approximately more than 1100oC. With the application of hydrates, chemically bound water in the hydrates sublimes during the endothermic reactions ensuring the unexposed side remains below the boiling point of water, this means that the more the hydrates, the longer the fire-resistance duration, which would be very useful in buildings or companies dealing with highly sensitive materials including in the oil industry.
Potential Energy source
Two factors make gas hydrates attractive as potential energy sources:
- The enormous amounts of methane that is apparently sequestered within clathrates, approximately 6.4 trillion tones on the deep ocean floor and even at shallow sediment depths within 2000 m of the earth’s surface (Buffett, 2004, pp. 185 – 199).
- The availability of hydrates in different geographical locations across the globe.
Natural gas would constitute the most environmentally friendly form of energy source if adopted since it emits the least amount of carbon dioxide per unit of energy compared to other sources of energy.
References
Azarinezhad, R, Chapoy, A, 2008, ‘can gas hydrates provide a solution to gas utilization challenges in Russian oil fields’, Russian Oil & Gas Technical Conference and Exhibition, Moscow, Russia.
Buffett, B, and Archer, D, 2004, Global Inventory of Methane Clathrate: ‘Sensitivity to changes in the deep ocean’, pp. 185 – 199, Earth Planet. Sci. Lett 2004.
Bishinoi P, R, and Clarke, M, (2005), ‘Natural Gas Hydrates’, Applied and industrial chemistry; industrial engineering and manufacturing. Calgary, Alberta, Canada.
Carroll John J, 2008, An Introduction to Gas Hydrates, Aqualibrium, Alberta, CANADA. Retrieved from, Web.
Claypool, G W, and Kaplan, IR, 1974, The Origin and distribution of methane in marine sediments, in Natural gases in marine sediments, edited by I.R. Kaplan, pp. 99-139, Plenum, New York, USA.
Edmonds, B, R Moorwood, and R. Szczepanski, 1996, A practical model for the effect for the salinity on gas hydrates Formation. Web.
Energy API, (2009), Natural gas hydrates may help fuel the future, Web.
Osegovic, J, 2008, Gas Hydrates, Knol: “Unit of Knowledge”, Web.
Sloan, E D, 1998, Clathrate hydrates of Natural gases, 2nd Ed. Marcel Dekker, New York, United States of America.
Sloan, E D, 2003, Fundamental principle and applications of Natural Gas Hydrates, 2nd Ed., Marcel Dekker, New York, NY. USA. Pp. 426, 353 – 368.
Stokelberg, V, Muller, H M, 1954, Zeitschrift fur Electrokemie58, pp. 1, 16, 83.