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Natural Gas Hydrate as an Upcoming Resource of Energy | OMICS International
ISSN:2157-7463
Journal of Petroleum & Environmental Biotechnology

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Natural Gas Hydrate as an Upcoming Resource of Energy

Amit Arora1, Swaranjit Singh Cameotra2* and Chandrajit Balomajumder1
1Department of Chemical Engineering, Indian Institute of Technology, Roorkee, India
2Institute of Microbial Technology, Chandigarh, India
Corresponding Author : Swaranjit Singh Cameotra
Senior Principal Scientist
Institute of Microbial Technology
Sector 39A, Chandigarh -160036, India
Tel: 91- 9041036750
E-mail: [email protected]
Received November 03, 2014; Accepted November 28, 2014; Published February 10, 2015
Citation: Arora A, Cameotra SS, Balomajumder C (2015) Natural Gas Hydrate as an Upcoming Resource of Energy. J Pet Environ Biotechnol 6:199. doi:10.4172/2157-7463.1000199
Copyright: © 2015 Arora A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

With increasing energy demand and depleting energy resources, gas hydrates may serve as a potentially
important resource for future energy requirements. Methane gas hydrates is one such source of methane gas which is trapped in crystalline ice like structure in permafrost regions and beneath the sea in outer continental margins. It is estimated that total amount of carbon in the form of methane hydrates, far exceeds the carbon content in all the fossil fuel reserves put together and hence these are supposed to be the future potential energy resource. Like several other countries, India has also started a national gas hydrate programme for focussed exploration and possible exploitation of this important natural gas resource. This review article briefly takes up the issues related to understanding of methane hydrates, their origin, occurrence, energy potential and its exploitation techniques.

Keywords
Gas hydrate; Methane clathrate; Carbon content; Energy resource
Introduction
Gas hydrates were discovered by Sir Humphrey Davy in 1881; he made a careful observation that water and chlorine can form a crystalline substance under certain conditions of temperature and pressure and chose to call it chlorine hydrate. Subsequently in 1930’s, it was realized that solid gas hydrate formations in the oil and gas transmission pipelines in the U.S. were responsible for the clogging of such pipelines. Since then, gas hydrate was considered as a nuisance and technologist around the world just hated this ice like compound. However, around 1970’s basic research on understanding the formation and dissociation of gas hydrates along with its characterization has made it easier for the scientists and the technologists to tackle this problem in a more scientific way. With the improved understanding about gas hydrates, today gas hydrates are known as one of the potential source of methane and therefore it is sometime called as fuel of the future. Gas hydrates (clathrate) are basically a solid and nonstoichiometric crystalline compound made up of water and low molecular weight hydrocarbons. In a clathrate, water molecule called host, arranges itself in a cage like formation that encapsulates a gas molecule known as guest molecule. At certain temperature and pressure condition, these guest and host interact with each other through Van der Waal’s interaction and thus stabilize the structure; there is no chemical bonding between the two. When the guest molecule is methane, it is said to be a methane hydrate. It occurs worldwide in the oceanic and polar sediments where temperature is low enough and pressure is sufficiently high to crystallize the prevalent methane and water into gas hydrates. Thermodynamically, hydrate formation is favoured at high pressure, low temperature and low salt concentration [1,2]. Such conditions can exist in ocean-bottom sediments at water depths below 500 m [3]. One volume of gas hydrates releases about 160 volumes of methane and 0.8 volume of fresh water at standard temperature and pressure (STP). Gas hydrates represent a tremendous reserve of natural methane in the earth [4,5]. For example, 1 m3 of hydrate disassociates at atmospheric temperature and pressure to form 164 m3 of natural gas plus 0.8 m3 of water [3]. With pressurization, gas hydrates remain stable at temperatures up to 291 K. Density of gas hydrae is 0.79 kg/L [6,7].
The worldwide organic carbon in the gas hydrates is estimated to be roughly about 10000×1015 grams which is almost double the carbon content in total fossil fuel (crude oil, natural gas and coal) reserves of the world. The comparison of the sources of global organic carbon is shown in Figure 1.
Historical Perspective
The history of natural gas hydrates has developed over three key periods as described below:
Period I: Hydrate as a laboratory curiosity
As discussed above, natural gas hydrates were first discovered by Sir Humphrey Davy in the early 1800’s [8]. Since then, gas hydrates were more of an academic curiosity for almost five decades. Scientists attempted to identify all of the compounds which can form hydrates along with hydrates compositions and their physical and chemical properties. During more than a century gas hydrates remained a mere niche curiosity. This did not change when by the end of the 19th century Villard [9,10] showed that also hydrocarbons like methane, ethane, ethylene or propane can form hydrates. First tabulated results date back to de Foucard [11], a collaborator of Villard.
Period II: Hydrate as a problem to the natural gas industry
Only in the late 20ies and early 30ies of the last century when in the United States transport of natural gas in pipelines started operation on a larger scale and at higher pressures natural gas hydrates began to form in gas pipeline and are blocking the pipelines. The first such example which blocked the pipes due to gas hydrate was given by Hammerschmidt [12] in 1934. Hammerschmidt in 1939 [13] also gave the first algorithm for calculating the amount of methanol to inhibit the formation of natural gas hydrates.
During this time, several research programme was launched which were aimed at finding techniques to inhibit gas hydrate formation within the pipeline and thus maintain a constant flow of gas. Such research programmes were later termed as “flow assurance” and taking a clue from ice inhibition, many workers [14] studied the effects of several chloride salts on hydrate formation and identified that these salts inhibit gas hydrate formation. In recent years, industries have shifted their focus on a different kind of inhibitors which is required in small quantity and instead of stopping hydrate formation completely; it slows down the hydrate growth process thus preventing formation of solid plugs in a given time scale.
Period III: Hydrate as a potential energy resource
About sixty years ago natural gas hydrates were found for the first time as gas hydrate deposit in Siberia. Since then they are seen as prospective fossil fuel reserves. Makogon [15] gave the background on the up to date sole continuous gas production from natural gas hydrates in the Messoyaka field, Siberia. Since then scientists realised that the low temperature and high pressure conditions necessary for hydrate formation should exist extensively around the globe, underneath deep oceans and in permafrost region. However, in the mid-1960’s Russian scientists [15] succeeded in locating hydrate saturated cores in exploratory drilling crew in Messoyakha in Siberian gas field. The presence of gas hydrates in oceanic sediments was first postulated on the basis of seismic observations [16,17]. In 1992, the Ocean Drilling Programme began intentionally locating for hydrate deposits, and samples were brought to the surface for further study [18]. The intensive exploration research by oceanographers and petroleum geologists during the last decade resulted into the discovery of widespread occurrence of gas hydrates in the continental margin areas [19].
Hydrate Fundamentals
Natural gas hydrates are formed at a moderate to high pressure of the order of 1-30 MPa and at low temperature (-5°-20°C). The methane within the marine sediments is either biogenic [20] or thermogenic [21] or mixture of these two. Clathrates are formed as per the following reaction:-
G + NHH2O = G. NHH2O
G is the guest molecule which is typically 90-99% methane in a natural gas hydrate sample. The most common guest molecules are methane, ethane, propane, isobutane, n-butane, nitrogen, carbon dioxide, and hydrogen sulfide, of which methane occurs most abundantly in natural hydrates [22]. NH is the hydration number, which is nothing but average number of water molecules per guest molecules in an unit cell of crystalline gas hydrate compound. NH also depends on resultant gas hydrate structure. For a stable gas hydrate crystal, hydration number varies from 5.66 to 17; a lower hydration number would mean higher saturation of gas in the sample.
Structure of gas hydrates
By applying X-ray diffraction methods von Stackelberg [23,24] gave the information of the detailed structure of these compounds. Stackelberg and his coworkers described two different structures, based on this information and applying statistical thermodynamics, van der Waals and Platteeuw [25] laid the basis for calculating gas hydrate phase equilibria. Parrish and Prausnitz in the early 70ies of last century [26] were the first to give additional assumptions on the occupation of the cages to gas mixtures including natural gases and to present a first computer applicable algorithm.
The structures I and II were first described by von Stackelberg, structure H was discovered by Ripmeester et al. [27]. The type of structure a guest substance will form depends on the size of the molecule and on the composition of a mixture. For instance, pure methane forms structure I, if we add less than one mol percent propane the structure changes to structure II, being formed by pure propane [28].
Three types of methane hydrate structures have been identified in natural sediments. These structures have been identified as sI, sII and sH hydrate. All the three hydrate structure has a cage which can be identified as pentagonal dodecahedra of water molecules enclosing methane. In between the dodecahedra are other cage which differs in its shape and size thus can accommodate guest molecule of different size. The three important structures are explained as follows:
Structure I (sI): Structure I is cubic. A pentagonal dodecahedral (12- sided) cage is centred at the corners of the unit cell and a rotated dodecahedron is in the centre of the cell. Tetrakaidecahedron cage are formed by association of 14 rings consisting of two hexagonal rings sitting at the top and bottom of the cage, in turn connecting 12 pentagon rings through its 12 edges. Pure methane forms sI hydrate in which it occupies all the large cages. Other gases of similar size like CO2, SO2, H2S which are the constituents of natural gas also forms sI hydrate which is shown in Figure 2. The Structure I consist of 2 small cages made up by 12 pentagonal surfaces called 512 and 6 larger cages of again 12 pentagonal and two hexagonal surfaces called 512 62 [29].
The unit cell consists of water molecules=(12×512 +2×512 62 = 46) [30].
Structure II (sII): Structure II is also cubic, in addition to common pentagonal dodecahedra, a large cage (hexakaidecahedral) consisting of 12 pentagons rings and 4 hexagonal rings is also present. Large cage diameter in sII hydrate is slightly bigger than the large cage diameter of sI hydrate. Higher hydrocarbons like ethane, propane, butane (all these gases are constituents of natural gas) forms sII hydrates which is shown in Figure 3.
Structure II consist of 16 small cages made up by 12 pentagonal surfaces called “512” and 8 larger cages of again 12 pentagonal and four hexagonal surfaces called 51264.
The unit cell consists of water molecules = (16×512+8×51264 = 136) [30].
Structure H (sH): sH hydrate structure. It has hexagonal crystals containing three 512 cavities, two small 435663 cavities, one large 51268 cavity, and 34 water molecules per unit cell, and contains even larger molecules such as 2,2-dimethylbutane in the larger cavities only. Each 51268 barrel-shaped cavity is surrounded by six 435663 cavities around its central ring of six hexagons which is shown in Figure 4 [31,32].
The Structure H consist of 3 small cages made up by 12 pentagonal surfaces called “512” and 6 larger cages of again 12 pentagonal and 1 hexagonal surfaces called 512 68 .
The unit cell consists of water molecules = (3×512 +2×435663 +1×512 68=34) [1,33,34].
Classification of Gas Hydrate Accumulation
Natural occurring hydrate reservoir varies both in terms of geologic structure and thermodynamic conditions.
Gas hydrate deposits have been classified into four major classes:
Class-1: These are made up of two layers – the hydrate layer and underlying two phase fluid zone containing free gas and liquid water which is shown in Figure 5. Class I accumulations are referred as “Hydrate capped gas reservoirs” [31].
Class-2: This is made up of two zones i.e. hydrate layer overlying the zone of mobile water zone.
Class-3: Hydrate deposits comprise of a single zone of hydrate interval and characterized by the absence of an underlying zone of mobile fluids.
Class-4: Hydrate deposits are referred to oceanic accumulations and involve disperse, low saturation hydrate deposits (< 10%) which lack confining geologic strata [32].
They are classified into following categories as shown in Figure 5.
Energy Potential of Gas Hydrates
Among all renewable (solar, wind, wave, nuclear, hydro, geothermal, bioprocess etc) and non conventional (gas shale, coal bed methane, basin centred gas, tight gas shale, gas hydrates etc.) energy resources, gas hydrates are regarded as one of the most suitable candidate for cleaner energy resources in this century. Methane is the dominant component among other hydrocarbon gases in sediments [34,35].
As per the USA department of energy, if only 1% of the methane stored in the hydrates could be recovered, it would be more than double the current domestic supply of USA of natural gas [36,37]. It was mentioned that the energy potential of methane hydrates is greater than that of the other unconventional sources of gas, such as coal beds, tight Shales, deep aquifers and conventional natural gas [38].
The current techniques for exploiting methane from gas hydrates are discussed as per follows:
Current techniques of extraction of gas from natural gas hydrates
The gases in the natural gas hydrate sediment are primarily methane molecules [39]. Methane hydrates are considered a major potential source of hydrocarbon energy and could be important in meeting natural gas demand in the future [40]. Natural gas hydrates are a vast potential, though not presently commercial, source of additional natural gas.
There is a need for the development of new, lower-cost technologies and approaches are required for economic production of methane gas from offshore hydrates. The production of natural gas from oceanic and permafrost sediments is currently being developed using methods such as depressurization, thermal stimulation, and injection of hydrate inhibitors [41]. The difficulty in recovering this source of energy is that the fuel is in solid form and is not amenable to conventional gas and oil recovery techniques [42]. Different mechanisms have been proposed for economically developing gas hydrates as an unconventional gas source [43,44].
The natural gas from gas hydrate can be produced via depressurization, thermal stimulation, chemical inhibitor injection. Among these, depressurization and thermal stimulation have been considered to be the most economical, though other methods are under investigation. The type of method depends on the reservoir characteristics. Due to less energy input for depressurization, this method has been studied more than the thermal stimulation. However, the efficacy of latter method needs to be investigated in more detail.
The four techniques which are used for the extraction of methane from the natural gas hydrates are illustrated as per following:
Depressurization: It is an extraction technique involving drilling of hole into the layer of hydrate and reducing the pressure beneath the hydrate equilibrium incorporating an endothermic process (i.e. absorbing heat from the surroundings) resulting in to release of methane gas from the hydrate flowing up the pipe [45-47]. The dissociation prevails until equilibrium is maintained at the lower temperature. Depressurization is advantageous because it is a most economical method and low energy consumption. It is assumed that free gas can be released by drilling hole down the natural gas hydrate as shown in Figure 6.
This technique is implemented for hydrates only in polar regions beneath the permafrost where the free gas is found to be present under the hydrate stability zone. Production of gas using this technique was numerically simulated to analyse various methods to explore free gas from hydrate zone [41]. The gas production rate is function of well pressure and reservoir temperature [48]. This method is most suitable to those deposits where widespread gas occurs in a closure below the hydrate cap [49].
For the depressurization process, the model behaves as a closed system with no boundaries. Hydrate decomposition is proportional to depressurization rate which follows a first-order kinetic model.
Specific heat of gas hydrate dissociation is about 0.5 MJ/Kg which exceeds specific heat of ice melting. Due to increased heat exchange, resevoir zone temperature falls below to equilibrium temperature of hydrate formation which stop the dissociation or the temperature falls to ice formation temperature which may lead to sharp drop in reservoir rock permeability, So, the soluion lies in inventing a technology which can maintain a stable gas hydrate dissociation within the porous media i.e. providing continuous heat to the reservoir.
Thus far, the only method that has been successfully used to economically produce gas from gas hydrates is the depressurization method.
Inhibitor injection: It is a method to suppress the creation of hydrates. This method associates with the injection of a chemical inhibitor which tends to disturb the gas hydrate equilibrium condition beyond the thermodynamic stability of the gas hydrate zone. The chemical inhibitor like methanol reduces the dissociation temperature [50,51] which shifts the pressure- temperature equilibrium of the outer hydrate zone, so as to disturb the stability of hydrates and thus hydrates decompose to free gas and water. The common thermodynamic organic inhibitors are methanol, monoethylene glycol, diethylene glycol. Dissolved salts such as NaCl, CaCl2, KCl and NaBr can also act as inhibitors. Schematic representation of Inhibitor Injection is shown in Figure 7.
Chemical inhibition injection method demands admirable porosity and moreover cost of various chemicals makes it an expensive technique. This method is homologous to put salt on icy road. Mostly gas industries make use of this technology to prevent inhibition in pipelines. The use of chemical inhibitors in gas hydrates is a sound technique for the prevention of gas hydrate in engineering applications, their use in the creation of natural gas hydrate is hampered due to its environmental impact.
An earlier work [52] studied the behavior of gas production from methane hydrate in porous sediment by injecting ethylene glycol solutions of different concentrations and at different injection rates in a one-dimensional experimental apparatus. The production efficiency is affected by both the ethylene glycol concentration and the ethylene glycol injection rate, and it reaches a maximum for an ethylene glycol concentration of 60 wt% [52].
Thermal stimulation: It is an extraction technique which uses methods like steam injection, hot brine solution, fire flooding and cyclic steam injection etc. that raises the temperature of the local reservoir outside the hydrate region. The rise in temperature will cause the dissociation of the hydrate, thus releasing free gas which can be collected. The methane released can then be pumped to the surface of the seafloor through another drill hole [51]. Schematic representation of the Thermal Stimulation is shown in Figure 8.
Tang et al. [53] states that the efficiency of the method is directly proportional to the latent heat of the brine solution, the rate at which Figure 6: Depressurization technique. the solution is injected and initial hydrate saturation.
Global Scenario
The occurrence of gas hydrates has been known since the mid of 1960s when gas hydrate were discovered in Russia. These are present in oceanic sediments along continental margins as well as in polar continental settings [54]. Four regions of gas hydrates have been mapped on the continental rise in the offshore regions between New Jersey and Georgia. Significant quantities of these have been detected in many regions of the Arctic including Siberia, the Mackenzie River delta and the north slope of Alaska. Gas hydrates are scattered as shown in the figure above, they are present at south eastern coast of United States on the Blake Ridge, in the Gulf of Mexico, western and eastern margins of Japan, the Middle America Trench and in the Cascadian basin near Oregon, Peru. Figure 9 shows the global distribution of natural gas hydrates:-
Recently, Shyi-Min Lu [56] has explained the global effort put up by various countries such United States of America, Japan, Canada, China, India and Taiwan for the development of gas hydrates. It is mentioned in the review that Japan could be the first country to develop methane hydrates and can start commercial mass production in the eastern Japanese Nankai Trough prior to 2018 according to Japan’s Methane hydrate R&D programme MH-21. Taiwan is also planning Methane hydrate exploration with the co-operation of countries like Germany and U.S. and can go for commercial production as soon as 2026. As per the global consumption and proven resources of fossil energy sources in 2012 conventional gas and oil can be only used for about another 57 years [57]. So, in the light of above circumstances many countries are joining hand together in order to exploit this untapped resource of energy.
Conclusions
An enormous amount of methane is available in the gas hydrates. Even a small percentage of which could meet the energy requirements of the world for centuries. If exploited properly gas hydrates will be the next generation energy resource. Several pilot scale production test have been completed in permafrost region and one test in marine gas hydrate deposit to test the possible gas recovery technology from hydrate bearing reservoir was conducted. Long term production test are planned in USA and Japan to establish the viability of production technologies However, there is a strong need to prepare a suitable technology for exploiting this energy resource.
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