Abstract

In search of non-conventional energy sources in the earth, the discovery of large gas hydrate accumulations in terrestrial permafrost regions of the Arctic and beneath the sea along the outer continental margins of the world's oceans creates the interest of gas hydrates as a possible energy resource. The combined information from Arctic gas hydrate studies shows that, in permafrost regions, gas hydrates may exist at subsurface depths ranging from about 130 to 2000 m. The presence of gas hydrates in offshore continental margins has been inferred mainly from anomalous seismic reflectors, known as bottom-simulating reflectors, that have been mapped at depths below the sea floor ranging from about 100 to 1100 m. Current estimates of the amount of gas in the world's marine and permafrost gas hydrate accumulations are in rough accord at about 20,000 trillion m³. Disagreements over fundamental issues such as the volume of gas stored within delineated gas hydrate accumulations and the concentration of gas hydrates within hydrate-bearing strata have demonstrated that we know little about gas hydrates.

Introduction

Gas hydrates are crystalline substances composed of water and gas in which a solid water-lattice accommodates gas molecules in a cagelike structure, or clathrate. However, whether gas hydrates will be a factor in contributing to the world's energy requirements depends ultimately on the availability of producible gas hydrate resources and the cost to extract them. Considerable uncertainty and disagreement prevail concerning the world's gas hydrate resource potential. Gas hydrate as an energy commodity is commonly grouped with other unconventional hydrocarbon resources that are either expensive to extract or require specialized technology for extraction. Except for gas hydrates, most unconventional natural gas resources are being commercially produced somewhere in the world. In most cases, the evolution of a nonproducible unconventional gas resource to a producible energy resource has relied on significant capital investment and technology development. To evaluate the energy resource potential of gas hydrates also will require extensive research and development programs. Today, most of the gas hydrate research community is focused on three fundamental issues: Where do gas hydrates occur, how do gas hydrates occur in nature, and why do gas hydrates occur in a particular setting? Relatively little has been done to integrate these distinct research topics or to evaluate how they collectively affect the ultimate resource potential of gas hydrates. Only after understanding the fundamental aspects of where-how-why gas hydrates occur in nature will be able to make accurate estimates of how much gas is trapped within the gas hydrate accumulations of the world.The technical and nontechnical factors controlling the ultimate resource potential of gas hydrates are identified and assessed. The fundamental questions of where do gas hydrates occur, how do gas hydrates occur in nature, and why do gas hydrates occur in a particular setting are individually reviewed and discussed. In addition, published gas hydrate volume assessments are summarized, and the production technology needed to extract the world's gas hydrate resources is assessed.

In recent years the topics of naturally occurring gas hydrates have attracted major interest worldwide due to the fact that they may play a dominant role as possible energy resources in the future. Prior to this natural gas hydrates were mainly viewed at as a source of operational problems in gas processing and transportation equipment. The historical background and development of gas hydrates and natural gas hydrates is reviewed as well as the necessary fundamental information about the structures of gas hydrates. One prerequisite of stable operation of gas processing plants as well as the allocation of gas deposit is the exact knowledge of the hydrate stability (equilibrium) data. Whereas a lot of data have already been measured and based on this reliable computation methods have been developed, there is still the necessity to measure equilibrium data to further improve the accuracy of the models. The organization of own measurements carried out with natural gases as well as key binary systems with water-electrolyte systems and also inhibitors are presented together with some representative results. Some review of the currently vast growing investigations on detection of natural gas hydrate locations, exploratory and first production efforts as well as some issues on possible hazards identified today in connection with naturally occurring gas hydrate deposits are highlighted.

GAS HYDRATE RESOURCE ASSESSMENT

As noted in the introduction, this article deals with the assessment of the geologic, engineering, economic, and political factors that control the ultimate resource potential of gas hydrates. This assessment is conducted mainly though the examination of several relatively well-characterized gas hydrate accumulations.

Where Do Gas Hydrates Occur?

Gas hydrates have been inferred to occur at about 50 areas throughout the world. However, only a limited number of gas hydrate accumulations have been examined in any detail. Described in the following sections are five of the best-known marine and on shore permafrost-associated gas hydrate accumulations in the world. They are located on the Blake Ridge along the southeastern continental margin of the United States, along the Cascadia continental margin off the Pacific coast of Canada, near the Nankai Trough off the eastern coast of Japan, on the North Slope of Alaska, and in the Mackenzie River delta of northern Canada. Discussions pertaining to the volume of gas within each of the gas hydrate accumulations described in the following sections are included in the energy resource assessment section of this article.

Methane Hydrate

What do you get when you combine water and swamp gas under low temperatures and high pressures? You get a frozen lattice like substance called methane hydrate, huge amounts of which underlie our oceans and polar permafrost. This crystalline combination of a natural gas and water (known technically as a clathrate) looks remarkably like ice but burns if it meets a lit match. Methane hydrate was discovered only a few decades ago, and little research has been done on it until recently. By some estimates, the energy locked up in methane hydrate deposits is more than twice the global reserves of all conventional gas, oil, and coal deposits combined. But no one has yet figured out how to pull out the gas inexpensively, and no one knows how much is actually recoverable. Because methane is also a greenhouse gas, release of even a small percentage of total deposits could have a serious effect on Earth's atmosphere. Research on methane hydrate has increased in the last few years, particularly in countries such as Japan that have few native energy resources. As scientists around the world learn more about this material, new concerns surface. For example, ocean-based oil-drilling operations sometimes encounter methane hydrate deposits. As a drill spins through the hydrate, the process can cause it to dissociate. The freed gas may explode, causing the drilling crew to lose control of the well.

Potential Hazards originating form natural gas hydrates

When investigating the possibility of producing natural gas from the offshore reservoirs one must also take into consideration possible hazards that can evolve from their recovery a well as from their mere existence and also natural dissociation. Natural occurring gas hydrates often are distributed very differently in the ocean sediments, as solid hydrate inclusions or finely dispersed in the sediments. In this case hydrates act as a cementing agent giving the necessary stability to the sediment itself. Small changes in local temperature profile can lead to dissociation and possibly to unstable conditions. There are hypotheses that huge slides of sub-sea sediments having occurred in the past are associated with the decomposition of hydrates also with possible impact on the world's climate (e.g., GEOMAR, Paull et al). Also it has been discussed that (uncontrolled) hydrate dissociation may be a potential hazard to the foundations of production platforms and pipelines [40] and drilling activities have reported instabilities of hydrates. A detailed analysis about drilling and production hazards caused by gas hydrates has been presented recently by Collett and Dallimore  showing that gas hydrates can and have create severe problems in oil and gas drilling such as well bore casing damage, uncontrolled gas leakage outside the bore casing due to dissociation, uncontrolled gas flow during drilling .They stress the utmost importance of understanding the geological formation and occurrence of natural gas hydrates and the need for detailed information of gas hydrate distribution, reservoir temperatures and pressures as well porosities and permeabilities to ensure safe handling. So there is still a long way to go until natural gas hydrate deposits will be developed as the same safe production fields as conventional natural gas fields are today. The recently established and very much tied up international efforts in research consortiums and technological developments represent a promising development in this direction.

Conclusion

Gas hydrates are the potential source for alternative fuel for world. To explore the gas hydrate as reliable source of alternate energy extensive research and developments are required. Affordability of the simple technique, reliable use and compatible cost are the main limitations for gas hydrates. All these limitations are subject to proper research and awareness in this area.

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